Biogas is a mixture of methane (CH4), carbon dioxide (CO2), hydrogen (H2) and hydrogen sulphide (H2S). It contains about 65% methane which as an excellent fuel. Biogas is obtained by the anaerobic fermentation of animal and plant wastes in the presence of water. Fermentation is carried out by anaerobic microorganisms in the presence of water. These microorganisms or bacteria decompose the complex carbon compounds present in bio-mass into simple compounds such as methane.
Types of Biogas Plants
There are two types of biogas plant being used in our country. Since in these plants mainly gobar is used, so they are also called gobar-gas plants. In some cases animal dung, human excreta, domestic sewage along with water serve as the raw material for those gas plants.
The two types of bio-gas plants are used are:
1. Fixed-dome type
2. Floating-gas holder type.
1. Fixed-dome Type Bio-gas Plant
This type of biogas plant consists of a digester, which is well like structure made of bricks. The roof of digester is dome-shaped. The dome of digester tank acts as a storage tank (gasholder) for biogas. Thus, the gasholder in this type of gas plant is fixed. The dome is provided with an outlet for the supply of biogas.
On the one side of digester tank is a mixing tank made above the around level and an inlet tank below ground level. On the other side of digester are outlet tank and an overflow tank. Mixing tank is connected with inlet chamber while overflow tank is connected with outlet tank.
Animal waste and water are mixed in equal proportion in the mixing tank to form slurry. The slurry goes into the digester to fill it partially so that dome is left free for the collection of bio-gas. It requires about two months for the new gas plant to start functioning. The animal dung undergoes fermentation by anaerobic bacteria to form bio-gas which gets collected in the dome. As more and more gas gets collected there, it exerts pressure over the slurry and forces the spent slurry to go into overflow tank through the outlet chamber. The spent slurry is rich in nitrogen and phosphorus compounds and forms good manure. The bio-gas collected in the dome is taken out through a pipe and used as required. Once the gas plant starts functioning more animal dung slurry is added to the digester and thus a continuous supply of bio-gas can be obtained.
The fixed dome type bio-gas plant can be constructed by using locally available materials in villages. Hence, it is cheap and easy to construct.
2. Floating Gas-holder Type Bio-gas Plant
This gas plant is quite similar to the fixed dome type but differs in one respect. Here the gasholder is not fixed but is mobile and floats over the dung slurry in the digester. The gasholder is made up of steel. The mixing tank, the digester tank and the overflow tank are made of bricks.
As more and more biogas is produced by the anaerobic fermentation of animal dung slurry by the action of bacteria, the gasholder rises. The movement of the gasholder is controlled by a control pipe. Since gasholder cannot rise beyond a certain point the pressure of gas over the slurry starts increasing. The pushes out the spent slurry into overflow tank thorough outlet pipe. This spent slurry is used as manure. The biogas collected in the gas holder is taken out through the gas control valve. IT may be mentioned here that human excreta and domestic sewage can also be used along with animal dung to obtain bio-gas.
Why Biogas is a good fuel?
The Biogas is considered as good fuel because of the following reasons:
1. Biogas is a clean fuel. It does not produce smoke during burning and hence does not cause much pollution.
2. Bio-gas has high calorific value. In other words, it produces large amount of heat per unit mass.
3. It is very convenient to use.
4. It cheaper than most of the other fuels.
5. It does not require any storage space in the houses. It is directly supplied through pipes from the gas plant.