Biogas

Biogas is produced from the anaerobic digestion of organic matter by naturally occurring bacteria. The composition of the biogas depends on the characteristics of the feedstock, and ranges between 50 - 60% methane (CH 4 ) and 40 - 50% carbon dioxide (CO 2 ) with traces of hydrogen sulphide (H 2 S).

Although the biogas is produced principally from the digester itself gas systems can be designed to collect gas from the head spaces of all process tanks, all of which will produce some biogas. This method means that there are no uncontrolled emissions to atmosphere from residual biogas.

Biogas is normally stored at near atmospheric pressure. Methane has a very low boiling point and is therefore difficult to liquefy. The UK does import liquid natural gas (LNG) in large tankers but it is then normally injected into the grid as a gas. Some vehicles in the UK are fuelled by compressed natural gas (CNG) which is stored in cylinders at high pressure (300 bar).

There are broadly three uses for biogas:

•  In a conventional boiler to produce hot water or steam.

•  In a stationary engine to produce power

•  As biomethane for vehicle fuel.