Environmental Engineering Reference
In-Depth Information
temperature in the digester will determine how much evaporation into the
biogas will occur. Low molecular weight siloxanes will evaporate to a higher
extent than others. Siloxanes can also be found in landfill gas due to the
evaporation of siloxanes in the landfill. During combustion, siloxanes form
siloxane oxide, which can cause damage to engines. The formed siloxane
oxide is insoluble and will form unwanted depositions on combustion
equipment.
Halogenated hydrocarbons are hydrocarbon molecules containing
chlorine, bromine or fluorine. They can be present in raw biogas due to
the volatilisation of halogen-containing material in landfills, for example.
Hydrogenated hydrocarbons can cause corrosion and acidification when
combusted, due to the formation of acids (e.g. hydrochloric acid).
Particles
Particles are often present in raw biogas. In many cases, particulates form
the nuclei onto which drops of water condense. Particles can cause wear of
equipment due to their abrasive properties.
14.2.2 Biogas utilisation and quality demands
How clean the biogas needs to be depends on how the biogas is going to be
utilised. The energy in biogas can be utilised as heat or heat and power or
can be converted to kinetic energy when used as vehicle fuel. Before
utilisation, cleaned and upgraded biogas can also be injected into the gas
grid. There are different aspects on quality demands for biogas utilisation.
The technical aspect is of course very important and relates to limitations on
the equipment used, but it is also important to know which gas the
equipment in question is certified for and how the equipment can be
adjusted for other gas qualities. In general, it can be said that the cleaner the
gas is, the lower the maintenance cost will be. Gas cleaning can thus
sometimes be a compromise between cleaning and maintenance costs. When
evaluating biogas composition and its impurities one cannot look at each
impurity individually since the different impurities also affect each other; for
example, carbon dioxide and hydrogen sulphide form acids when dissolved
in water and these acids then cause corrosion.
Quality demands on biogas for heat and power production
Boilers are used for heat production from biogas. Compounds that can
cause problems in a boiler are hydrogen sulphide, particles and siloxanes.
For condensing boilers, in which the flue gas is cooled down and the water
in the gas condensates, hydrogen sulphide will form sulphuric acid with
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