Environmental Engineering Reference
In-Depth Information
9.2.5 Gas utilisation in CHP units
When the main role of a biogas plant is electricity production, the second
most important source of energy consumption is the CHP unit. The CHP
consumes no heat; it is in fact the main 'producer' of heat in the form of
waste heat from the exhausts and the cooling water cycle. However, the unit
consumes not insignificant amounts of electricity during operation. Typical
values taken from German biogas plants show an electricity demand of
between 7.5 and 15 kWh
el
per MWh
Hi
of biogas produced for a range of
different CHP units. More important is the provision of effective cooling in
the cooling water cycle of the unit. If this cooling is necessary for all the heat
produced, the electricity demand will be at the upper end of the range. In
addition, transport losses must also be taken into account: these occur both
in cables between the CHP and the final consumer or the electricity grid and
in transformers.
9.2.6 Upgrading of gas to natural gas quality
Gas upgrading mainly involves the removal of carbon dioxide from the
biogas. This process is typically energy intensive and the energy demand
depends to a large extent on the technology used. The main source of energy
demand for all technologies is electricity, mainly for the operation of
compressors or pumps. The typical electricity demand for water scrubbers,
pressure swing adsorption and the majority of membrane technologies is
between 0.2 and 0.25 kWh
el
per m
3
of raw biogas. For chemical absorption
processes, the electricity demand can be lower but there is an additional heat
demand of up to 0.4 kWh
th
per m
3
raw biogas, depending on the technology
used. The energy consumption may also be influenced by further use of
upgraded biogas. Electrically driven compression may be required for
injection into gas grids (which typically operate at pressures of between 4
and 80 bar) or for supplying a vehicle filling station (which requires pressure
of up to 300 bar). For vehicle fuel supply, the energy demand for additional
compression has to be taken into account: this ranges from approximately
0.2 up to 0.35 kWh
el
per m
3
of biomethane. The lower value applies for
compression of the gas from the 30-40 bar supplied by the grid up to
300 bar; the upper value is for compression all the way from 1 to 300 bar.
The typical requirement is for compression from 1 to 250 bar, for which
0.31 kWh
el
per m
3
of biomethane is required. The heat generated by most
biogas upgrading technologies, especially from compressors, can be
recovered at temperatures between 50 and 100
8
C and can be used, for
example, for fermenter heating. This can be considered as a method of
supplying energy to the biogas plant.
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