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272 CHAPTER NINE
fuel comes from a sustainable source). Co-firing may require wood fuel preparation or
boiler modifications to maintain boiler efficiency.
9.2.2 Gasification
Gasification is a high temperature process which produces gas that can then be used in an
internal combustion engine or fuel cell.
Briefly, gasification was used as long ago as the early 1800s. The process was rather
crude and the fuel was most often coal. The gas product (town gas, Chap. 5) was used for
heating and lighting. For example, the gas was piped to street lights as early as 1846 in
England.
The gasification process produces a fuel gas from crops by heating them under carefully
controlled temperature, pressure, and atmospheric conditions. A key to gasification is using
less air or oxygen than is usually found in combustor. The product (biogas or fuel gas), like
natural gas, can be burned in high-efficiency gas turbines.
The gasifier is the heart of the gasification process (Chaps. 5 and 7). Gasifiers are
designed to process the fuel in a variety of ways consistent with the type of fuel, the end
use of the gas, the size of the process, and the source of oxygen. The oxygen may be intro-
duced as a pure gas or may come from air or steam. Some gasifiers operate under pressure,
others do not.
The simplest type of gasifier is the fixed bed counter current gasifier. The biomass is fed
at the top of the reactor and moves downward as a result of the conversion of the biomass
and the removal of ashes. The air intake is at the bottom and the gas leaves at the top. The
biomass moves in counter current to the gas flow, and passes through the drying zone, the
distillation zone, reduction zone, and the oxidation zone.
The major advantages of this type of gasifier are its simplicity, high charcoal burn-out,
and internal heat exchange leading to low gas exit temperatures and high gasification effi-
ciency. In this way also fuels with high moisture content (up to 50 percent by weight) can
be used. Major drawbacks are the high amounts of tar and pyrolysis products, because the
pyrolysis gas is not lead through the oxidation zone. This is of minor importance if the gas
is used for direct heat applications, in which the tars are simply burnt. In case the gas is used
for engines, gas cleaning is required, resulting in problems of tar-containing condensates.
In the conventional downdraft gasifier (sometime called the co-flow gasifier), biomass
is fed at the top of the reactor and air intake is at the top or from the sides. The gas leaves at
the bottom of the reactor, so the fuel and the gas move in the same direction. The pyrolysis
gasses are lead through the oxidation zone (with high temperatures) and or more or less burnt
or cracked. Therefore the producer gas has low tar content and is suitable for engine applica-
tions. In practice however, a tar-free gas is seldom if ever achieved over the whole operating
range of the equipment. Because of the lower level of organic components in the condensate,
downdraft gasifiers suffer less from environmental objections than updraft gasifiers.
Successful operation of a downdraft gasifier requires drying the biomass fuel to a mois-
ture content of less than 20 percent. The advantage of the downdraft design is the very low
tar content of the producer gas. However, disadvantages of the downdraft gasifier are: (a) the
high amounts of ash and dust particles in the gas, (b) the inability to operate on a number
of unprocessed fuels, often pelletization or briquetting of the biomass is necessary, (c) the
outlet gas has a high temperature leading to a lower gasification efficiency, and (d) the
moisture content of the biomass must be less than 25 percent by weight.
A more recent development is the open core gasifier design for gasification of small sized
biomass with high-ash content. The producer gas is not tar-free; it contains approximately
0.05 kg tar per kilogram of gas. In the open core gasifier the air is sucked over the whole cross
section from the top of the bed. This facilitates better oxygen distribution since the oxygen