Energy Fuels 2009, 23, 5765–5772 Published on Web 09/16/2009
: DOI:10.1021/ef900674b
Analysis of Cross-Linking Behavior during Pyrolysis of Cellulose for Elucidating Reaction Pathway Weerawut Chaiwat, Isao Hasegawa, Takaaki Tani, Kenshi Sunagawa, and Kazuhiro Mae* Department of Chemical Engineering, Kyoto University, Nishikyo-ku, Kyoto, 615-8510, Japan Received July 1, 2009. Revised Manuscript Received September 1, 2009
The analyses of structure change during pyrolysis of cellulose at different heating rates were investigated to clarify the existent behavior of the cross-linking reaction. For structural analyses of cellulose precursors, FTIR spectra and XRD patterns confirmed that the dehydration reaction to produce water and cross-liked precursor simultaneously occurred with the glycosidic reaction to produce tar during pyrolysis. On the basis of the assumptions that hydroxyl groups in cellulose converted to water, a new parameter index, so-called the degree of dehydration during pyrolysis, Xp, including its distribution in char (Xc) and tar (Xt), was determined. For the cross-linking analyses at low heating rate, cross-linking reaction may occur until approximately 360 °C where Xc reached its maximum, while the cross-linked tar seems to be released above 360 °C. For tar analyses, cross-linked dimer can be observed for slow pyrolysis, whereas tar products obtained by flash pyrolysis showed relatively higher yield in cellobiosan with no cross-linked volatiles. Finally, based on previous kinetic models, the modified pathway of cellulose pyrolysis was proposed by considering the proposed parameter related to the dehydration.
cellulose pyrolysis and confirm its reaction pathway under various pyrolysis conditions.6-14 Briefly, from previous various kinetic points of view, the mechanism of cellulose pyrolysis can be generally recognized by at least two main competitive pathways: dehydration, that is, cross-linking reactions; and depolymerization, that is, the splitting of glycosidic bonds. The effects of pyrolysis conditions on product yields and distribution can be explained by these two competitive pathways.15 However, Mamleev et al. showed the contradictory discussion on the possibility of dehydration of cellulose inside the solid matrix and the existence of active cellulose as anhydrocellulose generally mentioned in the B-S model. They concluded that neither considerable dehydration nor other reaction of the β-elimination seems to occur inside the cellulose matrix. The dehydration seems to possibly occur only when the ring undergoes the breakage. The two-phase model was finally proposed to explain all observable phenomena related to cellulose pyrolysis and oxidative decomposition.16-19 Throughout this contradictory discussion, the
Introduction Biomass has recently been considered as a renewable resource due to its environmentally benign characteristics. Cellulose, a major component in biomass, is often considered for investigations of pyrolysis behavior because of its simple structure. Since pyrolysis is a promising thermo-chemical method for energy production and consists of complex reactions, basic knowledge of kinetic behavior including its mechanism is needed for process development and reactor design. Antal and V arhegyi summarized various kinetic models of cellulose pyrolysis proposed by several research groups. A Brodio-Shafizadeh (B-S) Model, which shows the competitive formation of tar and char via the formation of the so-called anhydrocellulose or active cellulose intermediate, can achieve a good fit when thermal pretreatment at low temperature is applied.1 The B-S model, therefore, was occasionally modified by neglecting the formation of active cellulose in practical pyrolysis/gasification that occurred at high temperature.2-5 Moreover, the kinetics of secondary reactions, time profiles of gas evolution, and the approximation of intrinsic kinetics with several kinetic models have been investigated by many researchers to modify the model of
(8) Bradbury, A. G. W.; Sakai, Y.; Shafizadeh, F. J. Appl. Polym. Sci. 1979, 23, 3271–3280. (9) Blasi, C. D. J. Anal. Appl. Pyrolysis 1998, 47, 43–64. (10) Shafizadeh, F. J. Anal. Appl. Pyrolysis 1982, 3, 283–305. (11) Banyasz, J. L.; Li, S.; Lyons-Hart, J.; Shafer, K. H. Fuel 2001, 80, 1757–1763. (12) Luo, Z.; Wang, S.; Liao, Y.; Cen, K. Ind. Eng. Chem. Res. 2004, 43, 5605–5610. (13) Julien, S.; Chornet, E.; Tiwari, P. K.; Overend, R. P. J. Anal. Appl. Pyrolysis 1991, 19, 81–104. (14) Yamaguchi, Y; Fushimi, C; Tasaka, K; Furusawa, T; Tsutsumi, A. Energ Fuels 2006, 20, 2681–2685. (15) Klass, D. L. Biomass for Renewable Energy, Fuels, and Chemicals; Academic Press: San Diego, CA, 1998; pp 225-269. (16) Mamleev, V.; Bourbigot, S.; Bras, S.; Yvon, J; Lefebvre, J. Chem. Eng. Sci. 2006, 61, 1276–1292. (17) Mamleev, V.; Bourbigot, S.; Yvon, J. J. Anal. Appl. Pyrolysis 2007, 80, 151–165. (18) Mamleev, V.; Bourbigot, S.; Yvon, J. J. Anal. Appl. Pyrolysis 2007, 80, 141–150. (19) Mamleev, V.; Bourbigot, S.; Bras, M. L.; Yvon, J J. Anal. Appl. Pyrolysis 2009, 84, 1–17.
*To whom correspondence should be addressed.
[email protected]. ac.jp. (1) Antal, M. J., Jr.; Varhegyi, G. Ind. Eng. Chem. Res. 1995, 34, 703– 717. (2) V arhegyi, G.; Antal, M. J., Jr.; Jakab, E.; Szabo, P. J. Anal. Appl. Pyrolysis 1997, 42, 73–87. (3) V arhegyi, G.; Jakab, E.; Antal, M. J., Jr. Energ Fuels 1994, 8, 1345–1352. (4) Hajallgol, M. R.; Howard, J. B.; Longwell, J. P. Ind. Eng. Chem. Process Des. Dev. 1982, 21, 457–465. (5) Milosavljevic, I.; Suuberg, E. M. Ind. Eng. Chem. Res. 1995, 34, 1081–1091. (6) Lanzetta, M.; Blasi, C. D.; Buonanno, F. Ind. Eng. Chem. Res. 1997, 36, 542–552. (7) Broido, A.; Weinstein, M. Proceeding of the 3rd International Conference on Thermal Analysis; Wiedemann, Ed.: Birkhauser, Basel, 1971; pp 285-296. r 2009 American Chemical Society
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analyses of cellulose structure changed during pyrolysis; therefore, it should be more investigated to clarify the existent behavior of the cross-linking reaction. Structural analysis of pyrolyzed precursors, particularly in terms of cross-linking behavior, has been less investigated in literature for products estimation and mechanism clarification of cellulose, biomass, or even coal pyrolysis.20 Recently, our research group has focused on the examination of cross-linking behavior during pyrolysis and pretreatment of cellulose at low temperature by developing a new kinetic model and proposing new parameter indexes, so-called the degree of cross-linking.21-24 On the basis of the discussions on previous kinetic models and the investigations of cross-linking behavior with structural analysis, the generalized pyrolysis pathway of cellulose, then, should be further modified to be applicable and describable for all pyrolysis conditions with different temperature and/or heating rates. In this work, pyrolysis behavior of cellulose was preliminarily investigated using a thermogravimetric method with different heating rates. The structural properties of cellulose pyrolyzed at low temperature region were further studied using FTIR and XRD analysis. Dehydration reaction with cross-linking behavior, then, was examined with a new proposed parameter index, so-called the degree of dehydration during cellulose pyrolysis. The effect of the parameter on related structure properties was also investigated. Moreover, the distribution of the parameter in char and tar was analyzed to clearly understand its mechanism. Finally, the pyrolysis mechanism of two different heating rates was simply proposed by using the change of cellulose structure with pyrolysis temperature. The pyrolysis pathway based on the dehydration parameter including its cross-linking behavior was also modified from previous schematic models.
Figure 1. TG curves of cellulose samples pyrolyzed at various heating rates.
(CPP, JHP-2S). Samples of 0.5-1.0 mg were weighed and wrapped in metal foils of different Curie points to indicate their pyrolysis temperatures. Gaseous products were continuously analyzed using a gas chromatograph (GC; Shimadzu, GC-14A) directly connected to the CPP. Since the amount of pyrolyzed char obtained from TGA is too small for analysis, pyrolysis using a quartz tube reactor was carried out to obtain an adequate amount of pyrolyzed char for structural analysis. A cellulose sample of 50-100 mg in a platinum tray was placed at the middle of the reactor contacted to a temperature probe to control a temperature distribution. The sample was pyrolyzed in nitrogen with the flow rate of 100 cc 3 min-1 to various pyrolysis temperatures at a heating rate of 5, 10, 20 K 3 min-1 after holding at 110 °C for 30 min to evaporate the moisture in the samples. It was, then, immediately cooled down to room temperature after keeping at pyrolysis temperature for 10 min. Gaseous products were continuously analyzed by micro gas chromatography (VARIAN, CP-4900) connected to the reactor. Analysis of Pyrolyzed Precursors and Tar Products. The pyrolyzed samples were dried in vacuo at 70 °C for 24 h before analysis using an elemental combustion system (Costech Instruments Co., ECS-4010) for ultimate analysis, Fourier transform infrared spectroscopy (FTIR; Jeol, JIR-SPX60) for determination of functional groups in solid structure, particularly OH group; and X-ray diffraction (XRD; Rigaku, MultiFlex) for crystallinity examination of pyrolyzed cellulose. For tar analysis, condensed tar products, which adhered at the inner tube wall of the reactor outlet, were washed and collected with acetone. Acetone was used as the mobile phase and was fed at 0.5 mL 3 min-1 to a gel permeation chromatograph (GPC) equipped with a column (Showa Denko, Asahipak GF-310HQ) and a RI detector, to estimate the composition of tar products.
Experimental Section Samples. Filter paper cellulose powder (Advantec, 40-100 mesh) was used as the cellulose sample. It is a crystallized polymer that has a degree of polymerization of approximately 5000. Raw sample was dried in vacuo at 70 °C for 24 h prior to use. The elemental composition of filter paper cellulose powder consists of 42.9 wt % of carbon, 5.9 wt % of hydrogen, and 51.2 wt % of oxygen. Pyrolysis at Various Heating Rates and Temperatures. For preliminary investigation, pyrolysis at various heating rates (5, 10, 20, and 50 K 3 min-1) by using thermo gravimetric analyzer (Shimadzu, TGA-50) was conducted to study its effect on pyrolysis behavior. Raw biomass samples that weighed about 1.5-3.0 mg were heated from room temperature to 110 °C under inert atmosphere (N2), then the temperature was kept for 30 min to vaporize the moisture in samples before further heated with the same heating rate to 800 °C. The samples were, then, completely combusted with air at 800 °C to determine their carbon conversion and char yield. Flash pyrolysis at a heating rate of 3000 K 3 s-1 of cellulose sample was performed at different temperatures (280, 386, 423, 445, 485, and 590 °C) for comparison with TGA results, using a Curie-point pyrolyzer
Results and Discussion Preliminary Investigation of Cellulose Pyrolysis and Precursor Structure. Thermogravimetric analysis (TGA) and Curie-point pyrolyzer (CPP) was used to preliminarily investigate the effect of heating rate on the kinetic behavior of cellulose pyrolysis as shown in Figure 1. For slow pyrolysis carried out with TGA, the beginning temperature of cellulose decomposition were gradually shifted to lower temperature from approximately 350 to 300 °C when the heating rate decreased from 50 to 5 K 3 min-1. For pyrolysis with 5 K 3 min-1, the decomposition of cellulose was completely proceeded before reaching 400 °C, while it almost reached 450 °C for that of 50 K 3 min-1. This indicates that the pyrolysis with lower heating rate which caused longer reaction period required lower temperature to complete cellulose decomposition via glycosidic reaction. Compared to TG curves at heating rates of 5-50 K 3 min-1, flash pyrolysis with CPP at various temperature was also conducted. The decomposition curve showed nearly no difference compared to that of
(20) Solomon, P. R.; Serio, M. A.; Despande, G. V.; Kroo, E. Energ Fuels 1990, 4, 42–54. (21) Mae, K.; Maki, T.; Miura, K. J. Chem. Eng. Jpn. 2002, 35, 778– 785. (22) Sunagawa, K.; Hasegawa, I.; Mae, K. Science in Thermal and Chemical Biomass Conversion; CPL Press: UK, 2006(2); pp 1125-1135. (23) Ohmukai, Y.; Fujimoto, K.; Hasegawa, I.; Hayashi, S.; Mae, K. J. Chem. Eng. Jpn. 2008, 41, 312–318. (24) Chaiwat, W.; Hasegawa, I.; Kori, J.; Mae, K. Ind. Eng. Chem. Res. 2008, 47, 5948–5956.
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Figure 3. FTIR Spectra of cellulose pyrolyzed to different temperatures at various heating rates: (a) 5 K 3 min-1, (b) 10 K 3 min-1, and (c) 3000 K 3 s-1 (CPP). Figure 2. Gas formation rates and products yield distribution during pyrolysis of cellulose at various heating rates: (a) 5 K 3 min-1, (b) 10 K 3 min-1, and (c) 3000 K 3 s-1 (CPP).
region, the analyses of FTIR and XRD were then conducted in order to compare the structure of cellulose residues pyrolyzed at different heating rates. For FTIR spectra in Figure 3, absorptions between 3000 and 3600 cm-1 are typically ascribed to hydroxyl groups (OH) or adsorbed water. The spectra of OH peak intensity gradually decreased when the samples were treated to higher temperature. The OH peak intensity in Figure 3a was extremely dropped between the samples pyrolyzed at 5 K 3 min-1 to 320 and 340 °C. This sudden decrease in peak intensities was also shown between the samples pyrolyzed in a flash mode at 445 and 485 °C as shown in Figure 3c. However, the spectra of the residues at higher temperature then showed almost the same low intensities of the OH absorptions for all heating rates. This indicates that the dehydration to produce water such as cross-linking reaction was promoted during pyrolysis at low temperature region, which is in agreement with the results of thermogravimetric analysis. Through the analysis of solid crystallinity, XRD patterns of the samples treated with inert gas and air showed the peak intensities at 2θ = 20-25° as shown in Figure 4. The peak intensities were continuously decreased with increasing temperature. This indicates that the crystallinity of pyrolyzed residues gradually decreased, and then the sample structure became near amorphous at 340 and 485 °C for the samples pyrolyzed at low heating rate (Figure 4a) and in a flash mode (Figure 4c), respectively, because the glycosidic bonds may be randomly decomposed to produce tar when heating to higher temperature. Analysis of Cross-Linking Behavior and Dehydration with New Parameter Index. According to the results of products distribution and OH loss in FTIR spectra, it has been
50 K 3 min-1, but char yield obtained at high temperature was slightly lower from those obtained from slow pyrolysis. This indicates that the pyrolysis with the heating rate of 50 K 3 min-1 can be considered as nearly in flash mode because reaction time was too short for decomposition at lower temperature. Since the char yield obtained at lower heating rate was relatively higher than those of rapid heating rate, we may preliminarily conclude that the cross-linking reaction to release water and form char would occur during pyrolysis, especially at low heating rate. To investigate product gases released during pyrolysis at each pyrolysis temperature, micro gas chromatography was connected to the reactor for gas measurement. Figure 2 showed the evolving rate of gaseous products (left axis) and accumulated products yield (right axis) along with pyrolysis temperature. The results in Figure 2 showed that pyrolysis started to proceed from approximately 300 °C. Pyrolysis also showed the highest evolving rate of products at around 360 °C to produce mainly water and tar. Gaseous products, CO and CO2, were slightly released during pyrolysis, whereas hydrocarbon gases were hardly produced. When pyrolysis completely occurred above 400 °C, the final yields of 90 wt % of tar and 7-8 wt % of H2O were obtained from pyrolysis at low heating rates. This agrees that two competitive reactions during pyrolysis are glycosidic cleavage to release tar and cross-linking reaction to produce water. Since TG results showed the change of pyrolysis behavior due to its decomposition reaction during low temperature 5767
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Figure 6. The relationships between the degree of dehydration and pyrolysis temperature at different heating rates.
must be developed by further reasonable assumptions involving the amount of H2O formation. Each cellulose monomer has three OH groups in the structure. It indicates that 1.5 molecules of water (MW = 18 g 3 mol-1) are formed per one cellulose monomer (MW = 162 g 3 mol-1) when all hydroxyl groups are consumed by dehydration. Then, the maximum degree of cross-linking in the structure that can be possibly achieved when the highest yield of water was obtained by dehydration at 16.7 wt %. Since the cross-linking may not be always formed in the solid structure when water molecule was released during the dehydration, eq 1 would be modified to eq 2 for the definition of the degree of dehydration during pyrolysis, designated as Xp, instead of the degree of cross-linking. Yield of H2 O produced during pyrolysis Xp ¼ ð2Þ 16:7 wt %
Figure 4. XRD patterns of cellulose pyrolyzed to different temperatures at various heating rates: (a) 5 K 3 min-1, (b) 10 K 3 min-1, and (c) 3000 K 3 s-1 (CPP).
Through the determination of Xp, the relationships with dependent parameters have been further investigated. The parameter Xp was plotted with temperature at different heating rates as shown in Figure 6. It showed that the increasing rate of Xp was relatively higher, and its peak was shifted to lower temperature when decreasing the heating rate. Moreover, Xp at final char yield was relatively higher at 0.48 with lower heating rate at 5 K 3 min-1, while it decreased to 0.27 for flash pyrolysis at 3,000 K 3 s-1. The results confirmed that dehydration with cross-linking reaction to form char would proceed relatively more for slow pyrolysis. Figure 7a showed the relationship between Xp and the dimensionless ratio of the amount of hydroxyl groups in cellulose structure, OH/OH0, at different heating rates. The amount of hydroxyl groups, OH, for each pyrolysis temperature can be calculated from integration area of the decreased OH peak obtained from FTIR spectra. Meanwhile, OH0 shows the amount of OH groups obtained from raw cellulose sample. For OH/OH0 values more than about 0.6 at each heating rate, the values of OH/OH0 decreased with an increase in Xp. This agreed with above results that cross-linking formed to produce water was caused by the OH loss in cellulose structure. When Xp then reached the final value for each heating rate, OH/OH0 immediately decreased to zero at the final Xp. This indicates that OH groups in solid structure rapidly disappeared because glycosidic cleavage seemed to subsequently proceed to release tar containing OH groups at higher temperature. We, therefore, can understand that tar products released at higher temperature may contain hydroxyl groups which are different from tar obtained at lower temperature. Figure 7b showed the relationship
Figure 5. The relationship between O/C and H/C of cellulose char pyrolyzed to 300-700 °C.
confirmed that water was mainly produced from OH groups in cellulose structure during pyrolysis at 300-600 °C. Moreover, the relationship of O/C and H/C as shown in Figure 5, which was obtained by elemental analysis of pyrolyzed cellulose during 300-700 °C, also showed that dehydration to produce water seemed to mainly occur due to the slope nearly equal to 1:2 (O:H). Consequently, based on the assumptions: (i) one molecule of H2O can be produced from two molecules of OH group in cellulose structure; and (ii) H2O molecule produced by dehydration forms cross-linking in cellulose structure, the degree of cross-linking in cellulose structure during pyrolysis can be described as eq 1. Degree of cross-linking in cellulose structure ¼
Amount of cross-linking formed in cellulose during pyrolysis Amount of all OH groups in cellulose before pyrolysis 0:5
ð1Þ However, the amount of cross-linking in the structure is difficult to be exactly determined. Equation 1), therefore, 5768
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The parameter Xc can be approximately calculated from the disappearance of OH amount in solid char, which can be determined from the integral area of its characteristic peak obtained by FTIR. The amount ratio of OH loss in cellulose, which was pyrolyzed to each different temperature, can be quantitatively determined as (OH0 - OH)/OH0 or 1 OH/OH0. Since Xc is a specific parameter, which is based on the yield of released water as Xp, and also dependent on its char yield (1- x) at each pyrolysis temperature, the parameter index, Xc, then, can be determined by steps as described in eqs 4 and 5, respectively. According to the determination of Xc, it shows that the dehydration in char simultaneously forms cross-linking in the structure, the parameter Xc, therefore, can be also named as the degree of cross-linking. Xc The yield of water released to form cross-linking in pyrolyzed char ¼ 16:7 wt %
ð4Þ Xc ¼
ð1 - OH=OH0 Þ 0:167 ð1 - xÞ 0:167
¼ ð1 - OH=OH0 Þ ð1 - xÞ
However, the disappearance of OH in solid precursor can be indeed explained via two major reactions: (i) the dehydration to release water (from two OH groups) and create crosslinking in solid precursor, and (ii) the glycosidic cleavage to release noncross-linked tar which contains OH groups in the structure. Since Xc considered the amount of cross-linking only in solid char, the yield of char (1 - x) was multiplied with the weight loss of OH amount to accurately specify Xc as shown in eq 5. Moreover, the relationship between (1 - OH/ OH0) and (1 - x) was plotted in Figure 8 to confirm the validity of Xc. The graph shows that (1 - OH/OH0) has a linearly inverse variation with (1 - x), which can be written as eq 6. The constant variable in eq 6 can be considered as Xc described by multiplying (1 - OH/OH0) with (1 - x) as eq 5. This reasonably confirms that Xc determined by eq 5 can be properly represented as the degree of cross-linking formed in pyrolyzed char. 1 ð6Þ ð1 - OH=OH0 Þ µ ð1 - xÞ
Figure 7. Relationships between the Xp and (a) OH/OH0, (b) φ/φ0.
between Xp and the dimensionless ratio of cellulose crystalliinity, φ/φ0, at different heating rates, where φ0 shows the crystallinity of raw cellulose. The crystallinity of cellulose can be determined by the peak obtained from XRD patterns in Figure 4. It showed that the crystallinity of cellulose structure rapidly decreased when Xp increased, particularly, it became amorphous at Xp less than 0.1 for the sample pyrolyzed at 5 K 3 min-1. For flash pyrolysis, the crystallinity decreased and kept nearly constant at φ/φ0 = 0.4, then rapidly decreased to zero at Xp = 0.25. This can be concluded that the crystallinity of cellulose decreased more rapidly when it was pyrolyzed at lower heating rate because of not only the random cleavage of glycosidic bonds, but also the effect of cross-linking formation. Distribution of Xp in Char and Tar. Since the degree of dehydration during pyrolysis, Xp, shown in eq 2 indicates some extent of cross-linked structures formed during pyrolysis, it is necessary to deeply analyze the ratio of cross-linking structure distributed in two major products, char and tar, for clarification of pyrolysis mechanism. The parameter, Xp, therefore, can be preliminarily described by the summation of the degree of dehydration in char, Xc, and the degree of dehydration in volatile tar, Xt, as shown in eq 3. Xp ¼ Xc þ Xt
ð5Þ
In Figure 9, the relationships between Xp, which is distributed in char and tar as Xc and Xt, respectively, and the pyrolysis temperature for each different heating rate were plotted to further investigate the behavior of dehydration during pyrolysis of cellulose. For heating rate at 5 K 3 min-1, Xc showed its maximum (Xc-max) at approximately 350 °C, while it was obtained at higher temperature around 370 °C when heating at 20 K 3 min-1, as shown in Figure 9 parts a and b, respectively. Then, Xc contrastingly decreased to rather the same final value less than 0.1, which has an agreement with the final yield of char after pyrolysis to higher temperature. This indicates that, for pyrolysis with lower heating rate, the depolymerization via glycosidic cleavage to produce cross-linked tar may possibly start at relatively lower temperature because of its adequate accumulated heat. For the degree of dehydration in tar, the final Xt obtained at 5 K 3 min-1 was obviously higher than that obtained at 20 K 3 min-1. This shows that the rate of cross-linking
ð3Þ 5769
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Figure 8. The relationship between OH loss, (1 - OH/OH0), and char yield, (1 - x).
Figure 10. GPC chromatogram of tar obtained from pyrolysis at different heating rates.
Figure 11. The yield distribution of each tar product obtained during pyrolysis with different heating rates at each cellulose conversion: (a) x = 0.6, (b) x = 0.8, and (c) x = 0.95 at Xc-final.
cross-linking in volatile tar and/or the water release via the fragmentation of intermediates to produce other light gases. When focusing on flash pyrolysis by CPP, cross-linking reaction hardly occurred because Xc-max showed almost the same as Xc-final as shown in Figure 9c. This shows that it hardly produce cross-linked volatile in flash pyrolysis. Consequently, the parameter Xt obtained in Figure 9c would probably show the water release by fragmentation to produce light gases as indicated by Xt . Moreover, this agrees with the results of tar analysis with GPC method as shown in Figure 10, which showed only noncross-linked tar such as cellobiosan and levoglucosan, while the intension peak of cross-linked dimer can be observed only for the slow pyrolysis at the heating rate of 5 and 20 K 3 min-1. For further tar analysis, the yield distribution of each tar product obtained during pyrolysis, based on overall tar yield and the integral area of GPC peaks, were compared with different heating rates at each cellulose conversion as shown 0
Figure 9. Distributions of the degree of dehydration in char and tar products during pyrolysis: (a) 5 K 3 min-1, (b) 10 K 3 min-1, and (c) 3000 K 3 s-1 (CPP).
reaction for lower heating rate may be relatively higher. The cross-linking reaction seems to simultaneously occur with glycosidic cleavage to produce cross-linked tar with Xt-CL = Xc-max - Xc-final, while the rest of Xt, designated as Xt , may show the secondary dehydration to form intramolecular 0
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Figure 12. Structural mechanism diagram of cellulose pyrolysis with different heating rates.
Figure 13. Proposed pyrolysis pathway of cellulose based on dehydration and cross-linking behavior.
in Figure 11. At the same conversion, the total yield of tar obtained at higher heating rate was relatively lower, particularly at the conversion less than 0.8 as obviously shown in Figure 11 parts a and b. This is because the temperature was shifted to higher temperature for rapid pyrolysis, in which it could produce more volatile gases. On the other hand, only little amount of gases can be released by slow pyrolysis where it reached the same conversion at lower temperature. The gap of total tar yield with different heating rates was closer at higher conversion, particularly at x = 0.95 in Figure 11c, because the temperature of pyrolysis (500 °C) was high enough to release all gas products. The results confirmed previous studies that the release of gas products would be preferable at high temperature, especially with higher heating rate. Focusing on the distribution of each major tar product, particularly at Xc-final in Figure 11c, cellobiosan was preferable to be produced with more rapid heating rate, particularly in flash pyrolysis, while levoglucosan expressed the opposite tendency. Slow pyrolysis showed the existence of cross-linked dimer, while no cross-linked products were obtained by flash pyrolysis. This indicates that
the reducing-end of active oligomer in the partly crosslinked precursor could simply form levoglucosan in slow pyrolysis, while cellobiosan was produced relatively much more in flash pyrolysis, especially at high temperature, because of the random glycosidic cleavage in noncrosslinked precursor. Structural Mechanism and Modified Pathway of Cellulose Pyrolysis. Through above structural analyses of cellulose pyrolysis, particularly with the new dehydration index, the structural diagram of cellulose pyrolysis mechanism with temperature can be illustrated by comparing the pyrolysis at low and rapid heating rate as shown in Figure 12. It can be summarized that pyrolysis of cellulose may start with the cross-linking reaction to produce water at approximately 300 °C, while the depolymerization may also simultaneously occur to produce tar. Xc-max obtained from slow pyrolysis was relatively higher at lower temperature around 360 °C, which leads to relatively higher char yield at final temperature. For tar products, cross-linked dimer can be observed for slow pyrolysis, while high yield of cellobiosan can be obtained without cross-linked dimer in flash pyrolysis. 5771
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Through the proposed cross-linking parameter, pyrolysis pathway of cellulose, can be modified from the models proposed by previous researchers, particularly based on the discussion and contradiction on the B-S model and the two-phase model proposed by Mamleev et al.,16-19 as shown in Figure 13. Raw cellulose would be gradually changed their structure via two simultaneous competitive reactions, dehydration and glycosidic cleavage to form a socalled “partly cross-linked precursors” in solid phase. The precursor may be simultaneously cross-linked with an unstable intermediate, so-called active oligomer, which can be cleaved from the cellulose precursor during transition state. This leads to the release of water via dehydration until reaching Xc-max. The cross-linked precursor could be partly cleaved to form active cross-linked oligomer and finally became cross-linked dimer with the so-called Xt-CL, equal to Xc-max - Xc-final, while the remaining eventually turned into char product with Xc-final via char formation. For the rest of Xt, designated as Xt , the dehydration could proceed during the transition state to finally form other light gases such as CO2 via the active cross-linked oligomer. The cleaved noncross-linked (NCL) active oligomer could be rapidly produce cellulose monomer and oligomer such as levoglucosan and cellobiosan via unzipping transglycosylation. For the explanation of Xt with different heating rates, the mechanism of flash pyrolysis would pass through only active NCL oligomer to form NCL monomer. The water release via dehydration during flash pyrolysis may be caused by only the fragmentation to simultaneously form other light gases. For slow pyrolysis, aside from NCL monomer and cross-linked dimer in tar, intramolecular cross-linking may be also formed during transition state to produce cyclic compounds. Consequently, the proposed pathway as shown in Figure 13 can reasonably explain the simultaneous reactions occurred during cellulose pyrolysis under various conditions.
Conclusions The analyses of structure change during cellulose pyrolysis were investigated to clarify the existent behavior of crosslinking reaction. Cellulose pyrolysis at different heating rates was performed to investigate the relationships between pyrolysis behavior and formation of cross-linking in cellulose structure. With lower heating rate, the decomposition of cellulose during pyrolysis was relatively shifted to lower temperature with an increase in degree of dehydration because of the longer reaction period. For structural analyses of pyrolyzed cellulose precursors, FTIR spectra and XRD patterns also confirmed that dehydration to produce water and some extent of cross-liked precursor simultaneously occurred with the glycosidic reaction to produce tar during pyrolysis. Based on the assumptions that hydroxyl groups in cellulose converted to water, a new parameter index, so-called the degree of dehydration during pyrolysis, Xp, including its distribution in char (Xc) and tar (Xt), was proposed. For the cross-linking analysis at low heating rate, cross-linking reaction may occur until approximately 360 °C when Xc reached its maximum, while the cross-linked tar seems to be released above 360 °C. For tar products, cross-linked dimer can be observed for slow pyrolysis, while high yield of cellobiosan can be obtained without cross-linked dimer in Flash pyrolysis. Since flash pyrolysis hardly produced cross-linked volatile, the parameter Xt, therefore, would mostly show the water release by fragmentation to produce light gases as indicated by Xt . Through structural analyses and previous kinetic models, the modified pathway of cellulose pyrolysis can be described with the proposed parameter of dehydration.
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Acknowledgment. This work was financially supported by the Ministry of Education, Science, Sports and Culture of Japan through the Grant-in-Aid for Scientific Research (A) (Grant No. 19206083).
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