Alternative Fuel Sources - Hydrogen and Fuel Cells: Advances in Transportation and Power - page 107

Digestion may continue for about 10 days to a few weeks. The bacterial
action generates heat but in cold climates additional heat is normally re-
quired to maintain a process temperature of about 35°C. A digester can
produce 400 cubic meters of biogas with a methane content of 50% to 75%
for each dry ton of input. This is about two thirds of the fuel energy of the
original fuelstock. The effluent which remains when digestion is complete
also has value as a fertilizer.
FUEL FROM WASTES
A large part of municipal solid wastes (MSW), is biological material.
Its disposal in deep landfills furnishes suitable conditions for anaerobic
digestion. The methane that is produced was first viewed as a possible
hazard and this led to systems for burning it off. In the 1970s some use
was made of this product. The waste matter is miscellaneous in a landfill
compared to a digester and the conditions not as warm or wet, so the pro-
cess is much slower, taking place over years instead of weeks. The product
is called landfill gas (LFG) and is a mixture consisting mainly of CH 4 and
CO 2 .
A typical site may produce up to 300 cubic meters of gas per ton of
wastes with about 55% by volume of methane. In a developed site, the
area is covered with a layer of clay or similar material after it is filled, pro-
ducing an environment to encourage anaerobic digestion. The gas is col-
lected by pipes buried at depths up to 20 meters in the refuse. In a large
landfill there can be several miles of pipes with as much as 1000 cubic
meters an hour of gas being pumped out. The gas from landfill sites can
be used for power generation. Some plants use large internal combustion
engines, standard marine engines, driving 500-kW generators but gas tur-
bines could provide improved efficiencies.
The fuel gas from biomass gasifiers can be used to operate gas tur-
bines for local power generation. A gas-turbine power station is similar to
a steam plant except that instead of using heat from the burning fuel to
produce steam to drive the turbine, it is driven directly by the hot combus-
tion gases. Increasing the temperature in this way improves the thermo-
dynamic efficiency, but in order not to corrode or foul the turbine blades
the gases must be very clean which is why many gas-turbine plants use
natural gas.
One biomass conversion plant converts wood chips into a methane