Heat of wood pyrolysis

Fuel 82 (2003) 81–91 www.fuelfirst.com

Heat of wood pyrolysisq J. Ratha,*, M.G. Wolfingera, G. Steinera, G. Krammera, F. Barontinib, V. Cozzanib a b

Institut fu¨r Apparatebau, Mechanische Verfahrenstechnik und Feuerungstechnik, Technische Universita¨t Graz, Inffeldgasse 25, A-8010 Graz, Austria Dipartimento di Ingegneria Chimica, Chimica industriale e Scienza dei Materiali, Universita degli Studi di Pisa, Via Diotisalvi 2, I-56126 Pisa, Italy Received 12 February 2001; accepted 9 May 2002; available online 30 August 2002

Abstract The heat of pyrolysis of beech and spruce wood was investigated by means of a differential scanning calorimeter. Wide variations were found for the heat of the primary pyrolysis process, depending on the initial sample weight and on the conditions used in the measurements. However, reporting the heat of the primary pyrolysis process versus the final char yield resulted in a linear correlation. This strong dependency of the heat of wood pyrolysis on the final char yield, that is in turn highly sensitive to the experimental conditions, can explain the uncertainty of the data for the heat of wood pyrolysis reported in the literature. A possible explanation for the variability of the heat of wood pyrolysis depending on the final char yield seems to be an exothermic primary char formation process competing with an endothermic volatile formation process. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Pyrolysis; Heat of reaction; Differential scanning calorimetry

1. Introduction Detailed knowledge of the thermal conversion processes of wood for the production of energy becomes increasingly important, since wood is a renewable source. An improvement of the efficiency of the thermal conversion processes of wood may be achieved only by the thorough analysis of all the aspects of the problem. As reported in the literature [1,2], heat of pyrolysis of wood has an important influence on the course of thermal conversion. However, as partly summarized by Roberts [3] and shown in Table 1, the results reported in the literature [1,4 – 7] for the heat of pyrolysis of wood range from endothermic to large exothermic values. Secondary pyrolysis reactions between volatiles and primary char as well as autocatalytic effects due to impurities are usually assumed to be the reasons for these differences [3,8]. Differential scanning calorimetry (DSC) proved to be an effective technique for the obtainment of reliable values of the elementary heat of reaction in the absence of complicating phenomena, as heat or mass transfer limitations [9]. However, in the pyrolysis of wood secondary reactions may take place even in DSC experiments if the primary * Corresponding author. E-mail addresses: [email protected] (J. Rath), [email protected] (F. Barontini). q Published first on the web via Fuelfirst.com—http://www.fuelfirst.com

volatiles are not swept off the primary char. The extent of secondary reactions are reduced if low sample weights and/ or crucibles without a lid are used. These conditions promote a fast gas exchange between the reacting particles and their surroundings. As a matter of fact, quite different results are obtained for the heat of pyrolysis of wood in this kind of experiments with respect to conventional DSC runs with lid. The present study was focused on the analysis of the influence of experimental conditions on the measured heat of pyrolysis of wood in DSC experiments. Spruce and beech wood samples were used in experimental runs. The influence of the final char yield and of the radiating heat exchange on DSC results was also investigated.

2. Experimental 2.1. Techniques DSC experimental data were obtained using a Mettler DSC 25 calorimeter. Experimental runs were performed using pure nitrogen as the purge gas (300 ml min21). Typical total sample weights of about 2– 10 mg were used. The DSC runs were performed using aluminium crucibles. Depending on the experimental conditions desired, the crucible was used without a lid (thus resulting

0016-2361/03/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 6 - 2 3 6 1 ( 0 2 ) 0 0 1 3 8 - 2

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Nomenclature Ci cp,char cp,wood mchar mend m(T ) m0 HP HS Q Qchar Qrad QR Qs

Qwood t T Tshift X Yc ai DH1 DH2

mass of ith component of wood (hemicellulose, cellulose, lignin) (g) specific heat of char (J g21 K21) specific heat of wood (J g21 K21) weight of char sample, dry (mg) weight of wood sample after pyrolysis experiment, dry (g) sample weight at temperature T, dry (g) initial weight of wood sample, dry (g) heat of primary pyrolysis (J g21) heat of secondary pyrolysis (J g21) heat flow corrected by baseline from a run with an empty crucible (W) calculated heat flow for the heating of char (W) heat flow due to heat radiation effects in the DSC instrument (W) heat flow induced by pyrolysis reaction (W) calculated heat flow for the heating of the pyrolysing solid sample taking into account the transition from wood to char (W) calculated heat flow for the heating of wood (W) time (s) temperature of the sample inside the DSC instrument (K) temperature, denotes the shift from endothermic to exothermic heats of pyrolysis (K) conversion of the sample related to dry sample mass (– ) final char yield (– ) weight fraction of the ith component of wood (–) apparent reaction heat of char formation (J g21) apparent reaction heat of volatiles formation (J g21)

Table 1 Heats of wood pyrolysis reported in literature, (2) exothermic heat, (þ ) endothermic heat Material

Heat of pyrolysis (J g21)

Refs.

Wood Wood Wood Wood Average of three unextracted hardwoods Average of three unextracted softwoods

235 (total) 2115 to 2280 2340 190 2254.2 (total)

[1] [4] [5] [6] [7]

2446.3 (total)

[7]

in a 5 mm diameter surface available for mass transfer to the gas flow), or using a pierced lid (thus limiting the surface available for mass transfer to a 1 mm diameter hole). To assess the influence of the purge gas flow, a further data set was produced using crucibles without lid and purge gas flow rates of 80, 160, and 240 ml min21. For each of the different flowrate values used a preliminary calibration of the DSC was performed. Simultaneous thermogravimetric (TG) and DSC data were obtained using a Netzsch STA 409/C thermoanalyzer. A typical sample weight of 5 – 15 mg was used in experimental runs. Runs were carried out using a pure nitrogen gas flow of 80 ml min21 and aluminium crucibles. 2.2. Materials Beech and spruce wood particles of size between 250 and 1000 mm were used in experimental runs. Table 2 shows the results of the proximate and ultimate analysis and the specific gravity [10] of the wood used. The particles were prepared by means of a hammer-mill and sieving. All samples were dried for 2 hours at a temperature of 378 K in a drying oven. The wood particles were pressed to compact discs (about 5 mm diameter and 1 – 3 mm height) using a hydraulic press. 2.3. Procedures At the beginning of DSC or simultaneous TG –DSC runs, the sample was positioned in the aluminium crucible. Runs were performed starting from 348 up to 773 K, using a constant heating rate of 10 K min21. At the end of each run, the furnace was cooled to ambient temperature still providing a nitrogen purge gas flow. The char formed was weighted and a second run was performed on the char sample using the same experimental conditions (temperature range and heating rate). All the DSC results reported in this study are corrected by baselines obtained from runs with empty crucibles. Table 2 Proximate analysis, ultimate analysis, and specific gravity of spruce and beech wood

Volatiles Fixed C Ash C H O N S Specific gravity ( –) [10]

Spruce wood (wt% dry)

Beech wood (wt% dry)

86.97 12.78 0.25 50.19 6.10 43.54 0.16 0.01 0.37 (softwood)

88.15 11.46 0.39 49.59 6.06 44.08 0.26 0.01 0.68 (hardwood)

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3. Results and discussion 3.1. Results of simultaneous TG –DSC runs Fig. 1 shows the typical results of simultaneous TG – DSC run performed on spruce wood in the presence and in the absence of a lid on the crucible. The TG curves in Fig. 1(a) show the well-known behaviour of wood weight loss at low heating rates: a main weight loss step occurs at temperatures between 470 and 663 K (using a heating rate of 10 K min21) due to the primary pyrolysis process. A second low-rate weight loss takes place between 663 and 773 K which is

83

possibly due to primary char aromatization and dehydrogenation reactions [7,11,12]. The maximum temperature of TG – DSC runs was limited to 773 K since negligible weight loss takes place at higher temperatures, thus suggesting that the main pyrolysis process is completed at this temperature. The comparison of the TG curves shows that in the run without a lid a lower yield of char was obtained (19.5 vs. 24.3 wt% at 773 K), even if similar initial sample weights were used (7.952 mg for the runs with lid, 8.045 mg for the run without a lid). Possibly, the absence of the lid promotes evaporation and diffusion of primary volatiles, reducing secondary char formation reactions. Fig. 1(b) shows the

Fig. 1. (a) TG, (b) dTG and (c) DSC results from TG and DSC pyrolysis experiments, spruce wood, 373–773 K, 10 K min21.

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Fig. 2. DSC pyrolysis experiments, beech wood, 373 –773 K, 10 K min21 (see Table 4 for details on sample conversion).

comparison of the differential thermogravimetric curves (dTG) obtained. The DSC curves reported in Fig. 1(c) show a peak at a temperature where also the maximum differential weight loss rate has approximately its maximum. However, the comparison of the DSC curves in the figure shows that the absence of the lid has an important effect on both the shape of the heat flow peak and on the apparent baseline of the heat flow curve. The effect on the baseline was recognized to be the effect of radiative heat exchange, due to the different emissivity of the sample with respect to the empty crucible in runs without a lid [13]. On the other hand, the different shape of the heat flow peaks is due to differences in the main pyrolysis process. It seems to be influenced by several factors: the presence or the absence of a lid on the

crucible, the initial weight of the sample, and the final char yield. These results are confirmed in Fig. 2, that shows the results of DSC runs performed with beech wood samples. In order to achieve quantitative data on the heat of reaction from the DSC curves, it was necessary to obtain reasonable reference lines. The heat flow curves obtained from DSC measurements are the sum of two components: the heat flow necessary to heat the sample and the heat of reaction. To separate the first effect from the second one, a theoretical heat flow curve for sample heating was calculated from the available specific heat and conversion data. Fig. 3 shows the mass loss curves obtained for spruce wood in simultaneous TG – DSC runs using different initial sample weights (7.95 – 31.17 mg) in the presence and in the

Fig. 3. Experimental results from pyrolysis of spruce wood and fit curve for conversion, 373–773 K, 10 K min21.

J. Rath et al. / Fuel 82 (2003) 81–91

absence of a lid. The figure also shows the dimensionless sample conversion, defined as: XðTÞ ¼

m0 2 mðTÞ m0 2 mend

ð1Þ

where m0 is the initial sample weight, mðTÞ is the sample weight at temperature T and mend is the final sample weight (mend ; mchar, the mass of char produced in the run). Although different final char yields where obtained (ranging from 19.5 to 26 wt%), the conversion data show that the dimensionless conversion is only slightly dependent on the different experimental conditions. Thus, at least as a working hypothesis, a single dimensionless conversion curve can be obtained from the data in Fig. 3. This is used to estimate the heat flow curve Qs: Qs ¼ ½ð1 2 XðTÞÞm0 cp;wood þ XðTÞmchar cp;char 

dT dt

ð2Þ

where m0 is the initial weight of the wood sample and mchar is the weight of the char sample, dT/dt is the heating rate (10 K/min) and cp is the specific heat calculated according to formulas given in literature and reported in Table 3. The Qs curve was calculated accordingly for beech wood. Qs represents the heat flow necessary to heat the sample without considering any heat of reaction. Thus, in the absence of other complicating phenomena, the heat flow due to thermal effects of the reaction may be estimated subtracting Qs from the baseline corrected experimental DSC heat flow curve. However, since the heat flow curves obtained from runs using crucibles without a lid are different from those obtained from runs using crucibles with a lid, the determination of the heat of pyrolysis from the heat flow curves is discussed separately.

85

heat radiation effects in the DSC measurement cell when a lid is not used [13]. Fig. 5(a) shows the heat flow curve due to reaction thermal effects, QR, obtained subtracting the sum of Qrad and Qs from the baseline-corrected experimental DSC heat flow curve Q. The heat flow due to reaction thermal effects obtained for the runs without a lid shows an endothermic peak at temperatures between 450 and 682 K. An exothermic peak follows, at temperatures between 682 and 773 K. It is clear from the analysis of the TG curves in Fig. 1 that the first peak corresponds to the main pyrolysis process, while the second one is due to the small weight loss step between 663 and 773 K. The presence of two separate reaction steps with different heats of reactions was reported in previous studies (e.g. see Ref. [7]). The exothermicity of secondary char graphitization was also reported in Ref. [8]. The heats of pyrolysis of the primary endothermic process, HP, and of the secondary exothermic process, HS, were calculated as: HP ¼

1 ðTshift QR dt m0 T1

HS ¼

1 ðT2 Q dt m0 Tshift R

ð5Þ

where T1 denotes the lower peak integration temperature ðT1 ¼ 450 KÞ and T2 the upper peak integration temperature ðT2 ¼ 773 KÞ: The temperature Tshift indicates the change from endothermic to exothermic behaviour of the pyrolysis phenomenon. Tshift resulted dependent on the initial sample mass and the final conversion. The values obtained from the different experiments are given in Table 4. The reaction heats HP and HS calculated with this procedure are reported in Table 4. The comparison of Tables 1 and 4 shows that the results obtained from the procedure above are well in the range of those of previous studies.

3.2. Results of DSC runs without a lid 3.3. Results of DSC runs with a lid If crucibles without a lid are used, the influence of heat radiation effects in the Mettler DSC 25 must be also considered [13]. The procedure employed in this work is based on experimental results shown in Fig. 4(a) for spruce wood. The figure shows the heat flow curves obtained for a run performed with a spruce wood sample and the residual char (see procedures in the experimental section). The figure also gives the calculated heat flows Qwood and Qchar defined as: Qwood ¼ m0 cp;wood

dT dt

ð3Þ

Qchar ¼ mchar cp;char

dT dt

ð4Þ

where the cp values are calculated ones (Table 3). The difference Qrad between the calculated and measured heat flow for the heating of char was thus calculated and is reported in the figure. This difference can be explained by

The same procedure described above was used to obtain the heat flow curve due to thermal effects of the pyrolysis reaction from DSC experimental results obtained in the presence of a pierced lid. Fig. 4(b) shows a typical result obtained from a run with spruce wood. However, in these runs, radiation heat flow effects need not to be considered. Thus, the Qrad correction is not required and only Qs was subtracted from the experimental DSC heat flow curve. A typical result for the reaction heat flow is reported in Fig. 5(b), obtained from a run with spruce wood. A striking difference is present with respect to runs without lid: in the temperature range between 527 and 602 K (at the beginning of the primary pyrolysis), the apparent reaction heat flow is exothermic, and shifts to endothermic values between 602 and 658 K. At higher temperatures, corresponding to the char stabilization step, an exothermic peak is present as in the runs without lid. Thus it seems that the presence of a lid causes an

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Fig. 4. DSC pyrolysis experiments with spruce wood and char from spruce wood, 373– 773 K, 10 K min21, Mettler DSC 25, (a) crucibles without lid, (b) crucibles with lid.

exothermic thermal effect to prevail in the initial stage of the primary pyrolysis process. Roberts [3,8] suggested that secondary reactions of non-volatile primary decomposition products are exothermic and influence the global heat of wood pyrolysis up to 593 K. Possibly, the presence of a pierced lid, as well as a larger sample size, represents a

resistance to the flow of primary volatiles from the vicinity of the reacting particles towards the bulk gas. The presence within the sample of a higher quantity of lowvolatility primary products may cause a shift to an exothermic process due to the enhancement of secondary char formation reactions at low temperatures if the lid is

Table 3 Specific heat of wood and char Substance

Specific heat cp (J kg21 K21)

Refs.

Wood

cp;wood ¼ 1113:68 þ 4:8567ðT 2 273:15Þ

[14]

2

Char cp;char

0

380

122

0

1800

122 3

380 B 1800 B e T 2 1 C 7 T 21C 8314 6 C B C 7 6e T B e ¼ @ 380 A þ2e T @ 1800 A 5 5:75 4 T T

[15]

J. Rath et al. / Fuel 82 (2003) 81–91

87

Fig. 5. Heat of reaction, pyrolysis of spruce wood, 373 –773 K, 10 K min21, Mettler DSC 25, (a) crucible without lid, (b) crucible with lid.

used. This may also justify the differences in the weight loss curve. As a matter of fact, Table 4 shows that the overall value of HP resulted endothermic in all runs performed on spruce wood, while exothermic as well as endothermic values were obtained for beech wood. On the other hand, the values for HS are not significantly different with respect to those found in runs performed without a lid. 3.4. Influence of char yield Depending on initial sample weights and on the use of a lid, different final char yields were obtained in DSC runs, as shown in Table 4 and in Fig. 3. As a general rule, it can clearly be seen that runs performed on samples with higher initial sample weight resulted in higher char yields. Experiments performed using a lid also result in higher char yields compared with those without a lid, even if samples of the same initial weight were used. Both trends indicate that a higher resistance to the flow of the volatile

pyrolysis products from the sample to the bulk gas phase leads to a higher char yield. Table 4 shows that the values of the heat of pyrolysis of the second exothermic step, HS, is only slightly dependent on the final char yield. On the other hand, pyrolysis heat during the primary pyrolysis process, HP, shows a remarkable trend with respect to the final char yield of the sample. Fig. 6 reports the calculated values of HP as a function of Yc, the final char yield. A linear trend is found for both spruce and beech wood. Furthermore, the slope of the line is almost the same for both wood samples, though the intercept is different. Since HP shifts from endothermic to exothermic values, it is suggested that competitive exothermic and endothermic processes are present. The results obtained seem to confirm the findings of Mok and Antal [16]. These authors reported a linear dependency of the heat of pyrolysis on the final char yield in cellulose pyrolysis experiments at different pressures, and suggested that this was the result of the different conditions used in the experiments. Furthermore,

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Table 4 Heats of pyrolysis for spruce and beech wood, 373–773 K, 10 K min21, purge gas flow rate (nitrogen) 300 ml min21, Mettler DSC 25 Run

A B R C D E F G H I a

Type of wood

Spruce Spruce Spruce Spruce Spruce Spruce Beech Beech Beech Beech

Lid on crucible

No No No Yes Yes Yes No No Yes Yes

Initial weight (mg)

5.321 8.670 6.584 5.716 5.830 11.526 7.715 7.834 7.631 7.768

Final weight (mg)

0.958 1.691 1.220 1.275 1.347 2.824 1.373 1.387 1.725 1.919

Final char yield Yc (–)

0.180 0.195 0.186 0.223 0.231 0.245 0.178 0.177 0.226 0.247

Tshift (K)

682 668 672 663 660 658 656 652 (654)a (654)a

Heat of pyrolysis (J g21) HP

HS

total

387.3 241.5 315.0 162.1 70.6 41.9 145.3 147.8 285.6 2156.1

223.8 228.9 225.2 241.9 262.6 260.8 217.3 231.7 249.6 265.7

363.5 212.6 289.8 120.2 8.0 218.9 128.0 116.1 2135.2 2221.8

Estimated on the basis of the dTG results.

they found that the primary char formation process in cellulose pyrolysis is exothermic. These results are even more important if the variability of the final char yield is considered. As shown in Table 4, if the same experimental conditions are used the char yield increases as the initial sample weight is increased. Nevertheless, limited variations are present in the final char yield (less than ^ 1%) even for samples of the same initial weight. The limited reproducibility of the final char yield in pyrolysis processes is well known in the literature and is generally attributed to inhomogeneities of the material and of the sample structure [17]. However, in spite of these effects, the heat of pyrolysis data calculated for these experimental runs well agree with the proposed correlation of HP with respect to the final char yield, further confirming its validity and possibly explaining another reason for the high uncertainty of the heat of pyrolysis data reported in the literature.

3.5. Influence of gas flow rate To confirm the above findings, the influence of the gas flow rate on the pyrolysis heat of reaction was assessed performing experiments using spruce wood with crucibles without lid. Higher gas flow rates should enhance the mass transfer of primary pyrolysis products to the gas phase, thus contributing to a limitation of secondary reactions. Experimental runs were performed at flowrates of 80, 160, and 240 ml min21 (at 25 8C), while data in Table 4 were obtained using a flowrate of 300 ml min21, being the highest value allowable in the DSC device used. Table 5 shows the data obtained for the heat of the primary pyrolysis process, HP. No clear trend of HP was found with respect to the gas flow rate. However, in Fig. 7 the char yield is reported as a function of the initial sample mass and of the flow rate. The figure points out that, as expected, higher char yields are obtained when lower gas flow rates and samples

Fig. 6. Heat of pyrolysis for spruce and beech wood, dependency on the final char yield (bold solid lines indicate linear regression fits for the heat of primary pyrolysis HP, thin solid line indicates linear regression fit for the heat of secondary pyrolysis HS).

J. Rath et al. / Fuel 82 (2003) 81–91

89

Table 5 Heat of primary pyrolysis for spruce wood resulting from runs performed using different purge gas flowrates (nitrogen), all crucibles without lid, 373– 773 K, 10 K min21, Mettler DSC 25 Run

Flowrate (ml min21)

Initial weight (mg)

Final weight (mg)

Final char yield Yc (–)

Heat of pyrolysis HP (J g21)

J K L M N O P Q

80 80 160 160 160 240 240 240

7.047 6.251 5.779 7.158 5.731 7.370 6.465 5.227

1.474 1.285 1.134 1.420 1.124 1.435 1.232 0.949

0.2092 0.2056 0.1962 0.1984 0.1961 0.1947 0.1906 0.1816

240.7 279.5 276.5 264.3 275.8 247.9 309.4 343.0

of higher initial weight are used. As a matter of fact, reporting the data of Table 5 in Fig. 6, the primary pyrolysis heat shows the same trend as a function of final char yield, in spite of the different gas flow rates used in the experimental runs. 3.6. Accuracy of the measurements Although the main aim of the present study is to understand the underlying factors that determine the apparent values of the heat of pyrolysis of wood and not to assess specific values, it is important to estimate the accuracy of the heat of pyrolysis data obtained. The value of the secondary reaction heat HS resulted only weakly dependent on the final char yield, thus it was possible to estimate the uncertainty of the results obtained. The mean error for the HS values obtained is of ^ 32%. However, even if the relative error for HS is large, the absolute error is small, being of ^ 13 J g21. Errors are further reduced if data obtained for single wood types are considered: e.g. relative error is of ^ 20% for spruce wood,

and absolute error is of ^ 8 J g21. Clearly the slight dependence of HS on char yield shown in Fig. 6 can not be taken into account in these calculations, and this justifies the high values of relative error reported. Even more difficulties are present in the estimation of the errors in the measured HP values. Since these value show a high dependence on char yield and char yield shows an important dependence on experimental conditions but also on the characteristics of the sample used, it was not possible to draw general figures for the relative error of the measurement. However, some specific observations were possible: e.g. in runs F and G (see Table 4), performed in the same experimental conditions and resulting in almost the same final char yield, the difference in the estimated HP values is of less than 1.7%. Also the results obtained in runs L and N (see Table 5) show that under same experimental conditions nearly equal heats of pyrolysis were measured (HP: 276.5 J g21 for run L and 275.8 J g21 for run N, with a difference of 0.3%). However in runs C and D (see Table 4) the initial sample weights used were nearly the same (run C: 5.716 mg and run D: 5.830 mg) but the char yields were

Fig. 7. Final char yield as a function of initial sample weight and of purge gas flowrate, pyrolysis of spruce wood, 373–773 K, 10 K min21, Mettler DSC 25, all crucibles without lid.

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J. Rath et al. / Fuel 82 (2003) 81–91

slightly different (run C: 0.223 and run D: 0.231). Since a different final char yield was obtained, as expected a different pyrolysis heat was found (HP: 162.1 J g21 for run C and 70.6 J g21 for run D, HS: 2 41.9 J g21 for run C and 2 62.6 J g21 for run D), due to the correlation between these two quantities demonstrated in the present study. Nevertheless, as shown in Fig. 6, nearly all the results obtained fall within a 95% confidence interval when the dependence on the final char yield is considered, thus confirming the validity of the experimental results obtained.

lumped kinetic analysis of wood pyrolysis, it was never applied to the interpretation of reaction heat data. Obviously this is an oversimplified reaction scheme, the actual wood pyrolysis process being much more complex. However, since it proved to be useful in order to analyse kinetic data, it is interesting to apply the above model to reaction heat data. From the data in Fig. 6, DH1 and DH2, the apparent heats of the two lumped reaction pathways of primary pyrolysis can be determined:

3.7. Heat of the primary pyrolysis process

HP ¼ DH1 Yc þ DH2 ð1 2 Yc Þ

Several authors (e.g. see Refs. [11,18]) proposed to describe wood pyrolysis as a sum of independent reactions of the main wood macrocomponents (hemicellulose, cellulose and lignin). For each macrocomponent, a simplified lumped kinetic scheme of two competitive reactions was proposed to interpret the kinetic data: ð6Þ

where i indicates the wood macrocomponent considered (hemicellulose, cellulose and lignin). DHi,1 and DHi,2 denote the apparent reaction heats of the two lumped decomposition pathways of the component. This scheme was originally proposed by Kilzer and Broido [19] for cellulose, but is now widely used also for the other wood components [18]. Thus wood pyrolysis may be represented with the following lumped reaction scheme: ð7Þ

In Scheme (7), wood may be considered as the sum of the three different macrocomponents: WOOD ¼

3 X

Ci ¼

i¼1

3 X

ð8Þ

ai WOOD

CHAR ¼

chari ;

GAS ¼

i¼1

VOLATILES ¼

n X

gasi ;

i¼1 n X

ð9Þ

volatilesi

i¼1

and consequently: DH1 ¼

n X i¼1

In Eq. (11) Yc should be the char yield at the end of the primary pyrolysis process. However, this value was not available for all the runs reported in Fig. 6. Thus, the final char yield reported in Table 4 was used for data analysis. The results obtained for the heats of reaction are: (a) DH1 ¼ 2 3827 J g21 and DH2 ¼ 1277 J g21 for spruce wood; and (b) DH1 ¼ 2 3525 J g21 and DH2 ¼ 936 J g21 for beech wood. The validity of these results is limited to the heating rates and sample sizes used in the present work. Obviously, due to the assumptions used in the model and in data analysis, these are only rough estimates of the reaction heats of char and volatile formation processes. However, the simple model used suggests the contemporary presence of endothermic and exothermic competitive processes in primary wood pyrolysis. As evident from the data, the char formation process resulted exothermic, while the volatile formation process resulted endothermic for both wood samples. The different values of the apparent heat of reactions DH1 and DH2 for the two woods may arise from the different macrocomponent composition. This results in different values of the ai coefficients in Eq. (10). The competition of endothermic and exothermic processes may explain the presence of endothermic as well as exothermic global heat flows obtained from DSC measurements during primary pyrolysis (see the results in Fig. 5 at temperatures below Tshift).

i¼1

where ai is the weight fraction of the ith macrocomponent of wood. Thus: n X

ð11Þ

ai DHi;1 ;

DH2 ¼

n X

ai DHi;2

ð10Þ

i¼1

While this approach was widely recognized in the

4. Conclusions The heat of reaction of the wood pyrolysis process was investigated. Two reaction steps were identified, the first being the primary pyrolysis process and the second attributable to further reactions of the primary char. The heat of reaction of this second step was exothermic and almost independent of experimental conditions and char yield. On the other hand, wide variations were found for the heat of the primary pyrolysis process depending on the initial sample weight and on the crucibles used in the measurements. For both types of wood investigated, an increase in the final char yield resulted in a decrease of the total endothermic heat of pyrolysis, and

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eventually in a shift from an endothermic to an exothermic process. These results may be easily understood assuming that wood pyrolysis may be described by a competitive lumped reaction model, as it is widely used for kinetic data analysis. The variability of the heat of reaction can be explained if an exothermic primary char formation process competitive with an endothermic volatile formation process is hypotized. In this framework, the sensitivity of the overall apparent heat of reaction to the final char yield can explain the uncertainty of the data reported in the literature.

Acknowledgments ¨ AD (Austrian Scientific This work was supported by O Exchange Service) and by the Italian Ministry of Foreign Affairs in the framework of a bilateral corporation between Austria and Italy.

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