Pyrolysis - Biomass Gasification, Pyrolysis and Torrefaction - page 162

Temperature has a major influence on the product of pyrolysis. The
carbon dioxide yield is high at lower temperatures and decreases at higher
temperatures. The release of hydrocarbon gases peaks at around 450 C and
then starts decreasing above 500 C, boosting the generation of hydrogen.
Hot char particles can catalyze the primary cracking of the vapor released
within the biomass particle and the secondary cracking occurring outside the
particle but inside the reactor. To avoid cracking of condensable gases and
thereby increasing the liquid yield, rapid removal of the condensable vapor
is very important. The shorter the residence time of the condensable gas in
the reactor, the less the secondary cracking and hence the higher the liquid
yield.
Some overlap of the stages in the pyrolysis process is natural. For example,
owing to its low thermal conductivity (0.1

0.05 W/m K), a large log of wood
may be burning outside while the interior may still be in the drying stage, and
water may be squeezed out from the ends. During a forest fire, this phenome-
non is often observed. The observed intense flame comes primarily from the
combustion of the pyrolysis products released from the wood interior rather
than from the burning of the exterior surface.

5.4.2 Chemical Aspects
As mentioned earlier, a typical biomass has three main polymeric compo-
nents: (i) cellulose, (ii) hemicellulose, and (iii) lignin. These constituents
have different rates of degradation and preferred temperature ranges of
decomposition.

5.4.2.1 Cellulose
Decomposition of cellulose is a complex multistage process. A large number
of models have been proposed to explain it. The Broido

Shafizadeh model
(Bradbury et al., 1979) is the best known and can be applied, at least qualita-
tively, to most biomass (Bridgwater et al., 2001).
Figure 5.9 is a schematic of the Broido Shafizadeh model, according to

which the pyrolysis process involves an intermediate prereaction (I) followed
by two competing first-order reactions:

Reaction II: dehydration (dominates at low temperature and slow heating
rates)
Reaction III: depolymerization (dominates at fast heating rates).

Reaction II involves dehydration, decarboxylation, and carbonization
through a sequence of steps to produce char and noncondensable gases
like water vapor, carbon dioxide, and carbon monoxide. It is favored at low
temperatures, of less than 300 C (Soltes and Elder, 1981, p. 82) and slow
heating rates (Reed, 2002, p. II-113).