Environmental Engineering Reference
In-Depth Information
Table 14.1 Typical composition of landfill gas and biogas from an anaerobic
digester (injection of gases from non-conventional sources into gas networks)
Compound
Unit
Landfill gas
AD gas
Methane
mol-%
30-60
50-80
Carbon dioxide
mol-%
15-40
15-50
Nitrogen
mol-%
0-50
0-5
Oxygen
mol-%
0-10
0-1
mg/m
3
Hydrogen sulphide
0-1000
100-10000
mg/m
3
Ammonia
0-5
0-100
mg/m
3
Total chlorine
0-800
0-100
mg/m
3
Total fluorine
0-800
0-100
mg/m
3
Siloxanes
0-50
on which substrates and method were used for the biogas production.
Typical compositions of biogas and landfill gas are shown in Table 14.1.
Impurities occur in raw biogas for different reasons. Some impurities are
present in the substrate that is fed into the reactor; they will evaporate in the
digester and thus be present in the produced biogas. Examples of such
compounds are siloxanes. The amount that evaporates depends on how
volatile the compound is and on the temperature in the digester. Water is
found in raw biogas, the amount depending on the conditions in the
digester. Impurities can also be formed during anaerobic digestion. In
anaerobic digestion processes, degradable compounds are degraded into
smaller compounds by different microorganisms present in the digester. In
the final step of biogas formation in a digester, methane is formed either by
microorganisms digesting acetic acid or by microorganisms combining
hydrogen and carbon dioxide. Depending on how much these compounds
evaporate, they can be found in small amounts in the produced biogas.
Examples of such impurities are ammonia and hydrogen. Impurities can
also be added during the process. For example, oxygen (or air) can be added
in small amounts to remove hydrogen sulphide by oxidation and thus small
amounts of oxygen (and nitrogen) can be present in the biogas. The nitrogen
present in landfill gas occurs as a result of extracting the gas by lowering the
gas pressure, causing air to enter the landfill. While the oxygen in the air will
mostly be consumed by microorganisms in the landfill, nitrogen will remain
unchanged in the extracted landfill gas. The amount of nitrogen present in
landfill gas is thus dependent on how the gas is extracted from the landfill
and if a low pressure is used. In the following sections, the most common
impurities in raw biogas are reviewed in terms of their origin and how they
may affect biogas utilisation.
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