iogas is the gas resulting from an anaerobic digestion process. A biogas plant can convert animal manure, green plants, waste from agro industry and slaughterhouses into combustible gas.
► For further information on the biogas production and fermentation process please refer to the Biogas Portal on energypedia.
Biogas can be used in similar ways as natural gas in gas stoves, lamps or as fuel for engines. It consists of 50-75% methane, 25-45% carbon dioxide, 2-8% water vapour and traces of O2 N2, NH3 H2 H2S. Compare this with natural gas, which contains 80 to 90% methane. The energy content of the gas depends mainly on its methane content. High methane content is therefore desirable. A certain carbon dioxide and water vapour content is unavoidable, but sulphur content must be minimised - particularly for use in engines.
The average calorific value of biogas is about 21-23.5 MJ/m³, so that 1 m³ of biogas corresponds to 0.5-0.6 l diesel fuel or about 6 kWh (FNR, 2009).
The biogas yield of a plant depends not only on the type of feedstock, but also on the plant design, fermentation temperature and retention time. Maize silage for example - a common feedstock in Germany - yields about 8 times more biogas per ton than cow manure. In Germany, cow manure and energy crops are the main forms of feedstock. About 2 live-stock units (corresponding to about 2 cows or 12 rearing pigs) plus 1 ha of maize and grass are expected to yield a constant output of about 2 kWel (48kWhel per day. In the South Asian context, ESMAP uses a typical specific input-output relation of about 14 kg of fresh cattle dung (the approximate production of one cow on one day) plus 0.06 l diesel fuel to produce 1kWh electricity.
Gas Production Figures If the daily amount of available dung (fresh weight) is known, gas production per day in warm tropical countries will approximately correspond to the following values:
1 kg cattle dung 40 liters biogas
1 kg buffalo dung 30 liter biogas
1 kg pig dung 60 liter biogas
1 kg chicken droppings 70 liter biogas
If the live weight of all animals whose dung is put into the biogas plant is known, the daily gas production will correspond approximately to the following values:
cattle, buffalo and chicken: 1,5 liters biogas per day per 1 kg live weight
pigs, humans: 30 liters biogas per day per 1 kg weight
For yield ranges and methane contents for 33 different substrates, please see Gas Yields and Methane Contents for Various Substrates.
Some Additional Facts
Each kilogram of biodegradable material yields 0.4 m3(400 liters) of gas.
Gas lights consume around 0.1m3(100 liters) of gas in one hour[1].