Biomethane (also known as ‘green gas’) can be produced from a number of sources including biogas from anaerobic digestion, landfill gas and synthetic gas (‘syngas’) from the gasification of biomass. All these gases can be converted to biomethane by removing the CO2. Biomethane is a gas mixture that is predominantly methane (>97%). It has similar thermal characteristics to natural gas. Subject to meeting gas quality requirements biomethane is considered as pipeline quality gas and can be injected into the natural gas network and used in existing gas appliances. The raw gas is upgraded to pipeline quality by adding propane to increase the calorific value (CV), removing water vapour to safeguard pipelines and adding odorant for safety.
In the past landfill gas and biogas have been used to generate electricity, supported by the Renewables Obligation (RO). With a demanding EU target for renewable heat as well as renewable electricity and transport fuel, UK producers are now starting to convert the gas into biomethane and inject it directly into the gas distribution network. Gas injected in this way displaces fossil-derived natural gas giving savings in greenhouse gas emissions.
Anaerobic digestion (AD) is a natural biological process carried out by bacteria in the absence of air, by which organic material is broken down into stable fertiliser and useful biogas. These anaerobic bacteria are an integral component of nature’s waste management and are commonly found in soils and deep waters, as well as in landfill sites.
Biogas is the combustible gas created by anaerobic digestion. It is composed of approximately 60% methane (CH4), 40% carbon dioxide (CO2), and other trace levels of contaminants. The organic material can be from sources such as food waste, agricultural activities, domestic or industrial waste water treatment, and municipal solid waste.
Landfill gas production results from chemical reactions and microbes acting upon the waste in the landfill. The rate of production is affected by waste composition and the landfill geometry, which in turn influence the bacterial populations within the landfill, its chemical make-up, its thermal characteristics, the entry of moisture to it and the escape of gas from it. There are a wide range of physical conditions and biological ecosystems co-existing simultaneously within most sites. This heterogeneity, together with the varied nature of the contents, makes landfill gas production more difficult to predict and control than standard industrial bioreactors for sewage treatment.
Synthetic natural gas (or ‘syngas’) is a combustible gas created by the thermochemical process of gasification of organic material. It is composed predominantly of methane, hydrogen, carbon monoxide and carbon dioxide. Syngas has less than half the energy density of natural gas. Syngas is combustible and often used as a fuel source or as an intermediate for the production of other chemicals. Syngas can be extracted from a variety of organic materials (also known as ‘biomass’) including the biodegradable fraction of domestic and commercial wastes (including food waste, paper, card and wood), agricultural waste, sewage sludge and energy crops. Anaerobic digestion tends to be better suited to wetter resources and gasification to drier ones.
Official Information Portal on Anaerobic Digestion
DECC, Biomethane into the Gas Network: A Guide for Producers