Biogas Plants (GTZ, 1988, 85 p.) 3. Biogas plants 3.1 Feed methods 3.2 Plant types

Biogas Plants (GTZ, 1988, 85 p.)

3. Biogas plants

3.1 Feed methods

A distinction is made between batch and continuous plants.

Batch plants are filled completely and then emptied completely after a fixed retention time. Each design and each fermentation material is suitable for batch filling.

Large gasholders or a number of digesters are required for uniform gas supply from batch plants.

Continuous plants are filled and emptied regularly —normally daily. Each design is suitable for continuous operation, but the feed material must be flowable and uniform.

Continuous plants empty automatically through the overflow.

Continuous plants are more suitable for rural households. The necessary work fits better. into the daily round. Gas production is constant, and somewhat higher than in batch plants.

If straw and dung are to be digested together, a biogas plant can be operated on a semibatch basis. The slowly digested straw-type material is fed in about twice a year as a batch load. The dung is added and removed regularly.

3.2 Plant types

Three main types of simple biogas plants can be distinguished (see Figure 3):

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

Fig. 3: Simple biogas plants A Floating-drum plant B Fixed-dome plant C Fixed-dome plant with separate gasholder. The gas pressure is kept constant by the floating gasholder. The unit can be operated as a continuous overflow-type plant with no compensating tank. The use of an agitator is recommended. D Balloon plant E Channel-type digester with folia and sunshade

3.2.1 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.

3.2.2 Fixed-Dome Plants

A fixed-dome plant (Figure 4) 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.

Fig. 4: 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.

3.2.3 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.

Fig. 5: 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.