Page 191 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

168 Biomass Gasification, Pyrolysis and Torrefaction

temperature, heat consumed by endothermic reactions, and heat losses from
the reactor. In most cases, it is necessary to burn the noncondensable gases
and the char produced to provide the heat required. If that is not adequate,
other heat sources are necessary to supply the energy required for pyrolysis.
The following section discusses the heat requirement of reactions taking
place in a pyrolyzer.
The dehydration (reaction II) process is exothermic, while depolymeriza-
tion (reaction III) and secondary cracking (reaction IV) are endothermic
(Bridgwater et al., 2001). Among reactions between intermediate products
of pyrolysis, some are exothermic and some are endothermic. In general,
pyrolysis of hemicellulose and lignin is exothermic. Cellulose pyrolysis is

endothermic at lower temperatures (,400 450 C), and it becomes exother-
mic at higher temperatures owing to the following exothermic reactions
(Klass, 1998):
CO 1 3H 2 -CH 4 1 H 2 O 226 kJ=gmol (5.6)

CO 1 2H 2 -CH 3 OH 105 kJ=gmol (5.7)

0:17C 6 H 10 O 5 -C 1 0:85H 2 O 80 kJ=gmol (5.8)
CO 1 H 2 O-CO 2 1 H 2 42 kJ=gmol (5.9)
(All equations refer to a temperature of 1000 K, and C 6 H 10 O 5 represents
the cellulose monomer.)
For this reason a properly designed system initially requires external heat
only until the required temperature is reached.
Char production from cellulose (Eq. (5.8)) is slightly exothermic.
However, at a higher temperature, when sufficient hydrogen is produced by
reaction (Eq. (5.9)), other exothermic reactions (Eqs. (5.6 and 5.7)) can
proceed. At low temperatures and short residence times of volatiles, only
endothermic primary reactions are active (heat of reaction 2225 kJ/kg),
while at high temperatures exothermic secondary reactions (heat of reaction
20 kJ/kg) are active (Blasi, 1993).
In conclusion, for design purposes, one may neglect the heat of reaction
for the pyrolysis process, but it is necessary to calculate the energy required
for vaporization of products and for heating feedstock gases to the pyrolysis
temperature (Boukis et al., 2007).

5.6 PYROLYZER TYPES
Pyrolyzers have been used since ancient times to produce charcoal (Figure 5.2).
Early pyrolyzers operated in batch mode using a very slow rate of heating and
for long periods of reaction to maximize the production of char. If the objective
of pyrolysis was to produce the maximum amount of liquid or gas, then the
rate of heating, the peak pyrolysis temperature, and the duration of pyrolysis