Types of Biogas Plant

The biogas plants are built in different sizes and shapes depending on the process carried out. These plants are mainly classified as:

1. Continuous or batch type,
2. Dome and drum type.

Continuous or Batch Type Biogas Plants

In the continuous type of biogas plant the biomass is fed regularly to the digester and it delivers the biogas continuously. Continuous type biogas plants are of two types, namely:

1. Single stage continuous type biogas plant,
2. Two stage continuous type biogas plant.

Single stage continuous type biogas plant: In this type of plants the entire process of conversion of biomass into biogas (consisting of stage I of acid formation and stage II of methane formation) are carried out in the single chamber or digester without barrier. Such plants are simple in construction, easy to operate and control. It does not need any skilled workers.

Two stage continuous type biogas plant: These plants have two chambers for digestion of biomass. In the first chamber the biomass is fed in which the stage I of acid formation takes place. Then the diluted acids are only fed into the second chamber where the stage II of methane formation is carried out. The biogas so produced is collected from the second chamber.

Advantages of these plants are that it produce more biogas, requires smaller size of digestion chamber and lesser period of digestion as compared to single stage plants.

The disadvantages are that the process of digestion is complex, plant is costlier, difficult to operate and maintain and requires skilled workers. Single stage plants are preferred for small and medium size biogas plants while the two stage plants are preferred for larger size biogas plants.

Batch Type Biogas Plants

In case of batch type plants, the biomass feeding is done in batches with large time interval between the two consecutive batches. A digester is loaded with feed and given sufficient time (30 – 45 days) for digestion of biomass. After completion of digestion, the residue is emptied and it is again charged with the next batch of feed.

Therefore in batch type of biogas plants the rate of production of biogas is uneven. To overcome this difficulty, several digesters are used which are fed and emptied in sequential manner. Thus, it maintains the regular supply of gas. The silent features of batch type biogas plants are:

(i) Gas production is intermittent and unevenly paced.
(ii) Needs several digesters for continuously supply of gas.
(iii) The cost and space requirement is high.
(iv) Needs addition of fermented slurry to start the anaerobic fermentation.
(v) These plants have longer digestion period, thus they are suitable for hard fibrous materials.
(vi) Have operational and maintenance problems.

Dome and Drum Type Biogas Plants

Various models of dome and drum type of biogas plants have been developed by the various agencies. These are mainly of two types:

• Floating drum type biogas plants
• Fixed dome type biogas plants

Floating Drum Type Biogas Plants (Constant Pressure Type)

Many models of floating drum type biogas plants have been developed in various countries. One of the model is discussed here.

Floating drum type KVIC model: A common gobar gas plant suggested by Khadi and Village Industries Commission (KVIC) India, is described below. These are suitable for small scale gas production.

The plant consists of a digester made of masonary construction in the form of a well below the ground level and the floating gas holder, also called as dome, made of mild steel.

In the inlet tank animal waste slurry is prepared containing cow dung and water in the ratio as 1:1 to 1:1.25. The feeding of animal waste slurry is usually done once in a day.

The sludge comes out with the built up of gas pressure in the dome above the partition wall and flows out to the outlet tank through A C outlet pipe. This sludge is an excellent fertilizer which can be again fed to the soil.

At the top of the gas holder, the accumulated gas is drawn from the pipe through gas valve. The bifurcation of digestion chamber through a partition wall provides optimum conditions for growth of acid formers and methane formers as the pH value requirement for these bacteria are different.

Therefore, this gives a good yield of biogas. It operates naturally under constant pressure. The diameter of the digester of a gas plant ranges from 1.2 to 6 m and its height varies from 3 m to 6 m. The mild steel gas holders are prone to corrosion, thus needs painting at regular intervals.

This problem can be overcome by using fiber glass reinforced plastic (FRP) material for construction of gas holders, however it is costly.

Advantages of KVIC Plant

1. High gas yield
2. No problem of gas leakage.
3. Works under constant pressure naturally.
4. No problem of mixing of biogas with external air, thus no danger of explosion.

Disadvantages

1. It has higher cost
2. Heat is lost through metal gas holder.
3. Requires painting of drum to avoid corrosion at least twice a year.
4. Requires maintenance of pipes and joints.

Fixed Dome Type Biogas Plants

In fixed dome type biogas plants, the digester and the gas dome (gas collector) are combined and enclosed in the same chamber. These types of plants are best suited for batch type gas plants. These types of plants are more economical compared to floating dome type since only the masonry work is needed for their construction. The digester is usually built below the ground level, particularly for applications in regions having the cold climate.

The gas pressure inside the dome varies depending upon the rate of gas production and its consumption. However, the total volume of gas inside the dome remains constant due to which these are also called as constant volume type biogas plants.

Many variations of fixed dome type models are developed by various countries keeping in view the availability of local materials, the cost, ease of installation, maintenance and operation etc. Few of the fixed dome type biogas plants are described below.

1. Fixed dome type domestic biogas plant (Janta model or Chinese model): Figure represents the schematic diagram of a fixed dome type biogas plant called as Janta model or Chinese model of biogas plant. It is similar to KVIC model except that both the digester and gas holder are constructed in a fixed dome usually below the ground surface.

The mixture of biomass and water is supplied as feed to the digester through the inlet pipe. The mixture is stirred with a handle (not shown) to form slurry. The biogas generated in the digestion tank by anaerobic digestion is collected in the upper part of the digester.

A stirring arrangement is necessary in the digester if the raw material is a crop residue. The biogas so produced is delivered via the outlet gas pipe. When the gas is produced, the liquid level in the digester drops whereas level in the displacement tank rises.

The height difference between two levels helps in regulating the pressure of gas within the digester. The gas pressures obtained may be as high as 0.1 bar pressure above atmospheric.

Since these plants are constructed underground, their temperature of operation remains unaffected by the environment. Hence, these plants are suitable for winter operation. A manhole cover sealed by clay is provided at the top of digester dome for the purpose of its maintenance.

2. Flexible bag type biogas plant: In this the digester is made of flexible plastic material. The biomass with water and biomass slurry is supplied to digester from inlet pipe. After anaerobic digestion the biogas is collected in the upper part of the bag like dome of the digester which gets inflated. The digested slurry is discharged from outlet pipe.

Advantages of Fixed Dome Type Plant

(i) Cost of plant is less compared to floating drum type plant.
(ii) Loss of heat is negligible since these are constructed underground.
(iii) No corrosion problems as in fixed drum type.
(iv) It is maintenance free.

Disadvantages of Fixed Dome Type Plants

(i) Needs skilled labour to operate.
(ii) Gas production/m³ of digester volume is less.
(iii) Gas is produced at variable pressure.

Site Selection for Biogas Plants

Following points are considered while selecting a site for construction of biogas plants.

1. Should have minimum distance between the points of gas production and gas consumption.
2. Should be constructed in open space to utilize sun energy since the biogas production is high above 25° C temperature.
3. It should be constructed at least 10 in away from wells to avoid seepage of fermented slurry.
4. Sufficient quantity of water along with biomass is available at site for formation of biomass slurry.
5. Sufficient space is available for its operation and maintenance.

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