Mitigation of NOx with Pyrolysate Fragments of Solid Fuels ...

Mitigation of NOx with Pyrolysate Fragments of

Solid Fuels and Their Surrogates

Ibukun OLUWOYE, BSc (Hons), MSc

This thesis is presented for the degree of

Doctor of Philosophy

School of Engineering and Information Technology,

Murdoch University, Western Australia

2017

ii

Declaration

I declare that this thesis is my own account of my research and contains as its main content

work which has not previously been submitted for a degree at any tertiary education institution.

....................................

Ibukun OLUWOYE

February, 2017

iii

Supervisory Statement

We, the undersigned, attest that Higher Research Degree candidate, Ibukun OLUWOYE, has

devised and synthesised the experimental program, conducted experiments, analysed data,

performed computational quantum-mechanical calculations and has written all papers included

in this thesis. Professor Bogdan Z. DLUGOGORSKI and Dr Mohammednoor

ALTARAWNEH provided the necessary advice on the experimental program, project

direction and assisted with the editing of the papers, consistent with normal supervisors-

candidate relations.

....................................

Professor Bogdan Z. DLUGOGORSKI

February, 2017

....................................

Dr Mohammednoor ALTARAWNEH

February, 2017

iv

Dedications

To my families;

Both biologicals and within the congregation of Christ.

v

Acknowledgments

I appreciate GOD for the supreme assistance and strength – Dei sub numine viget.

The research work presented in this thesis reflects the help and support of many kind people

around me, only to some of whom it is possible to give noteworthy salutations here.

I would like to thank my supervisors Professor Bogdan Z Dlugogorski and Dr Mohammednoor

Altarawneh for their unwavering and exceptional support. Their understanding, mentorship,

encouragement, dedication and magnanimity have provided a good fundamental driving force

for the present study. Thank you again, your quick correspondences, corrections and gestures

achieved the intended.

I am grateful to Murdoch University for the award of a postgraduate research scholarship which

provided a valuable financial support for this research. This study has also been funded by the

Australian Research Council (ARC) and Dyno Nobel Asia Pacific, with grants of computing

time from the National Computational Infrastructure (NCI) and the Pawsey Supercomputing

Centre in Perth, Australia.

Special thanks to Dr Jeff Gore of Dyno Nobel Asia Pacific Pty Limited, as well as Murdoch

University’s Mr Kris Parker, Mr Andrew Foreman, Mr Kenneth Seymour, Mr Stewart Kelly,

Mr John Boulton, Dr Marc Hampton and Mr Iafeta Laava for their professional and technical

assistance.

vi

I acknowledge my gratitude to my fellow student colleagues and staffs of the Fire Safety and

Combustion Kinetics Research Laboratory: Zhe Zeng, Anam Saeed, Niveen Assaf, Ehsan

Mohammadpour, Nassim Zeinali, Kamal Siddique, Arif Abdullah, Jomana Al-Nu’airat, Alaa

Kamaluldeen, Sidra Jabeen, Sana Zahid, Pablo Fernández-Castro (University of Cantabria,

Spain), Dr Juita and Dr Jakub Skut. Courtesy of you all, I experienced an exciting and fun-

filled years of research and experiments.

I am highly privileged to receive the School of Engineering and Information Technology’s

Graduate Poster Prize (2014), as well as the Western Australian Joint Chemical Engineering

Committee (JCEC), i.e., the Institution of Chemical Engineers (IChemE) and the Engineers

Australia’s (EA) selected finalist Postgraduate Research Excellence Award (2016).

Ultimately, I value the benevolent support of my family members – omnia vincit amor.

vii

Abstract

This thesis presents a series of scientific studies exploring the thermal mitigation of nitrogen

oxides (NOx) with solid fuel (waste polyethylene) and biomass surrogate (morpholine). These

studies have revealed new mechanistic insights, have described the associated set of reactions,

and have developed a sustainable means of reducing industrially formed NOx, for example, in

combustion processes and during blasting of ammonium nitrate explosive.

Comprehensive quantum-mechanical calculations afforded the investigation of low-

temperature oxidation of polyethylene (PE) film under representative NOx environments.

Investigations of thermal reduction of NOx with recycled PE involved sample characterisation

techniques, such as, carbon-hydrogen-nitrogen-sulfur (CHNS) elemental analyses, ion

chromatography (IC) and inductively coupled plasma optical emission spectroscopy (ICP-

OES), as well as thermogravimetric experiments under practical NOx atmospheres. Innovative

application of the on-line Fourier transform infrared (FTIR) spectroscopy and NOx

chemiluminescence quantitated the reaction product species and established the removal

efficiency of NOx by fragments of pyrolysing PE.

Studies on modelled surrogates, i.e., morpholine, resolved the fundamentals of thermal

interaction of biomass with NOx. The experiment applied a flow-through tubular reactor

coupled with FTIR spectroscopy and chemiluminescence NOx analysis, at constant pressure,

reaction time, and representative fuel-rich condition. Furthermore, density-functional-theory

(DFT) calculations uncovered new mechanistic insights into initial nitration reactions, and

yielded their energetic characteristics, such as exothermic formation of nitro- and nitroso-

adducts.

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Results obtained in this study elucidated, for the very first time, initial products of low-

temperature reaction of NOx with polymers and biomass, and determined the rate parameters,

from the isoconversional degradation analysis, that are of great practical importance. A typical

NOx removal efficiency (by fragments of pyrolysing PE) amounts to 80 % at moderate

temperatures. Investigations employing a biomass surrogate revealed the sensitising role of

NOx in combustion of biomass fuels. NOx reduction by biomass occurs only at temperatures

in excess of 800 °C. The thesis also provides kinetic expression for process modelling. The

results of the present work prove that, carbon-hydrogen-type waste polymers and biomass

provide an effective means of mitigation of NOx emission in combustion processes.

Furthermore, the scientific approach can be optimised to deploy solid fuels in bulk ammonium

nitrate/fuel oil (ANFO)/emulsions explosive mixtures, so as to reduce the NOx formation

during detonation of nitrate-based explosives in mining operations.

ix

Table of Contents

Thesis Declaration ii

Supervisory Statement iii

Dedications iv

Acknowledgements v

Abstract vii

Table of Contents ix

Contribution towards Publication xiv

Glossary of Abbreviations and Technical Terms xvii

List of Figures and Tables xviii

1. Introduction 1

1.1. General Overture 2

1.2. Research Motivation 3

1.3. Objectives 5

1.4. Thesis Structure 7

1.5. References 9

2. Paper I: Anthropogenic Formation of NOx from Mining-Grade

Explosives and Its Abatement

12

x

2.0. Abstract 13

2.1. Introduction 14

2.2. Environmental Impact of Mining 18

2.3. Overview of Global NOx Formation 20

2.3.1. Anthropogenic NOx emissions 21

2.3.2. NOx in the environment and mining workplaces: processes and

health effects

24

2.4. Mechanism of NOx Formation During the Use of AN Explosives 26

2.4.1. Stoichiometrically unbalanced blasting 26

2.4.2. Deflagration of AN prills 31

2.4.3. Chemical gassing of emulsion blends 35

2.5. Effective control measures and abatement techniques 38

2.5.1. Operation guidelines for controlling NOx emission in blasting

operations

38

2.5.2. Abatement techniques 39

2.6. Perspectives and Future Outlooks 45

2.7. References 46

3. Methodology 63

3.1. Computational Techniques 64

3.2. Experimental Configurations 66

xi

3.3. Data Analyses 72

3.4. References 73

4. Paper II: Oxidation of Crystalline Polyethylene 75

4.0. Abstract 76


4.2. Methodology 79

4.2.1. Computational details 80

4.2.2. Molecular and crystallographic structural parameters of

polyethylene

80

4.2.3. Reaction modelling 83

4.3 Results and Discussion 85

4.3.1. Initiation reactions 85

4.3.2. Formation of hydroperoxide group 89

4.3.3. Hydroperoxide decomposition and other reactions 92

4.3.4 Kinetic considerations 96

4.4 Conclusions 100

4.5 References 101

5. Paper III: Oxidation of Polyethylene under Corrosive NOx

Atmosphere

109

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5.0. Abstract 110


5.2. Methodology 113

5.3 Results and Discussion 116

5.3.1. Initiation reactions and formation of radical species 116

5.3.2. Addition reactions and fate of consequential N-derivative species 118

5.3.3. Formation of organic nitrites and nitrates 121

5.3.4. Thermodynamics and reaction kinetics 124

5.4 Conclusions 130

5.5 References 131

6. Paper IV: Thermal Reduction of NOx with Recycled Polyethylene 138

6.0. Abstract 139


6.2. Experimental Section 142

6.2.1. Materials and sample characterisation 142

6.2.2. Experimental methods 145

6.3. Results and Discussion 147

6.3.1. Thermal decomposition of PE in NOx atmosphere and evolved

gas analysis

147

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6.3.2. Degradation kinetics 154

6.4. Conclusions 158

6.5. References 158

7. Paper V: Enhanced Ignition of Biomass in Presence of NOx 166

7.0. Abstract 167


7.2. Applied Methodologies 170

7.2.1. Materials and experimental setup 170

7.2.2. Computational methods 172


7.3.1. Effect of NOx on ignition temperature of morpholine 174

7.3.2. Initial steps in nitration of morpholine 177


7.5. References 182

8. Paper VI: Experimental Study on Thermal Reaction of Modelled

Biofuel (Morpholine) with NOx

191

8.0. Abstract 192


8.2. Applied Methodologies 194

xiv

8.2.1. Materials and experimental apparatus 194

8.2.2. Product sampling and analyses 196


8.3.1. Overview of ignition temperatures 198

8.3.2. Quantitative analysis of N-conversion at high temperatures 200

8.3.3. Comments on NOx reduction and formation of N2 203


8.5. References 206

9. Conclusion and Recommendations 212

9.1. Conclusion 213

9.2. Recommendations 217

10. Supplementary Document 221

Appendix I Supporting Information for Chapter 6 222

Appendix II Supporting Information for Chapters 7 and 8 247

xv

Contribution towards Publication

The work embodied in this thesis contains published joint-authored papers as listed below. I

am the principal contributor in the publications. I have included as part of the thesis a written

statement, endorsed by my supervisors, attesting to my significant contribution to the joint

publications.

Paper I: Atmospheric Emission of NOx from Mining Explosives: A Critical Review

Oluwoye, I.; Dlugogorski, B. Z.; Gore, J.; Oskierski, H. C.; Altarawneh, M.

Atmospheric Environment (submitted)

2015 impact factor: 3.459

Paper II: Oxidation of Crystalline Polyethylene

Oluwoye, I.; Altarawneh, M.; Gore, J.; Dlugogorski, B. Z.

Combustion and Flame 2015, 162, 3681-3690

DOI:10.1016/j.combustflame.2015.07.007


Paper III: Oxidation of Polyethylene under Corrosive NOx Atmosphere

Oluwoye, I.; Altarawneh, M.; Gore, J.; Bockhorn, H.; Dlugogorski, B. Z.

The Journal of Physical Chemistry C 2016, 120, 3766-3775

DOI: 10.1021/acs.jpcc.5b10466


Paper IV: Thermal Reduction of NOx with Recycled Polyethylene

Oluwoye, I.; Dlugogorski, B. Z.; Gore, J.; Vyazovkin, S.; Boyron, O.;

Altarawneh, M.

xvi

Environmental Science and Technology 2017

DOI: 10.1021/acs.est.6b05560


Paper V Enhanced Ignition of Biomass in Presence of NOx

Oluwoye, I.; Altarawneh, M.; Gore, J.; Dlugogorski, B. Z.

12th International Symposium on Fire Safety Science, Lund, Sweden.

published as a Special Issue in the Fire Safety Journal 2017

DOI: 10.1016/j.firesaf.2017.03.042


Paper VI: Experimental Study on Thermal Reaction of Modelled Biofuel

(Morpholine) with NOx

Oluwoye, I.; Dlugogorski, B. Z.; Gore, J.; Altarawneh, M.

(In preparation)

The list below itemises the publications to which I have contributed during the course of the

PhD study, but which fall outside the scope of this thesis.

i: Formation of Environmentally Persistent Free Radicals on α-Al2O3

Assaf, N. W., Altarawneh, M., Oluwoye, I., Radny, M., Lomnicki, S. M.,

Dlugogorski, B. Z.

Environmental Science and Technology 2016, 50 (20), 11094–11102

DOI:10.1021/acs.est.6b02601


xvii

ii: Phenol Dissociation on Pristine and Defective Graphene

Widjaja, H., Oluwoye, I., Altarawneh, M., Hamra, A. A. B., Lim, H. N.,

Huang, N. M., .Yin C. Y., Jiang, Z. T.

Surface Science 2017, 657, 10-14

DOI: 10.1016/j.susc.2016.10.010


iii: Inhibition and Promotion of Pyrolysis by Hydrogen Sulfide (H2S) and

Sulfanyl Radical (SH)

Zeng, Z., Altarawneh, M., Oluwoye, I., Glarborg, P., Dlugogorski, B. Z.

The Journal of Physical Chemistry A 2016, 120 (45), 8941-8948

DOI: 10.1021/acs.jpca.6b09357


iv Formation of PCDDs and PCDFs in the torrefaction of biomass with

different chemical composition

Gao, Q., Edo, M., Larsson, S. H., Collina, E., Rudolfsson, M., Gallina, M.,

Oluwoye, I.; Altarawneh, M., Dlugogorski, B. Z., Jansson, S.

Journal of Analytical and Applied Pyrolysis 2017, 123, 126-133

DOI: 10.1016/j.jaap.2016.12.015


xviii

Glossary of Abbreviations and Technical Terms

AEISG Australian Explosive Industry and Safety Group

AMD Acidic mine discharge

AN Ammonium nitrate

ANFO Ammonium nitrate/fuel oil

AP ammonium perchlorate

ADN ammonium dinitramide

AOP Acidity–oxidation potential measurements

ATR Attenuated total reflectance

BDH Bond dissociation enthalpy

BEZ Blast exclusion zones

CHNS Carbon-hydrogen-nitrogen-sulfur

DFT Density functional theory

DNP Numerical plus polarisation

DSC Differential scanning calorimetry

EPR Electron paramagnetic resonance

FMZ Fume management zones

FTIR Fourier transform infrared

GC-MS Gas chromatography mass spectrometry

GGA Generalised gradient approximation

HDPE High density polyethylene

HEDM High-energy-density material

HT-SEC High temperature size exclusion chromatography

xix

IC Ion chromatography

ICP-OES Inductively coupled plasma emission spectroscopy

IPCC Intergovernmental Panel on Climate Change

IR Infrared

LDPE Low density polyethylene

LST/QST Linear synchronous and quadratic synchronous transit

MMD Molecular mass distributions

NOx Oxides of nitrogen

Nr Reactive nitrogen

OB Oxygen balance

OECD Organization for Economic Cooperation and Development

PAC Photoacoustic calorimetry

PAN Peroxyacyl nitrates

PBE Perdew-Burke-Ernzerhof

PE Polyethylene

PM Particulate matters

PES Potential energy surfaces

PPM Parts per million

VOD Velocity of detonation

SOx Oxides of sulfur

STP Standard temperature and pressure

ST Shock tube

TGA Thermogravimetry apparatus

TMWL Temperature of maximum rate of weight loss

TST Transition state theory

1

CHAPTER 1

Introduction

This chapter sets the stage for the entire thesis, comprising

detailed expression of research motivation, purpose, as

well as concise outline of the dissertation

…

CHAPTER 1 — Introduction

2

1.1. General Overture

The recent expository report by the Intergovernmental Panel on Climate Change (IPCC)1

presented the global inventory of nitrogen oxides (NOx) in the environment. The report has

established that, the anthropogenic emission of NOx adds substantially to overall discharge of

reactive nitrogen species into the atmosphere. The term NOx refers to nitric oxide (NO),

nitrogen dioxide (NO2) and other higher oxides based on the various possible oxidation states

of nitrogen. Apart from its limited industrial and medical applications, NOx contributes to acid

rain, eco-eutrophication, photochemical smog, ground level ozone, greenhouse effect,

stratospheric ozone depletion, and health deterioration (e.g., chronic respiratory and obstructive

pulmonary dysfunction) of predisposed organisms.2-6

As industrial mitigation of NOx remains a prominent issue, most of the applied methods 3, 7-11

are designed solely for combustion systems. However, other emission sources, e.g. blasting of

nitrate explosive in mining activities, require adequate and reliable NOx reduction technology.

Practical technological limitations prevent the direct application of the post-combustion NOx

mitigation techniques to blasting. These limitations comprise: (a) the fast dispersion of

atmospheric air mixture, i.e., mixing of post-blast fume with air prevents its effective capturing,

scrubbing, treatment and/or reprocessing; (b) nitrate blasting agents exhibit incompatibilities

with materials (e.g., metal nitrides)12 that may serve, in the form of additives, to scavenge the

generated NOx.

This thesis presents a series of scientific studies exploring both fundamental and practical

aspects of the thermal mitigation of NOx with solid hydrocarbon fuel (waste polyethylene) and

biomass surrogate (morpholine). It aims at developing new sustainable technologies for


3

reducing industrially formed NOx, based on biomass and recycled plastics, especially for

applications relevant to blasting of mining-grade ammonium nitrate (AN) based high-energy

materials.

1.2. Research Motivation

Figure 1 presents the United Nations’ statistics on NOx emission per capita of different

countries across the globe.13 Accordingly, and in consistent with the Organization for

Economic Cooperation and Development (OECD) report,14 some countries (e.g. Australia)

display high emission (127 kg for the year 2007 and 75 kg for the year 2012) of NOx per capita.

This could be rationalised as a result of lack of mitigation of NOx emission in power plants,

some industrial processes, combustion activities involving untreated fossil, bushfires and

mining operations, especially large open cut mines that require significant movement of

materials (including overburden) by diesel-driven excavators and trucks.15-17


4

Figure 1.1. Global NOx emission per capita. The main source for NOx is burning of fuels,

particularly petroleum products. In some countries agriculture and burning of savannas is

also an important contributor.13 Anomalies associated with some countries such as Gabon,

Niue, Paraguay, St. Vincent and the Grenadines, and Zambia are explicitly discussed in the

cited reference.

Attention of environmental agencies and concerned citizens usually focuses on anthropogenic

combustion of fossil fuel and/or biomass as the primary source of human-related NOx emission.

Yet, according to some scientific studies available in open literature18-20 and in daily press,21,

22 mining operations contribute significantly to the local emission of NOx. The AN explosive

mixtures used in blasting processes may produce NOx because of imbalanced stoichiometry

and intricacies of geological conditions. For instance, in open-cast mining, measurements

reveal that the post-blast plume can comprise up to 500 parts per million (ppm) of NOx, –

primarily nitric oxide (NO) and nitrogen dioxide (NO2) – with the latter embodying the plumes

with their orange hue.18-20


5

The current World Health Organisation air quality guidelines for NO2 amount to a one hour

level of 200 µg/m3 (1.88 µg/m3 of NO2 = 10-3 ppm), and an annual average of 40 µg/m3.23 The

typical high concentrations of NOx in post-blast clouds, measuring between 5.6 to 580 ppm,

exceed the safe limits by around 30 to 3000 times. These high emission values can lead to

health deterioration of predisposed organisms.20 Consequently, emissions of NOx in mining

operations have come under intense scrutiny by chief inspectors of mines and explosives or

other health and safety authorities in the Australian jurisdictions, for example, the Chief

Inspector of Coal Mines and the Chief Inspector of Explosives in the State of Queensland.24

The clouds of NOx from the detonation sites do not dissipate rapidly, and may drift outside

mine boundaries into populated areas, in spite of stringent mining guidelines, causing protests

of local residents.

Thus, the prime motivation behind the present study was to fill the existing gaps in knowledge

and to find practical solutions to tackle the problems of NOx emissions from blasting of

ammonium nitrate (AN) based explosives by developing new abatement technologies, gaining

detailed knowledge of the fundamental mechanisms underpinning these technologies, and

estimating the contribution of AN explosives to the total anthropogenic NOx formation.

1.3. Objectives

The purpose of this research lies in its environmental and industrial implications. It aims to

investigate the thermal reactions relevant to mitigation of NOx with waste plastics and biomass

surrogate. To achieves this, the thesis targets fulfilling the following objectives:


6

i. Exploration of anthropogenic NOx formation: (a) To review the global NOx emission

channels, in light of consequential environmental impacts and mitigation methods. (b)

To introduce mining and infrastructure grade explosives, and overview of

environmental impact of mining. (c) To evaluate the pollutant loads from blasting

operations, to quantitate the NOx formation during detonation of ammonium nitrate

blasting agents with valid explanation of different underlying mechanisms, as well as

to assess their contributions with respect to the global nitrogen budget. (d) To present

various abatement technologies, prevailing challenges, and outlooks on future

developments regarding pollution-free blasting activities.

ii. Development of sustainable solid organic fuel for reduction of NOx emission: (a) To

characterise waste polymer pellets towards their applicability for the intended purpose.

(b) To measure the thermal decomposition pattern of neat and recycled polymer

samples under controlled and representative NOx environments. (c) To develop a

mechanistic insight explaining the formation of initial product, as well as to construct

the necessary isoconversional kinetic parameter governing the thermal decomposition

events. (d) To quantitate the NOx reduction efficiency of pyrolysing fragments of waste

polymer, and hence provide optimum operating conditions in combustion systems.

iii. Investigation of thermal interaction of biomass with NOx: (a) To gauge the sensitising

effect of NOx on biomass ignition/combustion. (b) To quantify the species produced

during pyrolysis and combustion of modelled biofuel in NOx presence. (c) To establish

the N-selectivity and NOx reduction profile as function of temperature. In order to

avoid the complex nature of biomass residues, to perform experiments using

representative model compounds, such as morpholine.


7

In agreement with principles of green chemistry and engineering,25, 26 thermal reduction of NOx

by recycled polyethylene assigns value to minimisation of solid wastes by providing an

ecofriendly exit route for excess plastic wastes in our society. Moreover, its application to NOx

formation in combustion plants will promote the growth of innocuous degradation products,

thereby enhancing low-emission technologies and effective energy recovery from polymeric

waste materials. Furthermore, the scientific approach can be optimised to deploy waste

polyethylene in bulk ammonium nitrate/fuel oil (ANFO)/emulsions explosive mixtures, so as

to mitigate the NOx formation during detonation of nitrate-based explosives, for example, in

mining operations. In view of the aforementioned tasks, the following section outlines the

specific outcomes of the thesis.

1.4. Thesis Structure

This dissertation represents a coordinated study assessing the thermal mitigation of NOx with

solid fuels and their surrogates. The following chapter presents a comprehensive literature

review designed to provide a balanced assessment of the contribution of mining-grade AN

explosives to anthropogenic NOx formation, to explain the mechanism of emission of NOx in

sensitisation and blasting of AN explosives, to describe the existing abatement technologies as

compared to those developed in this thesis, and to point out the gaps in knowledge.

Chapter 3 illustrates the main experimental and computational techniques employed within the

scope of this study.


8

Chapters 4 and 5 present systematic investigations of low-temperature interaction of

polyethylene with NOx, discovering pathways leading to the formation of initial nitro and

nitroso products. These pathways govern the mitigation of NOx by fragments of pyrolysed

polymers.

Chapter 6 develops technology for mitigation of NOx formed in thermal processes using waste

polyethylene. This chapter has two objectives: (a) to appraise the NOx reduction efficiency of

fragments of pyrolysing PE; and, (b) to construct a kinetic mechanism that operate during the

removal of NOx from the post-detonation gases.

Chapter 7 analyses the initial nitration steps and establishes the reasons for enhanced ignition

of biomass in presence of NOx. This enhanced ignition represents an unintended consequence

for process safety of using biomass for mitigation of NOx. This can be circumvented by proper

(and separate) venting of NOx exhausts in organic dust-laden areas.

Chapter 8 focuses on high-temperature reaction of biomass surrogate (morpholine) with NOx,

elucidating the N-conversion profiles, and NOx-reduction efficiency as compared to

conventional hydrocarbon fuels.

Chapter 9 draws the concluding remark of this thesis, and makes suggestions and

recommendations for future investigations.

Finally, Chapter 10 provides all necessary supplementary material.


9

1.5. References

1. Stocker, T. F.; Qin, D.; Plattner, G.-K.; Tignor, M.; Allen, S. K.; Boschung, J.; Nauels,

A.; Xia, Y.; Bex, V.; Midgley, P. M. Climate Change 2013: The Physical Science Basis;

International Panel on Climate Change (IPCC): Cambridge University Press Cambridge, UK,

and New York, 2014.

2. Seinfeld, J. H.; Pandis, S. N., Atmospheric Chemistry and Physics: from Air Pollution

to Climate Change. John Wiley & Sons: 2006.

3. Baukal, C. E.; Schwartz, R.; John Zink Company., The John Zink Combustion

Handbook. CRC Press: Boca Raton, Fla; London, 2001.

4. Galloway, J. N.; Aber, J. D.; Erisman, J. W.; Seitzinger, S. P.; Howarth, R. W.;

Cowling, E. B.; Cosby, B. J., The nitrogen cascade. Bioscience 2003, 53, (4), 341-356.

5. Glarborg, P.; Jensen, A. D.; Johnsson, J. E., Fuel nitrogen conversion in solid fuel fired

systems. Prog. Energy Combust. Sci. 2003, 29, (2), 89-113.

6. Williams, A.; Jones, J. M.; Ma, L.; Pourkashanian, M., Pollutants from the combustion

of solid biomass fuels. Prog. Energy Combust. Sci. 2012, 38, (2), 113-137.

7. Glarborg, P., Fuel nitrogen conversion in solid fuel fired systems. Prog. Energy

Combust. Sci. 2003, 29, (2), 89-113.

8. Boardman, R.; Smoot, L., Pollutant formation and control. Coal Sci. Technol 1993, 20,

433-509.

9. EPA, Nitrogen Oxide (NOx), Why and How They are Controlled (EPA 456/F-99-006R).

Clean Air Technology Center, United States Environmental Protection Agency: North

Carolina, 1999.

10. Hill, S. C.; Smoot, L. D., Modeling of nitrogen oxides formation and destruction in

combustion systems. Prog. Energy Combust. Sci. 2000, 26, (4-6), 417-458.


10

11. Smoot, L. D.; Hill, S. C.; Xu, H., NOx control through reburning. Prog. Energy

Combust. Sci. 1998, 24, (5), 385-408.

12. Bretherick, L.; Urben, P.; Pitt, M. J.; Battle, L., Bretherick’s Handbook of Reactive

Chemical Hazards, Vol. 1. Butterworth-Heinemann: 1999.

13. UN, NOx Emissions Per Capital. United Nations Statistic Division.

http://unstats.un.org/unsd/environment/air_nox_emissions.htm (January 10, 2017).

14. OECD, Sulphur oxides (SOx) and nitrogen oxides (NOx) emissions. In Environment at

a Glance 2015: OECD Indicators, Organization for Economic Cooperation and Development

Publishing: Paris, 2015.

15. DEE, National Pollutant Inventory, Emission Estimation Technique Manual for

Explosives Detonation and Firing Ranges. 3.1 ed.; Department of Environment and Energy,

Australia: 2016.

16. DEE, 2014/2015 data within Australia - Oxides of Nitrogen from All Sources.

http://www.npi.gov.au/npidata/action/load/emission-by-source-

result/criteria/substance/69/destination/ALL/source-type/ALL/substance-

name/Oxides%2Bof%2BNitrogen/subthreshold-data/Yes/year/2015 (January 10, 2017).

17. Weng, Z.; Mudd, G. M.; Martin, T.; Boyle, C. A., Pollutant loads from coal mining in

Australia: Discerning trends from the National Pollutant Inventory (NPI). Environ. Sci. Policy

2012, 19–20, (0), 78-89.

18. Attalla, M.; Day, S. J.; Lange, T.; Lilley, W.; Morgan, S., NOx Emissions from Blasting

in Open Cut Coal Mining in the Hunter Valley. CSIRO Energy Technology: 2007.

19. Attalla, M. I.; Day, S. J.; Lange, T.; Lilley, W.; Morgan, S., NOx emissions from

blasting operations in open-cut coal mining. Atmos. Environ. 2008, 42, (34), 7874-7883.

http://unstats.un.org/unsd/environment/air_nox_emissions.htm

http://www.npi.gov.au/npidata/action/load/emission-by-source-result/criteria/substance/69/destination/ALL/source-type/ALL/substance-name/Oxides%2Bof%2BNitrogen/subthreshold-data/Yes/year/2015




11

20. McCray, R. B. Utilization of a Small Unmanned Aircraft System for Direct Sampling

of Nitrogen Oxides Produced by Full-Scale Surface Mine Blasting. University of Kentucky,

2016.

21. Kelly, M., Toxic cocktail in fumes and dust. Newcastle Herald Newspaper April 13,

2013, pp 4-5.

22. Kelly, M., Effects of blasts worry residents. Newcastle Herald Newspaper May 13,

2013, pp 4-5.

23. WHO, Air Quality Guidelines: Global Update 2005: Particulate Matter, Ozone,

Nitrogen dioxide, and Sulfur dioxide. World Health Organization: 2006.

24. Queensland Guidance Note 20; Management of Oxides of Nitrogen in Open Cut

Blasting In DEEDI, Ed. The State of Queensland: Qeensland, 2011.

25. Anastas, P.; Eghbali, N., Green chemistry: principles and practice. Chem. Soc. Rev.

2010, 39, (1), 301-312.

26. Anastas, P. T.; Zimmerman, J. B., Peer reviewed: design through the 12 principles of

green engineering. Environ. Sci. Technol. 2003, 37, (5), 94A-101A.

12

CHAPTER 2

Paper I: Atmospheric Emission of NOx from Mining

Explosives: A Critical Review

Oluwoye, I.; Dlugogorski, B. Z.; Gore, J.; Oskierski H.C.; Altarawneh, M., Atmospheric

Emission of NOx from Mining Explosives: A Critical Review: Atmos. Environ.: Submitted (in

modified form) for publication 2017.

Overview of NOx forming mechanisms during the use of ammonium nitrate

explosives in mining operations

Transformation of nitrogen in NH4NO3

Chemical gassing of AN emulsion

Detonation of ANFO & emulsion

Decomposition of AN

Effect of sensitising and

nucleophilic agents

Positive oxygen balance as a result

of presence of moisture, intricacies

in mineral deposition and changes in

reaction stoichiometry

Post-blast cloud containing hazardous NOx species

The chapter presents a comprehensive focused review of one of the major

influences of mining activities on atmospheric chemistry. It provides a balanced

explanation on the contribution of AN explosives to anthropogenic NOx formation,

describing in detail the underlying mechanisms and abatement technologies as

compared to those developed in this thesis

…

CHAPTER 2 — Atmospheric Emission of NOx from Mining Explosives: A Critical Review

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2.0. Abstract

High-energy materials such as emulsions, slurries and ammonium-nitrate fuel-oil (ANFO)

explosives play crucial roles in mining, quarrying, tunnelling and many other infrastructure

activities, because of their excellent transport and blasting properties. However, these

explosives engender environmental concerns, due to atmospheric pollution caused by emission

of nitrogen oxides (NOx) from blasts that amounts up to 5 kg/(t AN explosive), on average.

This first-of-its-kind review provides a concise literature account of the formation of NOx

during blasting of AN-based explosives, employed in surface operations. We estimate the total

NOx emission from AN-based explosives as 0.05 Tg (i.e., 5×104 t) N per annum, which

compares to the total global anthropogenic NOx emissions of 41.3×106 t N/y. Although the

global budget is only on the scale of one per mille on a global scale, these emissions involve

concentrated discharges characterised by NOx concentration in the order of 500 ppm which can

have workplace and local effects. The review describes different types of AN energetic

materials for civilian applications, and summarises the essential properties and terminologies

pertaining to their use. Furthermore, we recapitulate the mechanisms that lead to the formation

of the reactive nitrogen species, and compare them with those experienced in other thermal and

combustion operations. We also review the mitigation approaches, including guidelines and

operational-control measures that minimise or manage the risk of post-blast NOx fumes during

mining operations. The review discusses the abatement technologies such as formulation of

new explosive mixtures, comprising secondary fuels, spin traps and other additives, in light of

their effectiveness and efficiency. We conclude the review with a summary of unresolved

problems, identifying possible future developments and their impacts on the environment with

emphasis on local and workplace loads.


14

2.1. Introduction

High-energy-density materials (HEDMs) have revolutionised the world’s mining,

infrastructure and aviation industries. These materials generally consist of explosives,

propellants and pyrotechnics. They generate high temperature and pressure via complex

combustion phenomena involving physicochemical phase changes, complemented by rapid

exothermic reactions.1-3 Technically, the term “explosion” defines a sudden release of energy

confined in (but not limited to) HEDMs, accompanied by dramatic discharge of expanding

gases4 that enables impacting work done on the surroundings.

HEDMs fall into two broad categories, namely, the low and the high explosives. The salient

features entrenched in this classification include the reaction velocities and pressures achieved

during the combustion processes.1, 5 Low explosives, otherwise known as fast-burning or

deflagrating substances, burn in a regular but less shattering manner, while exhibiting a

relatively low reaction velocities between 0.01 – 400 m/s, and bar-range pressures. Typical

examples of low explosives comprise black powders, smokeless powders, propellants and

pyrotechnic mixtures. On the contrary, high explosives demonstrate superior rates of reaction,

i.e., velocity of detonation (VOD) ranging from 1000 – 9000 m/s, as well as higher pressure of

explosion. High explosives generate supersonic shock waves and perform relatively higher

work on their surroundings. Sensitivity of high explosives to detonation serves as a convenient

criterion to subdivide them into three main classes:2, 6, 7

(i) Primary high explosives (primers) represent extremely sensitive materials that easily

explode with the application of shock, spark, fire, friction, impact and heat. They

remain dangerous to handle and find use in comparatively small quantities in primers,


15

detonators and percussion caps. Examples of primary high explosives include mercury

fulminate, lead azide, lead styphnate, silver azide, tetrazene, and diazodinitrophenol.

(ii) Secondary high explosives (base explosives) display relative insensitivity to

mechanical shock, friction or flame, but shock waves of primary explosives can set

them off, i.e., detonators. Common examples comprise nitro-glycerine, nitroglycol,

nitromethane, dynamite, TNT, RDX, PETN, HMX, etc.

(iii) Tertiary high explosives (blasting agents) exhibit the least sensitivity. They cannot

reliably detonate by application of practical quantities of primary explosive, requiring

a booster of secondary explosives. This class consists of oxidisers such as ammonium

nitrate (AN), ammonium perchlorate (AP), ammonium dinitramide (ADN), etc., and

their corresponding fuel-composite mixtures.

Tertiary high explosives excel other high explosives for their stability, demanding more energy

to set off. Blasting agents afford variety of civilian applications involving local displacement

of earth crust, structural demolitions and building implosions, spanning large-scale mineral

extraction, quarrying, tunnelling, and many other construction operations. AN-based blasting

agents display desirable compromise between production, handling, safety, economic, and

performance characteristics as compared to nitrated hydrocarbon compounds, i.e., molecular

explosives.

AN-fused high energy products include emulsions, slurries and ammonium-nitrate-fuel-oil

(ANFO) mixtures. Owing to their excellent transport and blasting properties, they represent

nearly 90 % of the total explosives employed worldwide in the civilian sector.8 A mixture of


16

a well-balanced bulk ANFO usually consists of 5.7 wt. % fuel oil in an oxidising AN matrix,

whereas, formulations of emulsion and slurry explosives consist of additional components that

improve the contact between the oxidising salt and the fuel, and sensitise these materials. For

instance, emulsion explosives contain droplets of supersaturated aqueous phase of AN

dispersed in continuous oil phase (mixture of fuel and emulsifier). The literature8-11 provides

further information on the composition, manufacturing and underlying properties of these

materials. Volatile oils give the greatest sensitivity towards the reaction. However, petroleum

fractions with low flash points pose hazards. Therefore, the optimum choice incorporates fuel

oils similar to those used in diesel engines, i.e., No 2 distillate fuel oil, biodiesel, etc. As seen

by comparing the exothermicity of overall Equations (a) and (b), the fuel component of AN

explosives increases their energy density.8, 12

NH4NO3 → N2(g) + 2H2O + ½O2(g), ΔHr = -1440 kJ/kg (a)

3NH4NO3 + CH2 → 3N2(g) + CO2(g) + 7H2O, ΔHr = -3900 kJ/kg (b)

Global (or overall) Equations (a) and (b) summarise a number of elementary steps that underpin

each of these reactions. We use the right-headed arrows in the equations to stress their

irreversibility. If a practical explosive formulation deviates from the stoichiometric

requirements defined by Equation (b), the resulting fuel rich and lean fuel conditions lead to

redox processes described in Equations (c) and (d), respectively.

2NH4NO3(s) + CH2 → 2N2(g) + CO(g) + 5H2O, ΔHr = -3400 kJ/kg (c)

5NH4NO3(s) + CH2 → 4N2(g) + CO2(g) + 11H2O + 2NO, ΔHr = -2500 kJ/kg (d)


17

Imbalanced stoichiometric reactions and intricacies in mineral deposits results in noxious

footprint of ammonium nitrate explosives. For instance, in open-cast mining, observations

reveal that, the post-blast plume can harbour more than 500 parts per million (ppm) of nitrogen

oxides (NOx),12-14 consisting mainly of nitric oxide (NO), which subsequently undergoes

atmospheric oxidation to form nitrogen dioxide (NO2) according to overall Equation (e).

2NO(g) + O2(g) → 2NO2(g), ΔHr = -57 kJ/mol (e)

Orange-hue post-blast fumes indicate the presence of NO2. Clouds of NOx from the detonation

sites do not dissipate rapidly, and may drift outside mine boundaries into populated areas, in

spite of stringent mining guidelines, causing protests of local residents. Managing exclusion

zones by industry adds to the cost of mine operations. The average emission flux of NOx from

blasts scales up to 5 kg/(t AN explosive).15-19

This critical review presents a comprehensive summary of the influence of mining activities

on atmospheric chemistry. We commence by introducing the main environmental impacts of

mining, and then provide a critical evaluation of mechanisms of NOx formation. The review

offers a balanced explanation on the contribution of AN explosives to anthropogenic NOx

formation as generalised in Figure 2.1, emphasising the relevant abatement technologies. From

this perspective, the review informs researchers and engineers across germane environmental

disciplines and interested citizens on how to tackle unconfined NOx emissions with best-

practice technologies.


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Figure 2.1. Outline of NOx forming activities during the use of ammonium nitrate explosives

in mining operations.

2.2. Environmental impact of mining

This section briefs the reader on the overall impact of mining on the environment, to set the

scene for subsequent discussion:

(i) Typical air contamination: This category includes the emission of harmful gases, dust,

explosive residues and particulate matters (PM).17, 20-32 The major gaseous load from

mining activities corresponds to NOx, sulfur oxides, carbon oxides, photochemical

oxidants, volatile organic compounds (VOCs) and methane.12, 17, 20, 33-35 Overall, these

emissions originate from either mobile, stationary or fugitive sources. The mobile

sources involve vehicles and excavation equipment with on-board abatement

techniques, such as catalytic converters. Stationary sources comprise large operations


19

such as drying, roasting and smelting that allow post-activity emission treatments,

whereas fugitive emissions result from rapid activities such as blasting and wind

erosion preventing the gas clean-up by venting technologies.36-40

(ii) Water resources impact: The environmental influence of mining on water resources

embodies quantity- and quality-related issues. The former refer to drop and alterations

in groundwater table.20 High water consumption in mining processes results in shortage

of local water supplies.37, 41, 42 In the interest of public concerns for aquatic life, the

quality of ground and surface water deteriorates as a result of suspended solids, leached

contaminants (basically heavy metals), erosion of soil and mine wastes, and acidic mine

discharge (AMD, formed when pyrite (FeS2) reacts with water and air to produce

sulfuric acid and dissolved iron).20, 39, 43

(iii) Land and soil infectivity: In addition to physical subsidence and topographical

disturbances,20, 41, 44-46 mining operations contribute to soil contamination via chemical

channels.47 The common contaminants include heavy metals and inorganic ions that

can persist long after mine remediation.47, 48 Other environmental pathways operate

through atmospheric deposition to affect the soil and water quality. The aftermaths of

soil pollution disturbs the biological food chain, progressing from absorption of

pollutants by plants to ingestion of plant material by individual organisms.

(iv) Miscellaneous issues: Other significant impact of mining includes thermal-related

losses, waste treatment and disposal, ecological imbalance, noise and vibration

pollution, resettlements, habitat loss and fragmentation, and economic waves.20, 37, 39, 41,

49-51


20

Blasting activities result in formation of dust, primary particles and gaseous (basically NOx)

emissions. The air quality remains highly vulnerable as, unlike water or soil, practical

considerations prevent reprocessing of atmospheric air mixtures.41 The post-blast fumes

require adequate on-site control measures to reduce their negative impacts on the surrounding

communities and the environment. The recent growth of mining industries, and continuous

shift from underground mining to open-pit excavations have exacerbated the fume hazards.

Other activities in mining operations also contribute to the formation of NOx species, however,

this review focuses on emissions pertinent to direct use of AN explosives.

2.3. Overview of global NOx formation

On a global scale, reactive nitrogen (Nr) species arise from various human-related activities

and natural terrestrial processes.52-55 Figure 2.2 demonstrates the speciation, environmental

interactions and global sources of NOx.56-59 The major Nr species comprise NOx, ammonia,

ammonium, nitrous oxide, nitrate and nitric compounds. Atmospheric NOx contributes

substantially (nearly half) to the overall balance of Nr species, by trailing only the emission of

ammonia. Also, Nr accumulates in the environment as a result of higher emission rates,

relatively to denitrification to nonreactive N2.56


21

Figure 2.2. Annual atmospheric emissions and interactions of NOx. The data on the

rightward figure, except for “others”, are extracted from IPCC 2014.66 All values are

reported in Tg N/y. Conversion details; 1 Tg = 1012 g or 106 tonnes. “Others” denotes

emission sources with unknown global estimates, which may include aircraft (≈ 0.7 Tg

N/y),67 oxidation of NH3 (≈ 3.0 Tg N/y)68 and dispersed use of nitrate explosives (estimated

to be ≈ 0.05 Tg N/y). Total anthropogenic emission correspond to 41.3 Tg N/y.

2.3.1. Anthropogenic NOx emissions

Of all the sources of anthropogenic NOx, combustion of fossil fuels induces the most emission,

mainly due to the constituent nitrogenous heterocyclic species in the fuels’ structure.70,71

During combustion processes, NOx and its precursors arise via the four routes: (a) the thermal

process involving the Zeldovich reaction between atmospheric nitrogen and oxygen as a result


22

of high combustion temperature (above 1500 °C); 72-74 (b) the fuel-N channel arising from

oxidation of nitrogen compounds chemically bounded with the fuel organic matter. This

corridor generates about 80 % of NOx emission in pulverised solid fuel combustion; 73,75-78 (c)

the prompt process characterising the reaction of atmospheric nitrogen with hydrocarbon

radicals to form NOx precursors via a series of gas-phase reactions; 73 (d) the heterogeneous

route occurring as the result of the reactions of solid carbonaceous substance, i.e., char

remaining after devolatilisation.70,78 The net amount of NOx formed from heterogeneous

reactions depends on the intrinsic reactivity and the internal surface area of the char, as well as

charring conditions.78-80 In addition, moderate nitrogen content of biofuels and lower

temperature of biofuel combustion (in comparison to fossil-fuel combustion) decrease the

release of NOx species in biomass firing.73,78,81-83

Other human-related emission sources include agricultural activities, emission of NH3 in

chemical industry (e.g. from the Haber-Bosch process) and use of nitrate oxidisers in aviation,

infrastructure and mining industries. Assessing the amount of NOx produced from a blast

depends on the relative position (distance and angle) of the observer and the prevailing weather

conditions.84 The Australian Explosive Industry and Safety Group (AEISG) has developed a

viable qualitative chart ,85 as seen in Table 2.1, to rate the post-blast NOx (typically NO2) on a

simple scale of 0 to 5, but without linking the fume appearance to NOx concentration.


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Table 2.1. Field colour chart for visual rating of blast-generate NOx.75

Level Colour Pantone number Typical appearance

Level 0

No NOx gas

Warm Grey 1C

(RGB 244, 222, 217)

Level 1

Slight NOx gas

Pantone 155C

(RGB 244, 219, 170)

Level 2

Minor yellow/orange gas

Pantone 157C

(RGB 237, 160, 79)

Level 3

Orange gas

Pantone 158C

(RGB 232, 117, 17)

Level 4

Orange/red gas

Pantone 1525C

(RGB 181, 84, 0)

Level 5

Red/purple gas

Pantone 161C

(RGB 99, 58, 17)

The pollutant release and transfer registers implemented since 1990s in all OECD countries86

do not provide estimates of the contribution of NOx from blasting to the total anthropogenic

emission of this gas. For example, the Australian National Pollutant Inventory (NPI) reports

combined emission of NOx from all activities at each mining facility, focussing on the amount

of each pollutant rather than on establishing links between the amounts and the underlying

discharge mechanisms.26,27 This is not a satisfactory situation. The increasingly prevalent open

cut mines in Australia move large amount of overburden, an effort that tends to govern the NOx

footprint of each mine. An additional, somewhat intractable, problem lies with the estimation

approach itself that assumes emission factors that do not account for episodes of large discharge


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of NOx fumes from blasting. For this reason, in this article we adopt an average yield of NOx

emission of 5 kg/(t AN explosive),25-29 as a first-order conservative approximation, and

consider the global annual (for the year 2016) AN production of 58.2 million tonnes,87 of which

35 % are deployed in the explosive industry,88 to arrive at a global annual emission of

approximately 0.1 Tg/y of NO (i.e. 0.05 Tg N/y, or 5×104 t N/y) generated as a result of mining-

related blasting. This number is rather small in comparison to the total emissions of

anthropogenic NOx from all sources that corresponds to 41.2×106 t N/y (cf. Figure 2.2). Thus,

in spite of its potential for large localised dispersive emission of NOx that may result in

fatalities, blasting is not a major contributor to the environmental inventories of this pollutant.

2.3.2. NOx in the environment and mining workplaces: processes and health effects

2.3.2.1 Direct NOx health effects

Accumulation of atmospheric NOx can lead to various environmental and bio-related hazards.

In the context of public health, NOx induces a number of chronic diseases. Low levels can

irritate eyes, nose, throat and lungs, possibly leading to coughing and bleeding, shortness of

breath, tiredness and nausea, and can also result in temporary build-up of lung fluids. Exposure

to high concentration of NOx can cause pulmonary oedema as well as acquired or type II

methemoglobinemia that exhibits symptoms of rapid burning, spasms and swelling of tissues

in the throat and upper respiratory tract, reduced oxygenation of tissues, build-up of lung fluids,

and possibly death.26,89 The current World Health Organisation air quality guidelines for NO2

allow one hour-level safe limit of 200 µg/m3 (1.88 µg/m3 of NO2 = 1 parts per billion), and an

annual average of 40 µg/m3.90 However, typical concentrations of NOx in post-blast clouds


25

can reach between 5.6 to 580 ppm, 12,23,24 exceeding the safe limits by around 30 to 3000 times.

Exposure of humans and wildlife to these high concentrations can lead to injuries24 and

fatalities.91

2.3.2.2 Atmospheric NOx processing and the consequential health impacts

The chemical reactivity of NOx in the presence of other atmospheric substances produces

secondary pollutants. For instance, ground-level ozone, abundant in photochemical smog,

arises as a result of chemical interaction of NOx with reactive hydrocarbons and other volatile

organic compounds (VOC), in a process facilitated by heat and sunlight.74 Likewise, oxidation

of NOx contributes to atmospheric formation of nitrate aerosols, 66,92-95 and notably, the mineral

particles and fugitive dusts synchronously present in post-blast fumes easily facilitate such

reactions. Concerns about effects on human health include damage of lung tissue and

dysfunction of respiratory system, such as emphysema and bronchitis. Furthermore, dissolved

N in the form of nitrate species in groundwater have negative impacts on human health.66,96,97

Short-term exposure to nitrate in drinking water (contaminated by groundwater) at levels above

the health standard of 10 mg/L nitrate-N remains a potential health problem, primarily for

infants and people previously subjected to kidney dialysis treatment. In the human body, NO3–

converts into nitrite, which can cause methemoglobinemia by interfering with the ability of

haemoglobin to take up O2. Most cases of methemoglobinemia occur after consuming water

with high concentrations of NO3–.98 Other health consequences of ingestion of nitrate at

elevated concentrations include respiratory infection, reproductive risks, alteration of thyroid

metabolism, and cancers induced by conversion of NO3– to N nitroso compounds (especially


26

to nitrosamine, under certain conditions of gastric achlorhydria in the oral cavity, bowel or

bladder) in the body.62,99,100

2.3.2.3. Impact to ecosystems

With respect to impact on ecosystems, the wet (e.g., acid rain) and dry deposition of NOx

increase the nitrogen contents in the soil and terrestrial water bodies. This leads to eco-

eutrophication and depletion of aquatic oxygen (hypoxia), as well as to rare anoxic conditions

(anoxic conditions refer to severe hypoxia). Excessive level of these conditions affects marine

life, causes leaf and root damage, and contributes to soil and water acidity.26,49,89 Other

environmental impact of NOx includes stratospheric ozone depletion and corrosive effects of

acid rain on life and property.

.

2.4. Mechanism of NOx formation during the use of AN explosives

Three distinct mechanistic pathways generate NOx during blasting of AN agents in various

mining, civilian and infrastructural activities, as explored in the following sections; i.e.,

stoichiometrically unbalanced blasting, deflagration of AN prills and gassing of emulsion

blends.


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2.4.1. Stoichiometrically unbalanced blasting

The following general reaction101 balances the chemical species appearing in the blasting

process:

(3x + 1)NH4NO3 + CxH(2x+2) → (3x + 1)N2 + (7x + 3)H2O + xCO2 (2.6)

Equations 2.7 and 2.8 recast Equation 2.6 for the fuel oil and carbon residue, respectively, the

two most common fuels comprised in the emulsion. In general, however, the fuel could be

carbon-based 8,101,102 or nonconventional, depending on fuel availablity.12,103-105

3NH4NO3 + CH2 → 3N2 + CO2 + 7H2O (2.7)

2NH4NO3 + C → 2N2 + CO2 + 4H2O (2.8)

Alterations in bulk material composition, wicking of fuel content (i.e., removal of fuel by

capillary suction of the surrounding rock), as well as intricacies of geological conditions such

as moisture in blast hole (wet blast condition) and the presence of mineral matter prompt the

formation of CO and NO. Manufacturers and end users often employ a characteristic property

called “oxygen balance” (OB) to describe the reaction stoichiometry. The OB value of the

constituent component of an explosive mixture expresses the amount of oxygen, in mass

fraction unit (i.e., g O2/(g compound)), in excess or deficiency with respect to stoichiometry.

It is calculated according to: 106

OB = 16 (O - 2C -

12

H + M)

MW

(2.9)


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where MW refers to the molecular weight in g/mol of a compound (e.g., 80 g/mol for NH4NO3),

and O, C, H and M correspond to the number of oxygen, carbon, hydrogen and metal atoms,

respectively. A well-formulated commercial explosive must maintain an OB value close to

zero. For example in ANFO mixture, AN has an OB of +0.20 (=16 × (3-1/2×4)/80), while the

fuel oil retains a unit of –3.4 (=16 × (0-2×1-1/2×2)/14). Hence, the optimised ratio of ANFO

for a fully balanced bulk formulation corresponds to 94.3/5.7 (solve for x, the mass fraction of

AN, x×0.2-(1-x)×3.4=0, x=0.943). OB influences detonation products, as illustrated in

example Reactions 2.10 and 2.11:

–OB

(excess fuel)

2NH4NO3 + CH2 → 2N2 + CO + 5H2O (2.10)

+OB

(lean fuel)

5NH4NO3 + CH2 → 4N2 + CO2 + 11H2O + 2NO (2.11)

The NO gas oxidises rapidly in the atmosphere (via Reaction 2.12 with a rate constant of 2.08

× 10-38 cm6/molecules2 s at 25 °C)107,108 into orange-coloured NO2, a relatively stable species

that exists in equilibrium with its colourless linear dimer, N2O4. The equilibrium constant for

Reaction 2.13 decreases with an increase in total pressure (P) of NO2 + N2O4, and amounts to

7.1 × 105 Pa-1 at P = 4.4 × 103 Pa and T = 25 °C.109

2NO + O2 → 2NO2 (2.12)

2NO2 ⇌ O2N–NO2 (2.13)


29

Figure 2.3 illustrates the effect of OB, fuel content and moisture on NOx formation from typical

ANFO blends. The literature also provides information on the aspect of other parameters such

as degree of confinement and emulsion additives.29,110


30

Figure 2.3. Effect of oxygen balance (a) and percent fuel oil (b) on NOx formation from

different ANFO blends, and the influence of water content (c) on NOx emission from 94/6

ANFO. Digitised data are sourced from Rowland and Mainiero’s study.18 NOx

concentrations are recorded in (L at STP)/(kg bulk explosive).


31

Furthermore, as shown in Figure 2.4a, the interaction of fuel with AN under fuel-lean

conditions leads to NOx species via the nitrous acid, nitro- and nitroso-hydrocarbon

intermediates. According to Oxley and co-workers, the NO2 generated during thermolysis of

AN reacts with the fuel content, producing additional NO.111

Figure 2.4. (a) NOx formation by interactions of fuel content of AN blasting agents during

stoichiometrically unbalanced detonation. Deficiency of fuel results in positive oxygen

balance. (b) Mechanism of NOx formation as a result of N transformation in AN deflagration

during blasting of bulk explosive mixtures. (c) NOx formation during chemical gassing of

emulsion explosives. Boxes denote the reactants and the dash-line circles the NOx products.

“D”, in pane (c), represents diffusion steps across the oil films.

2.4.2. Deflagration of AN prills


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Deflagration occurs in blasting of AN explosives when the fuel content is insufficient for a

complete chemical reaction, and as a result, the AN prill provides a nitrogen based fuel

source.110 This condition also arises as a consequence of fuel deficiency and/or mixture

inhomogeneity due to inadequate integration of ANFO. Literature comprehensively reviews

the properties of AN, especially those relevant to its use in explosive mixtures.8,9,112,113

Although various mode of decomposition chemistry of AN have been postulated,8,9,114 the

complete mechanism remains poorly understood. The current consensus of opinion explains

the thermal decomposition of solid AN to proceed by two channels in the condensed phase,

ionic and radical. Species that leave the condensed phase participate in further radical reactions

in the gas phases.13

Except for evaporation of water, during heating, AN remains thermally stable until the

temperature reaches the melting point. AN melts at around 169 °C, and then starts

decomposing as the ionic species react with each other.115 This begins by an endothermic

dissociation step (Equation 2.14), followed by decomposition of HNO3 and subsequent

oxidation of NH3 according to Equations 2.15, 2.16 and 2.17, respectively.10,14,15,111,116 A

broader list of reactions and their corresponding rate parameters in the liquid phase have been

provided by Skalis’s et al.,15 though, the decomposition period depends on imposed heating

rates and particle size.

NH4NO3 → NH3 + HNO3 (2.14)

HNO3+ HX → NO2+ + H2O + X-

where HX = NH4+, HNO3, H3O, and others

(2.15)

NH3+ NO2+ → products (N2O, H2O) (2.16)


33

NH3 + X-→ products (NO, NO2, H2O) (2.17)

The gas-phase mechanism becomes noticeable at higher temperatures, when species emanating

from the surface (above the melt front) undergo further radical oxidation and/or decomposition

to form a relatively more stable products. Analysis of the rapid decomposition (at 4800

°C/min) products of AN by in situ Fourier transform infrared (FTIR) spectroscopy identified

HNO3 as an intermediate species and revealed the formation of NO2 alongside N2O and

H2O.117,118 As HNO3 transforms into the gas phase,119 it experiences successive homolysis of

O‒N bond, acting as the chain initiation reaction as shown in Equation 2.18.14,120,121

HNO3 → OH + NO2 (2.18)

The complex gas-phase mechanism propagates via reaction of NH3 with the decomposition

products of HNO3 (from Equation 2.18), involving H and OH as the key chain carriers, whilst

NHx and NOx (x = 1 – 3) serve as the major participants in the redox process.16,18,19,120,122-124

The principal gas products comprise NO, NO2, N2O and H2O. Furthermore, reactions

involving NH3, NO, and water form AN aerosol, as follows:117,125

3NO + 3

2O2 → 3NO2 (2.19)

2NO2 + H2O → HNO2 + HNO3 (2.20)

NH3 + HNO3 → NH4NO3 (2.21)

Skalis et al.’s 15 semi-empirical model combines the condensed- and gas-phase sub-

mechanisms. Predictions from the model concur reasonably well with experimental

measurements.126,127 Kinetic rather than thermodynamic considerations govern the


34

decomposition of AN. This is evident from the results of experiments and the Skalis et al.

model and from the calculations of classical thermodynamics, say, using the Equilib program

in the Chemkin-Pro distribution.128 In general, the formation of NOx during deflagration of

AN prill depends on rates of condensed and gas-phase reactions that reflect the underlying

conditions of temperature, heating rates, and pressure and confinement. Figures 2.4b and 2.5

portray the simplified mechanism of NOx formation at moderate heating conditions and the

effect of heating rate on nitrogen selectivity in the gas-phase product species, respectively.

Figure 2.5. Impact of heating rate on nitrogen selectivity among gas-phase products. The

calculations were done by performing temperature-programmed decomposition of 1 mg AN

(pressure 1 atm, initial temperature 27 °C, bath gas helium, inlet volumetric flow rate 200

standard cm3 min−1). The temperature reached at 50 % AN conversion (T50) is represented by

a supplementary horizontal axis. Reproduced with permission from ref.15 Copyright 2014

Elsevier.


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2.4.3. Chemical gassing of emulsion blends

Sensitisation of emulsion explosives with chemically-generated nitrogen bubbles (~ 400 μm in

size)129 can also contribute to emission of NOx. A typical AN emulsion explosive involves a

concentrated aqueous phase of oxidiser salts, consisting of droplet 10-20 μm in diameter,

stabilised by an emulsifying agent; comprised in the continuous fuel phase (Table 2.2).8 In

practice, solutions of sodium nitrite (NaNO2) and acid (e.g. acetic; CH3COOH) are injected

into the pumped AN emulsion, during emulsion pumping into a blast hole. Once the bubbles

grow to their desired size and emulsion reaches its target density, setting off a booster induces

a shock wave to propagate through the emulsion column, adiabatically compressing the bubbles

and providing a large number of hot spots to engender the explosion.


36

Table 2.2. Typical composition of AN emulsion explosive. This type of formulations are

usually referred to as water-in-oil (W/O) emulsion.8

Components Percent quantity (%)

Ammonium nitrate 62

Sodium nitrate 6

Sodium perchlorate 5

Calcium nitrate 8

Water 11

Fuel oil (oil phase) 3

Waxes 2

Emulsifier 1.2

Atomised aluminium 0.8

Gassing agent 1.0

Oxygen balance Close to 0.0

As shown in Figure 2.4c, generation of N2 bubbles commences with molecular diffusion of

CH3COOH through the oil interface to droplets containing NO2- anions.130 Protonation of NO2

-

forms nitrous acid:

NO2- + H+ ⇌ HONO (2.22)

The nitrous acid diffuses out of sodium nitrite droplet into the emulsion phase with three

possible fates. It can react with strong nucleophilic species (X-) to from nitrosating agents of

the form ONX,8,131 which reacts with amine substrate, such as thiourea, to produce nitrogen

gas needed for the generation of hot spots. In practice, pH of the aqueous phase (see below) is


37

not high enough to deprotonated NH4+ and release ammonia. This limits the availability of

NH3 as substrate for N2 formation.

HONO + X- + H+ ⇌ ONX + H2O (2.23)

ONX + SC(NH2)2 → N2 + SCN- + X-+ 2H+ + H2O (2.24)

The nitrosating agent (ONX) can also decompose to form nitric oxide. For example, when

thiocyanate functions as a nucleophilic agent, nitrosyl thiocyanate (ONSCN) is formed as the

resulting nitrosating agent. Other types of sulfur-bound amine substrates include aminothiones

and thioacetamide.132,133 The unstable ONSCN decomposes at temperature approaching -60

˚C to form nitric oxide.

HONO + SCN- + H+ ⇌ ONSCN + H2O (2.25)

2ONSCN → 2NO + (SCN)2 (2.26)

In addition, SCN- reacts with ONSCN to form NO according to the following reaction: 134

ONSCN + SCN- → NO + (SCN)2

- (2.27)

Without nucleophilic species, nitrous acid follows the second fate to produce nitrosating agent,

dinitrogen trioxide (N2O3), under relatively mild acidic conditions (pH range ≈ 4-6) as a

product of reaction of nitrous acid with nitrite ion.135 Dinitrogen trioxide represents the

nitrosating agent that operates in practical emulsion explosives.

HONO + NO2- + H+ ⇌ N2O3 + H2O (2.28)


38

The third fate operates at high rate in acidic conditions, at pH of 2 and below, where nitrous

acid decomposes rapidly to form NOx via side Reaction 2.29:

2HONO ⇌ NO + NO2 + H2O (2.29)

As nitrosation (gassing) reactions accelerate at low pH, the dosing systems must include

devices that prevent intentional (but undesirable) addition of extra acid to speed up the gassing

process. Note that, one cannot eliminate completely the formation of NOx from HONO.

Hence, all gassed emulsions always contain a small amount of NOx. Most of that NOx remains

dissolved in the aqueous phase but a fraction may enter the nitrogen bubbles.130,135,136 Figure

2.4c demonstrates the NOx formation mechanism in chemical gassing of emulsion explosives.

2.5. Effective control measures and abatement techniques

2.5.1. Operation guidelines for controlling NOx emission in blasting operations

The Australian State of Queensland’s Guidance Note 2084 provides useful precautionary

measures for preventing, controlling and managing the formation of NOx fumes in open cast

mining. Note 20 comprises standards for manufacturing, storage time-limit, selection of proper

initiating devices, better design of mine shots and confinements, as well as appropriate planning

and personnel training. At present, all major manufacturers of AN explosives offer

formulations that can tolerate wet conditions, usually, with the resistance to these conditions

related to the percent loading of emulsion in prill/emulsion (heavy ANFO) mixes. In practice,


39

selection of appropriate explosive formulation, dewatering of holes prior to loading and

minimising the sleep time prior the detonation displays the largest impact on preventing the

fume formation.84 Moreover, from a managerial perspective, Note 20 recommends

considerations for establishing fume management zones (FMZ) and determination of blast

exclusion zones (BEZ) that account for effects of meteorological (e.g., wind speed and

direction, temperature, humidity, etc.) and geological conditions. FMZ represents an area

likely to contain fumes after blast, requiring workers to remain outside the zone. For industry,

managing exclusion zones adds to the cost of mine operations.

2.5.2. Abatement techniques

The NOx mitigation technologies employed in post combustion plants78,137-140 do not carry over

to blasting activities. This is because: (a) the post-explosion atmospheric air mixture do not

yield itself to capturing, scrubbing, treatment and/or reprocessing; (b) AN exhibits

incompatibilities with materials (Table 2.3)141 that could serve for scavenging NOx. However,

strategic research into NOx abatement led to the development of the following mitigation

technologies:


40

Table 2.3. Selected incompatibility of AN with some chemicals.118

(i) Alkalimetric neutralisation: One of the earliest studies on tackling the formation of

NOx in blasting operation involved the use of neutralising additives. Azarkovich142

proposed the addition of inexpensive substances (to AN explosive charges) that possess

the capacity of binding oxides of nitrogen. The author recommended additives that are

Substances Effects on AN

Powdered metals Most metals react violently or explosively with fused ammonium nitrate

below 200 °C. May also sensitise AN to shock.

Metal oxides Lower the ignition temperature of AN-fuel mixtures.

Metal sulfides Lead to runaway reaction, resulting in detonation at temperature below

40 °C, if the pH is less than 2.

Metal nitrite/nitrate Magnesium nitrate may desensitise AN. Contact of AN with potassium

nitrite causes incandescence.

Urea Dehydrated AN mixtures containing 35 – 39 % urea can produce a residue

capable of deflagration at 240 °C. The recommended processing

temperature for such formulation should not exceed 120 °C.

Ammonia Free ammonia may either stabilise, or tend to destabilise the AN salt

depending on the condition.

Halide salts Promote the thermal decomposition of AN. Lower the initiation

temperature sufficiently to give a violent or explosive decomposition, i.e.,

premature spontaneous ignition.

Acids Mineral acids destabilise. Concentrated acetic acid mixtures ignite on

warming.

Organic fuels Increases the heat of combustion. 2 – 4 % of organic fuel are used in

commercial explosives.


41

able to react as alkalis, viz., bases and salts of weak acids. Azarkovich et al. reported

the neutralisation reaction of NO2 occurring in a Dolgov bomb and an underground

chamber, especially for additives such as slaked lime Ca(OH)2, chalk CaCO3 and soda

Na2CO3, uniformly spread into the charge in small aliquots of 0.1 – 0.2 mass %, or

added into sand stemming. (Stemming denotes the practice of placing soil, sand or

rocks on the explosives in the blast hole.). Equations 2.30 – 2.32 suggest the operation

of the additives that reduce NO2 emissions by 40 – 80 %, as monitored by an

electromagnetically relayed air sampling vessel.142,143

Ca(OH)2 + 2NO2 + 1

2O2 ⇌ Ca(NO3)2 + H2O (2.30)

CaCO3+ 2NO2 + 1

2O2 ⇌ Ca(NO3)2 + CO2 (2.31)

Na2CO3 + 2NO2 + 1

2O2 ⇌ 2NaNO3 + CO2 (2.32)

(ii) Application of stabilising and scavenging additives: In the search of enhancing the

stability of AN, Oxley et al.10 screened a large number of formulations comprising

grounded sodium, potassium, ammonium and calcium salts of sulfate, phosphate, or

carbonates as well as certain high-nitrogen organic compounds such as oxalate,

formate, urea and guanidium salts at about 10 mol %. The authors revealed that, the

more stabilising the additive, the lower the N2O/N2 ratio, i.e., the more the N-selectivity

favours N2 formation. Although the Oxley et al.10 did not report measurements of NO

and NO2 concentration, the insight into the selectivity towards N2 suggests NOx

reduction. Opoku et al.144,145 inserted additive (non-ammonium) nitrates into the

nanocrystalline structure of AN, and studied their effect on formation of NOx in

deflagration of AN. The authors showed that additives, especially potassium nitrate


42

that forms 5 mol % K in the co-recrystallised AN salt, achieve up to 40 % reduction in

NO emission.

(iii) Reburning-like technique: In boilers, reburning abates NOx by using a supplementary

fuel to reduce NOx,146 to achieve an overall decrease in NOx emission of about 50 – 85

%. Reburning constitutes a three-stage process. In the first stage (primary zone), the

main fuel burns under slightly fuel-lean condition. The produced NOx then reacts with

supplementary hydrocarbon radicals in the “reburning zone” to form intermediate

nitrogenous species, and then molecular nitrogen (N2). Finally, addition of air in the

last stage (burnout zone) completes the combustion process by oxidising all unreacted

fuels and N-species.78,147 These interactions involve several elementary steps

summarised in the following overall equation:147

∑ CiHj + NO → HCN +... (2.33)

In excess NOx, the HCN decays through series of intermediates, and ultimately reaches

N2 via the reverse Zeldovich reaction (Equation 2.37):

HCN + O → NCO + H (2.34)

NCO + H → NH + CO (2.35)

NH + H → N + H2 (2.36)

N + NO → N2+ O (2.37)

Many practitioners had applied reburning-like technology to reduce NOx emission in

blasting of AN explosives adding supplementary fuel (usually solid) of higher oxidation


43

stability compared to fuel oil used in AN explosives. As seen in Figure 2.6, the fuel

with lower oxidation stability (fuel oil) participates solely in primary detonation

reaction, with the resulting NOx species subsequently reduced to nitrogen. However,

the excess fuel conditions, if they arise in the reburning of detonation-generated NOx,

may lead to formation of CO, as shown in Equation 2.10.

Figure 2.6. Conceptual schematic of the application of reburning-like method

during blasting. The arrows indicate the direction of oxidation. Atmospheric O2

completes the process by oxidising all unreacted fuels.

Sapko and co-workers25 demonstrated that the use of pulverised coal dust (mean

particle size of 74 μm) as an additive mitigates the total NOx emission by 10 – 50 %.

Likewise, biomass briquette and advanced reburning materials such as urea147 offer

better NOx-mitigation performance.10,25 Other potential fuel additives may include non-

hazardous waste polymers, such as polyethylene.148


44

(iv) Chemical trapping: Spin traps can provide an effective reduction of in the overall

release of nitric oxide in sensitisation (chemical gassing) of emulsion explosives. Spin

trapping reaction remove radical species, such as NO, by forming stable adducts.

Venpin et al. exploited the spin-trapping technique to develop NO scavengers to control

NOx emission from industrial processes, including sensitisation of emulsion

explosives.149-151 In their experiments, they applied aromatic ortho substituted nitroso

compounds, such as nitrosobenzene sulfonate (NBS), 3,5-dibromo-4-nitrosobenzene

sulfonate (DBNBS), 3,5-dimethyl-4-nitrosobenzene sulfonate (DMNBS) and 3,5-

dichloro-4-nitrosobenzene sulfonate, obtaining up to 70 % removal efficiency of NO

during sensitisation of AN emulsion blends. The reaction pathways leading to the

formation of N2 are shown in Equation 2.38 – 2.40:149

(2.38)

(2.39)

(2.40)

(v) Use of alternative oxidising agents: Even though the AN technologies have been

established for applications in civilian explosives, there exist at least one non-nitrogen


45

based formulation. Araos and Onederra152 replaced AN with oxygenated water

(hydrogen peroxide, H2O2, OW) and used micro balloons, rather than chemical gassing,

to avoid NOx formation both in detonation and in emulsion sensitisation. Such OW/fuel

blends detonate with similar heat release and VOD to AN explosives, 152-154 but without

NOx hazards. The technology requires further testing to verify its large-scale

performance.

Some noteworthy patents include formulating the blasting agent to contain from about 1 % to

about 20 % silicon powder,155 or appreciable amount of urea in the discontinuous oxidiser salt

phase,156,157 improved composition of hydrogen peroxide based explosives,158 application of

other nitrogen-free oxidisers,159 and appropriate gassing method.160

2.6. Perspectives and future outlooks

We have identified the possibility of the formation of secondary pollutants (nitrate aerosols) as

a result of atmospheric nitration of the mineral particles and fugitive dusts synchronously

present in post-blast fumes. Development of new sampling methodologies should include the

environmental monitoring of such species from blasting activities. Lightweight remote-

controlled drone-sampling instrumentation, as well as surface sampling, could facilitate the

measurements of nitrate aerosols, as well as other pollutants including supplementary Nr

species (e.g. HNO3, NH3, and N2O), hydrocarbons, nitrogenated analogues161 of dioxins and

secondary contaminants such as peroxyacyl nitrates (PAN)162. Understanding the mechanism

of the formation and the survival of these pollutants in detonation of heavy AN explosives is

crucial to achieve sustainable mining activities, as are further testing of OW emulsions and


46

bulking agents, and generating new knowledge on the exact mechanism of deflagration that

involves the surface burning of AN particles.

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CHAPTER 3 — Methodology

63

CHAPTER 3

Methodology

This chapter presents an overview of the main experimental and

computational techniques employed within the scope of this study. Details

of specific computational settings and experimental conditions are provided

in subsequent chapters.

…


64

3.1. Computational Techniques

Computational ab initio methods have been used in this dissertation to study all necessary

reaction steps. Ab initio (Latin for “from the beginning”) represents computation practices

derived directly from theoretical principles without the inclusion of experimental data. This

remains an approximate quantum mechanical calculation, reflecting only mathematical

approximations, such as using a simpler functional form for a function or finding an

approximate solution to a differential equation.1 The ab initio techniques solve the Schrödinger

equation to predict the energy and optimum conformational structure of a molecule. Equation

3.1 depicts the Schrodinger equation, where Ψ represents the wavefunction, and m, h, and V

denote the mass of particle, Planck’s constant and the potential energy, respectively.1-3

{-h2

8π2m∇2+V} Ψ(r, t )=

i h

2π ∂Ψ(r ,t)

∂t (3.1)

The accuracy of the final molecular energy relies on the level of assumptions made, as well as

the precision of the numerical simulation – generally termed as “theory”. This work applies

the density functional theory (DFT) within the framework of two selected model chemistries

as enumerated below:

i. Three-dimensional slab model enabled the investigation of solid-gas interaction of

polyethylene with oxidising species, such as atmospheric oxygen and NOx gases.

DMol3 package4, 5 supported this kind of theoretical simulations, assisting all structural

optimisations, and serving to calculate all energies and vibrational frequencies. In the

optimisation of all stationary points, the theoretical approach comprised the generalised


65

gradient approximation (GGA) as the exchange-correlation potential alongside with the

Perdew-Burke-Ernzerhof (PBE) functional. The total energy converged with a

tolerance of 1 × 10−6 Ha. The electronic core treatment included all electrons in the

calculations while deploying a double numerical plus polarisation (DNP) basis set4, and

a global cut-off of 4.0 Å. The applied Brillouin zone involved automatic generation of

2×3×6 and 1×2×1 k-points for the unit cell and periodic slabs, respectively, through the

Monkhorst-Pack grid scheme. The average computational cost, for each periodic slab

typically amounts to 20 h on a 32 parallel processing cores. Where necessary,

Tkatchenko and Scheffler’s semiemperical dispersion correction scheme6 offered the

necessary measures to validate the compromise between the cost of evaluation of the

dispersion terms and the need to improve non-bonding interactions in our system.

ii. Gas-phase reaction modelling allowed the development of theoretical insight into the

formation of initial nitration products during thermal reaction of biomass surrogate

(morpholine) with NOx. Gaussian09 suite of programs7 implemented the complete-

basis-set CBS-QB38 composite method to facilitate all structural optimisations, and

served to calculate all enthalpies as well as vibrational frequencies using the tight

convergence criteria and rigid-rotor/harmonic-oscillator approximations. In agreement

with previous studies9, 10, the adopted computational method offered an accurate

evaluation of thermochemical properties of all species studied herein.

Furthermore, each chapter involving ab initio calculations contains a detailed description of the

specific computational procedures.


66

3.2. Experimental Configurations

The experimental work of this thesis essentially investigates the conversion of NOx with

polyethylene (PE, selected solid fuel), and morpholine as the chosen biomass surrogate. The

following text describes the experimental rigs. Subsequent chapters elaborate the details of the

applied methodologies.

3.2.1. Experiments involving thermal reaction of recycled polyethylene with NOx

i. Characterisation techniques:

Fourier transform infrared (FTIR) spectroscopy: This method elucidated the major

functional groups present in the polymer samples, using the Agilent Cary 670

equipped with the attenuated total reflectance (ATR, germanium crystal, Pike

Technologies) accessory. Prior to analyses, samples were cryogenically pulverised

to achieve reproducible measurements. The FTIR recorded the spectra at 4 cm-1

resolution, with accumulation of 32 scans per spectrum. The bands at 2916 cm-1,

2848 cm-1, 1470 cm-1 and 718 cm-1 correspond to CH2 asymmetric stretch, CH2

symmetric stretch, CH2 scissors and CH2 rock modes, respectively.

Carbon-hydrogen-nitrogen-sulfur (CHNS) analysis: CHNS elemental analyser

(Australia National University, Australia) provided a means for rapid determination

of carbon, hydrogen, nitrogen and sulfur in the organic polymer matrices.

Pulverised samples were fed into the pre-calibrated instrument (EuroVector

EA3000) under dry helium and O2 purge, and the combustion furnace operated at


67

approximately 1100 °C. The reactor functioned in a dynamic flash combustion

mode, and contained both the oxidation zone represented by tungsten oxide and

copper wired reduction zone. Two repeated runs resulted in precise measurements

of the constituent elements.

Ion chromatography: This method determined the trace concentration of halogens

in polymers. Dissolved samples were injected into a DX-120 anion chromatograph.

Inductively coupled plasma emission spectroscopy (ICP-OES): Varian 715-ES ICP

OES (University of Newcastle, Australia) served to measure the metal

concentration in the sample. About 140 mg of the recycled PE were digested in

microwave, and then transferred via Varian SPS 3 sample preparation system for

analyses. High-purity ICP multi-element standard solution containing different

metal cations in 2.0 % HNO3 facilitated the necessary calibration.

High temperature size exclusion chromatography (HT-SEC) in TCB: The HT-SEC

(Université Claude Bernard Lyon 1, France) analyses operated on Viscotek system

(from Malvern Instruments) equipped with three columns (Polefin 300 mm × 8 mm

I. D. from Polymer Standards Service, porosity of 1000 Å, 100 000 Å and 1 000

000 Å). Sample solutions 200 µL in volume, with a concentration of 5 mg/mL,

were eluted in 1,2,4-trichlorobenzene (1,2,4 TCB) with a flow rate of 1 mL/min at

150 °C. The mobile phase was stabilised with 2,6-di(tert-butyl)-4-methylphenol

(200 mg/L). Online detection was performed with a differential refractive index

detector and a dual light scattering detector (LALS and RALS) for absolute molar


68

mass measurement. The OmniSEC 5.02 software facilitated the calculation of the

SEC parameters.

ii. Parametric and Temperature Profile of Experimental Setups

This aspect of the laboratory work involved three series of experiments. As shown in

Figure 3.1, a standalone thermogravimetry apparatus (TGA)11-13 has been employed

with the intention of eliminating any unwanted experimental artefacts that could be

carried over into the subsequent kinetic evaluations. Furthermore, hyphenated infrared

spectroscopy (TGA-FTIR, PerkinElmer Frontier) and chemiluminescence (TGA-NOx,

Thermo Scientific model 42i-HL) enabled the identification and quantitation of

evolved gases.


69

Figure 3.1. Schematic diagrams of adopted experimental configurations for the TGA-FTIR-

NOx analyses.

The reference standards for the mass and temperature calibration were selected

according to the desired experimental range. Table 3.1 lists the measured values as

compared to the expected quantities. The table also provides the calculated percent


70

error. During the entire duration of the study, replicate runs randomly carried out on

different days quantified for the instrumental drift and reproducibility of experimental

data.

Table 3.1. Verification of the Instrument’s (TGA-DSC) measured values

Weight (mg)

Expected Measured1 Measured2 Percent error

Certified mass 54.37 54.59 54.36 7.36 × 10-3

Temperature3 (C)


Indium 156.60 156.21 156.45 9.58 × 10-2

Zinc 419.47 417.12 419.20 6.44 × 10-2

Aluminium 660.10 662.67 660.30 3.03 × 10-2

Heat flow4 (J/g)


Zinc 108.37 115.70 110.20 0.17 × 10-1

Aluminium 400.10 410.02 400.80 1.74 × 10-1

1 Measured values before calibration.

2 Measured values after calibration.

3 Onset melting temperatures.

4 Enthalpy of fusion (∆Hf).


71

3.2.2. Experiments involving thermal reaction of morpholine with NOx

This section of exploration involved a flow-through tubular reactor typically employed in

similar studies.14-18 Figure 3.2 provides a schematic representation of the experimental setup

in which a digital syringe pump delivered the reactant fuel (morpholine; TCI, purity > 99.0 %)

into the preheated dilute stream of helium/NOx/oxygen flowing through the system. The

concentration of morpholine in the diluted stream corresponded to 1000 ppm. An electrically

heated three-zone horizontal split furnace accommodated the quartz reactor tube, and ensured

a well-defined isothermal regime. The use of helium (99.999 %) as primary dilution gas

facilitated better conductive heat transfer in the reaction zone, and also allowed quantitative

measurement of N2. The stream concentrations, stoichiometric conditions, and other

parametric details are provided in the subsequent relevant chapters.

Figure 3.2. Schematic representation of experiment rig for the study of the reaction of

biomass surrogate with NOx.


72

As illustrated in Figure 3.2, product analyses involved three parallel stages. Firstly, FTIR

spectroscopy (Perkin Elmer) facilitated online monitoring of product species exiting the tubular

reactor. The setup incorporated an electrically heated transfer line (3.175 mm ID × 300 mm)

linking the reactor to a small volume (100 mL) 2.4 m path-length online gas sampling cell

(Pike) placed inside the FTIR compartment. The spectrometer averaged 64 accumulated scans

per spectrum at 1 cm-1 resolution. Both the sample line and the sampling cell operated at 140

C, to avoid the condensation of product volatiles prior and during the analysis. Furthermore,

the chemiluminescence NOx analyser (Thermo Scientific model 42i-HL) monitored the

concentration of NO and NO2. Finally, the use of μGC (Agilent 490 micro-gas

chromatography, 20 m MolSieve-5A column, heated injection) assisted N2 quantitation. The

train of three impingers consisting of glass units filled with standardised 0.1 M H2SO4, 1 M

NaOH, and orange silica gel removed any contaminant species, e.g. ammonia and H2O that

could damage the GC column.

3.3. Data Analyses

Unless otherwise stated, Microsoft excel and Origin program served as the main numerical

analysis tools for data presented in this thesis. The results from analytical instruments were

processed in the respective software provided by the vendors. Regarding the FTIR spectra,

QASoft database enabled the semi-quantitation of product species by their characteristic IR

bands. Further details are provided in the subsequent chapters.


73

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for polyatomic molecules. J. Chem. Phys. 1990, 92, (1), 508-517.

5. Delley, B., From molecules to solids with the DMol3 approach. J. Chem. Phys. 2000,

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6. Tkatchenko, A.; Scheffler, M., Accurate molecular van der waals interactions from

ground-state electron density and free-atom reference data. Phys. Rev. Lett. 2009, 102, (7),

073005.

7. Frisch, M.; Trucks, G.; Schlegel, H. B.; Scuseria, G.; Robb, M.; Cheeseman, J.;

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Phys. 1999, 110, (6), 2822-2827.





74

10. Li, S.; Davidson, D. F.; Hanson, R. K.; Labbe, N. J.; Westmoreland, P. R.; Oßwald, P.;

Kohse-Höinghaus, K., Shock tube measurements and model development for morpholine

pyrolysis and oxidation at high pressures. Combust. Flame 2013, 160, (9), 1559-1571.

11. Brown, M. E., Introduction to Thermal Analysis: Techniques and Applications.

Springer Netherlands: 2001.

12. Haines, P. J., Principles of Thermal Analysis and Calorimetry. Royal Society of

Chemistry: 2002.

13. Höhne, G.; Hemminger, W.; Flammersheim, H. J., Differential Scanning Calorimetry.

Springer: 2003.

14. Bromly, J. H.; Barnes, F. J.; Mandyczewsky, R.; Edwards, T. J.; Haynes, B. S., An

experimental investigation of the mutually sensitised oxidation of nitric oxide and n-butane.

Symposium (International) on Combustion 1992, 24, (1), 899-907.

15. Bromly, J. H.; Barnes, F. J.; Muris, S.; You, X.; Haynes, B. S., Kinetic and

thermodynamic sensitivity analysis of the NO-sensitised oxidation of methane. Combust. Sci.

Technol. 1996, 115, (4-6), 259-296.

16. Bromly, J. H.; Barnes, F. J.; Nelson, P. F.; Haynes, B. S., Kinetics and modeling of the

H2-O2-NOx system. Int. J. Chem. Kinet. 1995, 27, (12), 1165-1178.

17. Glarborg, P.; Kubel, D.; Kristensen, P. G.; Hansen, J.; Dam-Johansen, K., Interactions

of CO, NOx and H2O Under Post-Flame Conditions. Combust. Sci. Technol. 1995, 110-111,

(1), 461-485.

18. Nelson, P. F.; Haynes, B. S., Hydrocarbon-NOx interactions at low temperatures—

1.Conversion of NO to NO2 promoted by propane and the formation of HNCO. Symposium

(International) on Combustion 1994, 25, (1), 1003-1010.

75

CHAPTER 4

Paper II: Oxidation of Crystalline Polyethylene

Oluwoye, I.; Altarawneh, M.; Gore, J.; Dlugogorski, B. Z., Oxidation of crystalline

polyethylene. Combust. Flame 2015, 162, (10), 3681-3690.

γ- path; formation of

aldehyde and oxetane

α- path; formation of

ketone

β- path; formation of

epoxide

Intermolecular path;

formation of alkoxy

and alkyl hydroxide

This chapter develops a systematic approach of investigating polymer-gas

reactions. The interaction of O2 with crystalline polyethylene have been

studied, with appropriate comparison to analogous reactions in gas medium.

This allows adequate investigation of the intended NOx-polymer reaction later

presented in Chapter 5 of this thesis

…

CHAPTER 4 — Oxidation of Crystalline Polyethylene

76

1

4.0. Abstract

Auto-oxidation of polyethylene (PE) is of a common occurrence and could be triggered by

several physical and chemical factors. In this study, for the first time, we report a

comprehensive theoretical account on the initial oxidation of crystalline PE at low temperatures

prior to its melting. We map out potential energy surfaces for large number of reactions, most

notably, initial abstraction by O2 molecules, formation of peroxy- and hydroperoxyl adducts,

unimolecular eliminations of HO2 and H2O as well as C-C bond fissions. Rate constants have

been estimated for all considered reactions over the temperature range of 300 – 800 K. We

have discussed noticeable similarities between the oxidation of PE and that of gas-phase

alkanes. Results presented herein provide new insights into the solid-state oxidation of PE and

germane crystalline polyolefins/paraffins and pure carbon-hydrogen-type polymers.


77

4.1. Introduction

Polyethylene (PE) holds wide range of applications in the society. As a nontoxic low-priced

polymer, PE resins have emerged as an important raw material for various industrial uses,

frequently deployed, in substantial amount, in medical, automobile, food, pipeline, civil, and

domestic sectors.1, 2 The key to the adaptability of PE lies in its adjustable semicrystalline

morphology, giving rise to its existence in various forms. Major types of PE include high

density polyethylene (HDPE), low density polyethylene (LDPE), linear low density

polyethylene (LLDPE), and ethylene-vinyl acetate copolymer (EVA).1, 3 Due to their elevated

calorific value and suitable combustion properties,1, 4, 5 used PE finds outlets in an extensive

array of recycling processes, such as thermal energy recovery via incineration technologies,

thermolytic conversion into liquid fuels6-9 and even pyrolytic conversion to low-carbon

hydrocarbon gases for application in hybrid rocket propulsion systems.10, 11 Practical

applications of thermal degradation of solid fuels often occur in oxidative environment,12 thus,

it is important to understand the thermo-oxidative degradation mechanism of PE.

The thermal decomposition of PE arises by homolysis of C–C bond, unzipping, in successive

enchained β-scission reactions, yielding monomer from the polymer. The decomposition then

proceeds by free radical mechanism involving multistep chain reactions. This consists of

consecutive H-abstraction and β C‒C scission, intramolecular hydrogen transfer (backbiting)

and intermolecular hydrogen shift (random scission). At high temperature under pyrolytic

conditions, PE undergoes both backbiting and random scission pathways.6, 13-16 The overall

relatively high activation energy for the inert decomposition of PE (150 – 260 kJ/mol) is linked

to a limiting step characterised by the initial random scission.6, 12, 17-20


78

In presence of oxygen, the reactions proceed differently with lower activation energy of around

80 – 143 kJ/mol.11, 12, 20, 21 Experiments on oxidation of low density PE and related polymers

usually involve irradiation at low temperature (photo-oxidative accelerated weathering) or a

temperature controlled air-circulated operation at moderate temperature of between 373 to 428

K (i.e., thermal ageing).21-24 Reactors and/or thermogravimetric analysers serve to perform

high temperature oxidation experiments above 473 K (thermo-oxidative degradation).12, 20

Different pathways exist for the oxidation mechanism of PE.21, 22, 25-30 In the most general

form, thermo-oxidative degradation begins by formation of radical sites initiated by induced

temperature, but UV radiation as well as existence of trace peroxides and hydroperoxides also

influence the initiation of photo-oxidative pathways. Oxygen reacts with the chain radical to

form a peroxy-radical intermediate during the propagation step:

Initiation: RH → R˙ + H˙ (a)

Propagation: R˙ + O2 → ROO˙ (b)

The peroxy radical abstracts hydrogen to form hydroperoxide and a radical centre. The former

yields an alkoxyl radical via a unimolecular or bimolecular decomposition mechanism. An

alkoxyl radical can effectively abstract a hydrogen to form a hydroxyl group. Intermolecular

and intramolecular H-abstractions lead to formation of new radical sites, thereby accelerating

the oxidative degradation.

ROO˙ + RH → ROOH + R˙ (c)

ROOH → RO˙ + OH (d)

ROOH + RH → RO˙ + R˙ + HOH (e)


79

RH + RO˙ → ROH + R˙ (f)

Amorphous zone of PE has more access to oxygen compared to the densely packed crystalline

regions. The amorphous zone oxidises first and then acts as a buffer, protecting the crystalline

region from attack during thermal degradation. Therefore, increasing the crystallinity lowers

the oxidation rate and increases the sensitivity of mechanical properties of PE towards

oxidation.31, 32

While oxidative decomposition of PE mainly occurs in liquid media upon melting, auto-

oxidation of solid PE at relatively low temperatures is a common phenomenon that could be

triggered by many physical or chemical factors. These factors encompass presence of

structural defects in PE, ultraviolet and ionising radiation, prompt heating, ultrasound, or direct

exposure to catalysis, singlet molecular or atomic oxygen and ozone.33

To this end, the present study deploys the density functional theory (DFT) to investigate initial

steps encountered in the oxidation of crystalline PE. We analyse the reaction mechanisms

pertinent to thermo-oxidative degradation of PE, and evaluate and compare the kinetic and

thermochemical parameters with limited experimental measurements from the literature.

Results presented herein provide improved insights into the solid-state oxidation of crystalline

polyolefins/paraffins and carbon-hydrogen-type polymers. Such knowledge is instrumental to

understand the auto-oxidation process of crystalline PE that underpins its natural ageing

mechanism under thermo- and photo-oxidative conditions.


80

4.2. Methodology

4.2.1. Computational details

DMol3 package34, 35 facilitates all structural optimisations, and serves to calculate all energies

and vibrational frequencies. In the optimisation of all stationary points, our theoretical

approach comprises the generalised gradient approximation (GGA) as the exchange-

correlation potential alongside with the Perdew-Burke-Ernzerhof (PBE) functional. The total

energy converges with a tolerance of 1 × 10−6 Ha. The electronic core treatment includes all

electrons in the calculations while deploying a double numerical plus polarisation (DNP) basis

set,34 and a global cut-off of 4.0 Å. Based on the subsequent thorough benchmarking against

analogous experimental and theoretical values from the literature, we have found that, the

adopted DFT functional and basis set offer a satisfying accuracy threshold for the system at

hand. We carry out integration of the Brillouin zone via automatic generation of 2×3×6 and

1×2×1 k-points for the unit cell and periodic slabs, respectively, through the Monkhorst-Pack

grid scheme. The average computational cost, for each periodic slab typically amounts to 20

h on a 32 parallel processing cores. Last of all, Tkatchenko and Scheffler’s semiemperical

dispersion correction scheme36 offers the necessary measures to validate the compromise

between the cost of evaluation of the dispersion terms and the need to improve non-bonding

interactions in our system.


81

4.2.2. Molecular and crystallographic structural parameters of polyethylene

Crystalline PE exhibits an orthorhombic crystal structure under all but the most exceptional

circumstances. Its unit cell contains all crystallographic data needed for a complete crystallite.1,

37 We generated the reference unit cell structure by optimising the orthorhombic crystalline

arrangement reported by Avitabile et al.38 The primitive cell exhibits a Pnam space group 39,

40 that consists of twelve atoms, equivalent to two -CH2- chains as shown in Figure 4.1.

Figure 4.1. Unit cell of ideal orthorhombic polyethylene; plan (left) and isometric (right)

views.

Tables 4.1 and 4.2 report optimised structural parameters, i.e., lattice constants a, b, and c, and

the fractional atomic coordinates for the optimised PE unit cell. Our calculated lattice

parameters are within the slightly scattered experimental measurements37, 38 and computed

values41, 42 available in literature. Note a negligible difference in the fractional atomic

coordinates, if compared with the analogous experimental values.38, 43, 44 Although, dispersion-

correction approach reduces the lattice constants somewhat closer to experimental values, it

also increases the computation time by approximately 20 %. As dispersion corrections display

importance in the modelling of some, but not all, physical systems.45 For the present


82

calculations, the application of the dispersion correction does not change the chemical

interpretations of the results.46 Therefore, our chemical reaction modelling excludes the

dispersion correction.

Table 4.1. Optimised geometrical parameter of crystalline polyethylene unit cell without (1)

and with (2) dispersion correction as compared to experimental values. Unit conversion: 1 Å

equals 0.1 nm.

Parameters Computational results Experimental measurements

This work LDA

41

GGA

41

GGA

42

Neutron

scattering

(4 K) 38

Neutron

scattering

(90 K) 38

X-ray

diffraction

37 (1) (2)

Lattice

constants

(Å)

a 7.803 7.211 6.549 7.304 8.28 7.121 7.161 7.400

b 5.251 4.776 4.446 5.017 5.64 4.851 4.866 4.930

c 2.566 2.555 2.515 2.565 2.57 2.548 2.546 2.543

Bond

length (Å)

C-C 1.532 1.526 1.5 1.53 1.523 1.578 1.574 1.530

C-H 1.104 1.101 1.12 1.11 1.110 1.100 1.070 -

Bond

angle (°)

C-C-C 113.8 113.6 113.5 113.5 113.0 107.7 108.1 112.0

H-C-H 105.9 105.9 105.4 106.1 105.8 109.0 110.0 -


83

Table 4.2. Calculated fractional coordinates of atoms in the unit cell of orthorhombic PE. ψ1

= 0.0399761, ψ2 = 0.0531014, ψ3 = 0.1788820, ψ4 = 0.0127033, ψ5 = 0.0279984, ψ6 =

0.2627310.

Atoms Fractional coordinates

C ± (ψ1, ψ2, 1

4)

H ± (ψ3, ψ4, 1

4)

H ± (ψ5, ψ6, 1

4)

C ± (1

2 – ψ1,

1

2 + ψ2, −

1

4)

H ± (1

2 – ψ3,

1

2 + ψ4, −

1

4)

H ± (1

2 – ψ5,

1

2 + ψ6, −

1

4)

4.2.3. Reaction modelling

A vacuum slab parallel to a plane defined by Miller index of (100) is constructed from the

optimised unit cell to represent a PE(100) surface. Figure 4.2 depicts optimised geometry of

the PE(100) surface. The vacuum slab has four repeated periodic layers. In all structural

optimisations, we relax fully the two top layers and molecules, while the two bottom layers

remain fixed at their corresponding bulk positions. A vacuum thickness of at least 10.0 Å (1

nm) separates each slab from its corresponding repeated images along the z axis. This

eliminates the interactions within the periodic images of the slab. A dipole correction, applied

along the direction perpendicular to the surface, minimises any surface deformation owing to

the dipole accumulation along the z direction.


84

Figure 4.2. Vacuum slab of crystalline PE(100). Unit conversion: 1 Å equals 0.1 nm.

Search for transition states deploys the complete linear synchronous and quadratic synchronous

transit approaches (LST/QST), as implemented in DMol3. Reaction rate constants are fitted to

the Arrhenius equation (i.e., k(T) = A exp(-Ea/RT)) in the temperature range of 298.15 – 800 K,

according to the classical transition state theory (TST):47

RT

ΔH

R

ΔS

h

Tkσk(T) B

e expexp

(4.1)

In Equation 4.1, σe signifies the reaction degeneracy number, i.e., set to unity in all considered

reactions (i.e. per C/H/O site). ΔS≠ and ΔH≠ refer to entropy and enthalpy of activation at

temperature T, in that order. The remaining symbols, kB, h, and R denote Boltzmann’s, Planck’s

and the universal gas constants, respectively. DMol3 package computes vibrational frequencies


85

from which it obtains the temperature-dependent thermochemical parameters (∆H, ∆S, ΔH≠

and ΔS≠).

4.3. Results and Discussion

4.3.1. Initiation reactions

Figure 4.3 presents energetics of two possible initiation pathways. Henceforth, we show only

the two top layers (unrestricted layers) of all modelled slabs. The C‒H bond dissociation

enthalpy (BDH) (1a) in PE amounts to 416.2 kJ/mol. This value concurs with the

corresponding BDH in secondary C‒H bonds in alkanes.48, 49 Direct abstraction of an H atom

by a triplet oxygen molecule (1b) entails a highly endothermic process of 351.1 kJ/mol at

298.15 K (all thermodynamic and kinetic parameters are calculated and discussed at this

temperature). Thus, it is unlikely that, direct H abstraction by the triplet oxygen molecule from

PE represents a significant initiation channel.


86

Figure 4.3. Initiation reactions in the oxidative decomposition of PE. The location of radical

site is indicated by the bolded C atom.

Figure 4.4 portrays detailed mechanisms for subsequent oxidation steps. In real oxidation

scenario, O/H radicals readily abstract an H from the CH site to form a radical centre. With

the appearance of a radical site, oxygen molecule reacts in two modes; namely, direct addition

and H abstraction. Figure 4.5 presents enthalpic profiles for these two reactions. Barrierless

addition of O2 (2a) to a –C(H)‒ site forms a highly reactive peroxy adduct intermediate. In an

ordered medium, such as a crystalline PE, there are restrictions to diffusion of oxygen.

Calculations yield the exothermicity of this reaction as 131.2 kJ/mol. Corresponding values of

the analogous reactions involving gaseous ethyl and butyl radicals amount to 141.8 kJ/mol and

136.0 kJ/mol, respectively.50, 51


87

Figure 4.4. Reaction pathway for thermo-oxidation of crystalline polyethylene. Reactive

hydrogen atoms are labelled α, β, and γ, according to their position relatively to the peroxy

group. Note the location of radical sites, for the figures that follow.

In the parallel competing pathway (2b), triplet oxygen abstracts an H-atom from the carbon

atom adjacent to the radical centre. This reaction leads to formation of an alkene molecule and

a hydroperoxyl radical. These two reactions constitute the major initiation channels in the low-

temperature combustion of alkyl radicals. Calculations yield the enthalpy of activation for the

abstraction channel as 212.5 kJ/mol and a modest reaction exothermicity of 44.2 kJ/mol, i.e.,

in a close agreement with the corresponding value for the formation of 1-butene + HO2 from

the n-butyl + O2 51 reaction. Based on enthalpic values included in Figure 4.5, the formation

of a peroxy adduct, via the addition channel (2a), represents a more preferred channel than the

competing abstraction corridor (2b).


88

Figure 4.5. Reactions between oxygen and the alkyl-type radical of PE. Enthalpy values are

calculated at 298.15 K and the bolded C atoms indicate radical sites.

Direct elimination of HO2 from peroxyalkyl (3) constitutes a principal corridor for the

formation of alkenes from peroxyalkanes at low temperatures.50 This reaction plays a crucial


89

role by generating HO2, a propagating radical that sustains the oxidation process via the

reaction sequence (HO2 + RH → H2O2 + R˙ → 2OH + R˙). In case of PE, our calculated value

reveals endothermicity of 87.0 kJ/mol, and a reaction barrier of 97.4 kJ/mol (TS2). For the

reactions of n-propyl radical with O2 molecules, concerted elimination of HO2 from RO2 adduct

implies endothermicity of 79.1 kJ/mol and a barrier height of 124.3 kJ/mol.50

4.3.2. Formation of hydroperoxide group

Apart from its contribution to the generation of alkenes and HO2 radicals, peroxy intermediate

branches also into three other channels. Isomerisation of peroxy radical consists of

intramolecular hydrogen transfer, driven principally by the conversion of alkyl peroxy species

into alkyl hydroperoxide radicals.52 A peroxy radical abstracts hydrogen atom intramolecularly

from α, β and γ positions (refer to Figure 4.4). Figure 4.6 represents potential energy surface

(PES) for these reactions. Abstraction of hydrogen atom from the α position (4c) does not lead

to formation of hydroperoxide species, rather yielding molecules of water and γ-alkyl ketone

radical in an exothermic reaction. Despite of our best attempts, we were unsuccessful in

locating a transition state for this reaction (4c). Previous studies attributed this reaction to

formation of radical species that dissociate in microseconds into a long-range complex between

a hydroxyl radical and an ester-type adduct.24, 53, 54

On the other hand, H-abstraction from the γ (4a) and β (4b) position are endothermic by 66.9

kJ/mol and 69.3 kJ/mol and require activation enthalpies of 82.6 kJ/mol (TS3) and 91.9 kJ/mol

(TS4), in that order. Barrier heights of TS3 and TS4 compare well with analogous values

encountered in the intramolecular hydrogen-transfer reaction induced by alkyl peroxy


90

adducts.52 For instance, the analogous gas phase reaction (CH3C(O2˙)H(CH2)2CH3 →

CH3C(O2H)HCH2C˙HCH3) has endothermicity of 68.2 kJ/mol and activation energy of 89.1

kJ/mol.52

Figure 4.6. Intramolecular formation of hydroperoxide and ketone. Enthalpy values are

calculated at 298.15 K and the bolded C atoms indicate radical sites. We could not locate the

transition state for Reaction 4c.

In the intermolecular path as shown in Figure 4.7, a peroxy species abstracts a labile hydrogen

from a neighbouring polymer chain (8). A computed reaction barrier displays prohibitively


91

high enthalpy value of 443.9 kJ/mol. From a view of a typical reaction coordinate, the

geometry of the neighbouring chain constitutes the only factor that could be at play here, to

make the barrier so high. The geometrical arrangement of PE might be highly constrained by

the crystal structure, rendering the hydrogen atom inaccessible. It follows that, hydrogen

abstraction from a neighbouring hydrocarbon chain carries negligible importance in auto-

oxidation of PE.

Figure 4.7. Formation of hydroperoxide by intermolecular abstraction of H atom. Enthalpy

values are calculated at 298.15 K and the bolded C atoms indicate radical sites.

However, at higher temperatures near the melting point, the chains acquire sufficient

conformational flexibility to render the hydrogen atoms more accessible between chains,

thereby substantially lowering the barrier. To provide a more realistic kinetic data germane to

Reaction 8 near melting point of PE, we invoke an alternative procedure to account for the

intermolecular H-transfer. Korcek et al.’s equation55 establishes a generalised linear


92

relationships among the activation energy, rate constant per hydrogen atom, and R‒H bond

strength for intermolecular H-abstraction by secondary peroxy radicals; Ea = 0.55 × (ΔH˚ −

62.5) and log(k) = 16.4 – (0.2 × ΔH˚ ), where ΔH˚ is expressed in kcal/mol. This relationship

allows obtaining approximate values of the activation energy and the rate constant for Reaction

8, from BDH of C-H bonds in PE of 416.2 kJ/mol.

4.3.3. Hydroperoxide decomposition and other reactions

Hydroperoxide from the γ pathway undergoes a O‒O bond dissociation and C‒C β scission

(5a1) to yield molecules of an alkene and an aldehyde. As shown in Figure 4.8, exothermicity

of this process amounts to 19.9 kJ/mol with 116.0 kJ/mol (TS5) as its enthalpy of activation.

The endothermic formation of alkyl oxetane (5a2) represents a competing pathway. This

reaction requires a barrier height of 89.0 kJ/mol associated with TS6. On the other hand,

hydroperoxide from the β channel decomposes through a similar barrier height of 92.9 kJ/mol

(TS7), assuming structural rearrangement to form an alkyl epoxide in exothermic reaction (5b)

of 27.1 kJ/mol.


93

Figure 4.8. Decomposition of intramolecular hydroperoxide species. Enthalpy values are


Figure 4.9 portrays conventional barrierless unimolecular decomposition that either yields

alkoxy radical (9a), through an endothermic reaction of 191.3 kJ/mol, or loses HO2 (9b), to

form alkyl radical with endothermicity of 292.7 kJ/mol. In comparison to similar reaction

types, values of BDH of nBuO‒OH and nBu‒OOH correspond to 189.1 kJ/mol and 293.7

kJ/mol respectively.56 The alkoxy radicals can ultimately generate alkyl hydroxide by


94

intramolecular hydrogen abstraction from γ (10a1) and β (10a2) sites. These reactions proceed

with modest activation enthalpies of 90.4 kJ/mol (TS8) and 133.0 kJ/mol (TS9), respectively,

as shown in Figure 4.10.

Figure 4.9. Decomposition of intermolecular hydroperoxide species. Enthalpy values are



95

Figure 4.10. . Formation of alkyl hydroxide species by intramolecular abstraction of H atom.

Enthalpy values are calculated at 298.15 K and the bolded C atoms indicate radical sites.

Barrierless addition of an oxygen molecule (6) at the γ radical site, formed by peroxy group

abstracting the H atom in Reaction 4a, is found to be exothermic by 98.5 kJ/mol and results in

the formation of peroxyalkyl hydroperoxide radical (a chain comprising peroxy radical on the

γ carbon with respect to the hydroperoxide group). This radical can isomerise by internal

transfer of hydrogen atom in an analogy to the well-documented process for the corresponding

gas-phase reactions of alkanes to form alkyl dihydroperoxide radicals.57 Such rearrangements

comprise mechanisms that are similar to that of α, β and γ H shift. Herein, we investigate

intermolecular abstraction of a hydrogen atom (7). The reaction is endothermic by 57.8 kJ/mol

and associated with a modest reaction barrier of 85.4 kJ/mol. Figure 4.11 shows the PES for

Reactions 6 and 7.


96

Figure 4.11. Formation of alkyl dihydroperoxide. Enthalpy values are calculated at 298.15 K

and the bolded C atoms indicate radical.

4.3.4. Kinetic considerations

Reactions passing through the initially-formed peroxy adducts play significant role in the

overall low-temperature oxidation of PE, as depicted in Figure 4.4. Thermal oxidation of PE

is typically modelled by adapting global reaction schemes.12, 20 Reaction rate constants in

oxidation of liquid or solid phase polymers originate from matching gas phase reactions with

corrections due to inversion or transposition of the polymers into the liquid or solid phase. In

this approach, Ea values are corrected (i.e. lowered) by subtracting condensation heats of

reactants. Correction in values of A factors reflects the loss of degree of freedom when shifting

from a gas to a solid or liquid phase. This loss is expressed by translational and rotational

entropy contributions according to the following expression:58


97

kliq/solid = kgas exp (-∆Hgas→liq/solid

RT) exp (

∆Sgas→liq/solid

R)

Likewise, rate parameters exist only for limited number of elementary kinetic steps27 and

mechanistic schemes built from the elementary reactions rarely involve reactive pathways

comprising intramolecular reactions.

We evaluate accuracy limits implicit in our calculated kinetic data based on a comparison with

corresponding experimental values obtained by Vana Sickle and co-workers,52, 59 for the

intramolecular 1,5-hydrogen shift during auto-oxidation of liquid 2-peroxypentane. Our

kinetic estimation for this reaction affords a barrier of 81.9 kJ/mol and log A of 13.1 s-1; i.e., in

agreement with the corresponding experimental assessment for this reaction, 82.4 kJ/mol and

11.5 s-1, respectively.

Figure 4.12 depicts Arrhenius plots for several reactions investigated in this work, and Table

4.3 itemises reaction rate parameters in terms of activation energies (Ea) and the Arrhenius pre-

exponential A factors. However, since molecular oxygen diffuses more easily in unordered

medium, we expect higher rates in the amorphous and defected regions of PE. Values in Table

4.3 represent the first set of kinetic data that are calculated specifically for PE. Literature

studies50, 52, 57, 60, 61 have adopted kinetic parameters of the corresponding gas-phase reactions.

The overall rate-limiting step revolves around formation and decomposition of hydroperoxides

that follow an experimental reaction rate expression of k(T) = 1.5 × 1010 exp(-73

(kJ/mol)/(R·T)) [L/(mol s)] and k(T) = 2.5 × 1014 exp(-146 (kJ/mol)/(R·T)) [s-1] respectively.21,

62 The latter accounts only for a typical unimolecular O‒O bond fission (ROOH → RO˙ +

˙OH). As listed in Table 4.3, the decomposition of hydroperoxides follow three additional

routes, in which the structural rearrangement of RO˙ lowers the activation energies. Baum63


98

explicitly followed the actual change in structure of PE during oxidation. At moderate

temperature, the rate of formation of hydroperoxides proves that, the adducts rapidly reach

their equilibrium concentration. Subsequently, their decomposition to free radicals accelerates

the oxidation process. Essentially, while our calculated values describe auto-oxidation process

in crystalline PE, they may also serve to model the oxidative decomposition of melted PE,

provided that they are adjusted by the transposition approach.

Reaction rate parameters for channels 2a, 4c, 6, 9a and 9b cannot be computed by the

conventional TST method because they appear barrierless. As an alternative approach, the

variational transition state theory64, 65 affords a robust procedure in which the reaction rate

constant is minimised as a function of temperature and reaction coordinate. However,

application of such methodology in our system proves highly challenging in terms of

computational cost. To the best of our knowledge, literature provides no application of the

VTST on a periodic-boundary system. Nonetheless, as a limiting case scenario, one can

assume the calculated enthalpies for these reactions as the energy term in the Arrhenius

equation. A factors for these reactions follow from those of analogous gas-phase reactions of

alkanes, corrected by the transposition method.


99

Figure 4.12. Arrhenius plots for the studied reactions.

1.0 1.5 2.0 2.5 3.0 3.5

-35

-30

-25

-20

-15

-10

-5

0

5

10

15 lo

g k

(s-1

)

1000/T (K-1)

2b

3

4a

4b

5a1

5a2

5b

10a1

10a2


100

Table 4.3. Kinetic parameters of reactions involving key radical intermediates.

* Estimated from linear relationship among R-H BDH, activation energy and rate constant for

intermolecular H-abstraction by secondary peroxy radicals.

4.4. Conclusions

This chapter has analysed the initial elementary steps occurring during the low-temperature

oxidation of PE, locating optimised geometries and estimating thermodynamic properties of all

plausible products and transition structures. Aldehydes, epoxides, oxetanes and ketones appear

as a consequence of intramolecular H-transfer from three adjacent ‒CH2‒ sites, on the same

hydrocarbon chain, to the outer oxygen atom in the initially-formed peroxy adduct. Secondary

alkoxy and hydroxy groups arise from intermolecular H-transfer from a neighbouring chain,

with a generalised linear relationship deployed to estimate the respective activation energy and

Reaction Ea (kJ/mol) A (s-1); for Reactions 2b

and 8, A [L/(mol s)]

2b: R˙+ O2 → Alkene + HO2 212.6 1.72 × 102

3: ROO˙ → Alkene + HO2 97.1 5.90 × 1012

4a: ROO˙ → Rγ˙OOH 84.0 1.67 × 107

4b: ROO˙ → Rβ˙OOH 91.3 4.41 × 109

5a1: Rγ˙OOH → Alkene + RO + HO˙ 111.5 1.30 × 1013

5a2: Rγ˙OOH → ROoxetane + HO˙ 93.5 1.67 × 1018

5b: Rβ˙OOH → ROepoxide + HO˙ 95.9 4.24 × 1015

8*: ROO˙ + RH → ROOH + R˙ 85.4 1.51 × 1011

10a1: RO˙ → Rγ˙OH 90.7 8.50 × 107

10a2: RO˙ → Rβ˙OH 135.0 1.36 × 1011


101

rate constant of H-transfer. Moderate activation energies accompany intermolecular reactions

if compared with intramolecular reactions. Thermochemical and kinetic parameters obtained

in this study provide an understanding of the solid-state auto-oxidation of PE and afford

insights into oxidation of other crystalline polyolefins.

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109

CHAPTER 5

Paper III: Oxidation of Polyethylene under Corrosive

NOx Atmosphere

Oluwoye, I.; Altarawneh, M.; Gore, J.; Bockhorn, H.; Dlugogorski, B. Z., Oxidation of

polyethylene under corrosive NOx atmosphe1re. J. Phys. Chem. C 2016, 120, (7), 3766-3775.

The paper has been reformatted for Australia English.

Nitroso ⇌ Oxime

Nitroxyl

Nitro ⇌ Aci form

Nitrous acid

Organic nitrite

Organic nitrate

Macromolecular Polyethylene

O2

NO2

NO

This chapter describes the low-temperature interaction of polyethylene

with NOx. It investigates the relevant pathways leading to the formation

of initial products, hence, furnishes insights into the experimental

observation reported in Chapter 6

…

CHAPTER 5 — Oxidation of Polyethylene under Corrosive NOx Atmosphere

110

1

5.0. Abstract

This chapter reports the results of comprehensive quantum-mechanical calculations of low-

temperature oxidation (i.e., below the softening point) of polyethylene (PE) film in a corrosive

NOx environment, by mapping out the potential energy surface for a large set of reactions,

developing thermokinetic parameters for each elementary reaction and discussing in detail the

most energetically-favourable paths. Remarkably, addition of nitric oxide (NO) and nitrogen

dioxide (NO2) results in formation of C-nitroso and nitro species, respectively, and

successively leads to internal tautomerisation or concerted elimination of nitroxyl (HNO) and

nitrous acid (HNO2). We demonstrate that, the presence of molecular oxygen sustains the

formation of O-nitroso compounds, organic nitrites and nitrates. Reaction rate parameters have

been established for all considered reactions over the temperature range of 300 K to 800 K.

The results presented herein provide new insights into the solid-state polymer-gas reactivity of

PE in NOx atmosphere pertinent to thermal recycling of materials laden with PE and to co-

combustion of PE with a nitrogen-rich fuel such as biomass. The results will find application

to systems that involve oxidative decomposition of germane crystalline polyolefins/paraffins

and pure carbon-hydrogen-type polymers induced by aggressive gases such as NO and NO2.


111

5.1. Introduction

Long-established and widespread use of polymers necessitates the study of their reactivity,

thermal stability, degradation and decomposition under conditions typical of their intended

function. Polyethylene (PE), for example, as one of the major non-toxic commercially

available polymer, serves in wide range of applications; from household utensils to industrial

and medical paraphernalia.2, 3 The deployment of PE had encouraged a great deal of research

on its pyrolysis, oxidation and chemical reactivity.4-10 This research has mainly aimed to

understand the decomposition behaviour of PE for application related to thermal recycling of

materials laden with PE and energy recovery of main stream municipal wastes, often loaded

with significant amounts of pure carbon-hydrogen-type polymers.

Regarding thermal studies, a clear distinction rests between thermal degradation and thermal

decomposition. The former describes a process wherein certain defining properties of a

polymeric material deteriorate under heat exposure, while the latter is characterised by a

variation in the chemical structure or an extensive change in chemical identity caused by heat.11

Although, to a reasonable extent, both phenomena for PE had been understood under oxidative

O2 conditions,12-15 the literature lacks detailed demonstrations on how PE decays in other

reactive fumes.

In the 1950s, Ogihara pioneered the study of reaction of nitrogen dioxide with PE.16, 17

Afterwards, Jellinek and collaborators investigated reactions of different classes of polymers

with air pollutants, such as oxides of sulfur (SOx) and nitrogen (NOx).18, 19 At low atmospheric

concentration and standard conditions, the aggressive pollutants (NOx) exert limited effect on

vinyl polymers. However, a slight increase in concentration and temperature induces a


112

pronounced interactions within their macromolecular structure.20 In fact, this kind of

conditions becomes practically important when industrial applications deploy PE in processes

characterised by elevated concentration of NOx. For instance, in high-voltage systems,

polymer insulators could suffer NOx attack during a coronal discharge.21-24 Likewise, a

discharge treatment of oxidative corona of PE films in air yields an unsaturated surface,

abundant in oxygenated species such as ketones, aldehydes, alcohols, and esters, with large

numbers of macroradicals,3, 25, 26 which can possibly react with oxides of nitrogen, produced

concurrently in the course of the same process.27, 28 Furthermore, in another type of surface

modification (hydrophilic plasma treatment), a stream of reactive nitrogen oxides implants

nitrogen-bearing functional groups on the surface of a polymeric material.29, 30 In addition,

oxidation of PE in fuming nitric acid (HNO3) also incurs the contributions of nitrogen-oxide

functionalities.31, 32 Furthermore, the polyethylene-enhanced selective catalytic reduction

(SCR) of NOx33 and co-combustion of PE with a nitrogen-rich fuel could serve as additional

examples for the co-existence of O2 – NOx – PE.

Primarily, NOx includes nitric oxide (NO) and nitrogen dioxide (NO2). Because of the toxicity

of NO and NO2, 34 their study remains a thematic research topic among environmentalists,

chemists and engineers. Gas-phase reaction of NOx with hydrocarbons attracts much interest,

as it plays an important role in combustion, as well as in atmospheric and tropospheric

chemistry.35-44 However, results from such findings cannot fully describe analogous

condensed-phase reactions. For instance, reaction of NO2 with alkene experiences different

competitive pathways in solutions.45 In gas-solid systems, molecules have insufficient

conformational flexibility that restricts mobility of constituent chains.


113

To this end, in the present chapter, we provide the first comprehensive theoretical evidence of

oxidative decomposition of PE under corrosive NOx atmosphere. By deploying the density

functional theory (DFT), from mechanistic and kinetic viewpoints, we analyse primary

initiation reactions of NOx with modelled PE species. Results presented herein provide new

physical insights into the solid-state oxidation of crystalline polyolefins/paraffins and carbon-

hydrogen-type polymers in NOx environment. In addition to the aforementioned systems, in

which NOx interferes with PE oxidation, our mechanistic and kinetic analyses are of broad

interest in understanding the role of nitrogen oxides during an auto-oxidation process of

crystalline PE, as apparent in its natural ageing mechanism under thermo- and photo-oxidative

conditions.

Finally, this work should not be confused with the so-called pinking or yellowing

(discolouring) of PE. This colour change arises from the reaction of NOx with a phenolic

antioxidant present in the resin. The surface oxidation leads to formation of quinone-like

structure that can be easily reversed in UV light.46, 47

5.2. Methodology

The reaction modelling rests on a similar approach applied in our recent theoretical

investigation of the oxidation of crystalline polyethylene by molecular oxygen.1 DMol3

package48, 49 enables all structural optimisations, and serves to calculate all energy and

vibrational frequencies. As illustrated in Figure 5.1, we develop a PE(100) surface defined by

the (100) Miller index, by employing an optimised form of a primitive orthorhombic crystalline

arrangement reported by Avitabile et al.50


114

Figure 5.1. Illustration of the adopted methodology. The prior study1 provides the complete

crystallographic structural parameters of the unit cell.

The vacuum distance between each periodic slab sufficiently eliminates any interactions within

the recurring images, while accommodating all target species, as well as dissociative products.

For the purpose of representation in this paper, we show only the major singular PE chain in

all potential energy surfaces (PES).

The DFT exchange-correlation potential employs the generalised gradient approximation

(GGA) of the Perdew-Burke-Ernzerhof (PBE) functional, with the tolerance on the

convergence of the total energy of 1 × 10−6 Ha. In the calculations, the electronic core treatment

includes all electrons while applying a double numerical plus polarisation (DNP) atomic basis

set, confined within a global cut-off of 4.0 Å. The integration of the Brillouin zone involves

automatic generation of 2×3×6 and 1×2×1 k-points for the unit cell and periodic slabs,


115

respectively, through the Monkhorst-Pack grid scheme. The average computational cost

typically amounts to 20 h (on 32 parallel processing cores) for each periodic slab. Our earlier

study1 revealed that, the selected choice of DFT functional and basis set offers a satisfying

accuracy threshold for the system at hand, even without semiemperical dispersion correction

scheme.

Furthermore, as implemented in DMol3, the complete linear synchronous and quadratic

synchronous transit approaches (LST/QST) locate all necessary transition states. Reaction rate

constants are fitted to the Arrhenius equation (i.e., k(T) = A exp(-Ea/RT)) in the temperature

range of 298.15 K to 800 K, according to the classical transition state theory (TST)51:

RT

ΔH

R

ΔS

h

Tkσk(T) B

e expexp

(5.1)

DMol3 package computes vibrational frequencies from which it obtains the temperature-

dependent thermochemical parameters. In Equation 5.1, we set the reaction degeneracy

number σe to unity (i.e. per C/H/O/N site) in all considered reactions. ΔS≠ and ΔH≠ refer to

entropy and enthalpy of activation at temperature T in that order, kB, h, and R symbolise

Boltzmann’s, Planck’s and the universal gas constants, respectively. The implicit accuracy

limit for the calculated reaction rate parameters have been validated by intramolecular 1,5-

hydrogen shift during auto-oxidation of liquid 2-peroxypentane. Our kinetic estimation for this

reaction affords a barrier of 81.9 kJ/mol and log A of 13.1 s-1; i.e., in reasonable agreement

with the corresponding experimental assessment for this reaction of 82.4 kJ/mol and 11.5 s-1,

respectively.52, 53


116


5.3.1. Initiation reactions and formation of radical species

Figure 5.2 portrays the energetics of the three initiation pathways. According to reaction (1a),

the bond dissociation enthalpy (BDH) of a secondary C‒H bond in PE amounts to 416.2 kJ/mol.

This value agrees well with the corresponding values for gas-phase alkanes; i.e., 410.0 – 416.0

kJ/mol.54, 55

RH → R˙ + H (1a)

Elevated temperature, UV radiation as well as mechanical or chemical factors may overcome

the sizable enthalpic requirement for the barrierless fission of a C‒H bond, permitting the

appearance of infinitesimal fraction of radical sites in the polymer matrix, ready for further

reaction.1

Earlier experimental studies speculated the initiation reactions to involve direct hydrogen

abstraction by NO2.19 Generally, such mechanism could take the form:

RH + NO → R˙ + HNO (1b)

RH + NO2 → R˙ + HONO (1c)

In this mechanism, the oxides of nitrogen abstract H atom from a polymer chain. Our

calculated results at 298.15 K (all reaction and activation enthalpies are discussed at this


117

temperature) elucidate (1b) as a highly endothermic process, requiring 342.9 kJ/mol, in like

manner to H-abstraction by triplet oxygen.1 Incurring a high reaction barrier of 380.7 kJ/mol

(TS1), it is unlikely that, direct H-abstraction by the NO free radical from PE represents a

significant initiation channel. However, direct secondary H-abstraction by a NO2 molecule

proves to be somewhat more feasible. It entails moderate reaction enthalpy (86.5 kJ/mol), over

a barrier height of 337.2 kJ/mol (TS2). Analogous abstraction of secondary hydrogen from

C3H8 incurs similar reaction enthalpy of 83.2 kJ/mol, but sufficient conformational flexibility

experienced in gas phase prompts a lowered barrier height of 136.1 kJ/mol.

Figure 5.2. Initiation reactions by H-dissociation (1a) and H-abstraction (1b and 1c).

Enthalpy vales are calculated at 298.15 K.

The presence of oxygen results in the generation of the reactive H/O radical pool in addition to

the formation of peroxy adducts. The latter triggers the formation of different stable species

such as aldehydes, epoxides, oxetanes, ketones and hydroxyl, mainly via intramolecular and

intermolecular H-transfer from adjacent ‒CH2‒ sites. As it is the case for alkanes,56 reactions

of H/O radicals with PE may also generate free alkyl sites. At lower temperatures,

hydroperoxyl radicals play a crucial role in driving the low-temperature oxidation cycle.57, 58


118

In subsequent steps, hydroxyl and hydroperoxyl radicals could add to vacant radical sites in

the PE chain. Consequently, oxidation of PE produces three main radical-type species; namely,

alkyl, hydroxyalkyl and hydroperoxyalkyl.1 In a rich NOx-environment, oxides of nitrogen

potentially react with three PE-type radicals as depicted in Figure 5.3. As the next sections

demonstrate, the reactivity of these radicals varies based on the characteristics of vicinal

functional groups.

Figure 5.3. Reaction pathways for primary initiations during oxidation of crystalline

polyethylene in NOx atmosphere.

5.3.2. Addition reactions and fate of consequential N-derivative species

The appearance of radical sites allows subsequent reactions. Remarkably, barrierless additions

of NO (2, 13 and 23) and NO2 (4, 15 and 25) are exothermically competitive. As shown in

Figure 5.4, these reactions lead to the formation of C-nitroso and nitro adducts, and respectively


119

release 164 – 192 kJ/mol and 383 – 411 kJ/mol, depending on the functionality of neighbouring

chemical groups, i.e. alkyl, hydroxyalkyl and hydroperoxyalkyl.

Figure 5.4. Addition of molecular oxygen, nitric oxide and nitrogen dioxide to different

carbon radical sites on a PE chain. Enthalpy vales are calculated at 298.15 K.

Generally, nitroso compounds convert into more thermodynamically stable oxime tautomers

by migration of an α H atom (i.e., from the same carbon atom attached to NO) to the nitroso

group.59 According to Figure 5.5, transformation of nitrosoalkyl in a PE chain (3a) reveals

exothermicity of 49.9 kJ/mol and a reaction barrier of 328.4 kJ/mol (TS3). However,

availability of hydroxyl (14) and hydroperoxyl groups (24), closely located on the same PE

chain, moderates the reaction barrier to 201.1 kJ/mol (TS4) and 231.1 kJ/mol (TS5),

respectively. On the other hand, with the exemption of nitromethanes, nitroalkanes exist as

highly versatile stable compounds.59, 60 As a result, in PE, tautomerisation of nitroalkyl into its


120

aci form (5a) proceeds via the enthalpy of activation of 285.6 kJ/mol with endothermicity of

73.1 kJ/mol. In addition, the presence of a hydroxyl group on the same chain (16) significantly

lowers the barrier to 132.4 kJ/mol, but existence of hydroperoxyl group (26) raises the barrier

up to 434.9 kJ/mol, with a significant change in the endothermicity of the reactions. Based on

results from our recent work on dehydrohalogenation of ethyl halides,61 we envisage that the

presence of the vicinal hydroxyl and hydroperoxyl functional groups alters the activation

energies via modifying the charge distribution environment around the transferred H atom.

Nevertheless, the exact effect of introducing these groups requires further scrutiny.

Figure 5.5. Tautomerisation of nitroso- and nitroadducts. Enthalpy vales are calculated at

298.15 K.


121

Owing to sizable activation enthalpy associated with the intramolecular H transfer in reactions

3a and 5a, the initially formed N-derivative species must have competing exit routes. In Figure

5.6, each of the two NOx species departs the PE chain simultaneously with a β-hydrogen atom.

This results in concerted eliminations of nitroxyl (3b) and nitrous acid (5b), with modest

endothermic reaction enthalpies of 112.7 kJ/mol and 84.0 kJ/mol, in that order. The reactions

lead to the formation of alkenes, over moderate barrier heights of 124.1 kJ/mol (TS9) and 165.4

kJ/mol (TS10).

Figure 5.6. Concerted elimination of nitroxyl (HNO) and nitrous acid (HONO) from the

initial N-derivative species. Enthalpy vales are calculated at 298.15 K.

5.3.3. Formation of organic nitrites and nitrates

As previously established, the presence of oxygen produces a peroxy adduct. Along this

corridor, NO or NO2 bond with the outer oxygen atom of peroxy species. In urban areas with

high NOx concentration, addition of NO/NO2 remains the major sink for ROO˙ radicals.40, 62, 63

In gas phase, considering the typical rate parameters for this set of reactions,64, 65 and excessive

NOx level, the pseudo-first-order rate will predominate over the unimolecular reactions of


122

ROO˙ species.1, 53, 66 Figure 5.7 illustrates the formation of alkylperoxy nitrites (7a and 7b)

and alkylperoxy nitrates (8), by respective exothermic addition of NO (-131.7 kJ/mol and 117.0

kJ/mol) and NO2 (-261.0 kJ/mol) to the peroxyl group. Alkylperoxy nitrite has two

conformers; namely, the cis- and the relatively less stable trans-structures, depending on the

arrangement of the ROO-NO link. Structural rearrangement of cis-alkylperoxy nitrite (9a2) is

exothermic by 128 kJ/mol, requiring a barrier height of 96.8 kJ/mol (TS11a). Likewise, similar

reaction for trans-alkylperoxy nitrite species (9b2) reveals exothermicity of 142 kJ/mol, and a

reaction barrier of 118 kJ/mol (TS11b). Furthermore, dissociation of NO2 from the alkylperoxy

nitrite conformers (9a1 and 9b1) or alkyl nitrates (10) remain less probable due to the relatively

high endothermicity of the reactions involved.


123

Figure 5.7. Formation of organic nitrites and nitrates. Reactions with a1 and b1 subscripts

represent NO2 dissociation from the cis- and trans-peroxy adducts respectively, according to

Figure 5.3. Enthalpy vales are calculated at 298.15 K.


124

The progress of NO or NO2 addition to an OH-functionalised PE chain (18a – 21) follows the

same profile, with slight alterations in energetics. However, in OOH-functionalised PE chain,

structural transformation of trans-peroxy nitrite adduct appears barrierless, expressing the

reaction as concerted elimination and re-addition of NO2. Also, dissociation of NO2 leads to

the formation of hydroxyalkyl peroxy, as the H atom of the OOH group preferentially migrates

to the adjacent alkoxy end.

The usual O2 oxidation route contributes to the formation of alkoxy species.1, 56 These species

combine with NO molecules to form alkyl nitrites. As shown in Figure 5.8, such reactions

exhibit exothermicity of 178.8 kJ/mol (11), and the presence of neighbouring hydroxyl and

hydroperoxyl functional groups reduces the exothermicity of the reactions to 140.5 kJ/mol (22)

and 113.1 kJ/mol (32), respectively.

5.3.4. Thermodynamics and reaction kinetics

Figure 5.9 presents changes of thermodynamic potentials of reactions involving addition of NO

and NO2 molecules as function of temperature. Over the considered temperature range, the

exothermic reaction enthalpies display weak dependence on temperature. However, on a ΔrG˚

scale, the reactions become gradually less spontaneous as temperature increases. Such

occurrences can be attributed to the orderliness of the system (negative change of entropy) due

to disappearance of gaseous species, and from a thermodynamic outlook, the spontaneity of

these reactions requires low temperature. In a more practical sense, at high temperature, the

rate of kinetic processes increases, and the system can more easily attain equilibrium, as both

forward and backward reactions operate at higher rates.


125

Figure 5.8. Addition of NO molecules to alkoxy species. Enthalpy vales are calculated at

298.15 K.


126

Figure 5.9. Variation of enthalpy (a), entropy (b) and Gibbs energy (c) with temperatures, for

all reactions attributed to addition of NO and NO2. Refer to Figure 5.3 for the corresponding

reaction sequences.


127

Reactions of polymers with NOx constitute an important factor responsible for deterioration

and chemical transformation of their macromolecular structures. Concerning carbon-chain

polymers containing no carbon‒carbon double bonds, pioneering studies16, 17 have confirmed

that, NO2 cannot abstract secondary and tertiary hydrogen atoms from PE and polypropylene

(PP) at 298 K, however at temperatures above 373 K, direct H-abstraction may occur.19, 20, 67

On the other hand, NO cannot abstract tertiary or allylic hydrogen from organic molecules, but

it readily reacts with free radicals to form nitroso compounds by addition to vacant radical

sites.20

Figure 5.10 depicts Arrhenius plots for several reactions comprising key radical intermediates,

and Table 5.1 assembles their respective reaction rate parameters in terms of activation energies

(Ea) and the Arrhenius pre-exponential A factors. Based on reaction rate constants reported in

Table 5.1, concerted eliminations of HNO (3b) and HONO (5b) dominate the fate of initially

formed nitroso- and nitroadducts, and tautomerisation of nitrosoalkyl compounds favours their

oxime derivatives. In case of nitroalkyls, the position of equilibrium lies towards the nitro

form. In addition, the rate parameters reveal that, alkylperoxy nitrites exhibit rapid structural

transformation into the corresponding alkyl nitrate species. While the rate parameters listed in

Table 5.1 remain valid for crystalline PE, they can also serve to describe similar reactions in

melted PE provided that, the Ea and A are accurately adjusted by considering the inversion or

transposition of the polymer into the liquid phase:

kliq = ksolid exp (-∆Hsolid→liq

RT) exp (

∆Ssolid→liq

R)


128

Unlike gas-to-liquid transposition,68 the energy term represents the heat of fusion required to

melt the polymeric material, and the positive change in entropy increases the A factor as the

system becomes more disordered in liquid phase.

Figure 5.10. Arrhenius plots for the studied reactions.

1.0 1.5 2.0 2.5 3.0 3.5-70

-60

-50

-40

-30

-20

-10

0

10

20

12

3

4

5

6

7

8

910

1112

1314

1516

1718

19202122

AB

C

D

E

F

G

H

I

J

K

LM

NO

PQ

RS

TU

V

log k

(s

-1)

or

(cm

3/(

moles

))

1000/T (K-1)

1b

1c

3a ()

3a ()

3b

5a ()

5a ()

5b

9a2

9b2

14 ()

14 ()

16 ()

16 ()

20a2

20b2

1 24 ()A 24 ()

26 ()

26 ()

30a2


129

Table 5.1. Kinetic parameters of reactions involving key radical intermediates.

As an extra note, hydroperoxide species constitute reactive intermediates in the oxidative

decomposition of polymers. Apart from their usual disappearance route,1 nitrogen oxides

Reaction Ea (kJ/mol)

A (s-1); for Reactions 1b and

1c, A [cm3/(mole s)]

1b: RH+ NO → R˙ + HNO 425.0 7.11 × 1019

1c: RH + NO2 → R˙ + HONO 383.6 1.57 × 1033

3aforward: RNO → RNOH 332.0 1.39 × 1014

3abackward: RNOH → RNO 382.0 1.71 × 1014

3b: RNO → R˙ + HNO 125.3 9.79 × 1011

5aforward: RNO2 → RNO2H 290.0 8.91 × 1014

5abackward: RNO2H → RNO2 220.0 1.09 × 1019

5b: RNO2 → R˙+ HONO 167.4 6.54 × 1011

9a2: cis-ROONO → RONO2 85.4 1.21 × 105

9b2: trans-ROONO → RONO2 104.3 4.36 × 103

14forward: OH‒RNO → OH‒RNOH 207.0 3.19 × 1015

14backward: OH‒RNOH → OH‒RNO 270.5 1.33 × 1017

16forward: OH‒RNO2 → OH‒RNO2H 133.8 3.50 × 1010

16backward: OH‒RNO2H → OH‒RNO2 67.0 3.87 × 1012

20a2: cis-OH‒ROONO → OH‒RONO2 64.3 1.20 × 108

20b2: trans-OH‒ROONO → OH‒RONO2 83.9 7.16 × 1010

24forward: OOH‒RNO → OOH‒RNOH 223.1 1.31 × 105

24backward: OOH‒RNOH → OOH‒RNO 289.1 1.08 × 108

26forward: OOH‒RNO2 → OOH‒RNO2H 437.2 1.73 × 1011

26backward: OOH‒RNO2H → OOH‒RNO2 358.8 1.15 × 1013

30a2: cis-OOH‒ROONO → OOH‒RONO2 52.5 9.55 × 108


130

induce autoaccelerated decomposition of polymeric hydroperoxides. For instance, in PP, NO2

dimer initiates ROOH decomposition, resulting in the generation of a thermally unstable

peroxy nitrite20, 69 that undergoes fast intracage conversion into nitrate as in (9), (20) or (30) of

our proposed mechanism.

5.4. Conclusions

This contribution has analysed the primary initiation reactions occurring during aggressive

low-temperature oxidation of PE. In the NOx-laden atmosphere, formation of diverse nitrogen-

containing species ultimately accompanies the free-radical processes in solid PE. Nitroso and

nitroalkyl species appear as a result of respective addition of NO and NO2 molecule to a carbon

radical site in PE chain. Availability of a neighbouring chemical functional group alters the

thermodynamic and kinetic properties of the reactions. Concerted elimination of HNO and

HONO from nitroso- and nitroadducts, respectively, requires moderate activation energies,

compared with unimolecular tautomerisation of the adducts. The presence of molecular

oxygen induces the formation of peroxy species, that subsequently bond with NO and/or NO2,

thereby serving as a potential source of organic nitrites and nitrates. In comparison with

previous studies that substantiated the addition of nitrogen oxides to solid polymer resins, the

current investigation delivers the concrete thermodynamic evidence of such events, and

provides the reaction channels describing the fate of consequential nitrogenous species. On

this note, the thermochemical and kinetic parameters obtained in this study offer an improved

understanding of the polymer-gas reactions, and present insights into similar reaction

sequences in other crystalline polyolefins.


131

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138

CHAPTER 6

Paper IV: Thermal Reduction of NOx with Recycled

Plastics

Oluwoye, I.; Dlugogorski, B. Z.; Gore, J.; Vyazovkin, S.; Boyron, O.; Altarawneh, M.,

Thermal Reduction of NOx with Recycled Polyethylene:. Environ. Sci. Technol. 2017, doi:

10.1021/acs.est.6b05560.

The paper has been reformatted for Australia English, and the Supporting Information is placed

as Appendix I in Chapter 10.

Fossil fuel combustion

Biomass firing

Power plants

Mining operationsDetonation of AN explosives

NOx from industrial

processes

Waste plastics

Low-emission combustion

Effective energy recovery system

Clean mine blasting operationsIsoconversional kinetics

Online IR and NOx scrutiny1

The chapter develops technology for mitigation of NOx formed in

thermal processes using waste polyethylene. It assesses the NOx

reduction efficiency of fragments of pyrolysing PE, providing the

kinetic evaluations for the thermal reactions

…

CHAPTER 6 — Thermal Reduction of NOx with Recycled Plastics

139

6.0. Abstract

This study develops technology for mitigation of NOx formed in thermal processes using

recycled plastics such as polyethylene (PE). Experiments involve sample characterisation, and

thermogravimetric decomposition of PE under controlled atmospheres, with NOx concentration

relevant to industrial applications. TGA–Fourier transform infrared (FTIR) spectroscopy and

NOx chemiluminescence serve to obtain the removal efficiency of NOx by fragments of

pyrolysing PE. Typical NOx removal efficiency amounts to 80 %. We apply the

isoconversional method to derive the kinetic parameters, and observe an increasing dependency

of activation energy with the reaction progress. The activation energies of the process span

135 kJ/mol – 226 kJ/mol, and 188 kJ/mol – 268 kJ/mol, for neat and recycled PE, respectively,

and the so-called compensation effect accounts for the natural logarithmic pre-exponential ln

(A/min-1) factors of ca 19 – 35 and 28 – 41, in the same order, depending on the PE conversion

in the experimental interval of between 5 and 95 %. The observed thermal delay of recycled

PE reflects different types of PE in the plastic, as well as possible molecular rearrangements

during the recycling processes. These measurements also provide data for kinetic modelling

of both isothermal and non-isothermal decomposition of PE in NOx atmosphere in practical

situations.


140

6.1. Introduction

Apart from its limited industrial and medical applications, oxides of nitrogen (NOx) contribute

to acid rain, eco-eutrophication, photochemical smog, ground level ozone, greenhouse effect,

stratospheric ozone depletion, and health deterioration (e.g., chronic respiratory and obstructive

pulmonary dysfunction) of predisposed organisms. Primarily, NOx consists of nitric oxide

(NO) and nitrogen dioxide (NO2). Typical anthropogenic emission of NOx (comprising mostly

NO) amounts up to 800 ppm in untreated combustion systems burning fossil/nitrogenous fuels

for power generation, and can soar as high as 500 ppm during decomposition of nitrate

oxidisers in some aviation propellants and mining-grade explosives.1-11

The quest for reliable NOx reduction has led to implementation of strict environmental

regulations and development of industrial-scale technologies. In combustion systems, these

technologies include fuel pre-treatment, combustion modification and post-treatment

techniques.12-14 Although post-flame gas clean-up remains viable, strategic modification of

combustion processes often controls NOx emission more economically.2,15,16

Noticeably, fuel reburning provides a commercially available technological retrofit that

appears more effective than alternative modifications of combustion processes. The process

abates NOx by using fuel as a reducing agent.17 Since 1950s, a number of practical and

experimental studies investigated the reburning technique,4,18-24 including a collateral effect of

SO2 reduction.25 Conceptually, reburning occurs in three stages (see Figure 6.1) to achieve

between 50 % and 85 % reduction in the total NOx emission. In the first stage (primary zone),

the main fuel burns under slightly fuel-lean condition. Afterwards, the produced NOx reacts

with supplementary hydrocarbon radicals in the “reburning zone” to form intermediate


141

nitrogenous species, and then molecular nitrogen (N2). Finally, addition of air in the last stage

(burnout zone) completes the combustion process by oxidising all unreacted fuels and reduced

N-species.2,4

Figure 6.1. Schematic diagram of the three stages of the reburning process.

Although, large-scale reburning systems usually rely on hydrocarbon gases (e.g., methane),

carbonaceous solid residues have proven equally suitable. Spliethoff et al. employed the

pyrolysate fragments of coal as a reburn fuel to mitigate NOx formation in a bench-scale

reburning facility, and attained good reduction efficiency of about 80 %.26 Likewise,

demonstrations of application of coal reburning in commercial boilers achieved excellent NOx

control averaged at around 70 % reduction efficiency.27-30 Biomass fuels such as wood, straw,

rice husk, bio-oil, sewage sludge, and carbonised municipal solid waste also provide effective

means of reducing NOx formation (50 – 75 %) in combustion systems.31-40 Yet surprisingly,

only a few studies assessed the performance of polymeric materials (e.g., waste tyres41-43) as a

secondary reburn fuel for NOx remediation.

NOx-rich flue gas

from primary

combustion zone

NO

HCN

NHi

NO

(ab

undan

t)

N2 + …

CHi

O2

(def

icie

nt)

Oxidation of

unreacted

hydrocarbons

and

intermediate

N-species

Primary fuel and air

Ash and flue gas

Gaseous; natural gas, C1-C3

Liquid; oil

Solid; coal, biomass,

sludge, polymers, etc.

Advanced reburning;

ammonia, urea supplements

Reburnfuel

Burnoutair


142

Unlike coal or biomass, plastics, such as polyethylene (PE), lack nitrogen functionalities that

can further contribute to fuel-NOx formation within the reburning zone. Owing to its high

calorific value, good volatile content and favourable combustion rate,44-46 waste PE finds

outlets in recycling processes such as thermal energy recovery, as it burns without prompting

environmental concerns, e.g., formation of pollutants and fire-related hazards.47,48

Measurements of pyrolytic conversion of PE to low-carbon gases in hybrid rocket

propulsion49,50 indicate the suitability of the polymer in reburning applications, and as a

potential NOx reductant during detonation of ammonium nitrate-based explosives in open-cut

mining operations.

To this end, the current study presents a series of experimental investigations of thermal

mitigation of NOx with PE. We hypothesis that, waste PE can potentially serve to reduce NOx

emission in industrial activities. We aim at solving two crucial environmental problems, to

develop a sustainable solution to NOx emission, and to provide additional means of dealing

with excessive plastic wastes in the society. In particular, we report the result of thermal

decomposition of PE in NOx atmosphere, providing a detailed analysis of reaction products.

The insights from the present study apply equally to abatement of NOx with materials laden

with PE, polyolefins/paraffins and pure carbon-hydrogen-type polymers, as well as to co-

combustion/pyrolysis of PE with nitrogen-rich fuels such as biomass and coal.


143

6.2. Experimental Section

6.2.1. Materials and sample characterisation

The experiments involved recycled polyethylene received in form of extruded black cylindrical

pellets, around 2.5 mm in diameter and 2 mm in length. We sliced the pellets into thin oblong

particles of uniform size distribution of between 250 μm and 400 μm. Sigma-Aldrich, USA,

provided neat low-density polyethylene, employed in control studies, to gauge the effect of

impurities in the recycled PE on its thermal and NOx-abatement behaviour. The attenuated

total reflectance (ATR) produced the IR spectra of the recycled PE, and a carbon-hydrogen-

nitrogen-sulfur (CHNS) elemental analyser yielded the elemental composition of the material.

Further analyses comprised the multi-detector high temperature size exclusion chromatography

(HT-SEC), ion (anion) chromatography (IC) and inductively coupled plasma optical emission

spectroscopy (ICP-OES) for determination of molecular mass distributions (MMD), halogens

and trace metals, respectively. Supporting Information (Chapter 10) includes details of these

analytical techniques. Table 6.1 itemises the molecular characteristics of the PE samples. The

FTIR spectrum of the recycled PE, documented in Figure 6.2, indicates no contamination with

other plastics, and Table 6.2 reveals the combined amount of metals and halogens, mostly Fe,

Ca and Cl, of less than 1 %. The recycled PE contains no sulfur, nitrogen, fluorine and

bromine-containing contaminants, confirming presence of no brominated flame retardants in

the sample.


144

Table 6.1. HT-SEC parameters of neat and recycled PE samples: Peak-average, number-

average and weight-average molar masses (Mp, Mn and Mw), as well as intrinsic viscosity [η],

radius of gyration (Rg based on the root-mean-square average distance of the components of

the molecule from the centre of gravity) and polydispersity index Ip in TCB solution.

Mp

(kg/mol)

Mn

(kg/mol)

Mw

(kg/mol)

[η]

(dL/g)

Rg(w)

(nm)

Ip = Mw/Mn

Neat PE 59.5 14.2 144 1.1 31.6 10.1

Recycled PE 77.1 24.4 105 1.9 34.9 4.3

Table 6.2. Elemental characterisation of the recycled polymer; basic elements from a CHNS

analyser, halogens from IC and trace metals from ICP-OES. Error margin for CHNS analysis

is 0.3 %.

Ultimate elements and halogens (wt. %)

C H N S Cl Br F

86.17 14.23 0.00 0.00 0.10 0.00 0.00

Average trace metals (wt. %)

Al Ca Fe Mg Mn Na Fe

0.10 0.24 0.03 0.05 0.02 0.11 0.25


145

Figure 6.2. FT-IR spectrum of the recycled polymer.

6.2.2. Experimental methods

Figure 6.3 illustrates the complete schematics of the three arrangements of the experimental

apparatus. A carefully calibrated (by melting point standards) PerkinElmer STA 8000 system

facilitated all thermogravimetric analyses (TGA), with the quantitation of the gases performed

by infrared spectroscopy (TGA-FTIR, PerkinElmer Frontier) and chemiluminescence (TGA-

NOx, Thermo Scientific model 42i-HL). All TGA experiments involved three replicable runs

(Figure S6.4) of 5.2 ± 0.2 mg samples, placed in alumina crucible, subjected to the nominal

heating rates of 5, 10, 15, and 20 K/min, bracketing temperatures between 25 C to 600 C in

argon, synthetic air and NOx (610 ± 5 ppm NO and 18 ± 3 ppm NO2 with balance of dry helium

gas) atmosphere. The continuous flow rate of the purge gases amounted to 80 mL/min, as

measured under the standard temperature and pressure (STP). Blank runs completed under

identical conditions to the production experiments yielded the thermal buoyancy corrections.


146

We have achieved excellent reproducibility of the TGA results (Figure S6.3 of Supporting

Information).

Figure 6.3. Schematics of adopted experimental configurations. A standalone

thermogravimetry apparatus (TGA, top) eliminates any unwanted experimental artefacts that

could be carried over into the subsequent kinetic evaluations. Hyphenated TGA–FTIR

(middle) and TGA–NOx analyser (bottom) enabled identification and quantitation of evolved

gases.


147

The TGA-FTIR and TGA-NOx measurements required larger sample sizes of 10.2 ± 0.2 mg to

improve the detection and quantitation of gaseous species. The FTIR spectrometer averaged

two accumulated scans per spectrum at 1 cm-1 resolution. This resulted in a temporal resolution

of 30 s, which we found sufficient even for the fastest temperature TGA ramp of 20 K/min.

The TGA-FTIR set-up incorporated an electrically heated transfer line (PerkinElmer TL 8500)

linking the TGA system to 100 mm path-length online gas sampling cell placed inside the FTIR

compartment. Both the sample line and the sampling cell operated at 220 C, to avoid the

condensation of product volatiles prior and during the analysis. An externally controlled

vacuum pump maintained a balanced flow throughout the system. The lag time between the

TGA and FTIR corresponded to 10 s, requiring a time-delay correction for comparison of the

mass-loss and the evolved gas-composition signals.


6.3.1. Thermal decomposition of PE in NOx atmosphere and evolved gas analysis

Figure 6.4 presents the characteristic thermogravimetric measurements of neat and recycled

PE, as the samples decompose under Ar, He-NOx and air atmospheres. In agreement with

previous studies, in inert argon purge, the pyrolysis proceeds in an apparent single-step process

that commences at around 446 °C (extrapolated onset temperature), attains a maximum at

around 477 °C, and then rapidly slows down owing to depletion of the material, to cease by

490 °C (extrapolated offset temperature). The residual char and ash constitutes about 0.8 %

for neat and 2 % for the recycled PE, respectively. Furthermore, the presence of oxygen


148

triggers multi-step, low-temperature, peroxy-channelled reactions, because of O2 addition to

radical sites in the polymer chains.51

Figure 6.4. TGA curves for the thermal decomposition of neat (a) and recycled (b) PE in

different purge atmospheres at a nominal heating rate of 10 K/min. Inset plots magnify the

respective normalised derivatives of the thermal events. NOx denotes a mixture of 610 ppm

NO and 18 ppm NO2 in balance of dry helium.

50 100 150 200 250 300 350 400 450 500 550 600

0

10

20

30

40

50

60

70

80

90

100

200 300 400 500 600

0.0

0.1

0.2

0.3

474 479

424

Mas

s per

cent

(%)

Temperature (°C)

Argon

NOx

Air

-(d

m/d

t)/m

o

Temperature (°C)

50 100 150 200 250 300 350 400 450 500 550 600

0

10

20

30

40

50

60

70

80

90

100

200 300 400 500 600

0.0

0.1

0.2

0.3

398

471

420

Mas

s p

erce

nt

(%)

Temperature (°C)

Argon

NOx

Air

477

-(d

m/d

t)/m

o

Temperature (°C)

(a)

(b)


149

Under an inert shield gas, PE decomposes into a series of lower molecular weight

hydrocarbons, mostly linear n-alkanes and n-alkenes,48,50,52 and this can be monitored by the

series of CHi bands, most easily at 2922 cm-1 that corresponds to CH2 asymmetric stretch; see

Figure 6.5. Other evolved functional group characteristics include CH3 asymmetric and

symmetric stretches, C–H out-of-plane bend, CH2 rocking vibration and symmetric stretch, as

well as CH3 asymmetric bend. These species emerge within the temporal range, corresponding

to ca 400 °C – 500 °C, correlating well with the thermal decomposition period of the samples

shown in Figure 6.4.


150

Figure 6.5. IR monitoring of hydrocarbon bands evolved during thermal decomposition of

PE samples (10 mg) in two purge atmospheres at nominal heating rate of 20 K/min. The four

inset plots represent the maximum evolution events with respect to absorbance values, at

different time t. List of observed absorbance peaks (cm-1) - 2922: CH2 asymmetric stretch;

2850: CH2 symmetric stretch; CH3 asymmetric bend; 915/990: out-of-plane C–H bend; 723:

CH2 rocking vibration. CH2 bands overlap with the CH3 asymmetric stretch at 2962 and the

symmetric stretch at 2873.

The presence of NOx, delivered in helium bath gas, accelerates the thermal decomposition of

PE, resulting in the maximum rate of pyrolysis shifting from 477 °C (or 479 °C in the case of

recycled PE) to 471 °C and 474 °C, for neat and recycled PE, in that order. This acceleration

occurs due to the oxidative nature of NOx. The onset and completion temperatures, under He-

NOx purge, correspond to 433 and 483 °C as well as 444 and 486 °C, for neat and recycled PE,

respectively. The observed delay in the thermal events of recycled PE suggests different


151

molecular branching structures53 (Figure S6.1 of Supporting Information) as well as possible

confining activity of the constituent inorganic residues. Inorganic residues can form micro-

glassy layers that may become impenetrable to volatiles, hence inhibit the thermal breakdown

of the underlying organic matrices.54

As shown in Figure 6.6,55 below the decomposition temperature, the barrierless exothermic

addition of NO and NO2 follows the formation of radical sites in condensed-phase PE. This

process results in formation of C-nitroso and nitro species, respectively, and leads to internal

tautomerisation or concerted elimination of nitroxyl (HNO) and nitrous acid (HNO2). This set

of reactions can disrupt the C‒C bonding configurations, and in turn lower the overall enthalpic

requirements for the homolysis of the C‒C bond and unzipping of the polymer in the successive

β-scission reactions.56

Figure 6.6. Reaction networks for interactions between condensed-phase PE and aggressive

NOx species. Bolded values represent reaction enthalpies and values in italic correspond to

the activation enthalpies. All values (in kJ/mol) relate to respective reactants at 298.15 K.55


152

The peak value of 2922 cm-1 (Figure 6.5) reduces substantially when similar amount (10 mg)

of PE decomposes under NOx atmosphere, signifying decrease in concentration of CHi

fragments and hence, gas-phase reactions between the PE pyrolysates and the NOx gases. This

interaction involves several chain steps that can be globalised as4,40:

∑ CiHj + NO → HCN + ... (6.1)

In excess NOx, the HCN decays through series of intermediates, and ultimately reaches N2 via

the reverse Zeldovich reaction, Equation (6.5):

HCN + O → NCO + H (6.2)

NCO + H → NH + CO (6.3)

NH + H → N + H2 (6.4)

N + NO → N2 + O (6.5)

In the absence of fuel-N in PE, the other NH3 pathways remain dormant. The detection limit

of the employed gas cell (100 mm path-length and 11.3 cm3 volume) restrains the online

elucidation of nitrogenous species. However, the chemiluminescence analyser monitors the

NO/NOx profile ensuing under the dynamic thermal conditions. As illustrated in Figure 6.7 (as

well as Figures S6.11 and S6.12 in Supporting Information), within the decomposition regime

of the polymer, the experimental measurements show significant reduction of NOx up to 80 %.

The heating ramp affects the reaction via the Arrhenius term in the rate expression,57 dictating

the composition of the volatile species.48,50,58 Even though the moderate heating rates alter the

NOx conversion profile in Figure S12, the maximum conversion efficiency remains


153

approximately the same (Figure 6.7b). As explained earlier, although the recycled PE displays

a delay in the evolution of volatile fragments, the maximum NOx reduction efficiency remains

the same when compared to that of neat PE samples.

Figure 6.7. Typical NOx conversion history during the thermal decomposition of PE samples

(10 mg) in NOx atmosphere under a nominal heating rate of 20 K/min (a), and a comparison

of maximum possible conversion efficiency as function of the dynamic temperature ramp (b).

The overall weighted mean for NOx conversion corresponds to 81 %. Online NOx data

treatment involves response analysis (time-lag corrections), but it does not account for the

spatial dispersion in the system.

0

20

40

60

80

100

2010

Max

imu

m N

OX c

on

ver

sio

n (

%)

Heating rate (K/min)

Neat PE

Recycled PE

5

320 340 360 380 400 420 440 460 480 500 520

0

10

20

30

40

50

60

70

80

90

100

Neat PE

NO

NOx

Sample temperature (C)

Mas

s p

erce

nt

(%)

0

10

20

30

40

50

60

70

80

90

100

NO

x c

on

ver

sio

n (

%)

(a)

(b)


154

6.3.2. Degradation kinetics

As expected, the decomposition of PE exhibits strong dependency on the heating rate (Figures

6.4 and S6.4). This allows the application of the isoconversional principle,57 to obtain the

activation energy as function of the conversion α. As illustrated in Supporting Information,

the calculations rely on the advanced isoconversional method of Vyazovkin57,59 (Equation 6.6)

that provides an accurate estimation of activation energy values from the temperature integral,

as compared to Lyon,60 Kissinger-Akahira-Sunose,61 and Starink’s62 approximate approaches.

For a series of runs performed at different heating rates, the appropriate activation energy value

minimises the following function:

∅(𝐸𝛼) = ∑ ∑𝐼(𝐸𝛼, 𝑇𝛼,𝑖)𝛽𝑗

𝐼(𝐸𝛼, 𝑇𝛼,𝑗)𝛽𝑖

𝑛

𝑗≠𝑖

𝑛

𝑖=1

(6.6)

The subscripts i and j represent integer numbers of different experiments performed under

varying heating programs, i.e., i equals 1 – 3 for replicates, and j corresponds to 1 – 4 for

heating rates, and I(E,T) denotes the Arrhenius temperature integral. Furthermore, the so-

called compensation effect permits the determination of the pre-exponential factor A by the

substitution of integral form of various reaction models g(α) into numerical approximation of

Equation 6.57,63 Although this method relies on the assumption that the thermal process obeys

single-step kinetics, it can also provide a reasonable estimation64 of Aα dependency on α,

provided that the processes underlying this dependency follow a general model of f(α) = (1-

α)n. Figure S6.6 indicates that, the thermal decomposition of PE remains consistent with the


155

Mampel first order model of f(α) = (1-α) over the range of measured activation energies,

satisfying this requirement. Supporting Information provides detailed discussion of the

adopted method to determine the pre-exponential factor A.

Previous investigators obtained the kinetic parameters for the pyrolysis of neat PE. 51 Their

reported isoconversional activation energies (150 – 240 kJ/mol) match with our estimated

values of 160 – 255 kJ/mol. The illustrated growth in Eα (Figure 6.8b) is a result of change in

reaction mechanism. The initial low value relates to the initiation process occurring at the

defect sites in the polymer chain. Eα varies as the limiting step of the degradation process shifts

towards other mechanisms, such as unzipping, backbiting, random scission and

branching.48,51,65,66 As shown in Figure 6.8, the presence of ca 600 ppm of NOx lowers the Eα

values for the decomposition of neat PE by 13 – 18 %. Returning to Figure 6.4, both the neat

and recycled PE experience acceleration of degradation (shift to lower temperature) under NOx

atmosphere. However, the effect manifests itself differently in the kinetics. For neat PE, the

drop in Eα (Figure 6.8a) represents a lower energy barrier in the presence of NOx, hence reaction

is faster. On the contrary, Eα increases in the case of recycled PE, but Figure 6.4 depicts

accelerating reaction. This means that, the mechanism of acceleration for recycled PE differs

from that of neat PE. The observed acceleration during decomposition of recycled PE under

He-NOx relates to a higher pre-exponential A factor. This factor depends on MMD, in

particular on higher number-average molar mass (Mn in Table 6.1) for recycled PE. The

recycled PE comprises longer linear chains (as also evident from the higher value of intrinsic

viscosity [η] for recycled polymer in Table 6.1). The longer the chain that breaks during the

reaction with NOx, the higher entropy change, and larger the pre-exponential A factor.

.


156

Figure 6.8. Kinetic parameters accompanying the thermal decomposition of neat (a) and

recycled (b) PE in argon and He-NOx baths. The grey trend-shades depict accuracy

thresholds of the evaluated parameters.

The isoconversional kinetic parameters (plotted in Figure 6.8 and assembled in Tables S6.4

and S6.5 in Supporting Information) substituted in Equation 6.767 provide a means to estimate

the time (tα) needed to achieve a target conversion α under isothermal condition (T0):

0.0 0.2 0.4 0.6 0.8 1.00

40

80

120

160

200

240

280

0.0 0.2 0.4 0.6 0.8 1.025

30

35

40

45

E (

kJ/

mo

l)

Conversion ()

Argon

NOx

ln (

A/m

in-1)

0.0 0.2 0.4 0.6 0.8 1.00

40

80

120

160

200

240

280

0.0 0.2 0.4 0.6 0.8 1.0

20

25

30

35

40

E (

kJ/

mo

l)

Conversion ()

Argon

NOx

ln (

A/m

in-1)

(a)

(b)


157

tα =

1β ∫ exp (-

Eα

RT) dT

Tα

0

exp (-Eα

RT0)

(6.7)

Figure 6.9 provides validation of the kinetic evaluations. At moderate temperatures, our

calculated Eα values predict well (according to Equation 6.7) the conversion-time required to

reach a target conversion during the isothermal operation.

Figure 6.8. Isothermal decomposition of neat (a) and recycled (b) PE under the He-NOx

atmosphere at three moderate temperatures. Continuous lines represent experimental results,

and dotted points denote the corresponding theoretical predictions (up to α = 0.95) as

evaluated from Equation 6.7.

In practical applications, strict isothermal processing of PE would not be possible, as heating

and cooling do not occur at infinite rate. In such situations, for process design under non-

isothermal conditions, one can rely on the isoconversional data presented in Tables S6.4 and

S6.5. The fundamental kinetic and conversion measurements revealed in the current

0 5 10 15 20 25 300.0

0.2

0.4

0.6

0.8

1.0

0 5 10 15 20 25 300.0

0.2

0.4

0.6

0.8

1.0

Co

nv

ersi

on

(

)

Time (min)

450 C

470 C

500 C

Time (min)

450 C

470 C

500 C

(b)(a)


158

contribution make it possible to optimise practical systems for removal of NOx by pyrolysates

from neat and recycled PE.

6.4. Conclusions

In conclusion, the pyrolysates of PE reduced NOx content by about 80 % with PE decomposing

under the nonisothermal heating conditions. The maximum NOx reduction efficiency of

recycled PE remains approximately the same relatively to that of neat samples. The injection

of waste PE into a reburning zone of combustors may significantly reduce the emission of NOx.

Reburning of NOx in practical combustors usually occurs in a temperature window between

1000 °C and 1200 °C. Therefore, a change of the reaction mechanism may occur at those high

temperatures. A subsequent study should address the high-temperature isothermal and flash-

particle heating conditions in drop-tube experiments. This will reveal possible effects of the

presence of oxygen and mass transfer on the reaction of NOx with PE pyrolysates under realistic

reburning temperatures (i.e., 1000 – 1200 °C).

The Supporting Information is included in Chapter 10 (Appendix I) of the thesis. It comprises

the assessments of calibration and repeatability of TGA experiments, thermal characterisation

of recycled PE, derivation of cited equations, as well as analysis of the compensation effect,

complete isoconversional kinetic data, enlarged time-resolved FTIR spectra, and complete NOx

reduction trends.


159

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166

CHAPTER 7

Paper V: Enhanced Ignition of Biomass in Presence of

NOx

Oluwoye, I.; Dlugogorski, B. Z.; Gore, J.; Westmoreland, P.R.; Altarawneh, M., 12th

International Symposium on Fire Safety Science, Lund, Sweden. Special Issue in the Fire Saf.

J. 2017, doi.org/10.1016/j.firesaf.2017.03.042.

NOx - O2O2atmosphere atmosphere

Biomass

residue

500 C

350 C

The chapter demonstrates the effect of emitted NOx on ignition of biomass

residues. It elucidates the low-temperature interaction of NOx with biomass,

establishing the necessary bases for N-conversion at high temperatures as

later describe in Chapter 8

…

CHAPTER 7 — Enhanced Ignition of Biomass in Presence of NOx

167

7.0. Abstract

This chapter presents an experimental study on a relative effect of nitrogen oxides (NOx) on

ignition temperature of morpholine, an important surrogate of biomass, to reveal the sensitising

role of NOx in combustion of biomass fuels and to gain mechanistic insights into this behaviour.

The experiments employed a flow-through tubular reactor, operated at constant pressure and

residence time of 1.01 bar and 1.0 s, respectively, and coupled with a Fourier-transform

infrared spectroscope. For a representative fuel-rich condition ( = 1.25), the concentration of

NOx as small as 0.06 % lowers the ignition temperature of morpholine by 150 °C, i.e., from

approximately 500 °C to 350 °C. The density functional theory (DFT) calculations performed

with the CBS-QB3 composite method, that comprises a complete basis set, characterised the

dynamics and energies of the elementary nitration reactions. We related the observed reduction

in ignition temperature to the formation of unstable nitrite and nitrate adducts, as the result of

addition of NOx species to morphyl and peroxyl radicals. Furthermore, the reaction of NOx

with low-temperature hydroperoxyl radical leads to the formation of active OH species that

also propagate the ignition process. The present findings quantify the ignition behaviour of

biomass residues under NOx–doped atmospheres, the result that is of great importance in

practical applications.


168

7.1. Introduction

Thermal interaction of biomass with oxides of nitrogen (NOx, mainly NO and NO2) has

important practical implications. While such interaction serves the intended industrial

purposes in low-emission combustion technologies,1-3 it has an unintended consequence of

degrading the safety of organic dust-producing plants. One reason for this is the particular way

in which NOx reacts with hydrocarbons, by accelerating their oxidation process.4,5 In other

words, NOx enhances the ignition of combustible materials, and this effect is generally referred

to as sensitisation via nitration reactions.6-10 The addition of NO and/or NO2, to fuel-oxygen

systems, significantly alters the overall kinetics by turning the usual low-temperature chain-

terminating steps of formation of peroxyl (RO2) radicals into a chain-propagating step.5

Premature ignition of biomass particles, as a result of the sensitising effect of NOx gases, can

lead to unexpected fires and dust explosions. The latter occur upon direct exposure of

combustible dust clouds (dispersed in air at concentrations above the flammability limit) to an

ignition source, such as electrical or electrostatic sparks, overheated powder particles, hot

surfaces, etc., within a certain degree of confinement.11-14 Wood dusts in the timber industry

display the same potential to explode as coal powders in underground mining operations.15,16

Minor quantities of NOx (e.g., from diesel engines) promote the primary ignition of small wood

particles. These enflamed particles prompt the formation of dust clouds creating conditions for

secondary deflagration or detonation. In fact, the industrial safety literature presents numerous

examples of explosions and fire accidents in wood-processing plants, for example,

sawmills.11,17-19 Biomass dusts form during harvesting, sizing, transportation, preparation,

storage, as well as through usage in combustion plants. Fire hazard arises when the dust

sediments (and suspensions) accumulate on hot surfaces of electrical, and mechanical devices,


169

such as conveyers, dryers, hot bearings and other machineries.20 Although, the storage

conditions in silos aggravate the propensity for low temperature ignition,21-23 local conditions,

such as NOx presence from plant equipment, constitutes a contributing factor. Examples of

biomass-related fire incidents involved storage facilities and power plants.24

The complex nature of biomass necessitates the use of representative model compounds to gain

an understanding of the ignition, combustion and pyrolysis of biomass-derived fuels. These

model compounds include hetero-element species, also known as surrogates, embodying the

well-defined N- and O-functionalities within their structures.25-27 Morpholine (C4H9NO, 1-

oxa-4-aza-cyclohexane) represents an excellent surrogate species for an oxygen and nitrogen-

containing biofuel, due to its unique heterocyclic structure, well-characterised and studied

behaviour, and a wide range of industrial applications. Its N-, C-, and O-content correspond to

that revealed by ultimate analysis of typical biomass feedstocks. Furthermore, morpholine is

widely used as an impregnating agent for fruit, cardboard, and paper, and as a fuel additive.

The molecule also bears structural resemblance to ethanol, dimethyl ether, ethyl amine, and

dimethyl amine.26 Several investigations have described its decomposition, combustion,

ignition and flame characteristics.26,28-33

Accordingly, the present work focuses on experimental and theoretical aspects of initiation of

thermal decomposition of morpholine, as a model for oxygen and nitrogen-containing

compounds present in real biomass fuel, exposed to NOx-laden atmosphere. Our results offer

insights into the low-temperature nitration reactions that govern the interaction of biomass with

NOx. The conclusions drawn from the present work apply equally to herbaceous, woody and/or

chaff biomass.


170

7.2. Applied Methodologies

7.2.1. Materials and experimental setup

Figure 7.1 provides a schematic representation of the experimental setup in which a digital

syringe pump delivered the reactant fuel (morpholine; TCI, purity > 99.0 %, 2.6 – 1.4 μL/min)

into the preheated combined stream of helium, NOx and oxygen gases flowing through the

system at combined rates of between 730 and 390 mL/min (at STP). The concentration of

morpholine in the diluted stream amounted to 1000 ppm on molar basis. An electrically heated

three-zone horizontal split furnace accommodated the quartz reactor tube, and ensured a well-

defined isothermal reaction zone (cf. Figure 7.1). The current experimental rig represents a

typical bench-scale flow-through tubular-reactor system employed in similar gas-phase kinetic

studies.34-38. The reactor operated at nominal Reynolds numbers of 7.8 (at 300 °C) – 2.8 (at

800 °C), indicating a laminar flow regime with an average hydrodynamic entry length of

approximately 0.9 % of the total reaction zone (300 mm). We maintained NOx concentration

at 620 ppm (600 ppm of NO and 20 ppm of NO2), adjusting the flow rate of O2 to maintain the

fuel-oxygen equivalence ratio of 1.25 (i.e., moderately fuel rich condition, also corresponds

to oxygen-fuel equivalent ratio λ = 0.8) as per the following stoichiometric reaction:

C4H9NO + 5.75O2 = 4CO2 + 4.5H2O + 0.5N2 (7.1)

We maintained a constant residence time of 1.0 s in the reaction zone, at ambient pressure of

1.01 bar, by regulating the flowrate. Charles’ temperature-volume relationship served to adjust

the gas flow rates metered by the mass flow controllers (MFC, operated at the room


171

temperature) but preheated just before the reactor entry. Similarly, we employed a mass

balance to convert the flow rate of the fuel needed for the predefined concentration at

isothermal reaction temperature, into the corresponding set-values at room temperature. The

relatively small volume of the annual space of zones 1 and 3 (Figure 7.1) makes a minor

contribution to the overall reaction in the system. The volume of the preheating and cooling-

down zones amounts to 4 % of the reaction zone.

Fourier transform infrared (FTIR) spectroscopy facilitated online monitoring of product

species exiting the tubular reactor. The setup incorporated an electrically heated transfer line

(3.175 mm ID × 300 mm) linking the reactor to a small volume (100 mL) 2.4 m path-length

online gas sampling cell (Pike) placed inside the FTIR compartment. The spectrometer

averaged 64 accumulated scans per spectrum at 1 cm-1 resolution. Both the sample line and

the sampling cell operated at 140 C, to avoid the condensation of volatile products prior and

during the analysis.

Figure 7.1. Schematic representation of experiment rig. Symbol “T” denotes thermocouples.

.


172

7.2.2. Computational methods

Gaussian 09 suite of programs39 served to study the structures of reacting species, transition

states, reaction products, as well as the dynamics and energetics of the nitration reactions

themselves.. The complete-basis-set CBS-QB340 composite method afforded all structural

optimisations, reaction enthalpies, as well as vibrational frequencies, using the tight

convergence criteria and rigid-rotor/harmonic-oscillator approximations. In agreement with

previous studies,30,33 the adopted computational method offers an accurate evaluation of

thermochemical properties of all species studied herein. The accuracy limit of the

computational basis set (CBS-QB3) usually settles within the mean absolute deviation of 6

kJ/mol from experimental values of gas-phase enthalpy.41-45 To confirm this, we have

computed the absolute mean error to be 4.5 kJ/mol for bond dissociation enthalpies (BDH) of

selected nitrogenated heterocyclic compounds (Table 7.1). Furthermore, the reported

exothermicity of O2 addition to o-morphyl (152 kJ/mol) and, m-morphyl (136 kJ/mol) concur

with the literature values of 148 and 136 kJ/mol, respectively.32


173

Table 7.1. Comparison of the calculated CBS-QB3 BDH with the literature values for selected

heterocyclic hydrocarbons. Bracketed values in italics correspond to the results an alternative

high-level composite G4 method. The second last column assembles the experimental

estimates obtained from photoacoustic calorimetry (PAC), acidity–oxidation potential

measurements (AOP) and shock tube (ST) techniques, as well as one computational value

calculated at the G2(MP2) level of theory.

Heterocyclic

species Homolytic BDH reaction

Calculated CBS-

QB3 (and G4)

BDH (kJ/mol)

Experimental 46,47

and theoretical 48

BDH (kJ/mol)

Absolute

error

(kJ/mol)

Morpholine

393.8

(386.6)

393.3

PAC

0.5

(6.7)

377.1

(371.0)

384.9±8.4

PAC

7.8

(13.9)

Piperidine

381.9

(376.0)

385.0±10.0

PAC

3.1

(9.0)

451.1

(444.3) N/A N/A

Pyrrolidine

385.9

(377.9)

377.0±10.0

PAC

8.9

(0.9)

387.1

(387.6)

384.3

Theory

2.8

(3.3)

Pyrrole

505.6

(498.2)

495.4 ±4.2

ST

10.2

(2.8)

403.0

(397.4)

405.8

AOP

2.8

(8.4)

Mean absolute error 4.5

(5.6)


174


7.3.1. Effect of NOx on ignition temperature of morpholine

As illustrated in Figure 7.2, the IR spectrum of vaporised morpholine (e.g. at 300 °C) contains

CH2 stretches (2840 – 2955 cm-1), CH2 scissors (1455 cm-1), CH2 wagging (1321 cm-1), and

oxazines signatures of C-O-C symmetric stretches (1237 cm-1, 1062 cm-1), asymmetric C-N-C

stretching (1320 cm-1) and aromatic N-H wag (712 cm-1). Previous studies on thermal

decomposition of morpholine28-33 have demonstrated the vulnerability of the heterocyclic ring

that readily opens up into linear adducts via homolytic cleavages and/or β-scissions at elevated

temperatures. Hence, we selected one of the aromatic group frequencies at 1132 cm-1 to

compute the fuel conversion, as indicated in Equation 7.2:

𝑋 = ⌊𝐴𝑏𝑠𝑖 − 𝐴𝑏𝑠𝑇

𝐴𝑏𝑠𝑖⌋ × 100 % (7.2)

where Absi denotes the initial concentration (proportional to absorbance value at 1132 cm-1) at

the reactor inlet, and AbsT stands for the concentration of the fuel at the reactor outlet. This

band represents a characteristic vibration of morpholine that does not appear in any of the

product species.


175

Figure 7.2. IR spectrum of vaporised morpholine. I: CH2 stretches (2840 – 2955 cm-1), II:

CH2 scissors (1455 cm-1), III: CH2 wagging (1321 cm-1), IV: oxazines signatures of C-O-C

symmetric stretches (1237 cm-1, 1062 cm-1), V: asymmetric C-N-C stretches (1320 cm-1), VI:

aromatic N-H wag (712 cm-1), and VII: selected group frequency (1132 cm-1).

Figures 7.3a, 7.3b, 7.3c, and 7.3d depict the IR profiles in He, He-NOx, He-O2, and He-O2-

NOx, respectively. The additional low-temperature peaks (1755 – 1960 cm-1) appearing in

experiments for He-NOx and He-O2-NOx bath gases correspond to NO. The figures elucidate

the structural identities of the products exiting the reactor. At relatively low temperatures

(denoted by black-coloured spectra), the fuel remains unchanged, i.e., no detectable chemical

addition, decomposition, or isomerisation reactions occur. However, as the operating

temperature increases, morpholine converts into acetylene (C2H2), ethylene (C2H4),

formaldehyde (CH2O), methane (CH4), carbon monoxide (CO), carbon dioxide (CO2),

hydrogen cyanide (HCN) and ammonia NH3, depending on the composition of the inlet gas

mixture (indicated in red in Fig. 7.3). NOx concentration of 620 ppm, selected for experiments,

represents that of untreated diesel engine exhausts.


176

Figure 7.3. IR profile of morpholine decomposition at different temperatures, in He (a), He-

NOx (b), He-O2 (c) and He-O2-NOx (d) atmospheres. Black coloured spectra correspond to

morpholine, with functional identities as described in the text of the chapter. Red coloured

spectra indicate obvious conversion to product species.

Figure 7.4 plots the fuel conversion, as well as the extrapolated onset ignition temperatures for

all experiments. The presence of NOx inhibits the decomposition of morpholine under

pyrolytic conditions. Under oxidative environment (i.e., in presence of O2), NOx reduces the

ignition temperature by 150 °C, i.e., from 500 °C to 350 °C. The trend is similar to that

experienced in other types of fuels.4,5,7-10,34-36 For instance, Bendtsen et al.4 reported that the

presence of NO or NO2 lowered the onset temperature for oxidation of methane (CH4) by 250

°C, at shorter residence time of 200 ms. This implies that, with an adequate heat/ignition

source, NOx can initiate premature atmospheric combustion of biomass residues. The present

measurements indicate that, safe operation of wood-working plants requires avoiding trace

concentration of NOx within the vicinity of dust-prone areas. This can be facilitated by proper

(and separate) venting of engine exhausts.


177

Figure 7.4. Conversion profile of morpholine, at the reactor outlet, at different temperature

and atmospheres (left). The points have been connected by basis-spline function, as guide to

the eye, and extrapolated for determination of the onset ignition temperatures (right). The

two tangential lines indicate an example of extrapolation of the onset ignition temperature for

He-NOx condition.

7.3.2. Initial steps in nitration of morpholine

The initial reaction of NO/NO2 with biomass produces nitro and nitro species (Equations 7.3

and 7.4) which then decompose to generate two new radical species RO· and NO2/NO3. The

latter, chain-branching reactions, underpin the sensitising effect of NO/NO2.49 Equation 7.5

expresses the biomolecular O-abstraction.7

R· + NO → RNO (7.3)

R· + NO2 → RNO2 (7.4)

R· + NO2 → RO· + NO (7.5)


178

Oxidative environments allow the biomolecular H-abstraction by triplet oxygen to form HO2

radicals (see Figure 7.5a), opening channels for the formation of peroxyl adducts (Reaction

7.6), an important combustion intermediates.50-54 Reactions 7.7 and 7.8 describe the

subsequent interaction of oxides of nitrogen with the peroxyl radicals.

R· + O2 → ROO· (7.6)

ROO· + NO → RO· + NO2 (7.7)

ROO· + NO2 ⇌ ROONO2 (7.8)

The thermal stability of peroxynitrate (RO2NO2) depends on the chemical nature of the R

backbone.55,56 It can dissociate back into peroxyl adduct, or preferably form alkoxy radical via

Reaction 7.9:57

ROONO2 → RO· + NO3 (7.9)

Previous studies35,38 described the importance of low temperature chemistry in understanding

the sensitisation effect of NOx during oxidation and ignition of hydrocarbons. In fact, these

calculations confirmed that, modelled results remain incomplete, as well as insensitive (to the

effect of NOx) without the inclusion of the low-temperature initiation in the overall mechanism.

Work is underway in our laboratory to produce a kinetic mechanism describing the initiation

reaction responsible for the enhanced ignition of morpholine by NO/NOx.

In the present context, initial decomposition of morpholine involves abstraction and direct

dissociation (under pyrolytic condition) of H atom and formation of three morphyl radicals,

namely, o-morphyl, m-morphyl and p-morphyl, depending on the relative position of the


179

radical site to the oxazine O atom. Figure 7.5 illustrates the enthalpies (ΔrH°298) of these

reactions, and of the barrierless addition of molecular (triplet) oxygen.

Figure 7.5. Radical initiation of morpholine decomposition by H-abstraction (a) and H-

dissociation (b) and formation of peroxyl adducts. Reaction enthalpies (ΔrH°298) and Gibbs

energies (ΔrG°298, in bold and italic) are reported in kJ/mol at 298.15 K (25 ° C).

HO2 radical remains highly stable at low temperatures. However, NO reacts with HO2

according to Reactions 7.10,58-60 generating OH, an active propagating radical sustaining the

oxidation process. Reactions 7.11 61,62 and 7.12 63,64 illustrate the interaction of NO2 with HO2.

The nitrous acid (HNO2) also dissociates and/or reacts with H/O radicals to form OH species.

HO2 + NO → OH + NO2 k(T) = 3.5 × 10-12 [cm3/molecule s] exp(250/T) (7.10)

HO2 + NO2 → HO2NO2 k(T) = 2.1 × 10-31 [cm6/molecule2 s] (T/298 K)-3.1 (7.11)

HO2 + NO2 → HNO2 + O2 k(296 K) = 5.0 × 10-16 [cm3/molecule s] (7.12)


180

The appearance of radical sites (Figure 7.5) enables subsequent addition reactions. Figure 7.6

depicts the addition of NO/NO2 to morphyl radicals under inert atmosphere, such as that of He-

NOx. These reactions proceed without a barrier, and lead to formation of C-nitroso and nitro

species, respectively. The nitro products convert into more thermodynamically stable

tautomers (i.e., aci forms) by migration of α-H atom (i.e., from the ipso carbon atom) to the

nitro group. The nitroso and nitro compounds, except for nitromethane, are highly stable,65,66

and this explains why we observe no reduction to the onset temperature of decomposition of

morpholine under the He-NOx atmosphere. Along the same line of inquiry, Venkatesh et al. 67

also reported higher inert decomposition temperatures for 2- and 4-nitroimidazoles, relatively

to that of imidazole. The authors have demonstrated that, the underlying Arrhenius constants

remain relatively large for the nitro compounds.

Figure 7.6. Pyrolytic nitration of morpholine with NOx. Reaction enthalpies (ΔrH°298) and

Gibbs energies (ΔrG°298, in bold and italic) are reported in kJ/mol at 298.15 K (25 °C). The

reversibility signs indicate the preferred reaction direction based on thermodynamic

stabilities.


181

Figure 7.7. Oxidative nitration of morpholine with NOx. Reaction enthalpies (ΔrH°298) and

Gibbs energies (ΔrG°298, in bold and italic) are reported in kJ/mol at 298.15 K (25 °C).

Bracketed digit implies corresponding trans conformation.

In presence of O2, hydrocarbons degrade along the peroxyl (ROO·) corridor50,51,68 into

oxidation products, such as aldehydes, epoxides, oxetanes, ketones and hydroxyl, with the first

step often involving the isomerisation (ROO·→ ·R’OOH). However, under NOx-containing

atmosphere (e.g. He-O2-NOx), as shown in Figure 7.7, NO/NO2 attaches to the outer oxygen

atom of peroxyl species, leading to formation of unstable organic peroxynitrites and

peroxynitrates, respectively. The addition channels release considerable amount of energy.

The heat feedback from the exothermicity of these reactions compensates the endothermicity

of reactions of Figure 7.4, sustaining the ignition process. Analogous reactions constitute the

major sink for ROO· in urban areas with high levels of NOx concentration.69-71 From a kinetic

viewpoint, considering the typical rate parameters for the addition of NO/NO2 to peroxyl

species,72,73 and the representative NOx level (i.e., ca 620 ppm), the bimolecular addition

reactions predominate over unimolecular isomerisation of ROO· adducts.


182

7.4. Conclusions

This chapter presented new results from experimental measurements and theoretical

calculations on the ignition of a representative biomass surrogate, due to sensitising effect of

NOx. Approximately 620 ppm of NOx reduces the ignition temperature of morpholine by 150

°C under a characteristic fuel-rich condition. The formation of organic nitrites and nitrates

plays an important role in decreasing the ignition temperature of fuel in NOx-laden oxidative

environments, but has negligible contributions in pyrolytic decompositions. The addition

reactions exhibit significant exothermicity. Further studies at varying fuel-oxygen equivalence

ratio, and NOx concentrations will help elucidate the limit of the sensitising effect.

Measurements of the ignition induction time involving actual biomass should provide further

insights into this behaviour. We recommend other experimental parameters for assessing the

risk of biomass ignition, e.g., thermogravimetrically-derived apparent activation energy (Ea)

and the temperature of maximum rate of weight loss (TMWL).20,22,23,74 The thermochemical

parameters of low-temperature intermediates identified herein will facilitate the development

of an improved reaction mechanism.

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191

CHAPTER 8

Paper VI: Experimental Study on Thermal Reaction of

Modelled Biofuel (Morpholine) with NOx

Oluwoye, I.; Dlugogorski, B. Z.; Gore, J.; Altarawneh, M. Manuscript in preparation as an

extension of Chapter 7.

Industrial processes such as

combustion plants

Biomass and biofuels

Interaction of morpholine

with NOx

The chapter inspects the thermal interaction of NOx with biomass fuels.

It focuses on high-temperature reaction of biofuel surrogate (morpholine)

with NOx, elucidating the N-conversion profiles, and NOx-reduction

efficiency as compared to conventional hydrocarbon fuels

…

CHAPTER 8 — Experimental Study on Thermal Reaction of Modelled Biofuel (Morpholine) with NOx

192

8.0. Abstract

This chapter investigates the thermal reaction of a biomass surrogate (morpholine), with oxides

of nitrogen (NOx). At conditions pertinent to industrial applications, we synergistically

employed experimental and analytical techniques to explore the effect of NOx on the pyrolytic

and oxidative thermal decomposition of morpholine. A laboratory-scale tubular laminar-flow

reactor enabled a flexible optimisation of reaction parameters within the temperature range of

300 C to 1100 C at different helium/oxygen/NOx reactive mixtures. Fourier transform

infrared (FTIR) spectroscopy, NOx chemiluminescence analysis and gas chromatography

(μGC) supported the assessment of major gas products as well as the N-conversion profiles.

The results indicate that, the presence of NOx lowers the ignition temperature of the fuel via

nitration processes. Furthermore, at high temperature, above 800 C, the fuel effectively

convert NOx (ca 80 %) via HCN/NH3 oxidation channel, depending on the abundance of O2 in

the system. These findings shed light on biomass-NOx interactions occurring in various

industrial processes, such as interaction of reactive nitrogen species with biomass residues

during thermal firing of solid fuels, and particularly, in fuel stagging technology utilising

biomass as the major reburn fuel.


193

8.1. Introduction

Practical interaction of biofuel and nitrogen oxides (NOx, mainly NO and NO2) occurs in

several industrial thermal processes. For instance, a broad assessment of biomass co-firing and

combustion framework reveals various NOx formation channels, categorically grouped as the

fuel, the thermal, and the prompt mechanisms. These channels serve as an active platform for

the NOx species (and its precursors) to react with volatile hydrocarbons, as well as the parent

biofuel. Such reactions can occur homogeneously within the gas-phase entities, and in form

of heterogeneous reaction with solid fuel residues.1-3

Furthermore, reburning technology serves in mitigating NOx formation during thermal

utilisation of solid fuels in combustion plants. The process basically involves the use of

secondary fuels (in a reburning zone) to reduce the NOx produced in a slightly lean-fuel

condition (primary zone) to intermediate nitrogenous species and molecular nitrogen (N2).4-7

Among other alternatives, it has been proven that the injection of organic-based materials such

as wood, straw, rice husk, bio-oil, sewage sludge, and carbonised municipal solid waste

effectively supports the reduction of NOx formation in this kind of combustion-modification

technology.8-17 Thus, it is essential to further understand the fundamental parameters

underpinning the thermal decomposition of biomass/biofuel in reactive NOx atmospheres, in

terms of product selectivity and NOx conversion efficiency.

The complex nature of biomass necessitates the use of representative model compounds in

understanding the conversion of nitrogen species during combustion and pyrolysis of biomass-

derived fuels. This includes hetero-element compounds, also known as surrogates, embodying

N- and O-functionalities within their structural entities.3,18 Morpholine (C4H9NO, 1-oxa-4-aza-


194

cyclohexane) has been revealed as an excellent surrogate candidate for an oxygenated and

nitrogen-containing biofuel compound due to its unique structure and wide industrial

applications.18 The molecule bears structural resemblances to ethanol, dimethyl ether, ethyl

amine, as well as dimethyl amine, and it is widely used as an impregnating agent for fruit,

cardboard, and paper, and as a fuel additive.18 Its N-, C-, and O-content correspond to that

revealed by ultimate analysis of typical biomass feedstocks. Several studies have described its

decomposition, combustion, ignition and flame characteristics.18-24

To this end, the present chapter focuses on experimental aspects of high-temperature

decomposition of morpholine, as a model for oxygenated nitrogen-containing compounds

present in the real biomass fuel, in different NOx-laden atmospheres. As it furnished

knowledge to the conversion of fuel-N in excess NOx conditions, it also aims to provide insight

into the interaction of biomass with NOx ensuing during thermal energy recovery from biofuels,

and in fuel reburning technology utilising herbaceous, woody and/or chaff biomass as the

secondary reburn fuel. In addition, it can also help describe the effect of NOx, arising from

detonation of ammonium nitrate explosives, on the ignition of lignocellulosic materials in

open-cut mines.

8.2. Applied Methodologies

8.2.1. Materials and experimental apparatus

We obtained the morpholine chemical from commercial source (TCI, purity > 99.0 %). Figure

8.1 presents a schematic representation of the experimental apparatus in which a digital syringe


195

pump delivered the reactant fuel (2.6 – 1.1 μL/min) into the preheated dilute stream of

helium/NOx/oxygen running through the reactor at combined rates of between 730 and 305

mL/min, at standard temperature and pressure (STP). The concentration of morpholine in the

diluted stream amounted to 1000 ppm on molar basis. An electrically heated three-zone

horizontal split furnace accommodated the quartz reactor tube, and ensured a well-defined

isothermal regime bracket a temperature range of 300 C and 1100 C. The use of helium

(99.999 %) as primary dilution gas allowed quantitative measurement of N2 formation. The

reactor operated at nominal Reynolds numbers of 7.8 (at 300 °C) – 1.8 (at 1100 °C), indicating

a laminar flow regime with an average hydrodynamic entry length of approximately 0.9 % of

the total reaction zone (300 mm).The NOx concentration remained constant at 620 ppm (600

ppm of NO and 20 ppm of NO2). During oxidative experimental conditions, we adjusted the

O2 concentration to deliver the fuel-oxygen equivalence ratio of 1.25 (i.e., moderately fuel

rich condition) as per the following stoichiometric reaction:

C4H9NO + 5.75O2 = 4CO2 + 4.5H2O + 0.5N2 (8.1)

We set a residence time of 1.0 s in the reaction zone at ambient pressure of 1.01 bar. The

reaction time corresponds to the average residence time in a practical reburning zone and a

typical experimental set-value in study of combustion of cyclic compounds.4,6 Furthermore,

we employed Charles’ temperature-volume relationships to adjust the gas flow rates metered

by the mass flow controllers (MFC, operated at the room temperature) but preheated just before

the reactor. Similarly, we employed a mass balance to convert the flow rate of the fuel needed

for the predefined concentration at isothermal reaction temperature, into the corresponding set-

values at room temperature. Due to the relatively small volume (4 % of the reaction zone) of

the annual preheating space of zone 1 (Figure 8.1), we conclude that the annual volume has


196

negligible contribution in the system. The products exit into various assemblages designed for

appropriate sampling and quantitation.

Figure 8.1. Schematic representation of experiment setup. Symbol “T” denotes

thermocouples.

.

8.2.2. Product sampling and analyses

As illustrated in Figure 8.1, gaseous product analyses involved three parallel stages. Firstly,

FTIR spectroscopy (Perkin Elmer) facilitated online monitoring of product species exiting the

tubular reactor. The setup incorporated an electrically heated transfer line (3.175 mm ID × 300

mm) linking the reactor to a small volume (100 mL) 2.4 m path-length online gas sampling cell


197

(Pike) placed inside the FTIR compartment. The spectrometer averaged 64 accumulated scans

per spectrum at 1 cm-1 resolution. Both the sample line and the sampling cell operated at 140

C, to avoid the condensation of product volatiles prior and during the analysis. Furthermore,

the chemiluminescence NOx analyser (Thermo Scientific model 42i-HL) monitored the

concentration of NO and NO2. Finally, the use of μGC (Agilent 490 micro-gas

chromatography, 20 m MolSieve-5A column, heated injection) assisted N2 quantitation. The

train of three impingers consisting of glass units filled with standardised 0.1 M H2SO4, 1 M

NaOH, and orange silica gel removed any contaminant species, e.g. ammonia and H2O that

could damage the GC column.

It must be noted that the spectroscopy technique25-27 has proven highly instrumental in the

detection and quantitation of the aforementioned gaseous nitrogenous compounds. QASoft

database (Infrared Analysis, Inc., Anaheim, CA) enabled semi-quantitation of the species by

their characteristic IR bands listed in Table 8.1. The accuracy of the measurements is highly

dependent upon the tolerance of the absorption coefficients of the reference spectra. This is

estimated to be within ±5 % in the QASoft library.


198

Table 8.1. List of gaseous species studied, and their selected unique IR frequencies.

Compounds Chemical formula Analysis frequency (cm-1)

Methane CH4 3019 – 3011

Ethyne (acetylene) C2H2 729 (Q-branch)

Ethylene C2H4 950 (Q-branch)

Formaldehyde CH2O 1747 – 1743

Carbon monoxide CO 2171 – 2157

Carbon dioxide CO2 2420 – 2207

Hydrogen cyanide HCN 712 (Q-branch)

Ammonia NH3 1047 – 1045

Nitric oxide1 NO 1901 – 1900

1 Concentration confirmed by chemiluminescence NOx analyser.


8.3.1. Overview of ignition temperatures

We selected one of the group frequencies of morpholine (at 1132 cm-1) for quantitative

monitoring of the fuel conversion according to Equation 8.2:

𝛼 = ⌊𝐴𝑏𝑠𝑖 − 𝐴𝑏𝑠𝑇

𝐴𝑏𝑠𝑖⌋ × 100 % (8.2)

where Absi represents the initial concentration (proportional to absorbance value at 1132 cm-1)

before the thermal reaction, and AbsT denotes the concentration of the fuel at the outlet of the


199

reactor. Figure 8.2 presents the conversion of morpholine under different gas-stream

conditions. It is evident that, the presence of NOx plays a crucial role in the ignition features

of the fuel. This can be attributed to the distinct way in which NOx reacts with hydrocarbons,

by accelerating their oxidation process.28,29 NOx enhances the ignition of organic fuels in a

process generally referred to as sensitisation via nitration reactions.30-34 The addition of NO

and/or NO2, to fuel-oxygen systems, significantly alters the overall kinetics by turning the usual

chain-terminating steps of formation of peroxy (RO2) radicals into a chain-propagating step.29

Figure 8.2. Conversion of morpholine at the outlet of the reactor, under different reaction

atmospheres. The extrapolated onset ignition temperatures amount to 581 °C, 613 °C, 494

°C, and 356 °C for He, He-NOx, He-O2, and He-O2-NOx conditions, respectively.

As shown in Chapter 7, the sensitisation of morpholine develops within the chain-propagating

NO/NO2 cycle.35 In the absence of oxygen, addition of NOx species to morphyl radicals

(formed as the result of H abstraction from the ortho, meta and para sites of morpholine)

operates via Equations 8.3 and 8.4:

300 400 500 600 700 800

0

20

40

60

80

100

Per

cen

t co

nv

ersi

on

(%

)

Temperature (C)

He

He-NOx

He-O2

He-O2-NO

x


200

R· + NO → RNO (8.3)

R· + NO2 → RNO2 (8.4)

Furthermore, the presence of O2 allows the formation of peroxy (ROO·) adducts and

hydroperoxyl radical (HO2),36-40 that subsequently interact with NOx in following reactions:

R· + O2 → ROO· (8.5)

ROO· + NO → RO· + NO2 (8.6)

ROO· + NO2 ⇌ ROONO2 → RO· + NO3 (8.7)

HO2 + NO → OH + NO2 (8.8)

HO2 + NO2 → HNO2 + O2 (8.9)

8.3.2. Quantitative analysis of N-conversion at high temperatures

This section focuses on the formation of N-products, but also describes the consumption of

some major hydrocarbons species. As shown in Figure 8.3, the principal reaction products

include acetylene (C2H2), ethylene (C2H4), methane (CH4), formaldehyde (CH2O), carbon

monoxide (CO), carbon dioxide (CO2), hydrogen cyanide (HCN), ammonia (NH3) and nitric

oxide (NO), depending on the composition of the inlet gas mixture.


201

Figure 8.3. Annotated IR spectra of product species during decomposition of morpholine at

different temperatures, in He (a), He-NOx (b), He-O2 (c) and He-O2-NOx (d) atmospheres.

Figure 8.4 plots the measured product species’ profiles over a wide range of temperatures, at

different atmospheres. The influence of these variables has been studied by measuring the

output concentration of the reaction zone of different nitrogenous species (NO, HCN, NH3),

carbonaceous species (in particular CO and CO2), and low molecular weight hydrocarbons

(C2H2, C2H4, CH4 and CH2O).


202

Figure 8.4. Mole-fraction profile of selected product species during decomposition of

morpholine at different temperatures, in He (a), He-NOx (b), He-O2 (c) and He-O2-NOx (d)

atmospheres. The initial concentration of morpholine amounted to 1000 ppm; reaction

residence time equalled 1 s; and initial concentration of NO in (b) and (d) corresponded to

600 ppm.

Previous studies have describe the pyrolysis and oxidative decomposition of morpholine.19-24

The interaction of NO relies on the action of the hydrocarbon radicals/fragments originated

from the decomposition and/or oxidation of the hydrocarbon fuel. Radicals from the

morpholine fuel can originate by two important mechanisms: oxidation and/or thermal

decomposition. The thermal decomposition (i.e., pyrolysis) of hydrocarbons occurs at high

temperatures. However, the presence of oxygen initiates relatively low-temperature


203

degradation. Hence, CHi radicals from the oxidation of hydrocarbons will be responsible for

the NO conversion at low temepratures.5 Comparing Figures 8.4a and 8.4b, the presence of

NOx somewhat reduced the concentration of the low molecular weight hydrocarbons. The

observed trend of CH2O match that of previous studies on its formation and destruction in

exhaust system of gas engines.41 The characteristic temperature regime for the oxidation

(Figures 8.4c and 8.4d) of the morpholine to CO and CO2 at similar reaction times differs

appreciably between the stream compositions. In the case of He-O2, the CO forms in relatively

lower concentration (at a temperature of approximately 500 °C) and rapidly increases to peak

at around 750 °C. Above this temperature, the CO concentration remains approximately

constant. He-O2-NOx results in higher CO concentration. However, the oxidation of CO to

CO2 is generally favoured at higher temperatures

The concentrations of NH3 and fuel-NOx appear negligible in all the experiments as compared

to the concentration of the total reactive nitrogenous species. Figure 8.4 shows that, the

interaction of NO with hydrocarbon radical increases HCN concentration. The subsequent role

of HCN remain highly dependent of the conditions that exist in the reaction zone. Reaction of

HCN with O2 recycled it into NO, while the total NOx (He-NOx + fuel NOx) converts it into

molecular nitrogen. The peak concentration of HCN, in oxidative conditions, also appears at

around 750 °C, after which it decreases significantly to its valley at higher temperatures.

Hence, the presence of oxygen in the reaction zone promotes the conversion of the intermediate

nitrogenous species to N2, since it is responsible for an increase of the radical pool.5,42-44


204

8.3.3. Comments on NO reduction and formation of N2

Equation 8.10 defines the transient NO conversion in terms of NO concentrations at the inlet

of the reactor [NO]𝑖,inlet, the outlet of the reactor [NO]𝑖,outlet, and the diminutive NO

concentration in the decomposition product of morpholine [NO]𝑖,fuel. The subscript i denotes

the gas composition, i.e., with and without O2.

NO conversion =[NO]𝑖,inlet − ([NO]𝑖,outlet − [NO]𝑖,fuel)

[NO]𝑖,inlet× 100 % (8.10)

Figure 8.5a depicts the NO conversion efficiency of morpholine in pyrolytic and oxidative (

= 1.25) conditions. The results show that, HCN, formed from fuel-N transformation and as an

intermediate, plays crucial role in overall conversion of NO. The oxygen content of

morpholine also allowed low temperature NOx reduction during pyrolytic conditions, in

agreement with former experimental study on furan (C4H4O).45 The maximum NO conversion

amounts to approximately 83 %, at 900 °C, after which it decreases to 80 % due to the activity

of the reduction scheme at higher temperatures.4,5,44


205

Figure 8.5. NOx conversion efficiency of morpholine (a), and N2 formation (b) in thermal

decomposition the fuel at different experimental conditions. Initial morpholine and NO

concentration are 1000 ppm and 600 ppm, respectively.

Likewise, the trend of N2 formation in Figure 8.5b confirms the overall reduction of NOx. The

NOx reduction efficiency of morpholine show an analogy with literature data. For example,

Biomass fuels such as wood, straw, rice husk, bio-oil, sewage sludge, and carbonised municipal

solid waste provide effective means of reducing NOx formation (50 – 75 %) in combustion

systems.8-17,46,47 In comparison to other conventional hydrocarbon fuels, morpholine reduction

efficiency surpasses that of acetylene, ethylene, ethane, natural gas, methane, and biogas.46,48-

51 However similar reduction efficiency had been reported for pyrolysate fragments of coal

and waste polymers.13,52-57

8.4. Conclusions

This chapter provided experimental measurements of thermal reaction of morpholine with NOx,

showing the ensuing products at different temperatures and gas stream compositions. The

reported NOx reduction efficiency elucidate morpholine as a good biomass surrogate with


206

efficiency values similar to that of actual biomass residues. In presence of oxygen, the

effectiveness of morpholine as NO reduction reagent compares well to that of C3H6. For

conventional hydrocarbon fuels, it has been demonstrated that the NO reduction efficiency

follows the order of C2H2 > C3H6 > CH4.48,58 Hence, morpholine should be more effective than

CH4, and better preferred to C2H2 that usually leads to the formation of NO2 rather than the

more desirable N2 or HCN. Finally, the O-content of morpholine permits NOx reduction at

relatively lower temperature, making it an excellent reburning fuel in O2-deficient

environment.

8.5. References




Combust. Sci. 2003, 29, (2), 89-113.




Combust. Sci. 1998, 24, (5), 385-408.

5. Alzueta, M. U.; Glarborg, P.; Dam-Johansen, K., Low temperature interactions between

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212

CHAPTER 9

Conclusion and Recommendations

This chapter concludes the entire dissertation, and lays out

some significant recommendations for future studies

…

CHAPTER 9 — Conclusion and Recommendations

213

9.1. Conclusion

This thesis contributed towards finding sustainable solutions that could alleviate the

unprecedented increasing anthropogenic emission of NOx, serving to mitigate adverse effects

on the environment. NOx discharge surfaces as a result of lack of adequate mitigation

techniques in power plants, oxidation of reactive nitrogen species (e.g. NH3 from the Haber-

Bosch system) in chemical and agricultural processes, combustion activities involving

untreated fossil, bushfires and mining operations, especially large open cut mines that require

significant movement of materials (including overburden) by diesel-driven excavators and

trucks. To tackle this problem, supplementary-fuel reburning remains a viable technological

approach through which an overall decrease in NOx emission of about 50 – 85 % can be

attained. Therefore, the present research work explored new sustainable technologies for

reducing industrially formed NOx, based on biomass and recycled plastics, particularly for

applications relevant to blasting of mining-grade ammonium nitrate (AN) based high-energy

materials.

The thesis reviewed the global NOx formation channels in light of consequential environmental

impacts. It quantitates NOx emissions during detonation of ammonium nitrate blasting agents

with valid explanation of different underlying mechanisms, and assesses their contributions

with respect to the global nitrogen budget. The estimated NOx emission from AN-based

explosives amounts to 0.05 Tg (i.e., 5×104 t) N per annum, in comparison to the total rate of

NOx of 41.3×106 t N/y, entering the environment from all anthropogenic sources. Although

small on a global scale, these emissions involve concentrated discharges characterised by NOx

concentration in the order of 500 ppm. With respect to the current World Health Organisation

air quality guidelines, the typical concentrations of NOx in post-blast clouds exceeds the safe


214

limits by around 30 to 3000 times. Furthermore, the chemical reactivity of blasting-related

NOx, in the presence of other atmospheric substances, produces secondary pollutants. For

instance, the mineral particles and fugitive dusts synchronously present in post-blast fumes can

easily facilitate the oxidation of NOx, contributing to atmospheric formation of nitrate aerosols.

The computational studies analysed the primary initiation reactions occurring during

aggressive low-temperature oxidation of PE. In the NOx-laden atmosphere, formation of

diverse nitrogen-containing species ultimately accompanies the free-radical processes in solid

PE. Nitroso and nitroalkyl species appear as a result of respective addition of NO and NO2

molecule to a carbon radical site in PE chain. It has been noted that, the availability of

neighbouring chemical functional groups alters the thermodynamic and kinetic properties of

the reactions. Concerted elimination of HNO and HONO from nitroso- and nitroadducts,

respectively, requires moderate activation energies, compared with unimolecular

tautomerisation of the adducts. However, the presence of molecular oxygen induces the

formation of peroxy species, that subsequently bond with NO and/or NO2, thereby serving as

a potential source of organic nitrites and nitrates. In comparison with previous studies that

substantiated the addition of nitrogen oxides to solid polymer resins, the current investigation

delivered the concrete thermodynamic evidence of such events, and provides the reaction

channels describing the fate of consequential nitrogenous species. On this note, the calculated

thermochemical and kinetic parameters offer an improved understanding of the polymer-gas

reactions, and present insights into similar reaction sequences in other crystalline polyolefins.

Although fuel reburning constitutes an effective technologies for NOx reduction, practical

applications are sometimes influenced by cost. This study has shown that, recycled plastics

offer a relatively effective and cheaper means of reburning. An extensive experimental study


215

characterised the behaviour of waste polymer pellets under pyrolytic low-temperature (<600

°C) conditions. In particular, TGA-based analyses measured the thermal decomposition

pattern of neat and recycled polymer samples under controlled and representative NOx

environments, and developed a mechanistic insight explaining the formation of initial product.

The limiting initial step of thermal decomposition involved H-dissociation to form an alkyl

radical. NOx then acts in the further decomposition steps by adding to the radical sites to form

double bonds in the oligomer chain, eliminating HNO or HONO, respectively. The well-

established isoconversional method constructed the kinetic parameters showing conversion

dependent apparent activation energies of 135 kJ/mol – 226 kJ/mol, and 188 kJ/mol – 268

kJ/mol, for neat and recycled PE, respectively. Also, the so-called compensation effect

accounted for the natural logarithmic of the pre-exponential factors ln (A/min-1 ) ranging

between ca 19 – 35 and 28 – 41, in same order., depending on the PE conversion in the

experimental interval between 5 and 95 %. The typical NOx reduction efficiency of pyrolysing

fragments of waste PE amounted 80 %, hence the injection of waste PE into a reburning zone

of combustors may significantly reduce the emission of NOx. The gathered kinetic data on the

thermal decomposition of PE in a NOx atmosphere, in the absence of mass transfer limitation

will serve to predict the polymer conversion (in NOx atmosphere) at elevated temperatures, to

plan the practical vertically-entrained drop-tube experiments.

In agreement with principles of green chemistry and engineering, thermal reduction of NOx by

recycled polyethylene assigns value to minimisation of solid wastes by providing an

ecofriendly exit route for excess plastic wastes in our society. Moreover, its application to NOx

formation in combustion plants promotes the growth of innocuous degradation products,

thereby enhancing low-emission technologies and effective energy recovery from polymeric

waste materials. Furthermore, the scientific approach can be optimised to deploy waste


216

polyethylene in bulk ammonium nitrate/fuel oil (ANFO)/emulsions explosive mixtures, so as

to mitigate the NOx formation during detonation of nitrate-based explosives, for example, in

mining operations. Application of waste PE in reducing NOx emissions from detonation of AN

explosives relies strongly on the heating condition. Since explosions are generally

characterised with flash heating rates of the order of 10,000 K/s, defragmentation of PE at such

conditions results in relatively low molecular weight hydrocarbon, which are expected to

further enhance the reduction efficiency of the system.

The second wing of this dissertation investigated the thermal interaction of biomass with NOx.

In which, low-temperature tubular reactor experiments gauged the sensitising role of NOx on

biomass ignition/combustion. We concluded that, this has an unintended consequence of

degrading the safety of organic dust-producing plants. For a representative fuel-rich condition

( = 1.25), the concentration of NOx as small as 0.06 % lowers the ignition temperature of

morpholine by 150 °C, i.e., from approximately 500 °C to 350 °C. The formation of organic

nitrites and nitrates, as the result of addition of NOx species to morphyl and peroxyl radicals,

plays an important role in decreasing the ignition temperature of biomass fuel in NOx-laden

oxidative environments, but has negligible contributions in pyrolytic decompositions. Other

contributing factor includes the reaction of NOx with low-temperature hydroperoxyl radical

leading to the formation of active OH species that also propagate the ignition process. The

present measurements indicate that, safe operation of wood-working plants requires avoiding

trace concentration of NOx within the vicinity of dust-prone areas. This can be facilitated by

proper (and separate) venting of engine exhausts.

Furthermore, our high temperature tubular reactor arrangements elucidated morpholine as a

good biomass surrogate with NOx reduction efficiency values similar to that of actual biomass


217

residues at conditions pertinent to industrial applications. The principal reaction products

include acetylene (C2H2), ethylene (C2H4), methane (CH4), formaldehyde (CH2O), carbon

monoxide (CO), carbon dioxide (CO2), hydrogen cyanide (HCN), ammonia (NH3) and nitric

oxide (NO), depending on the composition of the reaction mixture. In presence of oxygen, the

effectiveness of morpholine as NO reduction reagent compares well to that of propene (C3H6).

For conventional hydrocarbon fuels, it has been demonstrated that the NO reduction efficiency

follows the order of C2H2 > C3H6 > CH4. Hence, morpholine should be more effective than

CH4, and better preferred to C2H2 that usually leads to the formation of NO2 rather than the

more desirable N2 or HCN intermediate. Finally, the O-content of morpholine permits NOx

reduction at relatively lower temperature, making it an excellent reburning fuel in O2-deficient

environment. These findings shed light on biomass-NOx interactions occurring in various

industrial processes, such as interaction of reactive nitrogen species with biomass residues

during thermal firing of solid fuels, and particularly, in fuel stagging technology utilising

biomass as the major reburn fuel. In addition, it also help describe the effect of NOx, arising

from detonation of ammonium nitrate explosives, on the ignition of lignocellulosic materials

in open-cut mines.

9.2. Recommendations

The scientific studies embodied in this dissertation had led to a number of intriguing research

questions. The following paragraphs discuss some suggestion for potential future work.

We have identified the possibility of the formation of secondary pollutants (nitrate aerosols) as

a result of atmospheric nitration of the mineral particles and fugitive dusts synchronously


218

present in post-blast fumes. Therefore, development of new sampling methodologies should

include the environmental monitoring of such species from blasting activities. Lightweight

remote-controlled drone-sampling instrumentation, as well as surface sampling, could facilitate

the measurements of nitrate aerosols, as well as other pollutants including supplementary Nr

species (e.g. HNO3, NH3, and N2O), hydrocarbons, nitrogenated analogues of dioxins and

secondary contaminants such as peroxyacyl nitrates (PAN). Understanding the mechanism of

the formation and the survival of these pollutants in detonation of heavy AN explosives is

crucial to achieve sustainable mining activities, as are further testing of OW emulsions and

bulking agents, and generating new knowledge on the exact mechanism of deflagration that

involves the surface burning of AN particles.

The low-temperature non-isothermal measurements in Chapter 6 gathered the kinetic data on

the thermal decomposition of PE in a NOx atmosphere, in the absence of mass transfer

limitation, serving to predict the polymer conversion (in NOx atmosphere) at elevated

temperatures. The preliminary quantitative NOx reduction efficiency concluded that, the

injection of waste PE into a reburning zone of combustors may significantly reduce the

emission of NOx. Reburning of NOx in practical combustors usually occurs in a temperature

window between 1000 °C and 1200 °C. Therefore, a change of the reaction mechanism may

occur at those high temperatures. Hence, we recommend a subsequent study to address the

high-temperature isothermal and flash-particle heating conditions in drop-tube experiments,

with valid economic analysis. This will reveal possible effects of the presence of oxygen and

mass transfer on the reaction of NOx with PE pyrolysates under realistic reburning temperatures

(i.e., 1000 – 1200 °C). Along the same line of interest, the suitability of other complex-chain

polymer should be investigated, with respective to their NOx reduction efficiencies, and most

importantly, the likelihood of the formation of ecotoxic nitrogenated analogue dioxins, such as


219

diphenylamine (DPA), carbazole, phenoxazine, and phenazine. Figure 9.1 illustrates a

proposed laboratory-scale vertically-entrained drop-tube experimental setup for high-

temperature studies.

Figure 9.1. Proposed laboratory-scaled drop tube furnace reactor, equipped with variable

speed DC motor that allows the adjustment of feeding rate of pulverised polymer sample,

being carried downstream by inert (helium) gas. The symbol “T” refers to thermocouples.

Finally, regarding the sensitising role of NOx in biomass ignition, further studies at varying

fuel-oxygen equivalence ratio, and NOx concentrations would help elucidate the limit of the

sensitising effect. Measurements of the ignition induction time involving actual biomass

should provide further insights into this behaviour. We recommend other experimental

parameters for assessing the risk of biomass ignition, e.g., thermogravimetrically-derived

apparent activation energy (Ea) and the temperature of maximum rate of weight loss (TMWL).

Inert gas

NOx gas

T

T

Gas preheater

M

Iso

ther

mal

tem

per

atu

re z

on

e

Particulate filter

Product analysers;

FTIR, NOx analyser

Particle sampling and

characterisation

+ -

T


220

Finally, confirmation of interactions between fuel-N and NOx, (e.g., by experiments involving

isotopically labelled 15NOx), and quantitative measurements of nitro- and nitroso-adducts (e.g.,

by employing Tenax cartridge in thermal desorption unit, coupled with gas

chromatography/mass spectrometry setup) would constitute a significant advance in our

understanding of N-selectivity in high-temperature thermal reaction of morpholine with NOx.

Future work should assemble a complete reaction mechanism, validated by the experimental

conversion profile of morpholine, and the quantitative data on the formation of product species.

221

CHAPTER 10

Supplementary Document

This section provides the necessary supporting information

for some of the chapters included in this thesis

…

CHAPTER 10 — Supplementary Document

222

Appendix I: Supporting Information for Chapter 6

S6.1. Characterisation Techniques

Fourier transform infrared (FTIR) spectroscopy: This method elucidated the major

functional groups present in the polymer samples. We applied Agilent Cary 670 equipped

with attenuated total reflectance (ATR) accessory. Prior to analyses, samples were

cryogenically pulverised to achieve reproducible measurements. The FTIR recorded the

spectra at 4 cm-1 resolution, with accumulation of 32 scans per spectrum. The bands at

2916 cm-1, 2848 cm-1, 1470 cm-1 and 718 cm-1 correspond to CH2 asymmetric stretch, CH2

symmetric stretch, CH2 scissors and CH2 rock modes, respectively.

Carbon-hydrogen-nitrogen-sulfur (CHNS) analysis: CHNS elemental analyser provided a

means for rapid determination of carbon, hydrogen, nitrogen and sulfur in the organic

polymer matrices. Pulverised samples were fed into the pre-calibrated instrument

(EuroVector EA3000) under dry helium and O2 purge, and the combustion furnace operated

at approximately 1100 °C. The reactor functioned in a dynamic flash combustion mode,

and contained both the oxidation zone represented by tungsten oxide and copper wired

reduction zone. Two repeated runs resulted in 86.24 and86.10 wt. % for C, 14.25 and 14.20

wt. % for H, 0.00 and 0.00 wt. % for N as well as 0.00 and 0.00 wt. % for S.

Ion chromatography: Dissolved sample was injected into DX-120 anion chromatograph.

The result showed no trace of halogens, except for Cl present at < 0.1 %.


223

Inductively coupled plasma emission spectroscopy (ICP-OES): Varian 715-ES ICP OES

(University of Newcastle, Australia) served to measure the metal concentration in the

sample. About 140 mg of the recycled PE were digested in microwave, and then transferred

via Varian SPS 3 sample preparation system for analyses. High-purity ICP multi-element

standard solution containing different metal cations in 2.0 % HNO3 facilitated the necessary

calibration.

High temperature size exclusion chromatography (HT-SEC) in TCB: We performed HT-

SEC analyses using a Viscotek system (from Malvern Instruments) equipped with three

columns (Polefin 300 mm × 8 mm I. D. from Polymer Standards Service, porosity of 1000

Å, 100 000 Å and 1 000 000 Å). Sample solutions 200 µL in volume, with a concentration

of 5 mg/mL, were eluted in 1,2,4-trichlorobenzene (1,2,4 TCB) with a flow rate of 1

mL/min at 150 °C. The mobile phase was stabilised with 2,6-di(tert-butyl)-4-methylphenol

(200 mg/L). Online detection was performed with a differential refractive index detector

and a dual light scattering detector (LALS and RALS) for absolute molar mass

measurement. The OmniSEC 5.02 software facilitated the calculation of the SEC

parameters.


224

Figure S6.1. High temperature size exclusion chromatogram of neat (a) and recycled (b) PE,

and estimated molecular parameters. Symbols are explained in the caption to Table 6.1 in the

main manuscript. Ret. Vol. denotes the corresponding peak retention volume.

During recycling processes, the polymer chains of the recycled PE break, with chains of higher

molar mass displaying more probability of breaking. Therefore, the molecular mass

distributions (MMD) of the recycled PE reveals lower dispersity Ip values. Other contributions

to MMD include:

(a) Melting point: Our DSC result (Figure S6.2) showed that, the recycled PE displays a

higher melting point (123 °C) with broader peak, compared to the neat sample (116 °C).

Higher melting point suggests larger-molecular-mass, i.e., a higher Mp value. However,

when Mp varies only by around 18 kg/mol (our case here), there are no effects on melting

temperature.1,2 Rather, the structure of the PE samples induces this behaviour. The

intrinsic viscosity of the recycled PE (1.9 dL/g) exceeds that of the neat PE (1.1 dL/g).


225

This means that, the neat PE is more branched (short-chain branched LDPE) than the

recycled PE. More branches imply lower melting point and lower intrinsic viscosity.3

(b) Delayed decomposition of recycled PE: As shown in Figure S6.3, the relative

decomposition of the recycled PE is somewhat delayed due to the presence of more linear

chain. Neat PE is more branched, hence, its decomposition occurs earlier at the relatively

more sensitive branching-carbon sites. The alpha and beta scission mechanisms around

such carbons, during thermal degradation, have been well explained by Haney et al.4

S6.2. Assessments of Calibration and Repeatability of TGA Experiments

Prior to experiments, we carefully calibrated the instrument. The reference standards for the

mass and temperature calibration were selected according to the desired experimental range.

Table S6.1 lists the measured values as compared to the expected correspondence. The table

also provides the calculated percent error.

Table S6.1. Verification of the Instrument’s measured values.

Weight (mg)


Certified mass 54.37 54.59 54.36 7.36 × 10-3

Temperature3 (C)


Indium 156.60 156.21 156.45 9.58 × 10-2

Zinc 419.47 417.12 419.20 6.44 × 10-2

Aluminium 660.10 662.67 660.30 3.03 × 10-2


226

Heat flow4 (J/g)


Zinc 108.37 115.70 110.20 0.17 × 10-1

Aluminium 400.10 410.02 400.80 1.74 × 10-1

1 Measured values before calibration.

2 Measured values after calibration.

3 Onset melting temperatures.

4 Enthalpy of fusion (∆Hf).

During the entire duration of the study, we acquired replicate runs randomly at different dates

in order to check for the instrumental drift and reproducibility of experimental data. Figure

S6.2 shows a typical result of the thermal analysis, and Figure S6.3 depicts the selective

reproducibility of three experimental runs under different conditions (Table S6.2).

Table S6.2. Experimental conditions for the thermogravimetry analyses

Runs Sample Treatment Scan

condition

Purge

rate

Crucible Sample

mass (m)

Buoyancy

effect

1-3 Neat

PE

Sliced

(-400 μm)

25 – 620 C 80

mL/min

Alumina;

uncovered

5≤m≤5.4

mg

Corrected

1-3 Recycled

PE

Sliced

(-400 μm)

25 – 620 C 80

mL/min

Alumina;

uncovered

5≤m≤5.4

mg

Corrected


227

Figure S6.2. Thermal characterisation of recycled PE sample at nominal heating rate of 10

K/min under inert (argon) purge. The DSC signal of the simultaneous thermal analysers

(STA) cannot facilitate accurate quantitative analysis of thermodynamic properties. The

recycled PE sample dissolved completely in toluene (at 80 °C), indicating the absence of

cross-linked polymer.


228

Figure S6.3. Reproducibility of TGA results.


229

S6.3. Monotonic Behaviour of Heated PE Samples

Under non-isothermal conditions, in which a sample heats at a constant rate, the decomposition

pattern often relies on the applied heating rate. The conversion of the tested PE samples

exhibits an increasing monotonic behaviour with increasing temperature. This means that, NOx

is not incorporated permanently in the reacting solid, consistently with the mechanism of

Figure 6.5, included in the body of the article. Figure S6.4 portrays the dependence of α on

temperature. Herein, α refers to the fractional conversion of polymer sample, and it represents

the normalised form of mass loss data from the thermogravimetry analysis:

𝛼 =𝑚i − 𝑚t

𝑚i − 𝑚f (S6.1)

in terms of initial mass before the thermal analysis mi, instantaneous mass at any

temperature/time mt, and the final mass after the entire decomposition process mf.


230

Figure S6.4. The monotonic behaviour of samples studied in different purge atmospheres.

S6.4. Derivation of Cited Equations and the Compensation Effect

S6.4.1 . Isoconversional method

In solid-state reactions, the rate can be described in terms of three main variables: the

temperature T, the conversion α (already defined), and the pressure P, according to the

following equation:


231

𝑑𝛼

𝑑𝑡= 𝑘(𝑇)𝑓(𝛼)ℎ(𝑃) (S6.2)

Ignoring the pressure dependence h(P), the process rate hinges solely on the temperature

function k(T) and the conversion parameter f(α), otherwise termed as the reaction model.

Model-free or isoconversional methods rely on the assumption that, at constant conversion, the

reaction rate remains function of temperature only. Furthermore, Arrhenius equation expresses

the temperature dependence:

𝑘(𝑇) = 𝐴exp (−

𝐸

𝑅𝑇) (S6.3)

where the kinetic parameters A and E respectively represent the pre-exponential factor and the

activation energy, while R denotes the universal gas constant. In a non-isothermal experiment,

when temperature changes linearly with time, the heating rate β can be expressed as:

𝛽 =

𝑑𝑇

𝑑𝑡= 𝑐𝑜𝑛𝑠𝑡 (S6.4)

Combination of equations (S6.2), (S6.3) and (S6.4) yields:

𝛽𝑑𝛼

𝑑𝑇= 𝐴exp (−

𝐸

𝑅𝑇) 𝑓(𝛼) (S6.5)

Rearranging, and integrating, one obtains:


232

𝑔(𝛼) = ∫

𝑑𝛼

𝑓(𝛼)

𝛼

0

=𝐴

𝛽∫ exp (−

𝐸

𝑅𝑇) 𝑑𝑇

𝑇𝛼

0

=𝐴

𝛽𝐼(𝐸, 𝑇) (S6.6)

where g(α) is the integral form of the reaction model, and the corresponding limits of

integration, α and Tα follow from Figure S6.3. The integration of Equation (S6.6) eliminates

the need for a priori knowledge of the reaction model. However, the temperature integral I(E,T)

has no analytical solution. Consequently, according to Vyazovkin approach,5,6 for a series of

runs conducted at different heating rates, the Eα can be determined by minimisation of the

following function:

∅(𝐸𝛼) = ∑ ∑

𝐼(𝐸𝛼, 𝑇𝛼,𝑖)𝛽𝑗

𝐼(𝐸𝛼, 𝑇𝛼,𝑗)𝛽𝑖

𝑛

𝑗≠𝑖

𝑛

𝑖=1

(S6.7)

In this work, we experimented with four temperature ramps (5, 10, 15 and 20 K/min), repeating

each run three times. The activation energy can be estimated at any particular value of α, by

finding Eα for which the function attains a global minimum. The subscripts i and j represent

integer numbers of different experiments performed under varying heating programs, i.e., i

equals 1 – 3 for replicates, and j corresponds to 1 – 4 for heating rates. The temperature integral,

also known as the Arrhenius integral, corresponds to:

𝐼(𝐸𝛼, 𝑇𝛼) = ∫ exp (−

𝐸

𝑅𝑇) 𝑑𝑇

𝑇𝛼

0

(S6.8)

The presence of systematic errors (as a result of the integral approximation) can be eliminated

by performing the integration over a small segment of temperature:


233

𝐼(𝐸𝛼, 𝑇𝛼) = ∫ exp (−

𝐸

𝑅𝑇) 𝑑𝑇

𝑇𝛼

𝑇𝛼−∆𝛼

(S6.9)

In which α varies from 2∆α to 1-∆α, with a step of ∆α = (m+1)-1, and m defines the number of

equidistant values of α selected for the analysis. We applied the fourth order Senum and Yang

approximation7 for the integral function, and minimised Equation S6.7 by employing the solver

tool in Microsoft excel. The minimisation task has been repeated for each value of α to obtain

the dependency of Eα on α.

Alternatively, owing to other forms of numerical approximation of I(E,T), there have been

various standard integral isoconversional methods, in the form:

ln (

𝛽𝑖

𝑇𝛼,𝑖𝐵 ) = Const − 𝐶 (

𝐸𝛼

𝑅𝑇𝛼,𝑖) (S6.10)

The subscript i represents an integer number for the four heating ramps; i.e., i = 1 to 4.

According to the International Confederation for Thermal Analysis and Calorimetry (ICTAC),6

the use of B = 2 and C = 1 (as in the Kissinger-Akahira-Sunose method; KAS8) offers a

significant improvement in the accuracy of activation energy values. Also, setting B = 1.92

and C = 1.0008 (as proposed in Starink’s approach9) provides further improvement in accuracy.

Thus, at constant α, the plot of ln 𝛽𝑖/𝑇𝛼,𝑖𝐵 against 1/Tα,i yields a straight line whose slope (Figure

S6.4) can conveniently be used to evaluate Eα values.

Another accurate integral method was proposed by Lyon10:


234

𝐸𝛼,𝑖 = −𝑅 (

𝑑𝑙𝑛𝛽

𝑑1/𝑇𝛼,𝑖 + 2𝑇𝛼,𝑖) (S6.11)

with the differential term representing the slope of ln β versus reciprocal temperature 1/Tα

(Figure S6.5)

In other to estimate the errors in the activation energies, we applied combinations of different

replicates of the heating rates. The resulting values (at a specific conversion) then served to

compute the standard errors.


235

Figure S6.5. Parametric plots of ln 𝛽𝑖/𝑇𝛼,𝑖𝐵 against 1/Tα,i for the thermal decomposition of PE

samples in He-NOx.


236

S6.4.2 Compensation effect

The compensation effect relies on the determination of several kinetic triplets, namely, Ei, Ai

and fi(α) or gi(α) over a wide range of reaction models6. This can easily be achieved by

employing the integral analogue of equation (S6.5), i.e., Coats-Redfern equation:11

ln [

𝑔𝑖(𝛼)

𝑇𝛼2

] = ln (𝐴𝑖𝑅

𝛽𝐸𝑖) −

𝐸𝑖

𝑅𝑇𝛼 (S6.12)

For a constant heating rate, the slope and y-intercept of the plot of ln 𝑔(𝛼)/𝑇𝛼2 against 1/Tα

enable the evaluation of Ei and Ai. Table S6.3 lists the appropriate g(α) function for different

models.

Table S6.3. Set of kinetic models used in describing thermal decomposition of solids

i Reaction model Code f(α) g(α)

1 Power law P4 4α3/4 α1/4



4 Power law P2/3 2/3α-1/2 α3/2

5 One-dimensional diffusion D1 1/2α-1 α2

6 Mampel (first order) F1 1-α -ln(1-α)

7 Avrami-Erofeev A4 4(1-α)[-ln(1-α)]3/4 [-ln(1-α)]1/4



10 Three-dimensional diffusion D3 3/2(1-α)2/3[1-(1-α)1/3]-1 [1-(1-α)1/3]2

11 Contracting sphere R3 3(1-α)2/3 1-(1-α)1/3

12 Contracting cylinder R2 2(1-α)1/2 1-(1-α)1/2

13 Two-dimensional diffusion D2 [-ln(1-α)]-1 (1-α)ln(1-α)+α

14 Second order F2 (1-α)2 (1-α)-1-1


237

For kinetic parameter of associated with the models in Table S6.3, the plot of ln Ai versus Ei

will also yield a straight line (see Figure S6.6), with slope a, and y-intercept b.

ln 𝐴𝛼 = 𝑎𝐸𝛼 + 𝑏 (S6.13)

The substitution of isoconversional values of Eα allows the estimation of the dependence of ln

Aα over the entire conversion α. We stress that, theoretically, the compensation effect is not

affected by heating rates. Thus, we have repeated the evaluation steps for two different heating

rates (10 and 20 K/min), and confirmed similar values of the rate constant.


238

Figure S6.6. The compensation effect during thermal decomposition of PE samples in He-

NOx. Each point and each boxed number on the graph correspond to the ith kinetic

parameters, i.e., ln Ai and Ei derived for different reaction models in Table S6.3. Shaded

regions depict the range of experimental data. Models 6, 11 and 12 fall within the window of

the experimental results. As our approach to determine A is valid only for models described

by a general formula of f(α) = (1-α)n, the thermal decomposition of PE conforms with model

6 (Mampel, first order). Indeed, this model falls in the middle of both shaded regions.

S6.5. Complete Isoconversional Kinetic Data


239

Table S6.4. Kinetic constants for decomposition of neat PE sample in NOx according to

isoconversional methods

α

Eα isoconversional methods (kJ/mol) Reported parameters

Numerical

minimisation KAS1 Starink1 Lyon1

Eα2

(kJ/mol)

Standard

error

In (A/min-

1)

Standard

error

0.05 134.9 135.8 136.1 135.8 135.9 ± 2.1 19.9 ± 0.2

0.10 149.4 149.6 150.0 149.6 149.7 ± 2.7 22.1 ± 0.3

0.15 158.5 158.2 158.5 158.1 158.3 ± 3.0 23.5 ± 0.3

0.20 165.0 164.4 164.7 164.4 164.5 ± 3.2 24.5 ± 0.3

0.25 170.9 170.7 171.0 170.6 170.8 ± 3.2 25.5 ± 0.3

0.30 177.5 176.4 176.7 176.4 176.5 ± 3.4 26.5 ± 0.4

0.35 184.8 182.9 183.3 182.9 183.0 ± 3.5 27.5 ± 0.4

0.40 190.5 188.8 189.1 188.7 188.9 ± 3.5 28.5 ± 0.4

0.45 197.2 194.2 194.5 194.1 194.3 ± 3.5 29.4 ± 0.4

0.50 201.5 199.3 199.6 199.2 199.4 ± 3.5 30.2 ± 0.4

0.55 206.4 204.0 204.3 204.0 204.1 ± 3.5 31.0 ± 0.4

0.60 211.9 208.4 208.7 208.4 208.5 ± 3.4 31.7 ± 0.4

0.65 214.7 212.3 212.7 212.3 212.4 ± 3.3 32.3 ± 0.3

0.70 218.6 216.1 216.5 216.1 216.2 ± 3.2 32.9 ± 0.3

0.75 222.0 219.5 219.8 219.4 219.5 ± 3.1 33.5 ± 0.3

0.80 226.0 222.2 222.6 222.2 222.3 ± 3.1 33.9 ± 0.3

0.85 226.2 224.4 224.7 224.4 224.5 ± 3.0 34.3 ± 0.3

0.90 228.6 226.0 226.3 225.9 226.0 ± 2.6 34.5 ± 0.3

0.95 228.6 226.2 226.6 226.2 226.3 ± 2.0 34.6 ± 0.2

1 Average of three values, estimated from the results of three repeatable experimental runs at four

heating rates.

2 Average of the four models; standard error based on the standard deviation from the population mean.


240

Table S6.5. Kinetic constants for decomposition of recycled PE sample in NOx according to

isoconversional methods

α

Eα isoconversional methods (kJ/mol) Reported parameters

Numerical

minimisation KAS1 Starink1 Lyon1

Eα2

(kJ/mol)

Standard

error

In (A/min-

1)

Standard

error

0.05 188.1 188.1 188.4 188.0 188.1 ± 2.1 28.2 ± 0.2

0.10 220.2 220.0 220.3 220.0 220.1 ± 1.4 33.4 ± 0.2

0.15 231.8 231.6 231.9 231.6 231.7 ± 1.2 35.3 ± 0.1

0.20 238.3 238.1 238.4 238.1 238.2 ± 1.1 36.3 ± 0.1

0.25 241.2 241.0 241.3 241.0 241.1 ± 1.2 36.8 ± 0.1

0.30 243.6 243.4 243.6 243.3 243.4 ± 1.2 37.2 ± 0.1

0.35 246.0 245.9 246.2 245.9 246.0 ± 1.6 37.6 ± 0.2

0.40 249.3 249.2 249.5 249.2 249.3 ± 1.7 38.1 ± 0.3

0.45 252.9 252.8 253.1 252.8 252.9 ± 1.9 38.7 ± 0.2

0.50 257.3 257.2 257.5 257.2 257.3 ± 2.0 39.4 ± 0.2

0.55 261.8 261.7 262.0 261.7 261.8 ± 1.9 40.1 ± 0.2

0.60 266.4 266.3 266.6 266.3 266.4 ± 1.9 40.9 ± 0.2

0.65 269.4 269.2 269.5 269.2 269.3 ± 1.2 41.4 ± 0.1

0.70 271.3 271.1 271.4 271.1 271.2 ± 0.6 41.7 ± 0.1

0.75 273.2 273.0 273.2 272.9 273.1 ± 0.3 42.0 ± 0.1

0.80 273.5 273.2 273.5 273.2 273.3 ± 0.3 42.0 ± 0.1

0.85 272.6 272.3 272.6 272.3 272.4 ± 0.2 41.9 ± 0.1

0.90 270.9 270.7 271.0 270.6 270.7 ± 0.2 41.6 ± 0.1

0.95 267.9 267.6 267.9 267.6 267.7 ± 0.1 41.1 ± 0.1

1 Average of three values, estimated from the results of three repeatable experimental runs at four

heating rates.

2 Average of the four models; standard error based on the standard deviation from the population mean.


241

S6.6. Enlarged Time-Resolved FTIR Spectra

Figure S6.7. Time-resolved FTIR spectra acquired during thermal decomposition of neat PE

in argon.

Figure S6.8. Time-resolved FTIR spectra acquired during thermal decomposition of neat PE

in NOx.


242

Figure S6.9. Time-resolved FTIR spectra acquired during thermal decomposition of recycled

PE in argon.

Figure S6.10. Time-resolved FTIR spectra acquired during thermal decomposition of

recycled PE in NOx.


243

S6.7. Complete NOx Conversion Profile

The NOx analyser was pre-calibrated within the experimental range as shown below:

Table S6.6. Calibration of NOx analyser.

Precision NO

(ppm)

Precision

NOx (ppm)

Measured

NO (ppm)

Measured NOx (ppm) Error (% FS)

0 0 0 0 0.0

2030 2080 2030 2080 0.0

1000 1030 998 1030 -0.1

500 516 496 516 -0.2

250 258 244 258 -0.3

Figure S6.11 depicts the concentration trend of NO and NOx (NO + NO2), and Figure S6.12

shows the instantaneous NOx conversion according to Equation S6.14.

𝑋 =𝐹NO𝑥𝑖𝑛

− 𝐹NO𝑥𝑜𝑢𝑡

𝐹NO𝑥𝑖𝑛

× 100 % (S6.14)

𝐹NO𝑥𝑖𝑛= [𝑁𝑂𝑥𝑖𝑛

] ⋅ 𝑄𝑖𝑛 (S6.15)

𝐹NO𝑥𝑜𝑢𝑡= [𝑁𝑂𝑥𝑜𝑢𝑡

] ⋅ (𝑄𝑖𝑛 + 𝑄𝑣𝑜𝑙𝑎𝑡𝑖𝑙𝑒) (S6.16)

𝐹NO𝑥𝑖𝑛 and 𝐹NO𝑥𝑜𝑢𝑡

represent the molar flows in terms of the NOx concentrations and gas flow

rates entering (Qin = 80 mL/min at STP) and leaving the furnace, respectively. On the other

hand, 𝑄𝑣𝑜𝑙𝑎𝑡𝑖𝑙𝑒 corresponds to the estimated dilution term due to generation of volatile products

(from pyrolysing PE) in the furnace. Equation S6.17 yields the volumetric rate (in mL/min) of

volatile formation.


244

𝑄𝑣𝑜𝑙𝑎𝑡𝑖𝑙𝑒 = m

ρ=

��

𝑀𝑒𝑡ℎ𝑦𝑙𝑒𝑛𝑒⋅

𝑅𝑇

𝑃 (S6.17)

𝜌 denotes the average density of volatile species, taken as the density of ethylene for simplicity

sake, while �� signifies the decomposition rate of PE (in mg/min) as obtained from derivative

thermogravimetry data of neat (and recycled) PE decomposition in He-NOx. According to

Equation S6.17, the maximum volumetric rate of volatile product of PE (at 10 K/min) amounts

to 1.9 mL/min, i.e. approximately 2.4 % of the Qin.

Figure S6.11. Continuous NOx concentration profile during thermal of PE samples in NOx

atmosphere.

Recycled PE

0 1000 2000 3000 4000 5000 6000 7000 80000

100

200

300

400

500

600

700

5 K/min

NO

x c

on

cen

trat

ion

(p

pm

)

Time (s)

NO

NOx

0 1000 2000 3000 4000 5000 6000 7000 80000

100

200

300

400

500

600

700

5 K/min

NO

x c

on

cen

trat

ion

(p

pm

)

Time (s)

NO

NOx

0 1000 2000 3000 4000 5000 60000

100

200

300

400

500

600

700

10 K/min

NO

x c

on

cen

trat

ion

(p

pm

)

Time (s)

NO

NOx

0 1000 2000 3000 4000 5000 60000

100

200

300

400

500

600

700

10 K/min

NO

x c

on

cen

trat

ion

(p

pm

)

Time (s)

NO

NOx

0 1000 2000 3000 40000

100

200

300

400

500

600

700

20 K/min

NO

x c

on

cen

trat

ion

(p

pm

)

Time (s)

NO

NOx

0 1000 2000 3000 40000

100

200

300

400

500

600

700

NO

x c

on

cen

trat

ion

(p

pm

)

Time (s)

NO

NOx

20 K/min

Neat PE


245

Figure S6.12. Percent NOx conversion during thermal of PE samples in NOx atmosphere.

S6.8. References

1. Cossoul, E.; Baverel, L.; Martigny, E.; Macko, T.; Boisson, C.; Boyron, O.,

Homogeneous Copolymers of Ethylene with α‐olefins Synthesized with Metallocene Catalysts

and Their Use as Standards for TREF Calibration. In Macromolecular Symposia, Wiley Online

Library: 2013; Vol. 330, pp 42-52.

2. Nieto, J.; Oswald, T.; Blanco, F.; Soares, J. B.; Monrabal, B., Crystallizability of

ethylene homopolymers by crystallization analysis fractionation. J. Polym. Sci., Part B: Polym.

Phys. 2001, 39, (14), 1616-1628.

3. Striegel, A.; Yau, W. W.; Kirkland, J. J.; Bly, D. D., Modern size-exclusion liquid

chromatography: practice of gel permeation and gel filtration chromatography. John Wiley &

Sons: 2009.

0 1000 2000 3000 4000 5000 6000 7000 8000

0

10

20

30

40

50

60

70

80

90

100

5 K/min

NO

x c

on

centr

atio

n (

ppm

)

Time (s)

NO

NOx

0 1000 2000 3000 4000 5000 6000 7000 8000

0

10

20

30

40

50

60

70

80

90

100

5 K/min

NO

x c

on

centr

atio

n (

ppm

)

Time (s)

NO

NOx

0 1000 2000 3000 4000 5000 6000

0

10

20

30

40

50

60

70

80

90

100

10 K/min

NO

x c

on

centr

atio

n (

ppm

)

Time (s)

NO

NOx

0 1000 2000 3000 4000 5000 6000

0

10

20

30

40

50

60

70

80

90

100

10 K/min

NO

x c

on

centr

atio

n (

ppm

)

Time (s)

NO

NOx

0 1000 2000 3000 4000

0

10

20

30

40

50

60

70

80

90

100

20 K/min

NO

x c

on

centr

atio

n (

ppm

)

Time (s)

NO

NOx

0 1000 2000 3000 4000

0

10

20

30

40

50

60

70

80

90

100

20 K/min

NO

x c

on

centr

atio

n (

ppm

)

Time (s)

NO

NOx

Recycled PE

Neat PE


246

4. Haney, M. A.; Johnston, D. W.; Clampitt, B. H., Investigation of short-chain branches

in polyethylene by pyrolysis - GCMS. Macromolecules 1983, 16, (11), 1775-1783.

5. Vyazovkin, S., Advanced isoconversional method. J. Therm. Anal. 1997, 49, (3), 1493-

1499.

6. Vyazovkin, S.; Burnham, A. K.; Criado, J. M.; Pérez-Maqueda, L. A.; Popescu, C.;

Sbirrazzuoli, N., ICTAC Kinetics Committee recommendations for performing kinetic

computations on thermal analysis data. Thermochim. Acta 2011, 520, (1–2), 1-19.

7. Senum, G. I.; Yang, R. T., Rational approximations of the integral of the Arrhenius

function. J. Therm. Anal. 1977, 11, (3), 445-447.

8. Akahira, T.; Sunose, T., Method of determining activation deterioration constant of

electrical insulating materials. Res. Rep. Chiba Inst. Technol.(Sci. Technol.) 1971, 16, 22-31.

9. Starink, M. J., The determination of activation energy from linear heating rate

experiments: a comparison of the accuracy of isoconversion methods. Thermochim. Acta 2003,

404, (1–2), 163-176.

10. Lyon, R. E., An integral method of nonisothermal kinetic analysis. Thermochim. Acta

1997, 297, (1–2), 117-124.

11. Coats, A. W.; Redfern, J. P., Kinetic Parameters from Thermogravimetric Data. Nature

1964, 201, (4914), 68-69.


247

Appendix II: Supporting Information for Chapter 7 and 8

S7.1. Extrapolated Onset Ignition Temperatures

Figure S7.1. Conversion of morpholine at the outlet of the reactor, under different reaction

atmospheres. The extrapolated onset ignition temperatures amount to 581 °C, 613 °C, 494

°C, and 356 °C for He, He-NOx, He-O2, and He-O2-NOx conditions, respectively.

300 400 500 600 700 800

0

20

40

60

80

100

Per

cen

t co

nv

ersi

on

(%

)

Temperature (C)

He

He-NOx

He-O2

He-O2-NO

x

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