Types of Biogas Plant

Types of  Biogas Plant 

Three main types of simple biogas plants can be distinguished  Figure

•  balloon plants,
•  fixed-dome plants,
•  floating-drum plants.

 Balloon Plants

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.

Fixed-Dome Plants

A fixed-dome plant Figure 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.

Fixed-dome plant 1. Mixing tank with inlet pipe. 2. Digester. 3. Compensating and removal
tank. 4. Gasholder. 5. Gaspipe. 6. Entry hatch, with gaslight 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.

 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

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.

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

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.

Types of Biogas Plant

Types of  Biogas Plant 

Three main types of simple biogas plants can be distinguished  Figure

•  balloon plants,
•  fixed-dome plants,
•  floating-drum plants.

 Balloon Plants

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.


Fixed-Dome Plants

A fixed-dome plant Figure 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.

Fixed-dome plant 1. Mixing tank with inlet pipe. 2. Digester. 3. Compensating and removal
tank. 4. Gasholder. 5. Gaspipe. 6. Entry hatch, with gaslight 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.

 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

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.

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  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

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.

Biogas Plant - Digester types

 Biogas - Digester types

 

In this chapter, the most important types of biogas plants are described:

Fixed-dome plants Floating-drum plants Balloon plants Horizontal plants Earth-pit plants Ferrocement plants

Of these, the two most familiar types in developing countries are the fixed-dome plants and the floating-drum plants. Typical designs in industrialized countries and appropriate design selection criteria have also been considered.

Fixed-dome plants

The costs of a fixed-dome biogas plant are relatively low. It is simple as no moving parts exist. There are also no rusting steel parts and hence a long life of the plant (20 years or more) can be expected. The plant is constructed underground, protecting it from physical damage and saving space. While the underground digester is protected from low temperatures at night and during cold seasons, sunshine and warm seasons take longer to heat up the digester. No day/night fluctuations of temperature in the digester positively influence the bacteriological processes.
The construction of fixed dome plants is labor-intensive, thus creating local employment. Fixed-dome plants are not easy to build. They should only be built where construction can be supervised by experienced biogas technicians. Otherwise plants may not be gas-tight (porosity and cracks).
The basic elements of a fixed dome plant (here the Nicarao Design) are shown in the figure below.

Fixed dome plant Nicarao design: 1. Mixing tank with inlet pipe and sand trap. 2. Digester. 3. Compensation and removal tank. 4. Gasholder. 5. Gaspipe. 6. Entry hatch, with gastight seal. 7. Accumulation of thick sludge. 8. Outlet pipe. 9. Reference level. 10. Supernatant scum, broken up by varying level. Source: TBW

Basic function of a fixed-dome biogas plant, 1 Mixing pit, 2 Digester, 3 Gasholder, 4 Displacement pit, 5 Gas pipe Source: OEKOTOP

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);
• 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.

Fixed-dome plant in Tunesia. The final layers of the masonry structure are being fixed. Photo: gtz/GATE

Gas-Holder

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 Source: TBW

• 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.
• Deenbandhu, the successor of the Janata plant in India, with improved design, was more crack-proof and consumed less building material than the Janata plant. with a hemisphere digester
• CAMARTEC model has a simplified structure of a hemispherical dome shell based on a rigid foundation ring only and a calculated joint of fraction, the so-called weak / strong ring. It was developed in the late 80s in Tanzania.

Fixed dome plant CAMARTEC design Source: TBW

Climate and size

Fixed-dome plants must be covered with earth up to the top of the gas-filled space to counteract the internal pressure (up to 0,15 bar). The earth cover insulation and the option for internal heating makes them suitable for colder climates. Due to economic parameters, the recommended minimum size of a fixed-dome plant is 5 m³. Digester volumes up to 200 m³ are known and possible.
Advantages: Low initial costs and long useful life-span; no moving or rusting parts involved; basic design is compact, saves space and is well insulated; construction creates local employment.

Disadvantages: Masonry gas-holders require special sealants and high technical skills for gas-tight construction; gas leaks occur quite frequently; fluctuating gas pressure complicates gas utilization; amount of gas produced is not immediately visible, plant operation not readily understandable; fixed dome plants need exact planning of levels; excavation can be difficult and expensive in bedrock.

Installation of a Shanghai fixed-dome system near Shanghai, PR China Photo: L. Sasse

Fixed dome plants can be recommended only where construction can be supervised by experienced biogas technicians.

Floating-drum plants

Floating-drum plant in Mauretania Photo: gtz/GATE

The drum

In the past, floating-drum plants were mainly built in India. A floating-drum plant consists of a cylindrical or dome-shaped digester and a moving, floating gas-holder, or drum. The gas-holder floats either directly in the fermenting slurry or in a separate water jacket. The drum in which the biogas collects has an internal and/or external guide frame that provides stability and keeps the drum upright. If biogas is produced, the drum moves up, if gas is consumed, the gas-holder sinks back.

Size

Floating-drum plants are used chiefly for digesting animal and human feces on a continuous-feed mode of operation, i.e. with daily input. They are used most frequently by small- to middle-sized farms (digester size: 5-15m³) or in institutions and larger agro-industrial estates (digester size: 20-100m³).

Advantages: Floating-drum plants are easy to understand and operate. They provide gas at a constant pressure, and the stored gas-volume is immediately recognizable by the position of the drum. Gas-tightness is no problem, provided the gasholder is de-rusted and painted regularly.

Disadvantages: The steel drum is relatively expensive and maintenance-intensive. Removing rust and painting has to be carried out regularly. The life-time of the drum is short (up to 15 years; in tropical coastal regions about five years). If fibrous substrates are used, the gas-holder shows a tendency to get "stuck" in the resultant floating scum.

Water-jacket plant with external guide frame. 1 Mixing pit, 11 Fill pipe, 2 Digester, 3 Gasholder, 31 Guide frame, 4 Slurry store, 5 Gas pipe Source: Sasse, 1984

Water-jacket floating-drum plants

Water-jacket plants are universally applicable and easy to maintain. The drum cannot get stuck in a scum layer, even if the substrate has a high solids content. Water-jacket plants are characterized by a long useful life and a more aesthetic appearance (no dirty gas-holder). Due to their superior sealing of the substrate (hygiene!), they are recommended for use in the fermentation of night soil. The extra cost of the masonry water jacket is relatively modest.

Material of digester and drum

The digester is usually made of brick, concrete or quarry-stone masonry with plaster. The gas drum normally consists of 2.5 mm steel sheets for the sides and 2 mm sheets for the top. It has welded-in braces which break up surface scum when the drum rotates. The drum must be protected against corrosion. Suitable coating products are oil paints, synthetic paints and bitumen paints. Correct priming is important. There must be at least two preliminary coats and one topcoat. Coatings of used oil are cheap. They must be renewed monthly. Plastic sheeting stuck to bitumen sealant has not given good results. In coastal regions, repainting is necessary at least once a year, and in dry uplands at least every other year. Gas production will be higher if the drum is painted black or red rather than blue or white, because the digester temperature is increased by solar radiation. Gas drums made of 2 cm wire-mesh-reinforced concrete or fiber-cement must receive a gas-tight internal coating. The gas drum should have a slightly sloping roof, otherwise rainwater will be trapped on it, leading to rust damage. An excessively steep-pitched roof is unnecessarily expensive and the gas in the tip cannot be used because when the drum is resting on the bottom, the gas is no longer under pressure.
Floating-drums made of glass-fiber reinforced plastic and high-density polyethylene have been used successfully, but the construction costs are higher compared to using steel. Floating-drums made of wire-mesh-reinforced concrete are liable to hairline cracking and are intrinsically porous. They require a gas-tight, elastic internal coating. PVC drums are unsuitable because they are not resistant to UV.

Floating-drum plant in Burkina Faso Photo: gtz/GATE

Guide frame

The side wall of the gas drum should be just as high as the wall above the support ledge. The floating-drum must not touch the outer walls. It must not tilt, otherwise the coating will be damaged or it will get stuck. For this reason, a floating-drum always requires a guide. This guide frame must be designed in a way that allows the gas drum to be removed for repair. The drum can only be removed if air can flow into it, either by opening the gas outlet or by emptying the water jacket.
The floating gas drum can be replaced by a balloon above the digester. This reduces construction costs but in practice problems always arise with the attachment of the balloon to the digester and with the high susceptibility to physical damage.

Types of floating-drum plants

There are different types of floating-drum plants (see drawings under Construction):

• KVIC model with a cylindrical digester, the oldest and most widespread floating drum biogas plant from India.

  Fixed dome plant CAMARTEC design Source: TBW

• Pragati model with a hemisphere digester
• Ganesh model made of angular steel and plastic foil
• floating-drum plant made of pre-fabricated reinforced concrete compound units
• floating-drum plant made of fibre-glass reinforced polyester
• BORDA model: The BORDA-plant combines the static advantages of hemispherical digester with the process-stability of the floating-drum and the longer life span of a water jacket plant.