Page 182 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 5 Pyrolysis 159

Cellulose decomposes over a narrow temperature range of 300 400 C
(Figure 4.5A). In the absence of any catalyst, pure cellulose pyrolyzes pre-
dominantly to a monomer, levoglucosan (Diebold and Bridgwater, 1997).
Above 500 C, the levoglucosan vaporizes, with negligible char formation,

thus contributing mainly to gas and oil yields. Hemicelluloses are the least-
stable components of wood, perhaps because of their lack of crystallinity
(Reed, 2002, p. II-102). It decomposes within 200 300 C (Figure 4.5A).

Unlike cellulose, lignin decomposes over a broader temperature range of

280 500 C, with the maximum release rate occurring at 350 450 C (Kudo

and Yoshida, 1957). Lignin pyrolysis produces more aromatics and char than
produced by cellulose (Soltes and Elder, 1981). It yields about 40% of its
weight as char under a slow heating rate at 400 C (Klass, 1998). Lignin

makes some contribution to the liquid yield (B35%), which contains aqueous
components and tar. It yields phenols via cleavage of ether and carbon carbon
linkages (Mohan et al., 2006). The gaseous product of lignin pyrolysis is only
about 10% of its original weight.

5.3.2 Effect of Pyrolysis Temperature
During pyrolysis, a fuel particle is heated at a defined rate from the ambient
to a maximum temperature, known as the pyrolysis temperature. The fuel is
held there until completion of the process. The pyrolysis temperature affects
both composition and yield of the product. Figure 5.6 is an example of how,
during the pyrolysis of a biomass, the release of various product gases
changes with different temperatures. We can see that the release rates vary
widely for different gaseous constituents.
The amount of char produced also depends on the pyrolysis temperature.
Low temperatures result in greater amount of char; high temperatures result
in less. Figure 5.8 shows how the amount of solid char produced from the
pyrolysis of a grape bagasse decreases with increasing temperature, but the
heating value of the char increases with temperature. This happens because
the fixed carbon, which has a higher heating value, in the char increases
while the volatile content of the char decreases. The amount of noncondens-
able gas (CO 2 , CO, H 2 ,CH 4 ) increases with temperature.

5.3.3 Effect of Heating Rate
The rate of heating of the biomass particles has an important influence on the
yield and composition of the product. Rapid heating to a moderate temperature

(400 600 C) yields higher condensable volatiles and hence more liquid,
while slower heating to that temperature produces more char. For example,
Debdoubi et al. (2006) observed that during pyrolysis of Esparto, when the
heating rate increased from 50 C/min to 250 C/min, the liquid yield

increased from 45% to 68.5% at a pyrolysis temperature of 550 C.