What is biogas ?

Biogas is the product of the anaerobic digestion of organic matters. In absence of oxygen organic materials for example food waste, cattle manure and fish waste will degrade and release a gas called biogas. Biogas is composed mainly of methane (CH4) and carbon dioxide (CO2). It also contains hydrogen sulphide (H2S), Nitrogen (N) and water vapor (H2O).

The two main sources of biogas are biogas plants and landfills. A biogas plant receives organic materials and processes them in big air tight tank called also digesters or bioreactors. Materials are simply mixed together with water and stay for a period of 30 to 60 days. The digestion process involves bacteria that are naturally present in the cattle manure for example. Those bacteria can work at different temperature range. A classical digester has a working temperature around 37°C and is called mesophilic digester. Higher temperature can be used for example 55°C and the process is then called thermophilic and extreme thermophilic for even higher temperature. The liquid part after digestion also called digestate can be used as fertilizer on farmland.

Biogas from landfill is produced from the same process. Once the waste is buried and covered with soil the organic part that is around one third for European household waste will start to break-down in absence of oxygen. It is possible to collect this gas by drilling boreholes vertically through the landfill and vacuum the gas.

In both methods the gas extracted is a mix of several gases. It is possible to burn the gas directly in order to produce heat and electricity for example using a gas engine. The heat and electricity are then use on grid and for use in specific industries. A small part of the heat is also directly used to heat up the digester in the case of a biogas plants.

One of the most advanced uses for the biogas is biofuel. However biogas is not directly used for cars, buses or truck it need to be upgraded before. Upgrading of biogas means that the gas is clean out of carbon dioxide, water and hydrogen sulphide. The main component is therefore methane also called bio-methane. Bio-methane refers to the biological source of the methane but it is the same molecule of methane (CH4) also present in natural gas from non-renewable resources.

Typical composition for biogas

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