Production of biogas
As earlier mentioned bio gas is formed when organic material breaks down in an environment free from oxygen (anaerobic conditions) The process is called fermentation. Biogas has a heat value of 5-8 kWh/ Nm3 (normal cubic meter), depending on the content of methane. Normal cubic meter is a standard definition of a gas with the pressure 1013 mbar at the temperature 0°C. Most easy decayable organic material can be used as raw material when producing biogas.  
A large part of this material can be found in the agricultural sector partly in the form of manure, partly as waste and other rest products.
Biogas can also be found in the nature in eg. svamps, bogs, and in the stommach of chewing cattle. In the industry the aim is to imitate the process which takes place in the stomach of chewing cattle.