Biogas consists of 50-75% methane, 25-45% carbon dioxide, 2-8% water vapour and traces of O2, N2, NH3, H2, H2S. In comparison, natural gas contains 80 to 90% methane. Biogas can be used similarly to natural gas in gas stoves, lamps or as fuel for engines. The energy content of the gas depends mainly on its methane content. Hence, a high methane content is desirable. A certain carbon dioxide and water vapour content is not avoidable. But particularly for the use in engines, the sulphur content has to be minimized. The average calorific value of biogas is about 21-23.5 MJ/m³. This means 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, the fermentation temperature and the retention time. Maize silage for example, a common feedstock in Germany, yields about 8 times more biogas per tonne than cow manure.
Requirement of a Biogas
For the use in gas or diesel engines the gas has to fulfil certain requirements:
- High methane content: methane is the main combustible part of the gas.
- Low water vapour and carbion dioxide (CO2) content: The lead to a low calorific value of the gas. The water vapour can be lowered by condensation either in the gas storage or on the way to the engine.
- Low sulphur content: occurs mainly in the form of hydrogen sulphide (H2S). Through condensation and combustion, H2S is converted to corrosive acids. This can shorten the lifetime of a motor considerably and lead to serious damages. Desulphurization methods include the injection of a small amount of oxygen (air) into the headspace of the storage fermenter leads to oxidation of H2S to H2SO4 or elemental S by microorganisms and hence the elimination of a considerable part of the sulphur from the gaseous phase. Another option is the external chemical treatment in a filter using active materials such as iron-hydroxide Fe (OH)2 or activated carbon.
- Optimized steady fermentation process with continuous availability of appropriate feedstock is important to produce a gas of homogenous quality.