Page 323 - Synthetic Fuels Handbook

FUELS FROM WOOD 309

is 70 to 80 percent of the energy originally stored in the biomass. The producer gas can
serve in different ways: it can be burned directly to produce heat or can be used as a fuel
for gas engines and gas turbines to generate electricity; in addition, it can also be used as a
feedstock (syngas) in the production of chemicals, for example, methanol. The diversified
applications of the producer gas make the gasification technology very attractive.
A variety of biomass gasifiers have been developed and can be grouped into four major
classes: (a) fixed bed updraft or countercurrent gasifier, (b) fixed bed downdraft or cocur-
rent gasifier, (c) bubbling fluidized bed gasifier, and (d) circulating fluidized bed gasifier.
Differentiation is based on the means of supporting the biomass in the reactor vessel, the
direction of flow of both the biomass and oxidant, and the way heat is supplied to the reac-
tor. The processes occurring in any gasifier include drying, pyrolysis, reduction, and oxida-
tion. The unique feature of the updraft gasifier is the sequential occurrence of the chemical
processes: they are separated in relative position in the gasifier and therefore by time.
As an illustration, using the updraft gasifier (Fig. 10.2), biomass and air are fed in an opposite
direction. In the highest zone, biomass is heated up and releases its moisture. In the pyrolysis
zone, biomass undergoes a further increase in temperature and decomposes into hydrocarbons,
gas products, and char in the temperature range of 150 to 500°C. The major reactions are:
Biomass → C H + C H O + H O + CO + CO + H 2
y
x
y
z
x
2
2
Fuel
Gas

Drying

Pyrolysis

Reduction

Oxidation

Air
FIGURE 10.2 An updraft gasifier.