Environmental Engineering Reference
In-Depth Information
Cooling and/or compression
Cooling can be achieved by burying the biogas pipeline in the ground and
equipping the pipeline with water drains or condensate traps. Other
technical
installations for water compression cooling are demisters and
cyclones.
Adsorption
Adsorption can be achieved using silica, aluminium oxide, magnesium
oxide, activated carbon or zeolites. Regeneration is usually achieved by
increasing the temperature and/or decreasing the pressure. Usually, two
columns work in parallel - one is adsorbing while the other one is being
regenerated.
Absorption
Absorption can be done in glycol solutions (e.g. ethylene glycol, diethylene
glycol, triethylene glycol) that bind the water by absorption (regenerated by
heating) or by the use of hygroscopic salts. The salt is dissolved when it
absorbs water and the salt is usually not regenerated.
14.3.2 Hydrogen sulphide
Hydrogen sulphide can be removed in the digester itself or after the digester.
For some upgrading techniques it is very important to remove hydrogen
sulphide before upgrading since it will otherwise have a negative effect on
the upgrading plant; for example, in pressure swing adsorption (PSA) it will
be irreversibly bound to the adsorption material. For other upgrading
technologies it is less of a problem and for some it is removed (at least to
some extent) in the upgrading process itself. Hydrogen sulphide has a higher
solubility than carbon dioxide in water and will thus be removed together
with the carbon dioxide in the water scrubber. For more information about
biogas upgrading see Chapter 15. The technologies used for the removal of
hydrogen sulphide can be divided into biological, physical or chemical
methods.
Biological removal
If air/oxygen is injected into the digester, hydrogen sulphide will react with
oxygen and form elementary sulphur through biological oxidation. Some
sulphate dioxide may also be formed. The oxidation is catalysed by
Thiobacillus bacteria that are normally present in the digester. Usually, some
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