In this section we discuss landfill gas extraction systems and how the gas collected is used to create renewable energy and then becomes even more sustainable when the waste heat is then used in combined heat & power (CHP) system.
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 as Used in Landfill Gas Extraction Systems
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 as Used in Landfill Gas Extraction Systems
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.
Other uses of Landfill Gas
If an existing suitable high consumption rate gas energy user is located close to the landfill, it may be more cost effective to pipe the landfill gas to the factory and burn the gas where the heat is needed in order to produce a product, than to supply the national grid. In such cases this directly replaces natural gas (fossil fuel) as the gas source, with a sustainable and lower cost alternative.
On the continent the waste heat generated during power generation may used in CHP (Combined Heat and Power) schemes, also known as “district heating”. In these projects the low grade heat energy produced in the cooling jacket of the generator is used to heat homes and factories.
Suitably lagged and heat insulated pipes are laid in a town or district and the unwanted hot water is pumped from the waste-to-energy plant into the system. It is then a simple matter to direct the flow through radiators to the heat homes and factories connected into the system.
Until 2008 there were almost no active district heating scheme CHP schemes in operation in the UK. The best known district heating scheme in the UK for many years was the Byker District Heating Scheme in Newcastle-upon-Tyne, UK, and another in Sheffield, UK. (However, neither use landfill gas as the heat source.)
After 2008 some UK CHP schemes have been started, but most are related to Anaerobic Digestion Plant CHP and none are known to us that are from landfill gas EfW installations.
Additional equipment of course is provided for burning excess gas (gas flares), and for use during periods when (often partly due to atmospheric pressure changes), the quality of the gas at some landfills is not suitable (too low in methane) for flaring. Many sites have equipment to remove water from the gas, and reduce the very high degree of water saturation in the gas, before it is burnt.
All landfill gas generation systems are subject to a high corrosion potential due to the presence of warm to hot gas and air (oxygen) with corrosive impurities. In recent years the industry has done much to improve corrosion resistance, such that the best landfill gas generation plant rarely suffers down time due to this cause.
Some landfill sites yield useable landfill gas for no more than 5 years, while others have been generating for nearly 15 years and show little evidence of reducing gas flows.