Environmental Engineering Reference
In-Depth Information
Biogas Composition, Properties and Utilization
Raw biogas produced from anaerobic digestion is often around CH
4
55-65 % and
CO
2
35-45 % (Persson
2003
), with trace components of H
2
S and moisture. Methane
is extremely fl ammable. It combusts very cleanly with hardly any soot particles or
other pollutants, making it a clean fuel. But CO
2
, the non-combustible part of the
biogas, lowers the calorifi c value of the biogas (de Hullu et al.
2008
). On an average,
the calorifi c value of biogas is 21.5 MJ/m
3
whereas that of biomethane is 35.8 MJ/m
3
(Aebiom
2009
). Depending on the end use, different biogas treatment steps are
necessary. For some applications like cooking and lighting, it is important only to
purify biogas i.e. remove H
2
S and moisture, whereas in some, e.g. as vehicle fuel or
for grid injection, it is important to have a high energy content in the gas, the raw
biogas needs to be purifi ed as well as upgraded. The energy content of biogas is in
direct proportion to the methane concentration; hence by removing carbon dioxide
in the upgrading process the energy content of the gas is increased.
Need and Methods of Biogas Upgrading
Raw biogas produced from anaerobic digestion is often around 55-65 % methane
and 35-45 % CO
2
, with trace components of H
2
S and moisture. Raw biogas is there-
fore not ideal for use as a vehicular fuel. The traces of H
2
S produces H
2
SO
4
which
corrode the internals of pipes, fi ttings etc. The solution is to use biogas upgrading
process, whereby other constituents such as carbon dioxide and hydrogen sulphide
in the raw biogas stream are adsorbed in water, leaving above 90-95 % methane per
unit volume of gas (Vijay
1989
).
In different countries, there are different fuel quality standards for vehicle fuel
use. This upgraded gas is generally referred to as biomethane and this is the term
best representing upgraded biogas used in vehicles as opposed to the raw gas pro-
duced by the anaerobic digestion process. Gas upgradation and bottling is normally
performed in two steps where the main step is the process that removes the CO
2
and
H
2
S from the gas followed by its moisture removal and compression at 200 bar. The
processes used in biogas upgrading are reasonably well developed.
Despite several advantages, the biogas technology continues to suffer from cer-
tain limitations. Prime amongst these are the non-storability of biogas for longer
period and use nearby plant only. It can be overcome if biogas is compressed and
stored in cylinders like other gases, since liquefaction of biogas under normal condi-
tions is not possible. But, biogas contains a sizable amount of CO
2
, H
2
S and water
vapour, which have practically no use as fuel.
Presence of CO
2
in biogas poses following problems:
• It lowers the power output of the engine;
• It takes up space when biogas is compressed and stored in cylinder; and
• It can cause freezing problems at valves and metering points where the
compressed gas undergoes expansion during engine running.
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