Biogas is the organic gas mixture produced when microbes break down biomass without oxygen.
This process is called anaerobic digestion. Microorganisms decompose materials such as food waste, animal manure, sewage sludge, and energy crops inside an oxygen-free digester. The resulting gas is mainly methane and carbon dioxide, with small quantities of compounds such as hydrogen sulfide and water vapour.
Methane gives biogas its energy value. After cleaning, biogas can be burned for heat, used in an engine to generate electricity, or upgraded into biomethane for injection into a natural-gas network or use as vehicle fuel. The remaining digestate can sometimes be used as a soil amendment.
Biogas is often confused with syngas, which is produced by thermochemical processes such as gasification. Biodiesel is a liquid fuel made from oils or fats, while green hydrogen is produced by splitting water with renewable electricity.