Biogas Composition: Methane, CO₂ and Trace Gases

Biogas consists mainly of methane (CH₄) and carbon dioxide (CO₂), together with water vapour and smaller amounts of other gases. Its exact composition depends on the feedstock and production process. Methane provides the fuel value; the other constituents affect gas quality and the treatment needed before use.

Typical biogas composition

The IEA introduction to biogas and biomethane gives a broad methane range of 45–75% by volume, with carbon dioxide making up most of the remainder. The US EPA explanation of anaerobic digestion gives 50–75% methane. These are reference ranges, not a guaranteed specification for an individual plant.

Do not combine independently quoted maximum concentrations as though they describe one sample. A complete composition analysis must use the same measurement basis and account for the whole gas mixture.

Why composition varies

Biogas is produced by anaerobic digestion of organic materials. Feedstock and production pathway influence the mixture, so a general table cannot replace measurements from your plant. Gas sampled before treatment is also different from gas sampled after moisture removal or upgrading.

Raw biogas versus biomethane

Biomethane is produced by upgrading biogas to remove carbon dioxide and other unwanted constituents. The EPA guide to renewable natural gas from agricultural digesters explains this treatment process. The composition of raw biogas should therefore not be used as the specification for upgraded gas.

For treatment methods, see our biogas upgrading guide. This page explains composition; the upgrading guide covers how gas quality is changed.

Trace contaminants: siloxanes, nitrogen and oxygen

Trace constituents deserve attention even when methane and carbon dioxide account for almost all of the gas. The EPA overview of renewable natural gas from biogas identifies siloxanes as a potential equipment-damage concern, particularly for landfill and wastewater gas. Concentrations vary by source and site. Nitrogen and oxygen can also be relevant where air enters a collection system. Specify testing for the contaminants relevant to the feedstock and intended use, rather than assigning one trace-gas percentage to every digester.

How to read a biogas analysis

• Check wet or dry basis. A wet analysis includes water vapour; a dry analysis excludes it. Compare readings on the same basis.
• Check units. Major constituents are normally expressed as volume percentages. Small concentrations may be reported in parts per million by volume (ppmv): 1% equals 10,000 ppmv.
• Identify the sampling point. State whether the sample is from the digester, after cleaning, or after upgrading.
• Record measurement conditions. When comparing gas volumes, use consistent temperature and pressure reference conditions.
• Use the actual end-use requirements. Request the gas analysis needed by your equipment supplier or gas-network operator rather than relying on a generic composition table.

Methane percentage is not methane yield

Methane percentage tells you the share of methane in the gas. Yield tells you how much methane a feedstock or plant produces. For example, 100 m³ of dry biogas containing 60% methane contains 60 m³ of methane when both volumes use the same temperature and pressure conditions. A higher methane percentage alone does not prove a higher total methane output.

Frequently asked questions

What is the main gas in biogas?

Methane is usually the largest component. Carbon dioxide is the other major constituent.

Is biogas pure methane?

No. Raw biogas is a mixture. Upgrading removes carbon dioxide and other constituents to produce methane-rich biomethane.

Can I use this table to specify a biogas plant?

Use it for orientation. Base equipment selection and treatment requirements on measured gas quality and the intended end-use specification.

[Published November 2014. Rewritten October 2026.]