Biogas is produced through the microbial degradation of organic matter in humid conditions and the simultaneous absence of air (anaerobic environment) . In a biological decomposition process (digestion/fermentation) the organic biomass is converted primarily into the main components methane and carbon dioxide. This process occurs often in nature, such as in swamps and marshes or in the digestive tract of ruminants, while it is used systematically in biogas plants.
The final product is combustible biogas, a mixture essentially consisting of methane (50-75%), carbon dioxide ( 25-45%) as well as small amounts of water (2-7%), hydrogen sulfide, oxygen, nitrogen, ammoniac, hydrogen and other trace gases.
The methane in the biogas chemically corresponds to the natural gas and it is the main energy carrier. The energetic value of one cubic meter of biogas is about six kilowatt hours, with a 60 percent methane content. The average calorific value of a cubic meter of biogas therefore is approximately 0.6 liters of fuel oil.
The digestate (non-degraded biomass and minerals) which remains after the fermentation process has great fertilizing capacities.
A comparative overview of biogas yields of various substrates is published in “Biogas – an introduction”.