Page 252 - Synthetic Fuels Handbook

238 CHAPTER EIGHT

endothermic (Brown et al., 1952; Roberts and Clough, 1963; Demirbas and Kucuk, 1994),
but lignin pyrolysis is exothermic (Demirbas et al., 1996).
In fact, the pyrolysis of biomass is more complex than originally believed and the gen-
eral changes that occur during biomass pyrolysis are (Babu and Chaurasia, 2003; Mohan
et al., 2006):
1. Heat transfer from a heat source, to increase the temperature inside the fuel.
2. The initiation of primary pyrolysis reactions at this higher temperature releases volatiles
and forms char.
3. The flow of hot volatiles toward cooler solids results in heat transfer between hot vola-
tiles and cooler unpyrolyzed fuel.
4. Condensation of some of the volatiles in the cooler parts of the fuel, followed by second-
ary reactions, can produce tar.
5. Autocatalytic secondary pyrolysis reactions proceed while primary pyrolytic reactions
simultaneously occur in competition.
6. Further thermal decomposition, reforming, water gas shift reactions, radicals recombi-
nation, and dehydrations can also occur, which are a function of the process’s residence
time/temperature/pressure profile.
A process has been investigated for the saccharification of wood, involving prehy-
drolysis, lignocellulose pyrolysis, and tar hydrolysis. In this process, ground wood was
first prehydrolyzed to remove the more readily hydrolyzable hemicelluloses. The residual
lignocellulose was then pyrolyzed rapidly to provide a tar-containing levoglucosan and its
condensation products. The destructive reaction of cellulose commences at temperatures on
the order 52°C and is characterized by a decreasing polymerization. Thermal degradation
of cellulose proceeds through two types of reaction: a gradual degradation, decomposition
and charring on heating at lower temperatures, and a rapid volatilization accompanied by
the formation of levoglucosan on pyrolysis at higher temperatures. The glucose chains in
cellulose are first cleaved to glucose and from this, in a second stage, glucosan is formed by
the splitting off of one molecule of water. Since cellulose and levoglucosan have the same
elementary formula, C H O , a yield of 100 percent of the latter might be expected. The
10
6
5
initial degradation reactions include depolymerization, hydrolysis, oxidation, dehydration,
and decarboxylation (Shafizadeh and Stevenson, 1982).
Hemicellulose species react more readily than cellulose during heating (Cohen et al.,
2002). The thermal degradation of hemicellulose begins above 100°C during heating for
48 hours; hemicelluloses and lignin are depolymerized by steaming at high temperature for
a short time (Shafizadeh et al., 1976).
Lignin is broken down by extensive cleavage of b-aryl ether linkages during steaming
of wood under 215°C. It has been found that on analysis of the metoxyl groups after iso-
thermal heating of dry distilled wood, lignin decomposition begins at approximately 280°C
with a maximum rate occurring between 350 and 450°C and the completion of the reaction
occurs at 450 and 500°C (Sandermann and Augustin, 1963).
The formation of char from lignin under mild reaction conditions is a result of the
breaking of the relatively weak bonds, like the alkyl-aryl ether bond(s), and the consequent
formation of more resistant condensed structures, as has already been noted (Domburg et al.,
1974). One additional parameter which may also have an effect on the char formation is the
moisture content of the kraft lignin used.
Water is formed by dehydration. In the pyrolysis reactions, methanol arises from the
breakdown of methyl esters and/or ethers from decomposition of pectin-like plant materials
(Goldstein, 1981). Methanol also arises from methoxyl groups of uronic acid (Demirbas
and Gullu, 1998). Acetic acid is formed in the thermal decomposition of all three main