Biogas consists primarily of methane and carbon dioxide, but also smaller amounts of hydrogen sulphide, ammonia and traces of hydrogen, nitrogen, carbon monoxide, saturated or halogenated carbohydrates and oxygen may be present. The biogas is usually saturated with water vapour and may also contain particles and siloxanes. The energy content is determined by the methane content (1 kWh per m3 of biogas with 10% of methane).

The biogas can be used in as many applications as the natural gas (heating, combined heat and power systems, fuel cells).There may be different specifications for biogas to be used in different applications, especially, when biogas is to be used in stationary appliances or to be fed to a pipeline grid. The biogas needs purification to improve its quality in most cases.

Hydrogen sulphide and its oxidation products are the major ‘contaminants’ of the biogas (corrosive) with a maximum permitted concentration of 5 ppm.

Hydrogen sulphide reacts with most metals. Conditions of high pressure and temperature (prevailing during storage or usage of biogas) favour the reactivity of this contaminant. Sulphur dioxide also lowers the dew point (temperature to which a given volume of gas must be cooled, at constant barometric pressure, for water vapour to condense into water) in the stack gas. There are biological methods for hydrogen sulphide removal that can be applied in the anaerobic digester as well as other physicochemical methods applicable after biogas has been collected. The biological methods include the supply of small air amounts to activate the sulphide oxidising microorganisms (Thiobacillus) grown in a micro-aerophilic environment on CO2 (autotrophic). The hydrogen sulphide is converted to elemental sulphur but also to sulphate. A combination of biological filter (containing sulphide oxidising microorganisms) and a water scrubbing step can be used alternatively. Physicochemical methods include usage of iron containing compounds (iron chloride, iron oxide), activated carbon, water scrubbing, dimethylether of polyethylene glycol (or selexol) scrubbing and NaOH scrubbing. Iron chloride can be supplied into the digester which forms iron sulphide (insoluble) and is applied when hydrogen sulphide is produced at high concentrations.

Humidity should also be removed because the presence of water favours the formation of sulphur oxidation products. Water is condensed and frozen under conditions of high pressure during biogas storage.

Carbon dioxide must also be removed if the biogas has to meet the natural gas specifications. Especially if biogas has to be used a vehicle fuel, it must be enriched in methane. Suitable methods for carbon dioxide removal include water absorption, polyethylene glycol absorption (the carbon dioxide is better dissolved in selexol), carbon molecular sieves (a series of carbon columns is used to save energy required for pressure application) and membrane separation (with gas phase in both sides of the membrane — high pressure or with a liquid phase in the one side for absorption of the carbon dioxide while diffusing through the membrane — low pressure). Halogenated compounds (present in landfill biogas) and oxygen (due to air entrance when landfill biogas is collected) must be removed too. The requirements for removal of these constituents are reported in Table 12.3 depending on the biogas usage.