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Journal of Analytical and Applied Pyrolysis 101 (2013) 177–184

Contents lists available at SciVerse ScienceDirect

Journal of Analytical and Applied Pyrolysis

journa l h o me page: www.elsev ier .com/ locate / jaap

he effect of the biomass components lignin, cellulose and hemicellulose on TGAnd fixed bed pyrolysis

uisa Burhennea,∗, Jonas Messmera, Thomas Aichera, Marie-Pierre Laborieb

Fraunhofer-Institute for Solar Energy Systems ISE, Heidenhofstr. 2, 79110 Freiburg, GermanyInstitute of Forest Utilization and Work Science, University of Freiburg, Werthmannstr. 6, 79085 Freiburg, Germany

r t i c l e i n f o

rticle history:eceived 19 October 2012ccepted 13 January 2013vailable online 31 January 2013

eywords:iomassyrolysisGAixed bed

a b s t r a c t

Thermochemical conversion of biomass has been studied extensively over the last decades. For the design,optimization and modeling of thermochemical conversion processes, such as fixed bed pyrolysis, a soundunderstanding of pyrolysis is essential. However, the decomposition mechanism of most biomass typesinto gaseous, liquid, and solid fractions is still unknown because of the complexity of pyrolysis anddifferences in biomass composition.

The aim of this study was to find characteristic differences in the pyrolysis behavior of three widely usedbiomass feedstocks to optimize the performance of industrial fixed bed pyrolysis. This aim was achievedin three steps. First, devolatilization kinetics during pyrolysis of three biomass types was investigatedin a thermogravimetric analyzer (TGA). Then, a one-step multi-component pyrolysis model with threeindependent parallel reactions for hemicellulose, cellulose and lignin was derived to correlate the kineticswith single component decomposition and to identify their amount in the biomass sample. In a finalstep, the findings were tested in a fixed bed reactor at laboratory scale to prove applicability in industrialprocesses.

Three types of biomass were chosen for this investigation: wheat straw, rape straw and spruce woodwith bark. They represent biomass with a high cellulose, hemicellulose and lignin content, respectively.Since lignin is the most stable and complex of these three biomass components, its amount is assumedto be the main controlling factor in the thermochemical decomposition process.

The thermogravimetric (TG) curve of spruce wood with bark was found to shift to about 20 K highertemperatures compared to the TG curves of straw and rape straw. This result indicates that a higher acti-vation energy is needed to decompose woody biomass, which contains a higher amount and a differenttype of lignin than straw. Three wood decomposition phases were distinguished from the negative firstderivatives curves (DTG): a shoulder during hemicellulose decomposition, a peak during cellulose decom-position and a smaller rise during lignin decomposition. By comparison both herbaceous biomass typesdecomposed in only two phases at lower temperatures. The decomposition of the herbaceous, and woodybiomass samples was completed at about 830 K and 900 K, respectively, leaving only a solid residue ofash. The derived pyrolysis model estimated the composition and described the devolatilization curvesof each biomass with sufficient accuracy for industrial processes, although the same activation energyset, taken from the literature, was used for each biomass. In the fixed bed pyrolysis experiments similar

characteristics were found to those in the TGA experiments. Herbaceous biomass with a higher celluloseand hemicellulose content decomposed faster and produced a larger fraction of gaseous products thanwoody biomass with a higher lignin content. According to the assessment of the product distribution,performed after each experiment, woody biomass pyrolysis led to a larger fraction of solid productsthan herbaceous biomass pyrolysis. We conclude that industrial fixed bed pyrolysis can be optimized for

cks w

different biomass feedsto

∗ Corresponding author.E-mail address: luisa.burhenne@ise.fraunhofer.de (L. Burhenne).

165-2370/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jaap.2013.01.012

ith a specific composition of cellulose, hemicellulose and lignin.© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Plant biomass, a composite material mainly comprised of hemi-

cellulose, cellulose, lignin, is an ideal renewable resource for thegeneration of heat and power via thermochemical processes suchas fixed bed gasification. However, with the growing biomass needsfor both bioenergy and biomaterials, the availability of biomass

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78 L. Burhenne et al. / Journal of Analytica

eedstock for the thermochemical conversion processes is becom-ng more and more limited, especially in highly populated regions.herefore, technical processes need to be improved so that a wideange of biomass types, including biomass waste and side prod-cts, can be converted into synthesis gas that can be used directlyor heat and power production. In order to optimize any conver-ion process and to fine-tune process parameters to the specificeedstock characteristics, a detailed knowledge about the way inhich the biomass composition influences the conversion behavior

s important.Pyrolytic decomposition, the core reaction in all thermochem-

cal conversion processes, is crucial to the biomass decompositionnto gaseous, liquid, and solid fractions [1]. In fixed bed gasifica-ion, pyrolysis is caused by the heat of hot gases released duringartial combustion. The pyrolysis temperature and heating ratere given by heat transfer from the gas to the biomass particlen the one hand and by heat transfer within the particle itselfn the other hand. Since heat conductivity in biomass particles isnown to be very low, biomass pyrolysis during fixed bed gasi-cation can be compared to conventional slow pyrolysis. Hence,yrolysis temperatures, decomposition rates, product compositionnd yields are influenced by both heat transfer and chemical reac-ions. Both vary with the biomass composition since the threeolymeric constituents, namely lignin, cellulose and hemicellu-

ose, have different internal energy as well as heating values [2].s a result they differ markedly in their thermal stability with

ignin being the most stable of all structural components. Accord-ng to Yang et al. [3], lignin decomposes constantly in a wideange of temperatures compared to cellulose and hemicellulose,hich decompose in a small temperature interval between 573 K

nd 673 K due to their homogeneous structure. According to Yangt al. [3], lignin does not decompose completely until tempera-ures up to 973 K are reached. It was also found that pyrolysis ofoody biomass with high lignin content is an overall endother-ic reaction, whereas pyrolysis of herbaceous biomass, with lower

ignin content, involves an exothermic reaction [3]. Furthermore,he amount of lignin is assumed to be the main controlling fac-or for the pyrolytic decomposition rate, the product yields andomposition.

So far, numerous studies on the pyrolytic decomposition of theain components have been carried out [4]. Most have focused

n the pyrolysis of several milligrams of the pure biomass compo-ents lignin, cellulose and xylan or wood from specific tree species5]. However, the decomposition behavior of different types of realiomass, such as herbaceous and woody biomass, has rarely been

nvestigated with the aim of correlating it to the decompositionharacteristics of its main constituents. Such an approach could besed to optimize industrial processes.

In order to predict the decomposition behavior of any biomass,rst the characteristics of the single components need to be iden-ified [6]. Some authors have attempted to predict thermal andinetic behavior of biomass from its composition simply by theuperposition of the decomposition of the single components.ouhert et al. [7] have shown that the correlation between biomassas yields and the cellulose, hemicellulose and lignin fractions isot very accurate. Nevertheless, since the components differ sig-ificantly in their decomposition behavior [8], it is possible toetermine clear trends in decomposition time, reaction enthalpy,as yield and composition of any biomass according to its com-osition of lignin, cellulose and hemicellulose [1]. Grønli et al. [6]eveloped a one-step multi-component pyrolysis model consistingf three parallel, independent reactions with three activation ener-

ies: each one for cellulose, hemicellulose and lignin. Using thisodel approach, high temperature (>553 K) degradation of nineoody biomass types was described with accuracy. To the best

f our knowledge, this multi-component pyrolysis model has not

Applied Pyrolysis 101 (2013) 177–184

been used to compare pyrolysis of woody biomass to non-woodybiomass or to identify the amount of cellulose, hemicellulose andlignin in a biomass sample.

To compare the results of the TGA experiments with biomasspyrolysis in technical processes, some authors used installationssuch as packed bed pyrolysis reactors [3,8]. However, still in theseexperiments, only very small pure wood component samples wereanalyzed. There is still a gap between basic research on pyrolysison a micro-scale and the research on applied pyrolysis processes.Almost no published studies could be found in which results of TGAwith different biomass types were compared to packed bed reactorsusing large sample masses (>100 g).

The objectives of this work were to identify general trends in thedecomposition behavior of lignocellulosic biomass from biomasssources differing in cellulose, hemicellulose and lignin fractions,and then to express these trends in a simple pyrolysis model totest their relevance for the performance optimization of fixed bedpyrolysis.

Based on the literature reviewed above, we hypothesized thatthe lignin content of any biomass feedstock is the main control-ling factor in pyrolysis in industrial processes. In particular weproposed that a high lignin content leads to a slow decomposi-tion and low product gas yield and that devolatilization is initiatedat higher temperatures. We tested this hypothesis by pyrolysingthree widely used feedstock types, each with a characteristic highcellulose, hemicellulose and lignin contents in a TGA. In the TGAheat or gas transfer is not a limiting factor and therefore thermo-gravimetric behavior purely reflects the pyrolysis reactions. Wheatstraw, rape straw and spruce wood with bark was used to repre-sent biomass feedstocks with a high cellulose, hemicellulose andlignin content, respectively [1,9,10]. Then, we derived a theoreticalone-step devolatilization model to identify the biomass composi-tion and to relate it with the observations. To prove applicability ofthe findings on industrial processes, the same biomass feedstockswere pyrolysed in a fixed bed reactor that was purged with hotnitrogen as carrier gas.

2. Materials and methods

2.1. Feedstocks

Three types of biomass were chosen to represent feedstockswith different compositions: wheat straw for its high cellulosecontent, rape straw for its high hemicellulose content and sprucewood pellets with bark for its high lignin content. The wheat andrape straw are agricultural by-products from cultivation near Rotam See in Germany. The straw samples were provided by S + KGmbH Haus und Energietechnik. The spruce wood pellets weremade of Norway spruce with bark (Picea abies (L.) Karst.) har-vested in the Hochsauerland (forestry administration North-RhineWestphalia Germany) and provided by the Pusch Ag in Marien-rachdorf, Germany. All samples were provided as cylindrical pelletswith a diameter of 5–20 mm and a length up to 30 mm. Elemen-tal analysis of the biomass samples was conducted in the chemicallaboratory of University Freiburg using a fully automatized elemen-tal analyzer (VarioEL, Elementaranalysensysteme GmbH). Biomasscomposition in terms of C, H and O as well as the ash content isgiven in Table 1. The proximate analysis was provided by courtesyof the TEER Institute of RWTH Aachen following the standard DIN51719.

Biomass composition in terms of cellulose, hemicellulose and

lignin is derived by atomic balance of the components elemen-tal formula following the method of Ranzi et al. [1]. For celluloseand hemicellulose the elemental formulas for all biomasses areassumed to be C6H10O5 and C5H10O5, respectively. Lignin is a

L. Burhenne et al. / Journal of Analytical and Applied Pyrolysis 101 (2013) 177–184 179

Table 1Ultimate and proximate analysis of the biomass samples by wt.%.

C H O Moisture Volatiles Fixed carbon Ash

Wheat straw 43.15 6.21 45.92 4.2 73.2 18.2 4.39Rape straw 44.47 6.35 45.23 3.7 76.5 16.1 3.69Spruce + bark 46.43 6.21 42.19 6.9 68.9 19.3 4.88

Table 2Composition of dry biomass by wt.% calculated by atomic balance compared to Refs. [5,9,12].

Calculated by atomic balance Literature

Cellulose Hemicellulose Lignin Cellulose Hemicellulose Lignin Ref.

Wheat straw 38 36 22 38.0 29.0 15.0 [5]33.2 24.0 15.1 [9]28.8 39.1 18.6 [12]

Rape straw 36 37 24 37.6 31.4 21.3 [9]

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accounted for by the pre-exponential factor k0,i. The values forhemicellulose, cellulose and lignin found in the literature [6,14]that were applied in this study for the estimation of the biomasscomposition are listed in Table 3.

Table 3Parameters for the one-step three-component devolatilization mechanism of cellu-lose, hemicellulose and lignin from spruce [6].

Spruce + bark 42 27 26

roduct of polymerization of three types of monolignols, i.e. p-oumaryl, coniferyl, and synapyl alcohols, incorporated into ligninn the form of p-hydroxyphenyl (H type lignin), guaiacyl (G typeignin), and syringyl (S type lignin), respectively [11]. The elemen-al formulas of H, G and S type lignin are C9H10O2, C10H12O3 and11H14O4, respectively. Softwoods such as spruce contains mainly

type lignin, whereas straw contains about 45% of H, 46% of G and% of S type lignin [11]. Hence, the representing elemental formulaf lignin in spruce with bark is C10H12O3. The elemental formula ofignin and in both straws is C10.4H12.7O3.4, given by their propor-ions of each lignin type. The calculated cellulose, hemicellulosend lignin mass fractions are in line with values provided in theiterature [5,9,12] (Table 2). The component fractions calculated bytomic balance are very sensitive to changes of the input parame-ers, meaning that the biomass composition can change to a greatxtent with only a minor change in H, C or O ratio. Therefore, theiomass composition was additionally estimated by curve fittingxperimental data to a theoretical three-component devolatiliza-ion model based on input parameters from the literature.

Before conducting the experiments, the samples were oven-ried for 24 h at 105 ◦C. For the TGA experiments, the dry biomassellets were ground to a particle size smaller than 0.3 mm. Thisarticle size has been proven suitable for preventing heat and massransfer effects during the experiments [13].

.2. Devolatilization kinetics in TGA

Thermogravimetric experiments were carried out in a dif-erential thermogravimetric analyzer (PerkinElmer Pyris 1 TGA)rovided courtesy of FoBaWi Institute, Albert-Ludwigs University,reiburg. The precision of the temperature measurement was ±2 K.or each sample, 5 mg of dried and ground biomass was heatedrom 50 ◦C up to 700 ◦C under dynamic conditions at a heating ratef 20 K/min. The TGA experiments were carried out at atmosphericressure and the equipment was purged with a nitrogen flow of0 mlN min−1. Each experiment was repeated at least three times.

.3. Theoretical devolatilization model

Correlation analysis was performed between the differenteaction phases of the biomass and the single component decom-

osition using the multi-component model proposed by Grønlit al. [6] in order to estimate the biomass composition in termsf cellulose, hemicellulose and lignin. According to this model, theecomposition of biomass is described by three parallel reactions,

41.0 24.3 30.0 [5]50.8 21.2 27.5 [12]

each corresponding to the decomposition of one of the componentscellulose, hemicellulose or lignin [13]. Pure additive behavior of thecomponents and first order kinetics are assumed for modeling thedecomposition reaction of each component.

The overall conversion is expressed by the calculated normal-ized sample mass Xcalc = (m − mash)/(m0− mash) × 100 %, where m0is the initial sample mass, m the actual sample mass and mash themass of the ash in each sample, given in Table 1. The actual samplemass m at any time can be described by m(t) =

∑3i=1xim(t) + mash,

where xi is the mass fraction of cellulose, hemicellulose and lignin,respectively. Hence, the total normalized sample mass Xcalc canbe written as Xcalc = ∑3

i=1xim/(m0 − mash) × 100% = ∑3i=1Xi. Xcalc

varies from 0% to 100% and is calculated from the sum of the individ-ual normalized sample mass loss rates dXi/dt, each correspondingto the decomposition of component i = hemicellulose, cellulose andlignin.

dXcalc

dt=

3∑

i=1

dXi

dt= d

dt

∑3i=1xim

m0 − mash× 100% (1)

The conversion of each component Xi is described by a first orderkinetic reaction as:

dXi

dt= −ki(T)Xi = −ki(T)xi

m(t)m0 − mash

× 100% (2)

The rate coefficient ki(T) for each component i is describedwith the Arrhenius-type temperature-dependent equationki(T) = k0,iexp[−E(A,i)/RT], where EA,i is the apparent activationenergy. Grønli et al. [6] found activation energies EA,i that aresuitable for a wide range of woody biomass types. Differencesin the decomposition behavior of the biomass types in terms ofreaction temperatures and yields of solid products (char) were

EA,i (kJ/mol) k0,i (1/s)

Cellulose 236 1017.43

Hemicellulose 100 106.4

Lignin 46 100.58

180 L. Burhenne et al. / Journal of Analytical and Applied Pyrolysis 101 (2013) 177–184

pyroly

ccida

S

wfiact

D

2

rowoetatprgglwhwnflGa

Fig. 1. Process flow chart of the laboratory-scale

A nonlinear least squares algorithm was used to optimize thealculated correlation curve by identifying the mass fractions xi ofellulose, hemicellulose and lignin that minimize the function Sn Eq. (3). S is a function of the mean experimental DTG curvesXi,meas/dt and the calculated dXi,calc/dt values that can be calculateds follows:

=N∑

i=1

((dXi,calc/dt) − (dXi,meas/dt))2

N(3)

here N is the number of points on the resulting curve. For arst parameter estimation the component fractions calculated bytomic balance were taken (Table 2). The goodness of the optimizedurve was evaluated with the deviation between the measured andhe calculated curves at the optimal set of parameters (Eq. (4)).

eviation (%) =√

S × 100% (4)

.4. Fixed bed pyrolysis

To investigate pyrolysis in a fixed bed, a laboratory-scale testeactor was designed and built. In this laboratory test reactor, 500 gf biomass was heated up at a slow heating rate of about 20 K/minith hot nitrogen passing through the bed until a bed temperature

f 773 K was reached. In Fig. 1 the process flow chart of the pyrolysisxperimental setup is shown. The reactor consisted of a heat resis-ant pipe (DN 100) constructed from 1.4828 stainless steel withn inner diameter of 109.4 mm. The biomass entered the reactorhrough the filling air lock in the upper part, which contains of ainch valve (V1) and a manual gate valve (S1), and then fell into theeaction chamber. In the reactor, the biomass was held by a movablerid. The reaction chamber has a volume of 4 l. With the movablerid, the pyrolysis residues were removed through the char out-et after each experiment. The temperature profile in the reactor

as measured with eight thermocouples (TIR 1 to TIR 8) arrangedorizontally in the center of the reactor chamber. The carrier gasas heated with a spiral heating element (TIC 1) and an oven. The

itrogen volume flow rate for STP conditions was set by a massow controller (MFC type 1559A of MKS Instruments DeutschlandmbH). The hot gas was supplied to the reactor via a lance nozzlend flew upwards through the packed bed of biomass. The volatile

sis test unit for investigating fixed bed pyrolysis.

pyrolysis products along with the carrier gas left the reactor on theupper side and were washed in a canister filled with rapeseed oil(F 1). The gaseous products were cooled down with a thermostat.A sidestream of the product gas was cleaned and supplied to anonline gas analyzer (GA) in order to analyze the gas composition interms of CO, CO2, H2 and CH4. Other gases, e.g. O2, N2, alkanes, andalkenes, were analyzed with a gas chromatograph (GC). Pressuresensors were installed to detect any blocking in the washing andcooling sections. The volume flow of product gas was determinedthrough the N2 balance after gas analysis according to Eq. (5).

V̇i = xi

xN2

× V̇N2 (5)

where i = CO, CO2, CH4, H2. V̇N2 denotes the volume flow of nitrogen.V̇i and xi denote volume flow and volumetric fraction of componenti, respectively.

3. Results and discussion

3.1. Devolatilization kinetics in TGA

For the experiments in the thermogravimetric analyzer (TGA),the mean normalized sample mass X of all samples for one biomasstype was expressed as function of temperature between 50 K and700 K. The results are shown in Fig. 2.

The maximum standard deviation between the measured curveswas 1.88%, 1.07% and 1.66% for wheat straw, rape straw and sprucewith bark, respectively. The small change in sample mass below450 K was attributed to vaporization of moisture, and thereforeomitted from this study. The pyrolytic decomposition of all samplesbegan slowly at about 480 K with the rate increasing at about 510 Kfor the herbaceous biomass samples and at 530 K for the wood sam-ples. The thermogravimetric (TG) curve for wood with bark shiftedto higher temperatures, by about 20 K, compared to the TG curvesfor herbaceous biomass types. This result indicates that a higheractivation energy is needed to decompose woody biomass which

can be explained by the higher lignin content of the wood. Ligninis an aromatic polymer compound with three-dimensional links inan alkyl-benzene structure. During isolation, its structure is modi-fied [15]. Hence, the exact initial structure of lignin in wood is still

L. Burhenne et al. / Journal of Analytical and

Fh

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account three independent one-step devolatilization mechanisms

ig. 2. TG and DTG curves of three biomass types at a heating rate of 20 K/min. Leftand side y axis: TG curves; right hand side axis: DTG curves.

nclear. Composition, amount and type of lignin (H, G, S type) variesrom tree species to tree species, even in wood from different partsf the same tree. However, all lignin is known to be very stable andore difficult to decompose than cellulose or hemicellulose [16].

ignin decomposes slowly over a large temperature range up to pyrolysis temperature of 1173 K [8]. Another possible reason forhis temperature shift is the high amount of tannins which in spruceark can reach up to 10 wt.%. Tannins are also phenolic compoundserived from the same biosynthetic pathways as lignin. The impactf biomass extractives, e.g. tannin, and ash content on the pyrolyticecomposition will be investigated in subsequent studies.

The decomposition of the herbaceous and woody biomass sam-les was completed at about 830 K and 900 K, respectively. The solidesidue of the wheat straw, rape straw and spruce with bark sam-les was 5%, 4% and 9%, respectively. Comparing these amountso the proximate analysis of the biomass samples (Table 1) it cane seen that the solid residue of the herbaceous biomass samplesas only ash. The amount of solid residue of the woody biomass

amples however, is higher than their ash content. This indicatesn incomplete decomposition of the wood which was explained byhe higher lignin content of the woody samples.

The TG curves of wheat straw and rape straw were very similar,xcept that the rape straw decomposition peak occurred at higheremperatures than that of wheat straw. This is explained by theifferences in hemicellulose structure between both straw sam-les. Hemicellulose comprises a group of biopolymers which areranched polysaccharides [17]. They are more complex than theellulose structure. Hemicelluloses are thermally the most unsta-le components of biomass. Therefore, hemicellulose decomposesaster and at lower temperatures than cellulose and lignin.

In agreement with the literature [14], the first negative deriva-ive of the TG (DTG) curve of the spruce and bark samples showedhree decomposition phases: decomposition in the first phase com-

enced at 500 K and finished with a shoulder at 585 K. The secondhase, between 610 and 680 K, showed a peak at 644 K, indicat-

ng the maximal decomposition rate of 0.25 wt.% K−1. The thirdhase occurred between 690 and 880 K with a maximal decom-osition rate of 0.074 wt.% K−1 at a temperature of 830 K. Di Blasi14] reviewed a wide range of pyrolysis data and concluded that,or slow heating rates, hemicellulose decomposes in a temperatureange of 498–598 K. Therefore, we attributed this peak to hemicel-

ulose decomposition. The next phase, between 610 and 680 K with

peak at 644 K, was attributed to cellulose decomposition. Accord-ng to Di Blasi [14], cellulose decomposes between 598 and 648 K.

Applied Pyrolysis 101 (2013) 177–184 181

The third phase, indicated by a slight rise in mass loss rate between690 and 880 K (Fig. 2), may be attributed to lignin decomposition.Yet there is no mention of a rise due to lignin decomposition in theliterature [5,6,14]. Alternatively, this rise may be attributed to sec-ondary reactions of carbon-containing residues. Fisher et al. [18]have shown that, at temperatures above 673 K, a weight loss dueto thermolysis of carbon containing residues does takes place. Tounderstand the true nature of this decomposition rise at tempera-tures between 690 K and 880 K, further investigation is needed.

Both herbaceous biomass feedstocks decomposed in only twophases. The first regime was found between 500 and 670 K witha maximal decomposition rate of 0.275 wt.% K−1 at 600 K, and0.26 wt.% K−1 at 620 K for wheat straw, and rape straw, respectively.This regime was attributed to the decomposition of hemicellu-lose and cellulose, which can overlap at these temperatures [14].The second regime occurred with a much smoother and smallerrise at 760 K and 790 K for wheat straw and rape straw, respec-tively. The nature of this rise needs to be investigated further insubsequent studies, for the same reasons as stated above. Yanget al. [8] investigated the pyrolysis characteristics of pure hemi-cellulose, cellulose and lignin in a pack bed loaded with 2 g ofthe sample and a particle size of 0.05–0.1 mm. At a heating rateof 10 K/min they found maximum mass loss rates of 7 wt.% K−1,1 wt.% K−1 and 0.3 wt.% K−1 for cellulose, hemicellulose and lignin,respectively. In the TGA experiments carried out in this study,the maximum mass loss rates attributed to cellulose decompo-sition were 0.275 wt.% K−1, 0.26 wt.% K−1 and 0.25 wt.% K−1, themaximum mass loss rates attributed to hemicellulose decom-position were 0.15 wt.% K−1, 0.13 wt.% K−1 and 0.11 wt.% K−1 andthe maximum mass loss rates attributed to lignin decompositionwere 0.035 wt.% K−1, 0.02 wt.% K−1 and 0.074 wt.% K−1 for wheatstraw, rape straw and spruce + bark, respectively. The mass lossrates, much smaller than those reported in the literature, may becaused by the fact that Yang et al. [8] used pure biomass compo-nents, whereas, in this study, real biomass is used. Couhert et al[7] stated that decomposition of pure components differs fromreal biomass because the pyrolysis reactions are less hinderedby interactions with other components. However, in accordancewith the results reported in the literature [8], cellulose decompo-sition resulted in a much higher decomposition peak comparedto lignin decomposition. The maximum mass loss rates of cellu-lose and hemicellulose were similar for all biomass types, whereasthe peaks of lignin showed greater variations between the biomassgroups. This is explained by the different compositions of lignin,which depends strongly on the biomass type [16]. After pyrolysis ofthe wood + bark samples, 3.5% carbon-containing material was leftat 973 K, whereas herbaceous biomass samples were decomposedcompletely at about 830 K. In pyrolysis experiments with wood,Rath et al. [19] found a linear correlation between the reactionenthalpy of pyrolysis and the final yield of solid products. Char for-mation was found to be an overall exothermic reaction, competingwith the endothermic volatile formation processes during pyroly-sis. Studying the pyrolysis enthalpy of cellulose, Mok and Antal [20]also found a linear correlation between reaction enthalpy and finalchar yield. These studies [20] seem to confirm the results obtainedhere, and support the hypothesis that higher lignin content leadsto a higher char yield and, hence, a higher reaction enthalpy.

3.2. Theoretical devolatilization model

The theoretical decomposition behavior was described witha first order multi-component devolatilization model taking into

for hemicellulose, cellulose and lignin. Kinetic parameters, namelythe pre-exponential factors k0,j and the activation energies EA,i,were taken from the literature [6,14]. The mass fractions xi

182 L. Burhenne et al. / Journal of Analytical and Applied Pyrolysis 101 (2013) 177–184

F GA an( alues

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otdasb

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TCw

ig. 3. Optimization of calculated decomposition curves compared to measured Tc) spruce + bark. Measured values are represented by solid lines and simulated vemicellulose and lignin, respectively (dotted curves).

f cellulose, hemicellulose and lignin were obtained by fittinghe experimental DTG curves of each biomass sample to theevolatilization model. The calculated curves were optimized forll biomass samples by the minimizing the function S in Eq. (3). Astarting values, the component mass fractions calculated by atomicalance were taken (Table 2).

In accordance to the literature [1,9,10], wheat straw showedhe highest cellulose content, rape straw the highest hemicellu-ose content and spruce wood with bark the highest lignin content

ith deviations between the measured and the calculated curvesmaller than 4.3%, 5.8% and 4.6%, respectively (Table 4). The esti-ated cellulose mass fractions of all biomass samples were below

he reported values in the literature [5,9,12]. Cellulose decomposi-ion is the fastest of the three decomposition reactions and mayherefore be transport limited under the conditions of the TGA

xperiments, resulting in a decrease of the mass loss rate. Sincehe only adjustable parameters of the decomposition model are

able 4omposition of dry biomass, estimated by optimization of decomposition model int.%.

Cellulose Hemicellulose Lignin Maximaldeviation

Wheat straw 28.5 26.7 40.4 4.3Rape straw 21.2 37.3 37.8 5.8Spruce + bark 27.0 24.2 43.9 4.6

d DTG curves at a heating rate of 20 K/min: (a) wheat straw, (b) rape straw and(dashed lines) are superpositions of simulated decomposition curves of cellulose,

the mass fractions, an underestimation of the mass loss rate leadsto an underestimation of the cellulose mass fraction.

Lignin fractions were estimated significantly higher thanreported in the literature. This is mainly because the decomposi-tion model takes into account only the main biomass components,ignoring extractives. All other compounds which have comparabledecomposition characteristics to lignin, e.g. tannins, are accountedfor lignin in this model. The estimated hemicellulose factions of allsamples were in line with data from the literature (Table 4).

The estimated hemicellulose factions of all samples were in linewith data from the literature (Table 4).

The optimized correlation curves of all biomass samples areshown in Fig. 3 as superpositions of the simulated decompositioncurves of cellulose, hemicellulose and lignin. For both herbaceousbiomass samples, the calculated hemicellulose peak appeared atlower temperatures than in the measured DTG curves (Fig. 3). Thisis mainly because the used model parameters from the literaturewere given for woody biomass. The lower decomposition temper-ature indicates that the hemicellulose from herbaceous biomasshas a different structure and lower activation energy than hemi-celluloses from woody biomass. With deviations smaller than 4.3%,5.8% and 4.6%, the optimized correlation curves were in good agree-ment with the experimental values for the woody and non woodybiomass samples (Fig. 3). Hence, it can be assumed that the model

parameters reported by Di Blasi [14] and Grønli et al. [6] canbe applied for a wide range of woody and non-woody biomass.These parameters are also within the range of reported values forother lignocellulosic materials [13,14,21]. To improve the test of

L. Burhenne et al. / Journal of Analytical and Applied Pyrolysis 101 (2013) 177–184 183

Fig. 4. Volumetric product gas flow in lN/min from 500 g biomass at 773 K undernb

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itrogen atmosphere in a laboratory counter-current fixed bed reactor for pellets ofiomass samples wheat straw (*), rape straw (©) and spruce with bark (�).

his multi-step mechanism and to identify model parameters suit-ble for herbaceous biomass, the biomass composition in termsf cellulose, hemicellulose and lignin should be analyzed in fur-her studies. Nevertheless, for engineering purposes, the modelstimates biomass composition and describes decomposition of allamples with sufficient accuracy for industrial fixed bed processes.ence, this study confirmed that by describing pyrolysis with a one-

tep parallel reaction model, the single component characteristicsetermine the biomass decomposition. In the model, lignin decom-osition is the slowest reaction and hence, the main controllingactor of the overall pyrolysis process.

.3. Fixed bed pyrolysis

All three biomass samples were pyrolysed in the laboratory fixeded reactor. Each experiment was repeated at least three times.he total product gas flow was calculated by summing all mea-ured product gas flows determined according to Eq. (5). The meanroduct gas flow together with the mean bed temperature of eachiomass was plotted over time (Fig. 4). The standard deviation of theas release curves was between 9% and 12% and therefore, muchigher than in the TGA experiments. The temperature measure-ent was delayed by about 2 min due to pyrolysis char covering

he thermocouples.The decomposition of all biomass samples in the fixed bed

egan in the temperature range between 420 and 450 K, reach-ng maximum pyrolysis temperatures between 753 and 793 K.he initial temperature at which devolatilization commenced dif-ered between spruce samples and the herbaceous wheat and rapetraw samples. Both herbaceous biomass feedstocks showed a sim-lar devolatilization behavior: a rapid increase in the outgassingolume to a more or less constant volume flow reaching a max-mum of 6 lN/min and 5.2 lN/min at about 580 K for wheat strawnd rape straw, respectively. The outgassing of spruce with barkncreased continuously until a maximum of 3.5 lN/min at 680 K waseached. After reaching the maximum, the outgassing of the herba-eous samples was completed after about 14 min, whereas theutgassing of the spruce and bark samples decreased more slowlynd was completed after about 18 min. A maximum bed tempera-ure between 770 and 790 K was reached for all samples. Similar tohe TGA experiments, the herbaceous biomass samples revealed aecomposition peak that was higher, narrower and at lower tem-

eratures (580 K) than the peak of the woody biomass samples (at80 K). In the fixed bed as well and in the TGA experiments, thisehavior was ascribed to hemicellulose decomposition, occurringypically in the temperature range between 498 and 598 K, followed

Fig. 5. Pyrolysis product composition by wt.%, ash-free, in pyrolysis experiments of500 g pellets of three different biomass types, pyrolysed at 773 K.

by a high cellulose devolatilization between 598 and 648 K [14].Hemicellulose and cellulose decompose very rapidly compared tothe lignin polymers.

These results indicate that industrial fixed bed processes canbe optimized by increasing the process temperature and residencetime with increasing lignin content of the feedstock.

The product distribution of the herbaceous and woody biomasssamples was investigated for all experiments. The samples showeda liquid fraction of between 48% and 41% for rape straw and wood,respectively (Fig. 5). The high liquid yield is explained by the lowtemperatures in the fixed bed. It also indicates short residencetimes of the released gases. The gas phase around the solid particlesin the reactor is flushed by the carrier gas and time for relativelyslow secondary reactions such as tar cracking or homogeneous gasreactions was too short. Previous studies confirm this behavior [22].

The amount of solid products was calculated by subtracting theash proportion in the biomass feedstock. The percentage of char inthe biomass samples with lower lignin contents (wheat and rapestraw) was lower than in the samples with a higher lignin content(spruce + bark). The amount of product gas was calculated usingthe N2 balance after gas analysis. With a gas fraction of 20%, herba-ceous biomass produced more product gas than woody biomass,which only produced a gas fraction of 16%. The higher gas yield ofthe herbaceous biomass samples is explained by the lower lignincontent in herbaceous biomass (15–20 wt.%) compared to wood(25–30 wt.%). According to the literature [3], hemicellulose and cel-lulose release more gaseous products than lignin. The high mineralcontent in the straw biomass might also influence the total productgas volume. According to the literature [7], minerals lower the acti-vation energy of pyrolysis reactions, which accelerates exothermicthermochemical conversion reactions. The effect of the biomassash content on the pyrolytic decomposition will be investigatedin further studies. The reason for the larger fraction of solid prod-ucts produced during pyrolysis of the biomass samples with higherlignin content (spruce + bark) is that, up to 780 K, cellulose decom-poses almost completely and the hemicellulose decomposes toabout 20 wt.%, whereas lignin polymers decompose only to aboutof 60 wt.% [5].

According to the analysis of all permanent product gases (Fig. 6),the CO2 volume in the product gas of the two straw types wasnearly twice that of the CO volume. The difference in the ratio of

CO2 to CO for spruce with bark was lower. The CH4 volume didnot differ significantly between the samples. Both straw samplesshowed a higher H2 fraction compared to the wood samples. Inaddition to the direct release of gases during the pyrolysis process,

184 L. Burhenne et al. / Journal of Analytical and

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ig. 6. Product gas volumes (CO, CO2, CH4, and H2) in liters at standard conditionsor pyrolysis experiments of 500 g pellets of three different biomass types, pyrolysedt 773 K.

eterogeneous and homogeneous gas reactions can take place. Theigh amount of CO2 and CO is produced most probably by carbonxidation and reduction reactions, e.g. the Boudouard equilibrium.owever, with increasing temperatures, no increase in CO contentould be observed. This is attributed to very low residence timesf the gas in the reaction chamber. At around 773 K, the calculatedas exchange rate of nitrogen in the reaction chamber is approxi-ately 6 times per minute with a velocity of 4.6 cm/s. To confirm

nd quantify this correlation and to investigate the influence ofther secondary reactions, further studies are needed.

. Conclusion and outlook

The objectives of this work were to identify characteristic dif-erences in the pyrolysis behavior of three widely used biomasseedstocks from biomass sources differing in cellulose, hemicellu-ose and lignin fractions, and then describe these trends in a simpleyrolysis model to test their relevance for the performance opti-ization of fixed bed gasification. It was shown that the lignin

ontent of any biomass feedstock is the main controlling factor inyrolysis in industrial processes. A higher lignin content leads to

slower decomposition, a lower product gas yield and a higheremperature where devolatilization kicks in.

Therefore, devolatilization kinetics during pyrolysis was inves-igated for three biomass types in a thermogravimetric analyzerTGA). Wheat straw, rape straw and spruce wood with bark wassed to represent biomass feedstocks with a high cellulose, hemi-ellulose and lignin content, respectively. It was observed, thatore energy is needed to decompose the woody biomass which has

igher lignin content than straw, since the thermogravimetric (TG)urve of spruce wood with bark was shifted to higher temperaturesompared to the TG curves of straw and rape straw. A theoreti-al one-step devolatilization model was derived to estimate theass fraction of each component and to relate the observationsith the biomass composition. The pyrolysis model could describe

he devolatilization curves of each biomass with sufficient accuracyor industrial processes, although the same activation energy set,as used for each biomass. To prove applicability of the findings

o industrial processes, the same biomass feedstocks were pyro-ysed in a fixed bed reactor that was purged with hot nitrogen asarrier gas. In the fixed bed pyrolysis experiments, characteristicsere found similar to those in the TGA experiments. Herbaceous

iomass with a higher cellulose and hemicellulose content decom-

osed faster and produced a larger fraction of gaseous productshan woody biomass with a higher lignin content. Woody biomassyrolysis led to a larger fraction of solid products than herba-eous biomass pyrolysis. We conclude that industrial fixed bed

[

Applied Pyrolysis 101 (2013) 177–184

gasification can be optimized by adjusting residence times andtemperatures in the reactor according to specific composition incellulose, hemicellulose and lignin of the biomass feedstock. In par-ticular, it is recommended that woody biomass containing a higherlignin content is pyrolysed at higher temperatures and longer resi-dence times than herbaceous biomass with a lower lignin content.

Acknowledgements

The authors acknowledge gratefully the very valuable supportof Dr. Lisbeth Rochlitz, Christian Lintner and the help of all students.The Institute of Forest Utilization and Work Science (FoBaWi) at theAlbert-Ludwig University in Freiburg, is acknowledged for placingthe TGA equipment at our disposal.

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