Upgrading

Biogas from anaerobic digestion or landfill contains generally, in addition to a high percentage of methane, variable percentages of carbon dioxide (CO2), water vapour (usually water saturated), sulfur compounds, nitrogen, oxygen and siloxanes. The latter can be present in the biogas from sewage sludge or landfill as contained in detergents and cosmetics.

The table below compares the average composition of biogas from anaerobic digestion with landfill biogas and natural gas (from Biomethane Regions, 2012).

Parameter                  [unit]	Biogas	Landfill biogas	Grid natural gas (reference: Denmark)
Methane      [% vol]	60-70	25-65	89
Water v        [% vol]	saturated	saturated	0
Other hydrocarbons [% vol]	0	0	9.4
Hydrogen        [% vol]	0	0-3	0
Carbon dioxid     [% vol]	30-40	15-50	0.67
Nitrogen              [% vol]	≤1	5-40	0.28
Oxygen                [% vol]	≤0.5	0-5	0
Hydrogen sulfi     [ppmv]	0-4000	0-100	2.9
Ammonia             [ppmv]	≤100	≤5	0
Lower Heating Value (LHV)       [kWh/m3 STP]	6.5	4.4	11

The upgrading process removes the undesired components and any other impurities from biogas to obtain a relative enrichment in methane (≥ 95 %), making biomethane equal to natural gas from fossil origin. Depending on the starting composition of the biogas, its upgrading may include separation of carbon dioxide (and therefore the increase of the heating value), drying, removal of trace substances such as oxygen, nitrogen, hydrogen sulfide, ammonia, or siloxanes, as well as the compression at the pressure which is required for the specific usage of the gas. Under certain conditions, additional treatments may be necessary such as odorization (required for grid injection) or the adjustment of the heating value by adding propane. 

Available upgrading technologies are based on different physical or chemical principles related to gas separation (adsorption, absorption, permeation and cryogenic separation). The  upgrading technology is selected based on the compositional characteristics and volumes of biogas to be treated as  these variables affect the cost-efficiency of each technology. For small-medium sized systems, membrane technology is particularly interesting.