Biogas is generated by bacteria in the process of bio-degradation of organic material under
anaerobic (without air) conditions and constitutes a mixture of methane and other gases in following proportions:
Table 1. Biogas content [13]
| Gas | chemical formula | Volume |
|---|---|---|
| Methane | CH4 | 40 -70% |
| Carbon dioxide | CO2 | 30 -60% |
| Other gases | 1 -5% | |
| Hydrogen | H2 | 0 -1% |
| Hydrogen sulfide | H2S | 0 -3% |
Calorific value of 1 cubic meter of biogas depending on methane content is 20-25 MJ/ m3 which corresponds to combustion of 0,6 – 0,8 liters of benzene; burning of 1,3 – 1,7 kilograms of firewood or using 5 – 7 kW of electricity [13].
Biological fermentation process
In the process of fermentation in biogas plants methane producing bacteria degrade organic material and return the decomposition products in form of biogas and other components into the environment. Knowledge of this process is necessary for choosing design of a biogas plant, its construction and operation.
Substrate content and biogas yield
In principle all organic materials are subject to processes of fermentation and degradation. However in simple biogas plants it is advisable to digest only homogenous and liquid organic wastes: excrements and urine of cattle, pigs and birds, human faeces.
In more complex biogas plants it is possible to digest other types of organic waste such as plant residues and garbage. The volume of produced biogas depends on the type of substrate used and on the temperature of fermentation process.
Biogas utilization
Biogas can be utilized in any gas appliances in the same manner as natural gas. The most effective utilization of biogas is cooking, heating of premises, generating electricity and fuelling cars.