Page 187 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

164 Biomass Gasification, Pyrolysis and Torrefaction

TABLE 5.3 Rate Constants for Pyrolysis of Cellulose According to
Broido Shafizadeh Model
21
3
Reaction ðdm=dtÞ 5 A i ðV 2 V i Þe 2E i =RT A i (s ) E i (kJ/mol)
i
I—First degradation (active cellulose), Bradbury et al. (1979) 2.8 3 10 19 243
II—Dehydration (char 1 gas), Bradbury et al. (1979) 1.31 3 10 10 153
III—Depolymerization (tars), Bradbury et al. (1979) 3.16 3 10 14 198
IV—Secondary cracking (gas, char), Uden et al. (1988) 4.28 3 10 6 107.5

great commercial importance. For cellulose pyrolysis, Table 5.3 gives some
suggested reaction rate constants for reactions I, II, III, and IV.
If a log of wood is heated very slowly, it shows glowing ignition, because
reaction II predominates under this condition, producing mostly char, which
ignites in contact with air without a yellow flame. If the wood is heated
faster, it burns with a yellow flame, because at a higher heating rate, reaction
III predominates, producing more vapors or tar, both of which burn in air
with a bright yellow flame.

5.4.2.2 Hemicellulose
Hemicellulose produces not only more gas and less tar but also less char in
comparison to cellulose. However, it produces the same amount of aqueous
product of pyroligneous acid (Soltes and Elder, 1981, p. 84). Hemicellulose
undergoes rapid thermal decomposition (Demirbas, 2000), which starts at
a temperature lower than that for cellulose or lignin. It contains more com-
bined moisture than lignin, and its softening point is lower as well. The exo-
thermic peak of hemicellulose appears at a temperature lower than that for
lignin (Demirbas, 2000). In slow pyrolysis of wood, hemicellulose pyrolysis
begins at 130 194 C, with most of the decomposition occurring above 180 C

(Mohan et al., 2006, p. 126).
5.4.2.3 Lignin
Pyrolysis of lignin typically produces about 55% char (Soltes and Elder,
1981), 15% tar, 20% aqueous components (pyroligneous acid), and about
12% gases. It is more difficult to dehydrate lignin than cellulose or hemicel-
lulose (Mohan, 2006, p. 127). The tar produced from it contains a mixture of
phenolic compounds, one of which, phenol, is an important raw material of
green resin (a resin produced from biomass). The aqueous portion comprises
methanol, acetic acid, acetone, and water. The decomposition of lignin in

wood can begin at 280 C, continuing to 450 500 C and can reach a peak

rate at 350 450 C (Kudo and Yoshida, 1957).