Biogas cleaning - The Biogas: Science, Production and Applications - page 326

Hydrogen sulphide is also formed during digestion when proteins containing
sulphur, such as cysteine and methionine, are digested. In landfill gas,
hydrogen sulphide may also originate from gypsum. The presence of
hydrogen sulphide during utilisation of biogas can lead to corrosion since it
forms sulphuric acid in combination with water. The combustion of biogas
containing hydrogen sulphide will lead to emissions of sulphuric acid
formed during combustion. It is also important to note that hydrogen
sulphide is highly toxic and can result in serious health risks.

Oxygen and nitrogen

Since biogas is formed under anaerobic conditions, neither oxygen or
nitrogen are usually present in biogas but they can be found if air is allowed
(or manages) to enter somewhere in the system. If oxygen is present it will be
consumed; nitrogen in the raw biogas can thus be a sign of air leakage into
the digester. Sometimes air is consciously added to the process to reduce
hydrogen sulphide as a way of cleaning the biogas. It is more common for
nitrogen to be found in landfill gas, and there may also be small amounts of
oxygen, due to the fact that the extraction of landfill gas can cause a low
pressure in the landfill, which will result in air being sucked in. Oxygen can
lead to flammable mixtures being formed with the methane in the biogas and
oxygen content thus has to be carefully controlled.

Ammonia

Ammonia is an impurity often found in raw biogas; it is formed in the
digester during hydrolysis of materials containing proteins, such as
slaughterhouse wastes. High levels of ammonia in the digester can cause
inhibition of methane production in the digester.

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Volatile organic compounds

Volatile organic compounds are different types of organic compounds
found in various concentrations in biogas. The type of compounds and their
concentrations depend on the substrate used for biogas production;
examples are alkanes, siloxanes and halogenated hydrocarbons.
Siloxanes are compounds used in products such as fire retardants,
shampoos and deodorants. Siloxanes are molecules that are composed of
units of R 2 SiO, in which R is a hydrogen or a hydrocarbon group, Si is
silicon and O oxygen. If siloxanes are present in the substrate entering the
digester, these compounds will also be found in small amounts in the
produced biogas due to the fact that some of these compounds evaporate.
Siloxanes can thus be found in biogas produced from sewage sludge. The