Biogas is essentially a mixture of

Anaerobic digestion is the microbial breakdown of biodegradable organic matter in the complete absence of oxygen, and it proceeds in four stages: hydrolysis, acidogenesis, acetogenesis and methanogenesis. The carbon that enters such a system leaves it along two routes — part of it is reduced to its most hydrogenated gaseous form, and part of it is oxidised to its fully oxidised gaseous form. The bulk composition of any fuel gas produced by this route is therefore set by that carbon redox split, not by the gases of the surrounding atmosphere.

Applied to a biogas plant fed on cattle dung, agricultural residue or sewage, the reduced product is methane (CH4) and the oxidised product is carbon dioxide (CO2). On a dry basis a standard mixture runs roughly 55–65% methane by volume, with carbon dioxide making up most of the remaining 30–45%, while hydrogen sulphide and nitrogen appear only in traces. The raw gas leaving a digester is additionally water-saturated, so it carries water vapour that scrubbing removes before the gas is burnt.

Cross-check against the other gases named in the options:

• Nitrogen (N2) — about 78% of atmospheric air by volume and locked in a triple bond; the nitrogen a digester actually receives arrives bound in feedstock protein and is mineralised to dissolved ammonium rather than released as gas.

• Hydrogen (H2) — generated during acetogenesis but consumed within seconds by hydrogenotrophic methanogens, so it never accumulates into a bulk component of the finished gas.

• Helium (He) — chemically inert, radiogenic in origin from crustal rock, and never a product of microbial metabolism at any stage.

An independent check comes from combustion. Biogas burns only in its methane fraction, and the carbon dioxide fraction acts as an inert diluent that pulls the calorific value down to about 20–25 MJ/m3 against roughly 36 MJ/m3 for pure methane. That measurable dilution confirms that the second bulk component is a non-combustible, fully oxidised carbon gas.

Methane and carbon dioxide are therefore the two gases that essentially constitute biogas.