Environmental Engineering Reference
In-Depth Information
Carbon dioxide
Apart from methane, carbon dioxide is the main component in biogas. It is
formed during the decomposition of the different types of substrates used
for biogas production. The transformation of substrates into biogas is a
complex process that involves several steps and different types of
microorganisms. Carbon dioxide is formed in the different steps and acts
as an electron acceptor for the methane-producing bacteria. The carbon
dioxide will decrease the volumetric energy content in the biogas. If high
volumetric energy content is important (e.g. when the gas is used as a vehicle
fuel or injected into the gas grid), the carbon dioxide can be considered an
impurity that should be removed. For other applications, such as power and
heat generation, it does not usually cause problems. However, water that
condensates and carbon dioxide will form carbonic acid.
Water
Since water is always present during anaerobic digestion some of it will
evaporate in the digester and thus be present in the produced biogas. Biogas
leaving the digester is therefore always saturated with water. The amount of
water in the biogas depends on how much water can be present before the
gas becomes saturated with water. The water concentration corresponding
to saturation in biogas depends on the pressure and the temperature inside
the digester. Water in raw biogas can cause problems downstream from the
digester. It can, for example, cause corrosion in pipelines in the presence of
other compounds such as carbon dioxide, due to formation of carbonic acid.
Another negative effect is that water lowers the energy content of the gas
and can thus affect energy utilisation from the gas. Even in the case when
the raw biogas is dried before utilisation, water is formed when biogas is
combusted. Depending on the temperature and pressure downstream from
the combustion stage, water may condense and cause problems
in
downstream heat exchangers and exhaust components.
Hydrogen sulphide
Another common impurity in biogas and landfill gas is hydrogen sulphide.
Other sulphur-containing impurities may also be present in raw biogas, but
hydrogen sulphide is the most common. Hydrogen sulphide is formed by
bacteria that have the ability to reduce sulphate present in the digester to
hydrogen sulphide. These bacteria are present in the digester and compete
with methane-forming bacteria for the same substrate but, instead of
forming biogas, hygrogen sulphide is formed. Sulphate-rich substrates
include stillage, macroalgae and some substrates from the paper industry.
Search WWH ::
Custom Search