Environmental Engineering Reference
In-Depth Information
the substrate digested in the biogas plant, the methane content of the biogas
fluctuates between 50% and 75%. The second main component of biogas is
carbon dioxide (CO
2
) with a share between 25% and 50%. Other
components of biogas are water (H
2
O), oxygen (O
2
) and traces of sulfur
(S
2
) and hydrogen sulfide (H
2
S). If biogas is upgraded to biomethane with
approximately 98% methane in a biogas treatment plant, the biomethane
has the same properties as natural gas.
After simple desulfurization and drying, biogas can be converted to
electricity and heat in cogeneration units (combined heat and power (CHP))
or the biogas is burnt to produce heat. After treatment to natural gas grade,
the so-called biomethane can be used in all applications commonly known
for natural gas. Thus, biogas and biomethane produced from biogas are
flexible renewable fuels that can be stored. Motor fuel, electricity and heat
can be produced from them, which makes them important functions in the
context of sustainable energy supply. Besides, biogas can also replace
carbon compounds in plastic products.
Experts are not agreed as far as the importance of biogas for the
sustainable supply of energy is concerned. Basically, a difference should be
made between two different origins of the substrate on which biogas plants
feed: waste and energy plants. Whereas the untapped reserve of digestible
organic waste is enormous on a world scale, large unused areas of land on
which energy crops can be cultivated are also available. For Europe,
including the European succession states of the Soviet Union, it has been
calculated (Thra¨ n et al. 2007) that by the year 2020, 250 billion standard
cubic meters (m
3
N
) of biomethane from digested feedstock could be
produced, which would be enough to meet 50% of the present gas
consumption in the 28 European Union (EU) member states. These figures
illustrate that biogas can make a sizable contribution to the energy supply.
Besides, biogas is a versatile fuel: biogas produced from substrates by
digestion, is the only renewable fuel at present that is a viable alternative to
fossil natural gas and can be used for all purposes for which natural gas is
used and also by the same infrastructure. Thus, biogas technology can
contribute to solving the pressing questions of safe and sustainable energy
supply for electricity, heat and transport fuel.
1.2
Biogas technologies and environmental efficiency
Biogas is produced by anaerobic bacteria that degrade organic material to
biogas in four steps: hydrolysis, acidification, production of acetic acid and
production of methane. The product of the digestive process, raw biogas,
consists of 50-75% methane, 25-50% carbon dioxide and 2-8% other gases
such as nitrogen, oxygen and trace gases (e.g. hydrogen sulfide (H
2
S),
ammonia (NH
3
) and hydrogen). Before the biogas can be converted into
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