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Journal of Loss Prevention in the Process Industries 26 (2013) 45e51

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Journal of Loss Prevention in the Process Industries

journal homepage: www.elsevier .com/locate/ j lp

Explosions of carbon black and propane hybrid mixtures

Pawel Kosinski a,b,*, Roy Nyheim a, Vijayshankar Asokan a, Trygve Skjold a,c

aUniversity of Bergen, Department of Physics and Technology, Allegaten 55, 5007 Bergen, NorwaybCarbontech Holding AS, Bergen, NorwaycGexCon AS, Bergen, Norway

a r t i c l e i n f o

Article history:Received 18 October 2011Received in revised form8 September 2012Accepted 8 September 2012

Keywords:Hybrid mixturesCarbon blackPropaneExplosibilityNano-particlesAgglomeration

* Corresponding author.E-mail address: pawel@ift.uib.no (P. Kosinski).

0950-4230/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.jlp.2012.09.004

a b s t r a c t

Explosions of hybrid mixtures, i.e. mixtures containing more than one combustible phase, are not wellunderstood. Most studies in this area involve mixtures of common dusts and gases, such as coal andmethane, or polyethylene and ethylene. The present work focuses on explosions of carbon black parti-cles, i.e. almost pure carbon with a very low content of volatiles: this makes the process of explosion lessintense. However, addition of some quantities of combustible gases (here: propane) may sustaincombustion processes. Another important issue is the fact that the carbon black particles are smaller insize than most dusts encountered in the process industry. The experiments were carried out in a 20-Lexplosion vessel and the analysis of the results focuses on the maximum explosion pressures and themaximum rates of pressure rise as a function of carbon black and propane concentrations. In addition,some samples of unburnt dust were collected and analysed with a scanning electron microscope andwith thermo-gravimetric analysis.

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1. Introduction

The term carbon black (CB) describes materials consisting ofcarbon particles, often spherical and of colloidal size (see e.g.Donnet,1993). CB is usuallymanufactured using partial combustionor thermal decomposition of hydrocarbons. Most of theworld-wideproduction of CB is consumed by the tyre industry, but CB is alsoused as a pigment in printer and xerographic toners. The presentwork is concerned with the explosion hazard of CB: dust explosionspose a hazard whenever combustible material is present in theform of fine powder, there is a possibility of dispersing a sufficientmass of dust in air to form an explosive cloud within a relativelyconfined/congested volume, and there is an ignition source present.Both ignition sensitivity and rate of combustion increase withdecreasing particles size, primarily because the specific surface areaincreases. CB usually consists of individual particles with charac-teristic dimensions 20e200 nm.

When a cloud of organic dust is exposed to a source of thermalenergy, volatiles leave the solid particles and burn in the gaseousphase. In the case of CB, however, this mode of flame propagation isnot feasible due to the very low content of volatiles and flamepropagation relies on heat transfer, mostly through radiation,between the particles.

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This paper describes combustion in hybrid mixtures of CB andpropane in air. By introducing varying amounts of combustible gasin CB-air suspensions, it is possible to realize various modes offlame propagation, from purely heterogeneous to volatile flames.Finally the physics of the process when compared to explosions oftypical dusts is also different: the particles burn faster, but alsoreadily form agglomerates and deposits since the van der Waalsand electrostatic forces play a significant role. Particles of such lowinertia behave also in a special way in a turbulent flow: they tend tocentrifuge towards the “edges” of eddies and thus the gas/dustmixture is actually less uniform (see e.g. Kosinski, 2011).

Several researchers studied explosions in hybrid mixtures in thepast but there are still open questions regarding the phenomenainvolved, as well as the severity of hybrid explosions. Dufaud,Perrin, Traore, Chazelet, and Thomas (2009) investigated theexplosibility of vapour/dust mixtures related to pharmaceuticalproducts. They noted that research in this field was limited, eventhough this type of explosive atmospheres was frequentlyencountered in industry. The work by Denkevits (2007) is perhapsthe most relevant with respect to the present work. The main goalwas to analyse the explosibility of graphite dust dispersed in anatmosphere consisting of air and hydrogen in a 20-L bomb. Den-kevits measured engineering quantities such as the maximumexplosion pressure and the maximum rate of pressure rise, but thecarbon particles were about two orders of magnitude bigger thantypical CB particles. Chen, Sun, Wang, and Liu (2008) and Liu, Sun,and Chen (2007) presented results from hybrid explosions

Fig. 1. The maximum explosion pressure for propane concentration 2.2%.

Fig. 2. Maximum rate of pressure rise for propane concentration 2.2%.

P. Kosinski et al. / Journal of Loss Prevention in the Process Industries 26 (2013) 45e5146

involving methane and coal dust in a vertical chamber. The resultsfocused on flame propagation and temperature. Pilao, Ramalho,and Pinho (2006) also worked on a similar topic: hybrid cork/methane mixture explosions were investigated in a 22.7-L testchamber.

Amyotte, Pegg, Khan, Nifuku, and Yingxin (2007) studiedmoderation of dust explosions, and emphasized the necessity ofavoiding the formation of hybrid mixtures. Cashdollar (1996, 2000)looked into hybrid explosions with methane and coal dust. Ludwigand Roth (1996) modelled combustion process in hybrid mixturesof carbon, methane and air. Finally Garcia-Agreda, Di Benedetto,Russo, Salzano, and Sanchirico (2011) studied hybrid mixtures ofnicotinic acid dust, methane and air in the standard 20-L Siwekbomb, and identified five different regimes of combustion withrespect to solid vs. gaseous fuel concentration.

This paper focuses on hybrid explosions involving carbon blackparticles close to nanometre size, with very low volatile content.This makes the problem interesting both from the chemical andphysical point of view.

2. Experimental

The experimental tests were carried out in a 20-L explosionvessel of the USBM type (Cashdollar & Hertzberg, 1985), fitted witha standard dispersion system developed for the 20-L Siwek sphere(Siwek, 1997) from Kuhner. The apparatus and experimentalprocedure is described in detail in Skjold (2003). Prior to dustinjection, the chamber is evacuated to 0.4 bar absolute (bara) andthe dust reservoir is pressurized to 21 bara. Gaseous fuel was addedby the partial pressure method after evacuating the vessel suffi-ciently below 0.4 bara. A pneumatic valve separates the dustreservoir from the explosion chamber. It is possible to vary the levelof initial turbulence by triggering the ignition source at a specificignition delay time tvmeasured from the valve opens. Ignition delaytimes of 60 and 120 ms were used in the present study. It isassumed that both gaseous and solid fuels werewell mixed with airduring the injection process, prior to ignition. The turbulentmixture was ignited by either a 6 J electric spark discharge in thecentre of the vessel, or by volumetric ignition by a 1 kJ chemicaligniter. The use of energetic ignition sources can influence thepressure-time histories measured in 20-L explosion vessels(Cloney, Ripley, Amyotte, & Khan, 2012). Although the 1 kJ chemicaligniter used in the present study releases significantly less energythan the regular 2� 5 kJ igniters used for standardized tests, it doesnot represent a point-like ignition source.

Two piezoelectric pressure-sensors (Kistler) measured thepressure evolution inside the chamber during the explosionprocesses. The most important parameters obtained from each testare the maximum overpressure Pex and the maximum rate ofpressure rise, (dp/dt)m. For some tests, samples of dust residueweresampled for scanning electron microscopy (SEM) and thermo-gravimetric analysis (TGA).

3. Results

3.1. Introduction

The CB used in the experiments was of type ENASCO 350G fromTIMCAL Graphite & Carbon, with BET nitrogen surface area770 m2 g�1 and volatile content 0.3 per cent. The average particlesize was estimated to 50 nm based on SEM observations.

Pure CB turned out to be rather difficult to ignite as describedbelow, and in most tests a certain amount of propane was thereforeadded to the air in the 20-L sphere to form a hybrid mixtures. Thefollowing volume concentration of propane was used in the

experiments: 0.0, 2.2, 2.5, 3.0, 4.0, 4.5 and 5.0 per cent by volume.For comparison, the lower flammability limit (LFL) of propane-airmixtures is about 2.0e2.1 per cent, and the stoichiometricconcentration is 4.03 per cent.

3.2. CB explosions

A few initial tests were carried out with only CB and no gaseousfuel. A 1 kJ chemical igniter was used in all tests, and successfulignition was achieved in only one out of four tests. The measuredPex and (dp/dt)m values were 4.6 barg (gauge pressure) and23 bar s�1, respectively, for a nominal dust concentration of500 gm�3 and tv equal to 60ms. For comparison, the stoichiometricconcentration of pure carbon is about 100 g m�3.

When tv was increased to 120 ms it was possible to ignite CBmixtures in the concentrations range 250e500 g m�3, while noignition occurred outside this region. The values of Pex and (dp/dt)m

Fig. 5. The maximum explosion pressure for propane concentration 3.0%.Fig. 3. The maximum explosion pressure for propane concentration 2.5%.

P. Kosinski et al. / Journal of Loss Prevention in the Process Industries 26 (2013) 45e51 47

were still relatively low, ranging from 5.1 to 5.9 barg and from 44 to59 bar s�1, for the 250 and 500 g m�3, respectively. Turbulentquenching may explain the difficulties in igniting the samemixtures at shorter ignition delays. It was not possible to ignite anyof dust-air mixtures with the electric spark discharge.

3.3. Propane concentration: 2.2% and 2.5%

According to previous results for mixtures of coal dust, methaneand air, reported by e.g. Amyotte et al. (2007) and Cashdollar (1996,2000), addition of limited quantities of gaseous fuel to dustmixtures with concentrations below the lower explosion limit mayresult in flammable mixtures. The lean flammability limit can bepredicted by a linear relation between dust and gas concentrations.However, the situation is different for dusts with low volatilecontent (see e.g. Cashdollar, 1996).

The lower explosion limit of propane is around 2.0% and thepresent study focused on propane concentrations that were

Fig. 4. The maximum rate of pressure rise for propane concentration 2.5%.

somewhat higher than LFL: 2.2% and 2.5%. It was not possible toignite hybrid mixtures for propane concentration below thesevalues, in spite of the addition of CB. On this point the presentresults differ from observations reported for graphite (Denkevits,2007).

The following paragraphs present maximum explosion pres-sures and maximum rates of pressure rise obtained for various CBconcentrations. The results show the average of three repeatedtests. The results were rather stochastic and it was not alwaysstraightforward to derive unambiguous trends. The small particlesize may have contributed to the limited reproducibility, since theparticles tend to deposit on the connecting pipes and valves andthus change the course of explosions.

Figs. 1 and 3 show how Pex varies with the nominal dustconcentration for propane concentrations 2.2% and 2.5%, respec-tively. The figures show two curves for Pex for hybrid mixture of CB/propane/air (for tv equal to 60 and 120 ms) and two horizontal linesrepresenting pure propane/air mixtures.

Fig. 6. The maximum rate of pressure rise for propane concentration 3.0%.

Fig. 7. The maximum explosion pressure for propane concentration 4.0%. Fig. 9. The maximum explosion pressure for propane concentration 4.5%.

P. Kosinski et al. / Journal of Loss Prevention in the Process Industries 26 (2013) 45e5148

For lean gaseous mixtures, the addition of additional fuel(CB) increases the explosion pressures. For instance, fora propane concentration of 2.2% and tv equal to 60 ms, themaximum pressure is 7.9 bar, corresponding to nominal dustconcentrations in the range 80e100 g m�3. No explosions wererecorded for higher CB concentrations, where a possible expla-nation is deposition of CB in piping and nozzles due to its highcohesivity.

The ignition delay time does not influence the results for hybridmixtures in the sameway as for the CB dust explosions described inSection 3.2. Shorter ignition delay times, and hence more intenseturbulence, enhance the rate of combustion in hybrid mixtures,while similar flow conditions quenched the combustion in pure CB/air mixtures.

Figs. 2 and 4 show how (dp/dt)m varies with nominal dustconcentration for propane concentrations 2.2% and 2.5%. Up toabout 200 g m�3 CB in air the trend is similar as for Pex: more

Fig. 8. The maximum rate of pressure rise for propane concentration 4.0%.

violent explosions for hybrid mixtures, compared to pure gaseousmixtures. However, for higher CB concentrations the rate of pres-sure rise is reduced below the values observed for pure propaneexplosions, suggesting that heat loss to the CB particles influencesthe combustion process.

3.4. Propane concentration: 3.0%, 4.0%, 4.5% and 5.0%

This section shows results for tests where the concentration ofpropane was increased to 3.0, 4.0, 4.5 and 5.0%.

Figs. 5 and 7 shows Pex values as a function of nominal dustconcentration for 3.0 and 4.0% propane in air. The trend is similar tothe results reported in the previous section: addition of a modestamount of CB increases themaximumpressure for lean propane-airmixtures. Similar conclusions can be drawn from the rates ofpressure rise summarized in Figs. 6 and 8, where the effect of heatloss to an increasing amount of CB is evenmore pronounced. Figs. 9

Fig. 10. The maximum rate of pressure rise for propane concentration 4.5%.

Fig. 11. The maximum explosion pressure for propane concentration 5.0%.

P. Kosinski et al. / Journal of Loss Prevention in the Process Industries 26 (2013) 45e51 49

and 11 show results for Pex, and Figs. 10 and 12 show the corre-sponding results for (dp/dt)m, for fuel-rich mixtures.

For 4.0% propane one can observe a slight increase of explosi-bility for moderate amounts of CB. Also here a significant decreasein themaximum rate of pressure rise is obtained for higher nominaldust concentrations.

3.5. Particle analysis

CB samples were collected after selected experiments andanalysed with TGA and SEM.

Fig. 13 shows three SEM images of CB residue, where Fig. 13aeb correspond to the case where propane concentration was 2.5%,while the nominal CB concentration was 60 g m�3 before theexplosion. Both images reveal agglomerates of CB particles thatsupport observations from the previous sections. Fig. 13c shows anSEM image of a CB residue sample where the propane concentra-tion was 3.0% and the nominal dust concentration was 120 g m�3.

Fig. 12. The maximum rate of pressure rise for propane concentration 5.0%.

Significant differences between the samples were not observed:nevertheless, for the latter case the CB residue seemed to forma “spongy” structure.

TGA is a measuring technique in which a sample is subjected tohigh temperatures and its weight loss as temperature changes isrecorded. Fig. 14 shows the results from TGA measurements ofthree samples: (a) the original CB (before explosion), (b) CB afterexplosion for initial nominal dust concentration 60 g m�3, (c) CB

Fig. 13. Scanning electron microscope images of carbon black after explosions: a)Propane concentration: 2.5%, the nominal CB concentration: 60 g m�3 (for lowmagnification), b) Propane concentration: 2.5%, the nominal CB concentration:60 g m�3 (for high magnification), c) Propane concentration: 3.0% and CB nominalconcentration: 120 g m�3

Fig. 14. TGA analysis of three samples: (1) the original CB, (2) CB after explosion forinitial nominal dust concentration 60 g m�3, (3) CB after explosion for initial dustconcentration 120 g m�3.

P. Kosinski et al. / Journal of Loss Prevention in the Process Industries 26 (2013) 45e5150

after explosion for initial dust concentration 120 g m�3 (thepropane concentration did not influence these results and thus it isnot specified here).

The samples for initial dust concentration 60 g m�3 show lowerthermal stability than the original CB. The weight loss can beattributed to the release of free radicals that may have been formedduring the chemical reactions, or even due to the newly formedsurface groups. The samples where the dust concentration was120 g m�3 showed higher stability. The combustion process mighthave removed thermally weak structures (e.g. free radicals) on thesurface and created more thermally stable CB structures. Pleasenote, however, that TGA does not provide detailed informationabout material structure.

Fig. 15. Relation between dust and propane concentration that corresponds to themaximum rate of pressure rise, for two values of the ignition delay time, tv.

3.6. Summary

Fig. 15 shows a relation between the CB and propane concen-trations that resulted in the highest explosion pressures (Pex). Thefigure also compares results obtained for two values of the ignitiondelay time that describes the initial level of turbulence in thechamber as well as dispersion of the dust cloud. For the case whentvwas 60ms a trend can be observedwhere increase of the nominaldust concentration requires a decrease of propane concentration.This was not always true for tv equal to 120 ms, nevertheless, theremay be uncertainty for the lower values of propane concentrations.

4. Conclusions

For most hybrid mixtures of combustible dusts and gases usedin industry, both the dust and the gaseous fuel participate in theinitial processes: it is possible to obtain flame propagation evenwhen the concentration of gaseous fuel is below the lower flam-mability limit. Also for the case when the content of volatiles ishigh, the flame propagation in such a system resembles combus-tion of multi-component gas.

This was not the case in the current research: in order to startflame propagation, the propane concentration had to be above thelower explosion limit. Also it was difficult to ignite pure CB dust:this indicated the negligible amount of volatiles.

The SEM images revealed formation of agglomerates: thisphenomenon probably makes the course of explosion morecomplex than it would be for individual particles. Also this hadpractical implications in this research: for some cases it was notpossible to obtain ignition and a possible reason was deposition ofparticles in the valves, nozzles and pipes. Please note that SEMimages do not provide with detailed information regarding thechemical and physical properties of the residues samples. Thiswould require deeper studies, e.g. X-ray scattering techniques,Raman spectroscopy, Zeta potential etc. that was beyond the scopeof this research.

Future studies may include modelling by means of computa-tional fluid dynamics. This, however, is a challenging problem,since, in addition to themodelling of chemical reactions one shouldconsider possible particleeparticle and particleesurface interac-tions. These phenomena are especially important for describing theformation of the dust cloud inside the chamber.

Acknowledgements

The authors would like to thank Professor Rolf K. Eckhoff forfruitful discussions. The work was financially supported by Car-bontech Holding AS and the Norwegian Research Council.

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