Biogas is a mixture of methane (50-75%), carbon dioxide (25-45%) and small amounts of water (2-7%), as well as trace gases such as sulphur hydrogen, oxygen, nitrogen, ammonia and hydrogen.
Farm, municipal or industrial-based AD plants convert waste material into biogas. Waste/feedstock is pumped into a closed vessel (digester) which has been inoculated with suitable bacteria. Anaerobic (0% oxygen) conditions are then maintained in the vessel and the temperature is held at a constant value (typically 40oC).
The biogas produced can be upgraded to fossil ('natural') gas quality, but is normally used on site to generate heat and power electricity.
The AD process residue or digestate can be separated into a liquid fraction and a fibrous fraction.The liquid fraction can be returned to the land as a fertiliser and the solid fibre used as a soil conditioner.
Average composition of biogas
|Methane||CH4||50 - 75 volume - %|
|Carbon dioxide||CO2||25 - 45 volume - %|
|Water vapour||H2O||2 - 7 volume - %|
|Oxygen||O2||< 2 volume - %|
|Nitrogen||N2||< 2 volume - %|
|Ammonia||NH3||< 1 volume - %|
|Hydrogen||H2||< 1 volume - %|
|Hydrogen sulphide||H2S||20 – 20.000 ppm|
|[ppm: Parts per million; Volume. - %: volumetric percentage]|