Anaerobic Digestion
A combination of feedstocks are fed into a Digester where oxygen is excluded and bacteria breakdown the feedstock producing methane (CH4) and carbon dioxide (CO2). The digestion process can take place at low or high temperatures or a mixture of the two. The biogas is dried and vented into a gas engine connected to a generator to generate electricity. Heat can be taken off the engine to give combined heat and power. In addition to gas, digestate is produced which can be used as a fertiliser. It is also possible to utilise the gas as a fuel for vehicles.
The type of feedstock used will dictate the output from the plant. Cattle and pig slurry do not have very high biogas yields and will need to be supplemented with energy crops, particularly maize. Farm manures, surplus/waste vegetables and energy crops may be seen as more straightforward systems than those taking animal by-product material, sewage etc.
The other essential factor is an adequate grid connection. Whilst it is generally possible to establish such a connection, upgrading the grid can be extremely expensive and impact significantly on the economics of the project. It is important to establish grid issues at an early stage.
For more information download the AD FACT sheet