Biogas is an environment-friendly, clean, cheap and versatile fuel. It is produced by anaerobic digestion of degradable wastes such as cattle dung, vegetable wastes, sheep and poultry droppings, municipal solid waste, sewage water, landfill, etc. Presently, it is mainly used for cooking and lighting purposes in the rural areas. Biogas is used in stationary engines different agricultural operations. Its utilisation is also feasible in automobiles, and transportation purposes by enriching and compressing it within cylinders. Biogas can be converted into bio CNG after enrichment and bottling.

So far, biogas has been mostly used as fuel for cooking and running stationary engines. There is a great enhancement in its utilisation potential, particularly where bigger plants are in operation, e.g., institutional biogas plants in Gaushalas, dairy farms or community biogas plants in villages. Gaushalas are running generally on a charity basis and most of them are not in a sound financial position. Enrichment and bottling of biogas will help to improve this situation.

India has a vast potential of 6.38 X 1010 cubic metre of biogas per annum from 980 million tonnes of cattle dung produced. The Government of India launched a National Project on Biogas Development (NPBD) in 1981. By December 2004, a total of about 36.5 lakh family biogas plants had been installed under this programme all over India. This is about 30 per cent of the total 120 lakh family type biogas plants' potential. More than 3380 Community Biogas Plants (CBP), Institutional Biogas Plants (IBP) and Night-soil based Biogas Plants (NBP) have been installed all over the country with most reported satisfactory performance levels. The family biogas plants in the country are estimated to be saving 39.6 lakh tonnes of fuel wood per year. Besides, about 9.2 lakh tonnes of enriched organic manure are being produced every year from these plants.

A number of Gaushalas, dairies, and village communities with large numbers of cattle have the potential of installing biogas enrichment and bottling systems. In urban areas, large quantities of biogas can be produced in sewage treatment plants using anaerobic digestion. The Okhla Sewage Treatment Plant, New Delhi, is an example where more than 10,000 cubic meter of biogas is produced every day. Due to rising cost of petroleum products and environmental concerns, it has become imperative to make use of local resources as an alternate to petroleum fuels. Therefore, there should be worldwide efforts to explore and make use of biogas as an alternate fuel in vehicles.

Biogas comprises 60-65 per cent methane, 35-40 per cent carbon dioxide, 0.5-1.0 per cent hydrogen sulfide and water vapour. It is almost 20 per cent lighter than air. Like Liquefied Petroleum Gas (LPG), it cannot be converted to a liquid state under normal temperature. Removing carbon dioxide and compressing it into cylinders makes it easily usable for transport applications, say three-wheelers, cars, pick up vans, etc., and also for thermal applications. Already, CNG technology has become easily available and, therefore, bio-methane (enriched biogas) - which has nearly the same combination as CNG - can be used for all applications for which CNG is used.

A variety of processes are available for enrichment, i.e., removing CO2, H2S and water vapour. Commonly, CO2 removal processes also remove H2S. One of the easiest and cheapest methods involved is the use of pressurised water as an absorbent liquid. In this method, biogas is pressurised and fed to the bottom of a scrubber column where water is sprayed from the top. In a counter-currently operated absorption process, the carbon dioxide and hydrogen sulfide present in the biogas is absorbed in the down going water and the methane goes up and is collected in a vessel. Although water requirement in this process is high, it is the simplest method of removing impurities from biogas.

Biogas contains a large proportion (about 40 per cent by volume) of carbon dioxide, a heavier and non-combustible gas and some fraction of hydrogen sulphide. Hence, removing these undesirable gases to save compression energy and space in bottle and stop the corroding effect can enrich the biogas. The scrubbing system is found to enrich methane about 95 per cent or more, depending upon the biogas inlet and water injection pressure. Biogas can be used for all applications designed for natural gas, assuming sufficient purification.

Enriched biogas is made moisture free by passing it through filters, after which it is compressed up to 200-bar pressure using a three-stage gas compressor. Compressed gas is stored in high-pressure steel cylinders as is the case for CNG. There is large potential of this technology in buses, tractors, cars, auto rickshaws, irrigation pump sets and in rural industries. This will help to meet our energy demand for rural masses, thus reducing the biogas scrubber burden of petroleum demand, moving towards energy security and will further improve our economic status by creating employment generation in rural areas.

Cylinders filled from one 120 m3 biogas bottling plant = 8 cylinders /day

(Capacity 6 kg/ cylinder)

As 6 kg CNG cylinder = 6 litre Petrol

So, gas filled in these cylinders will be equivalent to = 6 x 8

= 48 litres of Petrol/day

or Diesel/Petrol savings = 17520 litres/annum.

= 876000 Rs./annum (Approx.)

= 9 lakh Rs./annum (from one bottling plant)

Therefore, from only one biogas bottling plant, enriched biogas filled cylinders will be able to replace fuel worth Rs. 9 lakh annually in the country. Increasing the number of bottling plants will subsequently increase the diesel/petrol savings. The entire cost can be recovered within two to three years from the installation of the plant.