The composition of biogas varies depending upon the origin of the anaerobic digestion process. Landfill gas typically has methane concentrations around 50%. Advanced waste treatment technologies can produce biogas with 55–75% CH4 or higher using in situ purification techniques As-produced, biogas also contains water vapor, with the fractional water vapor volume a function of biogas temperature; correction of measured volume for water vapor content and thermal expansion is easily done via algorithm.
In some cases biogas contains siloxanes. These siloxanes are formed from the anaerobic decomposition of materials commonly found in soaps and detergents. During combustion of biogas containing siloxanes, silicon is released and can combine with free oxygen or various other elements in the combustion gas. Deposits are formed containing mostly silica (SiO2) or silicates (SixOy) and can also contain calcium, sulfur, zinc, phosphorus. Such white mineral deposits accumulate to a surface thickness of several millimeters and must be removed by chemical or mechanical means.
Practical and cost-effective technologies to remove siloxanes and other biogas contaminants are currently available.| Compound | Chem | % |
|---|---|---|
| Methane | CH4 | 50–75 |
| Carbon dioxide | CO2 | 25–50 |
| Nitrogen | N2 | 0–10 |
| Hydrogen | H2 | 0–1 |
| Hydrogen sulfide | H2S | 0–3 |
| Oxygen | O2 | 0–0 |
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