Editing Biogas (section)

== Composition ==
{| class="wikitable floatright" style="text-align: center;"
|+ '''Typical composition of biogas'''
|-
! [[Chemical compound|Compound]]
! [[Molecular formula|Formula]]
! width="100" | Percentage by volume
|-
| [[Methane]]
| {{chem|CH|4}}
| 50–80
|-
| [[Carbon dioxide]]
| {{chem|CO|2}}
| 15–50
|-
| [[Nitrogen]]
| {{chem|N|2}}
| 0–10
|-
| [[Hydrogen]]
| {{chem|H|2}}
| 0–1
|-
| [[Hydrogen sulfide]]
| {{chem|H|2|S}}
| 0–0.5
|-
| [[Oxygen]]
| {{chem|O|2}}
| 0–2.5
|-
| colspan="3" style="font-size: 0.85em; padding: 4px 0 4px 6px; text-align: left; background-color: #f2f2f2;" | Source: ''www.kolumbus.fi, 2007''<ref>[http://www.kolumbus.fi/suomen.biokaasukeskus/en/enperus.html Basic Information on Biogas] {{webarchive |url=https://web.archive.org/web/20100106022729/http://www.kolumbus.fi/suomen.biokaasukeskus/en/enperus.html |date=6 January 2010}}, www.kolumbus.fi. Retrieved 2.11.07.</ref>
|}

The composition of biogas varies depending upon the substrate composition, as well as the conditions within the anaerobic reactor (temperature, pH, and substrate concentration).<ref>{{cite web|url=https://cran.r-project.org/web/packages/biogas/vignettes/predBg_function.pdf|title=Predicting methane and biogas production with the biogas package|last=Hafner|first=Sasha|date=2017|website=CRAN}}</ref> [[Landfill gas]] typically has methane concentrations around 50%. Advanced waste treatment technologies can produce biogas with 55–75% methane,<ref>{{cite web|url=http://www.oaktech-environmental.com/Juniper.htm|title=Juniper|access-date=15 May 2015|archive-url=https://web.archive.org/web/20150430064354/http://www.oaktech-environmental.com/Juniper.htm|archive-date=30 April 2015|url-status=dead}}</ref> which for reactors with free liquids can be increased to 80–90% methane using [[in-situ]] gas purification techniques.<ref>{{cite journal |first1=B. |last2=Herndon |first2=F. G. |last3=Jewell |first3=W. J. |last4=Cummings |first4=R. J. |last5=White |first5=T. E. |title=In situ methane enrichment in methanogenic energy crop digesters |journal=Biomass and Bioenergy |volume=6 |issue=4 |last1=Richards |pages=275–282 |year=1994 |doi=10.1016/0961-9534(94)90067-1 |bibcode=1994BmBe....6..275R |hdl=1813/60790 |hdl-access=free}}</ref> As produced, biogas contains water vapor. The fractional volume of water vapor is a function of biogas temperature; correction of measured gas volume for water vapour content and thermal expansion is easily done via simple mathematics<ref>{{cite journal |first1=B. |first2=R. |first3=T. |first4=W. |title=Methods for kinetic analysis of methane fermentation in high solids biomass digesters |last1=Richards |journal=Biomass and Bioenergy |volume=1 |issue=2 |pages=65–73 |year=1991 |doi=10.1016/0961-9534(91)90028-B |last2=Cummings |last3=White |last4=Jewell |bibcode=1991BmBe....1...65R |hdl=1813/60787 |hdl-access=free}}</ref> which yields the standardized volume of dry biogas.

For 1000&nbsp;kg (wet weight) of input to a typical biodigester, total solids may be 30% of the wet weight while [[volatile suspended solids]] may be 90% of the total solids. Protein would be 20% of the volatile solids, carbohydrates would be 70% of the volatile solids, and finally fats would be 10% of the volatile solids.

=== Contaminants ===
==== Sulfur compounds ====
Toxic, corrosive and foul smelling [[hydrogen sulfide]] ({{chem|H|2|S}}) is the most common contaminant in biogas. If not separated, combustion will produce [[sulfur dioxide]] ({{chem|SO|2}}) and [[sulfuric acid]] ({{chem|H|2|SO|4}}), which are corrosive and environmentally hazardous.,<ref name="AbatzoglouBoivin2009" /> Other sulfur-containing compounds, such as [[thiols]] may be present.

==== Ammonia ====
[[Ammonia]] ({{chem|NH|3}}) is produced from organic compounds containing nitrogen, such as the [[amino acids]] in [[proteins]]. If not separated from the biogas, combustion results in [[NOx|{{chem|NO|x}}]] emissions.<ref name="AbatzoglouBoivin2009"/>

==== Siloxanes ====
In some cases, biogas contains [[siloxanes]]. They 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 other elements in the [[combustion gas]]. Deposits are formed containing mostly [[silica]] ({{chem|SiO|2}}) or [[silicates]] ({{chem|Si|''x''|O|''y''}}) and can 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 available.<ref name="BiogasSiloxaneRemoval">{{cite web|url=http://biogas-digester.com/biogas-technology-lowers-costs.html|title=New Landfill Gas Treatment Technology Dramatically Lowers Energy Production Costs|last=Tower|first=P.|author2=Wetzel, J.|author3=Lombard, X.|date=March 2006|publisher=Applied Filter Technology|access-date=30 April 2009|archive-url=https://web.archive.org/web/20160102055113/http://biogas-digester.com/biogas-technology-lowers-costs.html|archive-date=2 January 2016|url-status=dead}}(broken link)</ref>