Composition
| Compound | Chem | % |
|---|---|---|
| Methane | CH4 | 50–75 |
| Carbon dioxide | CO2 | 25–50 |
| Nitrogen | N2 | 0–10 |
| Hydrogen | H2 | 0–1 |
| Hydrogen sulphide | H2S | 0–3 |
| Oxygen | O2 | 0–0 |
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% methane, which for reactors with free liquids can be increased to 80-90% methane using in-situ gas purification techniques As-produced, biogas also contains water vapor. The fractional volume of water vapor is a function of biogas temperature; correction of measured gas volume for both water vapor content and thermal expansion is easily done via a simple mathematic algorithm which yields the standardized volume of dry biogas.
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.
Read more about this topic: Biogas
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