The main constituents of biogas are:

Concept — Biogas is produced when microorganisms decompose organic matter without oxygen. Its composition is dominated by a combustible fuel gas and a non-combustible carbon-containing gas, with smaller amounts of other gases.

Application — Methanogenic activity supplies methane, while conversion of organic carbon also releases carbon dioxide. Therefore, among the listed combinations, methane and carbon dioxide are the principal constituents.

• Methane and nitric oxide: nitric oxide is mainly associated with oxidation of nitrogen in combustion or atmospheric processes, not with the bulk gas mixture from anaerobic digestion.

• Methane, hydrogen and nitric oxide: hydrogen may appear only in small or intermediate amounts, while nitric oxide is not a principal anaerobic-digestion product.

• Methane and sulfur dioxide: sulfur dioxide is an oxidized sulfur gas; raw biogas can contain traces of reduced sulfur compounds such as hydrogen sulfide instead.

Cross-check — Standard biogas descriptions place methane at roughly one-half to three-quarters of the mixture and carbon dioxide as the other major fraction. This independently confirms methane and carbon dioxide.