Page 161 - Synthetic Fuels Handbook

FUELS FROM COAL 147

insufficient to produce complete combustion and the presence of water as steam favors the
reactions that yield fuel-gas. However, to raise the efficiency of a power plant, commercial
gasifiers must recover the heat released during gasification. Most designs do this by cooling
the gasifier with water, to produce steam and fuel-gas. The fuel-gas itself may have to be
cooled and reheating reduces the overall efficiency of the plant.
The countercurrent fixed bed (up draft) gasifier consists of a fixed bed of carbonaceous
fuel (e.g., coal or biomass) through which the “gasification agent” (steam, oxygen, and/or
air) flows in countercurrent configuration. The ash is either removed dry or as a slag. The
slagging gasifiers require a higher ratio of steam and oxygen to carbon in order to reach
temperatures higher than the ash fusion temperature. The nature of the gasifier means that
the fuel must have high mechanical strength and must be noncaking so that it will form
a permeable bed, although recent developments have reduced these restrictions to some
extent. The throughput for this type of gasifier is relatively low. Thermal efficiency is high
as the gas exit temperatures are relatively low. However, this means that tar and methane
production is significant at typical operation temperatures, so product gas must be exten-
sively cleaned before use or recycled to the reactor.
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
efficiency. 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 pyrol-
ysis gas is not led 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.
The cocurrent fixed bed (down draft) gasifier is similar to the countercurrent type, but
the gasification agent gas flows in cocurrent configuration with the fuel (downward, hence
the name down draft gasifier). Heat needs to be added to the upper part of the bed, either
by combusting small amounts of the fuel or from external heat sources. The produced gas
leaves the gasifier at a high temperature, and most of this heat is often transferred to the
gasification agent added in the top of the bed, resulting in energy efficiency on level with
the countercurrent type. Since all tars must pass through a hot bed of char in this configura-
tion, tar levels are much lower than the countercurrent type.
Drawbacks 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, (d) the moisture content of the biomass must be less than
25 percent to maintain the high temperature and the mineral content (ash yield) should also
be low and nonslagging, and (e) the feed must have uniform particle size.
In the fluid bed gasifier, the fuel is fluidized in oxygen (or air) and steam. The ash is
removed dry or as heavy agglomerates that defluidize. The temperatures are relatively low
in dry ash gasifiers, so the fuel must be highly reactive; low-grade coals are particularly
suitable. Fluidized bed reactors feature extremely good mixing with good heat and mass
transfer. Gasification is efficient and typically exceeds 90 percent of the feedstock, often
falling in 95 to 99 percent range of carbon being converted. Ash is carried with gas and
separated from the gas in cyclones.
The agglomerating gasifiers have slightly higher temperatures, and are suitable
for higher rank coals. Fuel throughput is higher than for the fixed bed, but not as high
as for the entrained flow gasifier. The conversion efficiency is rather low, so recycle
or subsequent combustion of solids is necessary to increase conversion. Fluidized
bed gasifiers are most useful for fuels that form highly corrosive ash that would
damage the walls of slagging gasifiers. Biomass generally contains high levels of
ash-forming constituents.