Biogas is formed by anaerobic digestion, that is, the microbial degradation of organic material such as farm wastes or energy crops. This process is sometimes referred to as ‘biomethanation’.
Biogas is mainly composed of methane and carbon dioxide, but may also contain small amounts of nitrogen, hydrogen and carbon monoxide, as well water vapour and contaminants such as hydrogen sulphide and siloxanes.
After a relatively striaght-forward clean-up process, biogas (sometimes called biomethane or renewable natural gas (RNG)) can be used as a fuel. Biogas has been used as a fuel for many centuries, and in the UK, the city of Exeter used biogas for street lighting as early as 1895.
The production of biogas not only provides a ‘renewable’ source of energy, it also offers a use for waste products that might otherwise be difficult to dispose of.
The waste products from biogas generation iteslf are water and fermented organic material which can be used as a high-quality fertiliser.
Typically, anaerobic digestion requires a heat source, and so biogas is often used to fuel combined heat and power (CHP) plant that produces both electricity and heat. The heat can be used for the anaerobic digestion process and can also be used to pasteurise animal-derived waste so that it can be used as fertilisers.
Combined heat and power plant can feed electricity into the national grid, and so biogas installations may qualify for payments under the renewable heat incentive (RHI) scheme (other than biogas from landfill).
Biogas can also be cleaned of carbon dioxide and fed into the national grid.
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