Producer gas in power generation
Practical Action
Air gasification, although producing gas with the lowest calorific value (typically 4.2 MJ/m3),
is a much more widely used technology than other gasification techniques. This is because
oxygen production and usage incurs an extra cost and hazard and also involves the
complexity of having multiple reactors.
A producer gas unit consists of a number of components:
Gasifier
The gasifier is usually cylindrical in shape for strength and ease of manufacture. Fuel is fed
by gravity from the upper hopper section into the lower fire zone section. There are three
basic kinds of generator, named from the airflow direction. Down-draught units have a
restriction around the hearth causing an increase in air velocity and higher operating
temperature which tends to gasify tars which would otherwise be a problem when using green
wood as fuel. Updraught gasifiers are efficient producers of industrial type heating gas where
higher tar contents are no problem.
Cross-draught gasifiers have a small intensely hot fire zone with air fed from a nozzle or
tuyere. The refuelling lid and ash removal door must be air tight, and gas is sucked out by
the engine through a lateral delivery pipe. With a restricted fire zone, it is not necessary to
protect the casing with refractory or stainless steel lining, but it is essential to use evenly
sized well packed fuel if gasification is to be efficient. A fourth type is a combination of the
above.
Cleaning section
On leaving the gasifier, producer gas must be cleaned of impurities such as soot, ash,
unburnt fuel dust and tar in order to prevent engine damage. The design and sequence of
components will depend upon requirements of fuel and engine, but must not offer excessive
resistance to the gas flow if engine starvation is to be avoided.
Primary removal of coarse material may be effected by a simple expansion box, a baffle box
or a cyclone, in which particles drop out of the gas stream when the gas changes direction
and loses velocity.
Secondary cleaning, which may follow the cooling section, may be a dry or wet process. The
gas may be filtered dry through sisal, wood, wool, felt, paper, fabric, or close-set brush
bristles, and electrostatic filters are also on the market. In wet filtration, the filter element is
wetted with oil (usually diesel or kerosene) or water. Water is not very effective against tar but
may reduce the incidence of explosions. Wet filtering is also called scrubbing or washing.
The choice of filters should not be made on technical efficiency alone but rather on possible
operational reliability. Experience with agricultural tractors has indicated that for example,
although modern paper air filter elements may be capable of high efficiency, the odd hole
may cause serious and costly breakdowns in equipment and operations under practical third
world conditions.
Cooling section
Cooling the gas will increase its density, so allowing a greater charge (by mass) per cylinder,
and exposed piping when cooled will become less of a fire hazard. After the primary cleaning
unit, the gas may then be passed through a radiator unit mounted at the front of the engine.
The cooler may be by-passed by means of butterfly valve for starting from cold if
condensation is found to clog dry filters. A fine wire gauze or felt cloth placed before the
engine as a "security filter" will clog up and provide warning of failure of the cleaning-cooling
sections when the engine stalls.
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