Biogas

end uses of Biogas
The farm biogas facility on the island of Orø produces electricity and heat both for the farm itself and for a nearby institution.
The shiny cylinder in the foreground is a silencer.
Biogas is flammable and can therefore be
used both in energy production and transport.
The taxes levied on the fuel it replaces
usually determine what the end use of the gas
will be.
The simplest use of the gas is in a gas boiler,
where the gas produces heat that can be used for
heating purposes. The efficiency (annual efficiency
rating) is approx. 85%. With this type of
application, the fuel that is typically replaced is
heating oil. The method is currently only used on
smaller plants.
Under Danish conditions it usually makes
better economic sense to use the gas to drive an
engine generator system. The engine can be an
ordinary combustion engine. The engine drives a
generator that produces electricity. Residual heat
from the engine and from the flue gas system can
be captured and used for heating purposes.
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Of the total energy content of the biogas, 30-
40% is typically converted into electricity that
can be sold to the grid and 45-55% is converted
to heat (less the heat used in the process) that can
be sold to, for example, a smaller central heating
plant. The relationship between how much is
converted to electricity and how much to heat
depends on the size of the engine. The overall
conversion efficiency is usually about 85%. At
this efficiency level, biogas can replace both coal
in power stations and the fuels used by a heating
plant (oil, natural gas or biomass).
The biogas can also be used for transport and
for this purpose the carbon dioxide is often removed
from the gas to reduce its volume. The transport
option is not used in Denmark, but is widely
used in Sweden where bioenergy taxation levels
are more favourable for this end use. This end use
replaces liquid fuels such as petrol or diesel.