Biogas Plant Types and Design
Digester types in industrialized countries
To give an overview, we have chosen
three fictitious designs as they could be
found in, for example, Europe. The
designs are selected in a way that all the
typical elements of modern biogas
technology appear at least once. All
designs are above-ground, which is
common in Europe. Underground
structures, however, do exist.
Mixing pit varies in size and shape
according to the nature of substrate. It is
equipped with propellers for mixing
and/or chopping the substrate and often
with a pump to transport the substrate
into the digester. At times, the substrate
is also pre-heated in the mixing pit in
order to avoid a temperature shock
inside the digester.
Figure 11: Control glasses for an industrial
digester for solid organic waste, TBW,
Germany
Fermenter or digester is insulated and made of concrete or steel. To optimize the flow of
substrate, large digesters have a longish channel form. Large digesters are almost always
agitated by slow rotating paddles or rotors or by injected biogas. Co-fermenters have two or
more separated fermenters. The gas can be collected inside the digester, then usually with a
flexible cover. The digester can also be filled completely and the gas stored in a separate
gas-holder.
Gas-holder is usually of flexible material, therefore to be protected against weather. It can
be placed either directly above the substrate, then it acts like a balloon plant, or in a separate
’gas-bag’.
slurry store for storage of slurry during winter. The store can be open (like conventional
open liquid manure storage) or closed and connected to the gas-holder to capture remaining
gas production. Normally, the store is not heated and only agitated before the slurry is
spread on the field.
Gas use element is in Europe in 95% of the cases a thermo-power unit which produces
electricity for the farm, the grid and heat for the house, greenhouses and other uses. The
thermo-power unit has the advantage, that the required energy can be produced in any
mixture of gas and fossil energy. It can, therefore, react to periods of low gas production and
high energy requirements or vice versa.
Figure 12: Concrete digester with two chambers (one heated, one unheated for storage)
Source: TBW
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