Although I cover some technical subjects, I try to keep this blog as accessible as possible to everyone. Where it’s not really practical to avoid using terms that not everyone will know, I’ll include a short introduction to the term here, and link to it from relevant posts.
Biomethane is biogas that has been upgraded to a methane (CH4) content of >95%.
Biogas is gas produced from the rotting of biomass in the absence of oxygen, and it consists of typically around 60-65% methane, with the rest of the gas being CO2 plus a few impurities. The largest source of biogas in the UK presently is capped landfill sites, followed by sewage plants.
More and more biogas is coming from purpose built Anaerobic Digestion (AD) plants. These plants may be built to take wastes, such as farm wastes or food wastes, or the feedstock may be purpose grown crops such as maize (this is very common in Germany for example).
Most biogas is burned directly in generators using an internal combustion engine, to generate electricity. However, the gas can also be upgraded to biomethane using a variety of technologies to remove the CO2 and impurities. Biomethane can be injected into the gas grid, or liquefied for easy transportation.
Biomethane offers large greenhouse gas savings compared to the fossil fuels it can replace. Upgrading biogas uses some energy, reducing the overall GHG saving, as does compressing or liquefying the gas, and transporting it. If the feedstock is a purpose grown crop, then this will also have an energy requirement, and may cause indirect land use change (ILUC). Thus GHG savings of 60% versus fossil fuels are typical.
In some cases the GHG savings from using biomethane may be over 100%. This can occur when biomethane is made using wastes that would otherwise have rotted in the open air (e.g. some farm wastes) releasing methane direct to the atmosphere. Methane is a GHG with around 20x the warming potential of carbon dioxide, so capturing methane that would have gone to the atmosphere, and burning it to produce CO2, actually reduces each carbon molecule’s warming effect.