Landfill gas is extracted from the landfill by exerting a small negative suction pressure on boreholes driven into the waste after landfilling, or through wells built up progressively while the waste is being deposited.

A network of landfill gas extraction pipework is laid, and built into the capping of the landfill, through which the gas is extracted. All these pipes lead to a "manifold" and connect to the landfill gas compound.

Great care is needed to avoid the presence of low points where the build-up of water in the pipework can easily form a blockage to air flow at these low suction pressures. At low points, which must be designed into the system at suitable intervals, so called "knock-out pots" are installed.

These allow condensation to drain out of the pipework through a pre-primed water trap, without allowing pipe suction pressure to be lost through this water drainage point.

The blower

The suction is carried out by what is usually called a "blower", as it "blows" the gas into the power generation plant, at the same time developing suction at the inlet. The blower is located in the Landfill Gas Compound, and will usually be set such that it may either deliver gas into the generation set, or into a flare, or both.

Only the landfill gas which is of adequate quality (ie that it contains enough methane to burn satisfactorily) is fed into the generator.

The generator

The generator is usually based upon an internal combustion engine similar to a diesel engine. In fact the first landfill gas generating sets were just modified Caterpillar Diesel sets, with coupled dynamo (electric power generator winding) units. (However, turbines can also be used to burn the gas and hence to provide the motive force for electrical generation. One early set in the 1980s used an old English Electric jet fighter turbine and it worked very well - barring some initial corrosion problems.)

The electric power produced is fed into the local power grid through sophisticated (and expensive) equipment which synchronises the alternating current generated to match exactly with the with the phase and frequency of the grid.

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