Function
A fixed-dome plant comprises of a closed,
dome-shaped digester with an immovable,
rigid gas-holder and a displacement pit, also
named ’compensation tank’. The gas is stored
in the upper part of the digester. When gas
production commences, the slurry is displaced
into the compensating tank. Gas pressure
increases with the volume of gas stored, i.e.
with the height difference between the two
slurry levels. If there is little gas in the gas-
holder, the gas pressure is low.
Digester
The digesters of fixed-dome plants are usually
masonry structures, structures of cement and
ferro-cement exist. Main parameters for the
choice of material are:
• Technical suitability (stability, gas- and
liquid tightness);
Figure 2: Basic function of a fixed-dome
biogas plant, 1 Mixing pit, 2 Digester, 3
Gasholder, 4 Displacement pit, 5 Gas pipe
Source: OEKOTOP
• cost-effectiveness;
• availability in the region and transport
costs;
• availability of local skills for working
with the particular building material.
Fixed dome plants produce just as much gas as floating-drum plants, if they are gas-tight.
However, utilization of the gas is less effective as the gas pressure fluctuates substantially.
Burners and other simple appliances cannot be set in an optimal way. If the gas is required
at constant pressure (e.g., for engines), a gas pressure regulator or a floating gas-holder is
necessary.
Gas-Holder
Figure 3: Fixed-dome plant in Tunesia.
The final layers of the masonry structure
are being fixed.
Photo: gtz/GATE
The top part of a fixed-dome plant (the gas
space) must be gas-tight. Concrete, masonry
and cement rendering are not gas-tight. The
gas space must therefore be painted with a
gas-tight layer (e.g. ’Water-proofer’, Latex or
synthetic paints). A possibility to reduce the
risk of cracking of the gas-holder consists in
the construction of a weak-ring in the masonry
of the digester. This "ring" is a flexible joint
between the lower (water-proof) and the upper
(gas-proof) part of the hemispherical structure.
It prevents cracks that develop due to the
hydrostatic pressure in the lower parts to
move into the upper parts of the gas-holder.
Types of fixed-dome plants
• Chinese fixed-dome plant is the archetype of all fixed dome plants. Several million
have been constructed in China. The digester consists of a cylinder with round
bottom and top.
• Janata model was the first fixed-dome design in India, as a response to the Chinese
fixed dome plant. It is not constructed anymore. The mode of construction lead to
cracks in the gasholder - very few of these plant had been gas-tight.
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