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Renewable Energy 35 (2010) 232–242

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Renewable Energy

journal homepage: www.elsevier .com/locate/renene

Fundamentals, kinetics and endothermicity of the biomass pyrolysis reaction

Manon Van de Velden !, Jan Baeyens a,b,*, Anke Brems c, Bart Janssens c, Raf Dewil c,d

a Department of Chemical Engineering, University College London, London, United Kingdomb School of Engineering, University of Warwick, Coventry, United Kingdomc Department of Chemical Engineering, Katholieke Universiteit Leuven, Campus De Nayer, Sint-Katelijne-Waver, Belgiumd Chemical and Biochemical Process Technology and Control Section, Department of Chemical Engineering, Katholieke Universiteit Leuven, Heverlee, Belgium

a r t i c l e i n f o

Article history:Received 14 December 2008Accepted 22 April 2009Available online 4 June 2009

Keywords:BiomassPyrolysisKineticsHeat of pyrolysisExperimental and operating conditions

* Corresponding author at: School of EngineeriCoventry, CV4.7.AL, United Kingdom. Tel.: þ32 16 532

E-mail address: baeyens.j@gmail.com (J. Baeyens).! Started the research but died accidentally in 200

0960-1481/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.renene.2009.04.019

a b s t r a c t

The paper reviews the pyrolysis of biomass constituents and possible secondary reactions. Biomasspyrolysis yields mostly liquid and solid fuel, easy to store and transport.

Relevant working conditions for experiments and large-scale operation are: (i) biomass parti-cles< 200 mm, (ii) a particle heating rate of at least about 80 K min�1 and (iii) a reactor environmentwhere the internal resistance to heat penetration is smaller than the external resistance to heat transfer(Biot-number, Bi< 1).

The circumstances of TGA and DSC experiments meet these requirements and fully determine thereaction kinetics and endothermicity of the pyrolysis reaction. The reaction rate constant and the heat ofreaction are essential parameters in the design of a pyrolysis reactor. For most of the biomass speciestested, the first order reaction rate constant is large and >0.5 s�1. The heat of reaction ranges from 207 to434 kJ kg�1. All results tie in with literature data, although the reader is cautioned in using literature datasince experiments were not always performed under relevant testing conditions.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Energy from biomass is recognised as the renewable energysource with the highest potential towards sustainable developmentin the near future [1–3]. Biomass provides already 14% of theworld’s primary energy production [4], but is largely squandered byinefficient use and unsustainable exploitation. To exploit the fullpotential of this energy source, new approaches and moderntechnologies are needed.

Biomass includes all organic matter that is available ona renewable basis: energy crops and all kinds of organic wastes.Now and in the near future waste products from agriculture andforestry, easy and cheap to collect, will dominate as source for bio-energy. In the European Union wood waste accounts for 94% of thecurrently used biomass for energy [5]. In the medium and longterm, residues from agriculture and forestry, and energy crops willrespectively further develop the bio-energy industry [5,6].

Solid biofuels have a low energy density, which limits thecommercial applications to locations close to the place of

ng, University of Warwick,834; fax: þ32 16 538729.

7.

All rights reserved.

production. One way to solve this problem is the conversion of thefeedstock into liquid fuel. These liquids have a higher energydensity (Table 1) and are easy to store and to transport.

The two most important methods to convert biomass into liquidfuel are (i) the biochemical conversion by the enzymatic activity ofmicro-organisms (to e.g. ethanol) and (ii) the thermochemicalconversion by heat or oxidation. The biochemical conversionrequires a feedstock that contains sugars or carbohydrates anda water content in excess of 40%. The thermochemical methods arebest suited for dry biomass (moisture content< 10%) which is richin lignin (this substance is less suited for biochemical conversionsince it is hardly broken down by enzymatic activity), such as woodand agricultural wastes. Three main routes exist to thermochemi-cally convert biomass in energy: combustion, gasification andpyrolysis [8] (Fig. 1).

This paper targets pyrolysis as conversion technology because ofits efficient energy production and the important advantage thatmainly liquid fuels and solid char are formed, both easy to store andto transport. Pyrolysis oil moreover contains various chemicalswith specific high-quality and added-value applications [2]. Theexpectations for pyrolysis in general, and for bio-oil in particular,are hence considerable [2,3].

Pyrolysis is the thermal decomposition in the complete absenceof an oxidizing agent (air or oxygen), or with such a limited supply

List of abbreviations

A Pre-exponential factor of the Arrhenius equation (s�1)am Specific surface of a particle per unit weight (m2 kg�1)C Reaction heat (J kg�1)cp Specific heat capacity (J kg�1 K�1)d Diameter (m)D Thermal diffusivity of biomass particle (m2 s�1)E Energy (J)Ea Activation energy (J mol�1)h Heat transfer coefficient (W m�2 K�1)DHr Reaction heat of pyrolysis (J kg�1)k Global, apparent (action rate constant (s�1))kp Heat conductivity of the particle (W m�1 K�1)kr Intrinsic or chemical reaction rate constant (s�1)kht Rate of heat penetration (s�1)m Mass (kg)Q Heat (J)R Universal gas constant (8,31 J mol�1 K�1)r Radial position in a particle with radius R (m)

t Time (s)T Temperature (K)X Fractional conversion (–)Bi Biot-number (–)b Heating rate (K s�1)r Density (kg m�3)s Time for total reaction (s)

Subscripts0 InitialN EndB, b Biomassg Gas phasein Inputc Core of the particlemin Minimums Surroundingp Particleout Output

M. Van de Velden et al. / Renewable Energy 35 (2010) 232–242 233

that combustion or gasification do not occur to any appreciableextent. The proportion of products formed is strongly dependent onthe reaction temperature and time, and on the heating rate. Incomparison to gasification, pyrolysis occurs at relatively lowtemperatures (673–873 K). High temperatures and long residencetimes promote the formation of gas, while low temperatures andlong residence times promote the formation of char. Optimum toproduce oils are medium temperatures and short residence times,and thus high heating rates [9].

Fast pyrolysis occurs in seconds only, so reaction kinetics, phasetransitions, heat and mass transfer play important roles. It istherefore critical to subject the biomass particles immediately tothe optimum reaction temperature and to limit their exposure tolower temperatures, because this will promote the undesiredformation of char. One way to do this is to use small particles ina fluidized bed, as will be described later. Another possibility is totransfer the heat very quickly but only to the surface of the particle,as applied in ablative pyrolysis.

The pyrolysis process produces mainly vapours and char. It isessential to separate the char from the vapour as quickly as possibleto prevent side reactions, since char acts as a cracking catalyst. Charparticles are recovered by a gas–solid separation technique (mostlycyclones). After cooling and condensation, a brown, low viscosityliquid is obtained with a heating value about half that of conven-tional fuels.

To achieve high oil yields the process parameters are to beaccurately controlled. The essential parameters include [2,7]:

� a very fast heating and heat transfer to the reaction surface;� a reaction temperature around 773 K and temperatures of the

vapour phase of 673–723 K;

Table 1Bulk density, mean heating value and energy density of biomass and thermo-chemically derived fuels [7].

Energy carrier Bulk density(kg m�3)

Heating value(GJ t�1)

Energy density(GJ m�3)

Straw w100 20 2Sawdust w400 15 6pyrolysis oil w1200 17 20.4char w300 30 9

� short residence times, for vapours less than 2 s;� a fast char separation and cooling of the vapours, to avoid

secondary cracking.

The main product, bio-oil, is obtained with a maximum yield of60–70 wt% (dry matter), together with char and gas [9]. These by-products can be burnt externally (to dry the biomass, in co-combustion etc.) or in the process, to supply the endothermicreaction heat of pyrolysis. Char in itself has a heating value(z30 MJ kg�1) [2,3,8] comparable to petroleum cokes (‘‘petcoke’’),and can externally be valorised. The moisture content of thepyrolysis feedstock shall not exceed 10 wt% to limit the watercontent of the bio-oil.

The heart of the fast pyrolysis process is the reactor. Although itprobably represents only 10–15% of the total capital cost of anintegrated system, almost all research and development arefocused on the reactor, since subsequent techniques (separation,condensation etc.) are known as physical and chemical processes.Also the rest of the process (biomass reception, storage, handling,drying and milling, product storage and possible product upgrad-ing) applies known techniques [2].

Within the common pyrolysis reactor designs, mostly fluidizedbeds are applied, operated in either bubbling or circulating mode:these systems can meet the basic requirements to achieve high oilyields i.e. short residence time, fast heat transfer and fast separationof vapour and char. The technological strength and the marketattractiveness (Fig. 2) show that the circulating fluidized bed (CFB)has the highest commercial potential. The expected breakthroughof the CFB is conditioned by a better understanding of the hydro-dynamics (flow regimes, residence time) and reaction kinetics ofthe biomass pyrolysis at different heating rates [10].

pyrolysis

gasification

combustion

bio-oil

fuel gas

heat

storage

turbine

engine

boiler

chemicals

electricity or CHP

Fig. 1. Alternative thermochemical conversions of biomass [2].

Fig. 2. Status of the pyrolysis reactors [7].

M. Van de Velden et al. / Renewable Energy 35 (2010) 232–242234

The production of bio-oil by pyrolysis is a technology on theedge between development and demonstration. The most impor-tant technological issues that remain to be solved before large-scaleuse becomes possible, are related to the complex kinetics andreactor hydrodynamics [10]. The study of the basic kinetics is thepriority of the present research, which intends to make an exper-imental and theoretical contribution to the development of pyrol-ysis. It therefore concentrates on the determination of (i) theoperating conditions required in relevant experimental researchand large-scale pyrolysis of biomass; (ii) the kinetic constants ofbiomass pyrolysis; and (iii) the reaction heat of biomass pyrolysisand other thermal parameters required in the heat balance of thepyrolysis reactor.

2. Biomass pyrolysis reactions

Biomass mainly consists of three components: 30–60% cellulose,20–35% hemicellulose (polysaccharides) and 15–30% lignin(a polymer of methoxy substituted cyclic organics), together withsome resins and minerals. Pyrolysis of biomass is very complex dueto the diversity, the heterogeneity and the limited thermal stabilityof some of the components. Pyrolysis of biomass yields about theproducts expected from the pyrolysis of its three constituentcomponents separately, despite synergetic effects [11]. The study ofthe individual components thus forms the basis of the expectedreaction pathways and determines the occurring primary andsecondary reactions. The order of these reactions, the reaction rateand the yields depend on parameters such as heating rate,temperature, pre-treatment, catalytic effects etc. [2,10,12,13]. Thestudy of the reactions and the effect of the parameters are of criticalimportance to drive the reactions to high yields of the desiredproducts and to avoid side reactions.

2.1. Cellulose

Cellulose, (C6H12O5)n, is the main component of plant cell wallsand consists of anhydroglucose units, connected by glycosidebonds. Of all lignocellulotic components, the thermal decomposi-tion of cellulose is mostly investigated and best understood. TheWaterloo-mechanism (Fig. 3) [14] is widely accepted to representthe reaction pathway, yielding the expected products and definingthe effect of temperature and heating rate.

cellulose active cellulose (low degree ofpolymerisation)

fra

depo

de

Fig. 3. The primary decomposition of cellulose a

The three competitive primary reactions are (i) the fragmenta-tion to hydroxyacetaldehyde and other carbonyls, acids and alco-hols; (ii) the depolymerisation to levoglucosan and other primaryanhydrosugars; and (iii) the dehydration to char, gases and water.

Dehydration dominates at low temperatures (<623 K) and slowheating rates. At higher temperatures, depolymerisation and frag-mentation are dominant. Depolymerisation dominates at temper-atures between 573 and 723 K, while fragmentation has itsoptimum at around 873 K. However, inorganic species such ascations, bases, acids and salts can have a major impact on thesereactions [15,16]. The selectivity of cellulose for fragmentation anddepolymerisation can be as high as 70–80 wt% [17].

2.2. Hemicellulose

Hemicellulose binds the cellulose microfibrils of the cell wall. Itvaries in structure and composition (mainly xylan) and has a lowthermal stability [18,19,20]. It is decomposed in a way similar tocellulose: by dehydration at low temperatures (<553 K) anddepolymerisation at higher temperatures [15]. Dehydration yieldsanhydride fragments, water soluble acids, char, gases and water,while depolymerisation yields volatile organics, levoglucosan andother anhydrohexoses, levoglucosenone and furans [18]. Here too,inorganic impurities cause fragmentation effects, but xylan isspecifically sensitive for cations, because of its ion exchange sites.

2.3. Lignin

Lignin is the strengthening component of the cell wall and ismainly present in woody biomass [19]. It is a polymer of hydroxyand methoxy substituted propyl phenol units [18]. The verycomplex structure depends on the plant species and due to itsstructural diversity, pyrolysis yields various products (such ascatechols, vanillins, aromatic carbonhydrates). At low temperatures(<773 K) dehydration dominates, while at higher temperaturesa diversity of lignine monomers is formed. Above 973 K themonomers are decomposed and enter the vapour phase. Lignin isthermally more stable than cellulose and hemicellulose and yieldsmore char and a higher fraction of aromatic compounds [15,19].

In a pyrolysis reactor the biomass is subjected to the wholetemperature range and therefore all reactions mentioned aboveoccur to some degree, in function of the heating rate of the wholebiomass particle, and thus of the heat transfer and particlediameter.

2.4. Secondary reactions

Secondary reactions can occur in the vapour phase or betweenvapour and solid phase to form mostly gas. This will considerablyreduce the desired oil yield [21,22]. Two secondary reactions are ofimportance for pyrolysis: cracking (>973 K) and the water–gas shiftreaction (<1083 K): H2Oþ CO 4 H2þ CO2. These secondary reac-tions are however limited at temperatures below 923 K, residencetimes <2 s for the gases and vapours and when char is quickly

gmentation

lymerisation

carbonylcompounds,e.g. hydroxyacetaldehydes, acids and alcohols

anhydrosugars,e.g. levoglucosan

char, gases and waterhydration

ccording to the Waterloo-mechanism [14].

M. Van de Velden et al. / Renewable Energy 35 (2010) 232–242 235

separated. These are the working conditions for commercialpyrolysis. Reforming, carbon–steam and carbon–carbon dioxideconversions are negligible under these conditions [14].

The global pyrolysis mechanism of biomass is mainly based onthe pyrolysis of cellulose, although results are similar for wood andother biomass species [10,11]. The Waterloo concept of Fig. 4 isrecognised to represent the global mechanism.

3. Materials and methods

3.1. Biomass samples

The specific biomass species were selected because of previousliterature references, or because of their potential in Europe:spruce, pine, poplar, eucalyptus, sawdust, sewage sludge, straw andthe non-harvested stalks and leaves of sunflower and corn. Thesamples were crushed and dried for over 2 h at 105 �C and again30 min just before thermal processing.

3.2. The kinetic constants of biomass pyrolysis

The kinetic constants of the pyrolysis of biomass are determinedby thermogravimetric analysis. A Hi-res TGA 2950 Thermogravi-metric Analyzer was used at atmospheric pressure with a nitrogenflow of 50 ml min�1. The TGA scale was made of platinum and theweight was registered every 0.5 s with an accuracy of 0.001 mg.

Following the discussion which is presented in Section 4, allexperiments were performed at 100 K min�1 and were repeated atleast three times. Calculations use the average results, with a stan-dard deviation of maximum 5%. The resulting TGA-profiles plot theweight loss in function of the temperature.

3.3. Reaction heat

A DSC Q10 with Refrigerated Cooling System operating withina temperature range from 183 to 823 K was used to measure theheat requirement of the biomass pyrolysis reaction. The experi-ments were performed from 313 to 823 K, under atmosphericpressure and with nitrogen supplied at 50 ml min�1. The DSC scaleswere aluminium and were not reused. The DSC was calibratedregularly with indium as reference material. The obtained DSCcurves give the heat flux (W g�1) in function of the temperature orin function of the time (constant DT/Dt¼ 10 K min�1).

3.4. Specific heat of the biomass

The specific heat capacity of sawdust, corn residue and poplarwere determined at 25 �C using a calorimeter (Leybold Heraeus).The temperature increase of n-hexane was measured after additionof preheated biomass.

Fig. 4. The biomass pyrolysis concept of the University of Waterloo [23].

4. Results and discussion

4.1. The required operating conditions for relevant experimentalresearch and large-scale pyrolysis of biomass

4.1.1. Specific operating conditionsThe specific working conditions essential to achieve maximal oil

yields have already been described in the introduction of this paperand include a reaction temperature around 773 K, temperatures inthe vapour phase around 673–723 K; a high heating rate and highheat transfer at the reaction surface; a short residence time, forvapours less than 2 s and a rapid cooling of the vapours andseparation of the char, to avoid secondary cracking.

Essential for the fast decomposition and high oil yields is the fastheating of the biomass particle. Slow pyrolysis decreases the oil yieldconsiderably in favour of gas and char. Moreover long residence timesfavour secondary cracking of the oil. Short residence times, rapidcooling of the oil and rapid separation of the char are hence required.

This fast heating of the whole biomass particle to a uniformtemperature requires a high heat transfer. If the heat supply wouldbe insufficient, important thermal gradients would occur leading toslow pyrolysis and/or incomplete reactions in the core of theparticle. Smaller particles have a larger specific surface and aremore mobile within the reactor mass, which enhances the heattransfer. Also, temperature gradients in small particles are expectedto be negligible, as will be shown below.

When choosing the experimental and large-scale parameters,the thermal characteristics of the biomass, the reaction kinetics andthe heat transfer will have to be jointly considered. Some basicprinciples of the reaction kinetics and conversion of solid particles,of heat transfer in powder reactors and of heat penetration insidethe particle allow to determine these conditions in anticipation,which makes it possible to program the experiments in a relevantand effective way. This analysis allows at the same time to criticallyevaluate the literature data: research has not always been per-formed under these optimum conditions (for example [12]), withresults that could lead to wrong conclusions.

Recommendations concerning the computational aspects ofkinetic analyses were made in a series of articles [24–28], whereboth one-step/multi-step kinetic schemes and parallel/consecutivereactions are considered. Analysis of the data hereafter by themethod of Flynn, Wall and Ozawa reported in [24] demonstratesthat the biomass pyrolysis can be considered as a one-step reaction,thus simplifying the treatment, but nevertheless requiring anassessment at different heating rates. The analysis of the effect ofheating rate is presented below.

4.1.2. The influence of temperature gradients in the biomass particleFast pyrolysis and its associated maximum oil yield can only be

achieved by fast and uniform heating of the biomass particles. Tomeet these targets, avoiding thermal gradients in and around theparticle is very important. Application of the basic principles ofboth heat transfer to the particle and of heat conduction within theparticle, allows to determine the required working conditions.

The temperature uniformity throughout the biomass particles canbe determined by the heat conduction law of Fourier, here applied innon-stationary regime and for the simple case of a spherical particle:

vTvt¼ kp

rpcp

v2Tvr2 þ

2r

vTvr

!¼ D

v2Tvr2 þ

2r

vTvr

!(1)

with kp: heat conductivity of the biomass at temperature T(W m�1 K�1), cp: specific heat capacity of biomass at temperature T(J kg�1 K�1), rp: density of the biomass (kg m�3), D: thermal diffu-sivity of the particle (m2 s�1).

M. Van de Velden et al. / Renewable Energy 35 (2010) 232–242236

The solutions of this unsteady thermal condition are simplewhen easy boundary conditions can be applied, as will be usedfurther in the text. Solutions are very difficult or impossible forcomplex boundary conditions and/or bodies of irregular shape. Thegeneral solutions are therefore commonly expressed by a dimen-sional analysis. For a sphere of radius r, initially at temperature T0

and suddenly exposed to surroundings at Ts, the temperaturedistribution, at any time t and position x, is given by:

Ts � TTs � T0

¼ f�

hrkp;D

tr2;

xr

�(2)

with h: external heat transfer coefficient at surface of sphere(W m�2 K�1).

Since it is important to know the evolution of the temperature inthe core of the sphere (Tc at x¼ 0) and introducing the Biot-number(¼ (r/kp)/(1/h)) reduces equation (2) to:

Ts�Tc

Ts � T0¼ f

�Bi;D

tr2

�(3)

For large (Bi / N) or very small (Bi / 0) values of the Biot-number, solutions are graphically given by Carslaw and Jaeger [29].For applications of biomass particles in a fluidized bed, Biot-numbers are intermediate (Bi¼ 0.2–1, see further in the text) andextreme conditions do not apply. For these intermediate values ofthe Biot-number, results are presented by Heisler [30] in the formof charts, expressing (Ts� Tc)/(Ts� T0) in terms of Dt/r2 (the Fourier-number) with 1/Bi as parameter. Illustrative application of the chartis given in Table 2, with characteristic properties of sawdust at773 K, i.e. kp¼ 0.15 W m�1 K�1 [31], cp¼ 1150 J kg�1 K�1 andrp¼ 700 kg m�3, and for an average fluidized bed heat transfercoefficient at the surface of the sphere i.e. 500 W m�2 K�1.

Since fast pyrolysis requires the reaction to take place within 2–2.5 s, it is clear that only very small particles will meet theconditions of fast heating to a uniform temperature. In this case,particle sizes above 200 mm diameter (radius 100 mm) already take0.38 s to warm up, i.e. nearly 15% of the proposed reaction time of2–2.5 s, thus starting to jeopardize the ideal condition of fastpyrolysis.

According to the previous calculations, it can be assumed thatthe heating rate throughout the particle is uniform when heatingvery small particles (�200 mm) at a rate b (K s�1): simplifiedboundary conditions can hence be applied to Eq. (1), i.e. vT=vt ¼ b

for r¼ 0 to R(¼ dp/2) and vT=vr ¼ 0 for r¼ 0 (in the core).The solution is given by Carslaw and Jaeger [29] as:

DTmax ¼ ðTR � TcÞ ¼ bd2p=ð24DÞ (4)

with TR: T at the surface of the particle (at r¼ R) (K), Tc: T in thecore of the particle (at r¼ 0) (K).

Table 2Time required for the core of a spherical saw dust particle to nearly (99%) reach thetemperature of the surroundings (773 K) where the particle is suddenly submergedin.

r (mm) Bi-number Dt/r2 t (s)

50 0.17 13 0.017100 0.34 7 0.38200 0.67 5 1.08400 1.33 2.2 2.2

It is assumed that a condition where the core temperature has reached 99% of thetemperature of the surroundings (Ts z 773 K) meets conditions of uniformtemperature T throughout the sphere (i.e. Tc z 0.99 Ts). The initial T of the sphere is293 K and D z 1.86� 10�7 m2 s�1.

Eq. (4) can be applied for various values of b and with e.g. thecharacteristic properties of sawdust mentioned above. The resultsare a set of curves in function of the diameter as shown in Fig. 5.

These calculations show that the temperature differencesbetween the biomass surface and core are very limited, certainlywhen considering that the surrounding temperature is 773 K andthat the heating rate will vary between minimally 1.5 K s�1 in a TGA(thermogravimetic analysis) experiment, to a few hundred K s�1 ina bubbling or circulating fluidized bed.

To obtain a fast heating of the whole particle, it is appropriatethus to use small biomass particles (e.g. <200 mm for saw dust) andno significant thermal gradient will occur in these small particles:the core and the surface of the biomass particle will behave boththermally and kinetically in a similar way.

4.1.3. The influence of heat transfer to the biomass particlesFast pyrolysis requires a fast heat transfer to the biomass. This

heat transfer is conditioned by the degree of gas and solid turbu-lence achieved in the reactor [32,33]. The heat transfer coefficientdepends on the gas–solid contacting mode. It ranges from10 W m�2 K�1 for a static bed, to 50–100 W m�2 K�1 in a fixed bedwith forced gas circulation (as in TGA), and several hundreds ofW m�2 K�1 for bubbling and circulating fluidized beds. Thisexplains why the fluidized beds are specifically considered as toptechnologies for biomass conversions. The heat transfer coefficientin a CFB can be estimated from the results of Kobro and Brereton[34]. In a CFB operating with 100 mm sand as bed material ata recommended gas velocity of 5 m s�1 and a solid circulation rateof 200 kg m�2 s�1, the convective heat transfer coefficient is620 W m�2 K�1.

Since not only external convection but also internal conductionis important, the overall picture is expressed by the Biot-number,which represents the internal resistance to heat penetrationdivided by external resistance to heat transfer and is written asfollows:

Bi ¼ r=kp

1=h

For the 200 mm sawdust particle (r¼ 100 mm) at 773 K, thevalues are as follows:

h ¼ 300 W m�2 K�1 Bi ¼ 100� 10�6 � 300=0:15 ¼ 0:2

h ¼ 500 W m�2 K�1 Bi ¼ 100� 10�6 � 500=0:15 ¼ 0:33

h ¼ 700 W m�2 K�1 Bi ¼ 100� 10�6 � 700=0:15 ¼ 0:47

Fig. 5. Maximum temperature difference (DTmax) between the surface and core of thesawdust particle in function of the particle diameter at different heating rates (b).

M. Van de Velden et al. / Renewable Energy 35 (2010) 232–242 237

The Biot-number is <1 in all cases, implying that the externalresistance associated with convectional heat transfer largely domi-nates. Bi is only>1 for coarse particles and/or too high values of h. Thisresult confirms Fig. 5 where increasing DTmax values are noticed withincreasing b values (achieved at high convection heat transfer rates).

4.2. The selection parameters for the conversion of biomassin a powder reactor

4.2.1. The kinetic expression for the conversion of an individualbiomass particle

Occurring reactions during pyrolysis are endothermic, as will befurther determined from DSC (differential scanning calorimetry)measurements. As illustrated in Fig. 4, pyrolysis of biomass prog-resses along a reaction path with primary and secondary reactions,each reaction with its specific expression for the rate of productformation [35,36]. The present paper considers the overalldecomposition rate of the biomass particle only, expressed by a firstorder reaction. The overall reaction rate constant, k, is however thesum of the reaction rate constants of the parallel primary reactions.The experimental data moreover enable the reader to apply a morecomplex kinetic assessment, if so desired. In the present

Fig. 6. Time for complete conversion as function of the b

assessment, the kinetics are expressed by a continuous reactionmechanism for the small particles under consideration: thetemperature distribution across the small particle is quasi-uniformso that the pyrolysis reaction proceeds at the same rate everywherethroughout. The slowest step in the process is the heat transferthrough the external film around the particle. For small particles,the time needed to reach a certain conversion (including completeconversion) increases therefore in a way that is approximatelylinear with the particle size dp [33]. For relatively large particles, theinternal heat conduction resistance is larger, leading to tempera-ture gradients inside the particle, as mentioned above. The rate ofpyrolysis is now limited by the slower heat conduction process,whereby the time needed to reach a particular (or full) conversionincreases as dp

2. This distinction is demonstrated in Fig. 6 where thetime of complete conversion, as determined from TGA experiments,is illustrated for sieved biomass fractions of sawdust (0–200 mm)and eucalyptus (0–600 mm).

The continuous reaction model can hence certainly be used forthe description of the conversion of smaller biomass particles,expressed for a first order reaction as:

1� X ¼ expð�ktÞ (5)

iomass particle size for (a) sawdust, (b) eucalyptus.

Table 4The total heat requirements for pyrolysis.

The total heat requirementfor pyrolysis sample

Heat requirement for pyrolysis (J g�1)

pruce 342eucalyptus 400poplar 207sawdust 434corn 267sunflower 292straw 375sewage sludge 385

0,025

M. Van de Velden et al. / Renewable Energy 35 (2010) 232–242238

The apparent and measured rate constant, k, should beexpressed in terms of the rate of heat penetration (kht) and theintrinsic or chemical reaction rate constant (kr) [33,37–39]:

1=k ¼ 1=kr þ 1=kht (6)

with kht: rate of heat penetration, equal to ham/cp (s�1), h: heattransfer coefficient from reactor bulk to the particle (W m�2 K�1),am: specific surface area of the particle per unit weight (m2 kg�1),cp: specific heat capacity of the particle (J kg�1 K�1).

Heat transfer limitations are negligible in operations achievinga high value of kht and reaction rates are then kineticallycontrolled only (k z kr) if additionally the internal heat conductionresistance is negligible (small particles). Experiments shouldtherefore be preferably performed under such conditions. Suchhigh values of kht and thus of h can only be obtained in bubbling orcirculating fluidized beds. Since the use of small particles isrequired and the residence time needs to accurately controllable,the circulating fluidized bed is the most appropriate reactor forbiomass pyrolysis.

As will be shown in the experimental results in further sections,the reaction rate constant is a strong function of the temperature,and follows the Arrhenius equation (7). The activation energy hasa constant but different value for each specific biomass.

k ¼ A expð�Ea=RTÞ (7)

Table 3 compares kht and k for a few target samples, wherebyequation (6) is used to determine the error on k if no heat transferlimitations are accounted for. The error is insignificant, even at hightemperatures, indicating that virtually no external heat transferlimitations are present under the experimental conditions used.

4.2.2. The influence of the heating rate in TGA experimentsThe external heating rate b (K s�1) can be programmed in TGA

experiments, and literature data are available for b values between0.033 K s�1 and 1.67 K s�1. Experimental values of k vary signifi-cantly, even for a single biomass species. The influence of theheating rate is of paramount importance since the heat transferredmust be sufficient to supply the total endothermal heat of reaction(C, J kg�1). A too low heating rate (low value of b) will retard thepyrolysis reaction and will yield an apparently lower value of thereaction rate constant. The anticipated definition of the requiredminimum b value can prevent faulty experiments and relatedexperimental data.

During TGA-pyrolysis, reactions take place between approx. 473and 673 K (average 573 K). The relationship between sufficient heatsupply and reaction yield can be expressed in terms of kinetic andthermal data, as determined below. The first order kinetics relateconversion (X), reaction rate constant (k) and time (t), according toexpression (5). Since the heat supplied should at any time and forany conversion X exceed the heat consumed, a heat balance can bemade. The heat supplied during the reaction equals m0cpbt (J),with m0 the initial amount of biomass (kg). The char yield is onaverage 30 wt%. Heat requirements (C) were measured by DSC

Table 3Effect of the heat penetration upon the reaction rate constant (k).

T (K) Saw dust (85 mm) Eucalyptus (37.5 mm) Corn (50 mm)

k kht error onkr (%)

k kht error onkr (%)

k kht error onkr (%)

573 0.01 5.88 0.2 0.01 26.67 0.05 0.02 24.00 0.1673 0.12 5.47 2.1 0.21 24.81 0.8 0.27 22.33 1.2773 0.68 5.12 11.8 1.52 23.19 6.1 1.59 20.87 7.1

(see Section 4.2.4). The required heat of reaction is hence C.m0 (J).The heat balance then results in equation (8a).

m0cpbt � 0:7m0XC (8a)

Inserting the value of t from Eq. (5), the general equation (8b) isobtained.

�cpb lnð1� XÞ=k � 0:7XC (8b)

or

k � �cpb lnð1� XÞ=ð0:7CXÞ (8c)

This equation can thus be applied to any values of conversion(X). In the present research, overall values of k are measured, i.e. atnearly complete conversion of the biomass (Table 4). Equation (8c)is hence applied using these experimental k values and a conver-sion of 95% is assumed to be the measure of complete pyrolysis.Equation (8c) is then reduced to:

k � �4:5cpb=C (8d)

It should be noted that for very low conversions (X / 0) a morerestrictive condition on k is obtained since the initial reaction rate ishighest. This condition of very small conversions will however notbe applied in the present treatment since only overall k values areknown.

This approach was tested using the TGA results of Section 4.2.3below, obtained at different heat supply rates and for variousbiomass species. Illustration for spruce is given in Fig. 7, whichclearly indicates that relevant k values can only be obtained ifb exceeds 80 K min�1. Application of Eq. (8d) at an average 573 K,with a cp of 1.08 kJ kg�1 K�1 (at 573 K) and with the experimentallydetermined C values (Table 4) demonstrates that the theoreticalvalue of b should exceed 79 K min�1, in line with the experimentalfindings.

Similar calculations were performed for the different biomassspecies, using the experimentally determined char yield and C

0

0,005

0,01

0,015

0,02

0 20 40 60 80 100 120 140heating rate (K min-1)

k (s

-1)

Fig. 7. The effect of the heating rate upon the reaction rate constant for spruce.

Table 5Required minimum heating rates for an accurate measurement of the reaction rateconstant.

Biomass k at 573 K (s�1) bmin (K min�1)

spruce 0.0200 79eucalyptus 0.0138 64poplar 0.0116 28sawdust 0.0111 56corn 0.0242 75sunflower 0.0856 289straw 0.0350 152sewage sludge 0.0067 30

M. Van de Velden et al. / Renewable Energy 35 (2010) 232–242 239

values. A heating rate b of 80 K min�1 is sufficient to meet therequired reaction heat (Table 5) for most biomass species. Allexperiments were therefore performed at 100 K min�1. To accu-rately measure the kinetics, heating rates should exceed100 K min�1 for sunflower residue and straw.

The previous calculations demonstrate that literature dataobtained at lower heating rates (e.g. [12]) should be consideredwith caution and should not be compared to results obtained underrelevant conditions.

4.2.3. Kinetic analysisThe obtained TGA-profile plots, shown in Fig. 8, are similar for

all tested biomass samples. The results show that pyrolysis occursin the temperature range between 473 and 673 K, depending on thetype of biomass. This observed temperature range was also found inliterature data [10,12,40].

It is obvious that the pyrolysis reaction produces a solid residueor char, noticed by a final limited loss of weight per unit of time.This char consists of minerals and the organic coking-residue of the

Fig. 8. TGA plots for (a) spruce, eucalyptus, poplar and sawdust, and (b) corn,sunflower, straw and sewage sludge.

biomass and represents 25–35 wt%, with the exception of corn withonly 10 wt% residue, and sunflower and sewage sludge, where thechar fraction is significantly higher. The high ash content of sewagesludge is related to its well-known high content of minerals. Theresults correspond fairly with previous literature data, i.e. 21% charfor spruce [40], 30% for straw [40,41] and on average between 19and 26% [42].

From the dynamic TGA experiments, the reaction rate constant kcan be determined by Eqs. (9)–(11), as used in e.g. [10–12,43].

dXðtÞ=dt ¼ k�Xp � XðtÞ

�(9)

with

XðtÞ ¼ ðm0 �mðtÞÞ=m0 (10)

Xp ¼ ðm0 �mNÞ=m0 (11)

with m0: initial weight of biomass (at t¼ 0) (mg), mN: residualweight of biomass after the reaction (mg), m(t): weight of biomassat time t during the experiment (mg).

As the weight of the sample is continuously registered, X and Xp

are known parameters and the reaction rate constant can bederived from Eq. (9) at every moment. Because of the knowntemperature profiles of the TGA (known DT/Dt), the reaction rateconstant is also known as a function of the temperature. TheArrhenius Equation (7) can be applied after transformation andplotting of the results as ln(k) versus 1/T. The activation energy andthe pre-exponential factor can be derived from respectively theslope and the intersection of the obtained straight line with theln(k)-axis.

Results for the different biomass species at b¼ 100 K min�1 areillustrated in Fig. 9 and summarised in Table 6, together with someliterature data.

spruce eucalyptus poplar sawdust

-7

-6

-5

-4

-3

-20,0015 0,0016 0,0017 0,0018 0,0019 0,002

1/T (K-1)

lnk

corn sun flower straw sewage sludge

-7

-6

-5

-4

-3

-20,0015 0,0016 0,0017 0,0018 0,0019 0,002

1/T (K-1)

lnk

a

b

Fig. 9. Temperature dependency of the rate constant of biomass pyrolysis at a heatingrate of 100 K min�1 for (a) spruce, eucalyptus, poplar and sawdust, and (b) corn,sunflower, straw and sewage sludge.

Table 6Kinetic constants from own experiments and literature.

Sample Ea (kJ mol�1) A (s�1) at 100K min�1

k (s�1)at 773 K

Ownexperiments

Spruce 68.4 3.47� 104 0.824Eucalyptus 86.4 1.06� 106 1.52Poplar 54.1 1.00� 103 0.219Sawdust 75.8 9.12� 104 0.684Corn 77.0 2.55� 105 1.59Sunflower 63.9 2.48� 104 1.19Straw 76.3 3.16� 105 2.21Sewagesludge

45.3 8.95� 101 0.078

Thurner andMann [43]

Oak 106.5 2.47� 106 0.156

Gorton andKnight [44]

Hard wood 89.52 1.48� 106 1.31

Nunn et al. [45] Gum tree,hard wood

69.1 3.64� 104 0.775

Reina et al. [12] Forest wood 95.4 2.40� 105 0.0852Di Blasi and

Branca [46]Beech 95.4 2.4� 105 0.156

Table 7Specific heat at 298 K.

Sample cp (kJ kg�1 K�1)

Poplar 1.2Sawdust 1Corn residue 0.9

M. Van de Velden et al. / Renewable Energy 35 (2010) 232–242240

The activation energy of a specific biomass is nearly constant(and representative for the biomass type) at different heating rates(Fig. 10), as can be seen from the parallel ln(k) versus 1/T lines.Fig. 10 shows however that the pre-exponential factors, and thusthe reaction rate constant depend on the heating rate. When theheating rate increases, the pre-exponential factor A (and thus k)increases first quickly to then reach a maximum value: at lowheating rates insufficient reaction heat is supplied for the endo-thermic reaction, as was previously discussed. Although theobtained k values are nearly constant when obtained at highheating rates (b) during the TGA experiments, the b values are stillof the order of 1–2 K s�1 only (due to TGA limitations). In fastpyrolysis, biomass particles are subjected to heating rates in excessof 100 K s�1. Section 4.2.2. discussed the influence of the heatingrate on the biomass conversion in view of balancing the heat supplyrate with both the heat required for pyrolysis and progress ofconversion. This balance is met at b w 1–2 K s�1. Heating rates inexcess of 100 K s�1 can only be examined in pyrolysis reactors suchas fluidized beds. This study is ongoing at present, where the batchpyrolysis of biomass is studied. Results will be reported in the nearfuture and are essential to evaluate the possible extrapolation ofthe current data and kinetic expressions to systems with a fastheating rate.

From these A and Ea values, the rate constant of the pyrolysisreaction can be calculated at any temperature, and values at 500 �C

50 K min-1 80 K min-1 100 K min-1 120 K min-1

-6,5

-6

-5,5

-5

-4,5

-4

-3,5

-3

-2,5

-20,0015 0,0016 0,0017 0,0018

1/T (K-1)

lnk

Fig. 10. Temperature dependency of the reaction rate of the pyrolysis of spruce atdifferent heating rates.

(the optimum T in pyrolysis reactors) are included in Table 6.Literature data should be used with caution: although data for k, Aand Ea are given, both the method and procedure used, and theheating rates are generally not included. It is however clear that theEa values are in the range of those of the biomass components attheir respective wt%: 195–213 kJ mol�1 for cellulose, 105–111 kJ mol�1 for hemicellulose and 34–65 kJ mol�1 for lignin [47],each present in biomass for around respectively 45, 30 and 25%.Moreover, it should be mentioned that the k values, with theexception of poplar and sludge, exceed 0.5 s�1 corresponding toa fast reaction of the biomass. A high conversion can thus be ach-ieved in short reaction times so that the possible occurrence of sidereactions is limited.

4.2.4. Thermal analysisThe thermal parameters are necessary to formulate the heat

balance of the pyrolysis reaction. Since pyrolysis, as every crackingreaction, is an endothermic reaction, the reaction heat is to besupplied indirectly (heat exchangers) or directly (e.g. by preheatingthe bed material or the fluidization gas). Usually the combustion ofpyrolysis gas supplies the required heat (C, J g�1) and thecombustion gases form the oxygen-free carrier gas. The heatrequired to pyrolyse biomass contains two components (Eq. (2)):the heat required to heat the biomass to the temperature at which

Fig. 11. DSC curves (10 K min�1) for (a) spruce, eucalyptus, poplar and sawdust, and (b)corn, sunflower, straw, sewage sludge.

Fig. 12. Heat requirement for pyrolysis in function of the temperature for (a) spruce,eucalyptus, sawdust, poplar, and (b) corn, sunflower, straw, sewage sludge.

M. Van de Velden et al. / Renewable Energy 35 (2010) 232–242 241

pyrolysis occurs and the heat required for the endothermic pyrol-ysis as such. Hence the required heat can be written as follows:

C ¼ mbcp;bðTr � T0Þ þmbDHr (12)

The temperature at which pyrolysis occurs, the specific heatcapacity cp and the reaction heat DHr (J g�1) required for thepyrolysis itself were experimentally investigated and will be dis-cussed hereafter.

The specific heat capacity cp (J g�1 K�1) of the biomass samplesunder scrutiny is presented in Table 7. Since cp values increase withincreasing temperature, the value at 500 �C will be about 15%higher than the experimental value at 298 K. Results are close toliterature data, which at 773 K vary between 1.1 kJ kg�1 K�1 [48] to1.2 kJ kg�1 K�1 [49].

The obtained DSC curves, giving the heat flux (W g�1) in func-tion of the temperature or in function of the time (constant DT/Dt¼ 10 K min�1) are shown in Fig. 11.

The DSC curves show that the heating process is clearly in thedomain of heat requirement, which confirms the endothermiccharacter of pyrolysis. Integration of these heat fluxes over time(Eq. (13)) gives the total heat requirement C (J g�1) as a function ofthe temperature (Fig. 12). This includes the heat required forheating the biomass as well as the heat for the pyrolysis reaction.The endothermic peak at about 100 �C, due to the evaporation ofresidual water, was eliminated before integration. When thepyrolysis is completed, the heat requirement decreases. Decliningvalues before the end of the reaction and beyond the normaltemperature range of pyrolysis can be explained by the occurrenceof secondary reactions (e.g. the aggregation of the char).

dCdt¼ cp

dTdtþ DHr/C ¼

Zt

0

�cp

dTdtþ DHr

�dt (13)

The calculated heat requirements for pyrolysis of the testedbiomass species are presented in Table 4. These values are theaverage of three experiments, with a deviation of approximately6%. The end temperature of pyrolysis was obtained from the TGAexperiments.

The results are in the same order of magnitude of literature data:500 kJ kg�1 for straw [38], 400–450 kJ kg�1 for cellulose [41,50],235 kJ kg�1 [51] and 418 kJ kg�1 [52] for unspecified wood and 120–360 kJ kg�1 for various wood species [42].

5. Conclusions

Energy from biomass is recognised as the renewable energysource with the highest potential in the near future. Within thethermochemical conversion techniques, pyrolysis offers majoradvantages through yielding mostly liquid and solid fuel, easy tostore and transport.

Although the operating conditions for a maximum oil yield arewell-known, the kinetics of the biomass conversion need to bedetermined. The paper considered (i) the required workingconditions for relevant experimental research; (ii) the thermogra-vimetric measurement of the kinetic constants and (iii) the reactionheat of the biomass pyrolysis.

A review of the pyrolysis of the main constituents of biomass i.e.cellulose, hemicellulose and lignin, and of the possible secondaryreactions stresses the value of the biomass pyrolysis concept of theUniversity of Waterloo.

To program experiments in a relevant and effective way, and tocritically assess previous literature results, it was demonstratedthat tests should be carried out (i) with biomass particles smallerthan approx. 200 mm and (ii) in a reaction environment where theparticle heating rate is about 80 K min�1 and where the internalresistance to heat penetration is smaller than the external resis-tance to heat transfer (expressed in the condition of Biot-number,Bi< 1).

TGA and DSC experiments on a wide variety of biomass particlesdetermine respectively the reaction kinetics and the Arrheniusdependency, and the endothermicity of the reaction. For most ofthe biomass species, the reaction rate constant is >0.5 s�1, corre-sponding with a fast reaction. The endothermic heat of reactionranges from 207 to 434 kJ kg�1. These results tie in with literaturedata, although the reader is cautioned in the use of literature datasince experiments were often performed at non-representativetesting conditions.

Both kinetic and thermal data are essential in the design ofpyrolysis reactor, determining respectively the required residencetime and the amount of heat to be supplied to the reactor. Their usewill be illustrated in a further paper where the design strategy ofa 10 MW (bio-oil) pyrolyser is presented.

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