Biogas Generation

Biogas Generation

Biogas is produced by anaerobic decomposition of organic wastes by suitable bacteria. It contains (55-65) percent methane, (30-40) percent carbon dioxide and the remainder are impurities like H₂S, H₂,H₂ gases.

The main source of production of biogas are crop residue, wet cow dung, vegetable wastes, water hyacinth, algae, poultry or piggery droppings, human waste, bagasse , rice-husk etc. Any organic material of animal or plant which is easily bio-degradable can be the source of biogas production.

Cattle dung can produce 0.037 m³ of biogas per kg of cow dung. The calorific value of gas is 21000 to 23000 kJ / kg or about 38000 kJ /m³ of gas. The material from which biogas is produced retains its value as fertilizer or as animal feed which can be used after certain processing.

Biogas can be produced by digestion pyrolysis or hydro-gasification. Digestion is a biological process that occurs in absence of O₂ and in presence of anaerobic organisms at atmospheric pressures and temperatures of 35°C-70°C. The container in which the digestion takes place is called digester.

Bacteria used for production of biogas can be divided into two major groups based on their oxygen requirements to grow. The bacteria which grows in presence of oxygen is called aerobic bacteria and the other which grows in the absence of oxygen is called anaerobic bacteria. When organic matter undergoes fermentation, the anaerobe bacteria extracts oxygen by decomposing the biomass at low temperatures upto 65°C in the presence of moisture. (80% to 95%), the gas so produced is called biogas.

Principles of Biogas Production from Waste Biomass :

As discussed above, the biogas production from waste biomass is achieved by the action of anaerobic bacteria in presence of moisture and in the absence of oxygen. The conversion process is called bio-digestion or anaerobic fermentation. The bio – chemical process takes place in three stages as shown in Figure A. These are as follows :

1. Stage I-Hydrolysis : Firstly the biomass having complex compounds such as fats, proteins,  carbohydrates etc are broken down into simple water soluble organic compounds through the influence of water called hydrolysis.
2. Stage II (Acid formation) : The micro-organism of anaerobic and facultative group (which grows in absence of  O₂). called acid forming bacteria produce mainly the acetic acid and propionic acid at low temperature of about 25°C with release of CO₂. In certain cases, the acid may be produced in such large quantities that all the biological activity is arrested. Thus, it becomes necessary to control the pH value of mixture.
3. Stage III (Methane formation) : In this stage the anaerobic bacteria called as methane formers converts the organic acids formed in stage II into biogas having its main constituent’s as methane and CO₂with other small traces of H₂S, H₂ and N₂ etc. These methane formers are sensitive to pH changes.

The biogas production depends on the environment maintained in the digester.