Biogas
Designs
http://www.build-a-biogas-plant.com/Types-of-Biogas-Designs.html (15.10.2013)
There
are a number of different types of biogas designs.
The information here is drawn from a paper titled Biogas Plants by
Ludwig Sasse. The designs being discussed
here are relatively simple in design and are directed more at domestic and smaller scale rural installations rather than large
scale commercial designs.
Firstly
one must distinguish between batch and continuous plants.
Batch
biogas designs are filled completely and then emptied completely after fixed
retention time.
Continuous
biogas designs are filled and emptied regularly normally daily. Each design
is suitable for continuous operation, but the feed flow must be flowable and
uniform.
Three
Basic Types of Biogas Design
Fig1. A – Floating drum plant B – Fixed
dome plant C -Fixed-dome
with separate gasholder. The gas pressure is kept constant by the floating
gasholder. The unit can be operated as a continuous flow plant with no
compensating tank. The use of an agitator is
recommended. D – Balloon type of biogas design E – Channel typedigester with
folia and sunshade.
Balloon
Plants
Fig 2. Balloon
Digester as seen in Costa
Rica
A balloon plant consists of a plastic or rubber
digester bag, in the upper part of which the gas is stored.
The inlet and outlet are attached direct to the skin of the balloon. When the
gas space is full, the plant works like a fixed-dome plant – i.e., the balloon is not
inflated; it is not very elastic. The fermentation slurry is agitated slightly
by the movement of the balloon skin. This is favourable to the
digestion process. Even difficult feed materials, such as water hyacinths, can
be used in a balloon plant. The balloon material must be UV-resistant.
Materials which have been used successfully include RMP (red mud plastic), Trevira and butyl.
Advantages: Low cost,
ease of transportation, low construction (important if the water table is
high), high digester temperatures, uncomplicated cleaning, emptying and maintenance.
Disadvantages: Short
life (about five years), easily damaged, does not create
employment locally, little scope for self-help. Balloon
plants can be recommended wherever the balloon skin is not likely to
be damaged and where the temperature is even and high. One variant
of the balloon plant is the channel-type digester
with folia and sunshade as seen in Figure 2 above.
Fixed-Dome
Plants
Fig 3. Fixed dome plant. 1. Mixing tank with inlet pipe 2.
Digester 3. Compensating and removal tank 4. Gasholder 5. Gaspipe 6. Entry hatch, with gastight seal
and weighted 7. Difference in level = gas pressure in cm WC 8. Supernatant
scum; broken up by varying level 9. Accumulation of thick sludge 10.
Accumulation of grit and stones 11. Zero line: filling height without gas pressure.
A fixed-dome plant (Figure 3) consists of an
enclosed digester with a fixed, non-movable gas space. 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, therefore the volume of the digester should not exceed 20 m³. If there is
little gas in the holder, the gas pressure is low. If the gas is
required at constant pressure (e.g., for engines), a gas pressure regulator
or a floating gasholder is required. Engines require a great deal of gas, and
hence large gasholders. The gas pressure then becomes too
high if there is no floating gasholder.
Advantages:
Low construction cost, no moving parts, no rusting steel parts,
hence long life (20 years or more), underground
construction, affording protection from winter cold and saving space, creates employment
locally.
Disadvantages:
Plants often not gaslight (porosity and cracks), gas pressure
fluctuates substantially and is often very
high, low digester temperatures.
Fixed-dome plants can be recommended only where construction can
be supervised by experienced biogas technicians.
Floating-Drum
Plants
Fig 4. Floating drum plant. 1. Mixing tank with inlet pipe 2. Digester 3.
Overflow on outlet pipe 4. Gasholder with braces for breaking up surface scum
5. Gas outlet with main cock 6. Gas drum guide structure 7. Difference in level
= gas pressure in cm WC 8. Floating scum in the case of fibrous feed material
9. Accumulation of thick sludge 10. Accumulation of grit and stones 11. Water
jacket with oil film.
Floating-drum plants (Figure 5) consist of a
digester and a moving gasholder. The gasholder floats either direct on the
fermentation slurry or in a water jacket of its own. The gas collects in the
gas drum, which thereby rises. If gas is drawn off, it falls again.
The gas drum is prevented from tilting by a
guide frame.
Advantages: Simple,
easily understood operation, constant gas pressure, volume of stored gas
visible directly, few mistakes in construction.
Disadvantages: High
construction cost of floating-drum, many steel parts liable to corrosion,
resulting in short life (up to 15 years; in tropical coastal regions about five years for
the drum), regular maintenance costs due to
painting. In spite of these disadvantages, floating-drum plants are always
to be recommended in cases of doubt. Water-jacket plants
are universally applicable and especially easy to maintain. The drum won’t
stick, even if the substrate has a high solids content. Floating-drums
made of glass-fibre reinforced plastic and highdensity polyethylene have been
used successfully, but the construction cost is higher than with steel.
Floating-drums made of wire-mesh-reinforced
concrete are liable to hairline cracking and are intrinsically porous. They require a
gaslight, elastic internal coating. PVC drums are unsuitable because not
resistant to UV. The floating gas drum can be replaced by a balloon above the
digester. This reduces construction costs (channel
type digester with folia), but in practice problems always arise with the
attachment of the balloon at the edge. Such plants are still being tested under
practical conditions.