Biogas Essay

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Biogas is a clean environment friendly fuel. Raw biogas contains about 55–65% methane (CH4), 30–45% carbon dioxide (CO2), traces of hydrogen sulfide (H2S) and fractions of water vapours.
A typical composition of biogas can be seen on the following table:
Table 1: Biogas typical composition
Component Formula Concentration (% by vol.)
Methane CH4 55-70
Carbon dioxide CO2 30-45
Nitrogen N2 0-5
Oxygen O2 <1
Hydrocarbons CnH2n+2 <1
Hydrogen sulfide H2S 0-0.5
Ammonia NH3 0-0.05
Water (vapour) H2O 1-5
Siloxanes CnH2n+1SiO 0-50 mg/m3
The main compound in the Biogas mixture is Methane (CH4), which is responsible for the combustibility of the biogas. Methane burns according to the following exothermic combustion equation:
CH4 + 2O2 → CO2 + 2H2O + 192 Kcal/mol
The complete combustion of 1m3 of CH4 provides about 8.570 kcal of heat.
Biogas also contains significant amounts of Carbon Dioxide (CO2), which is non-combustible. Carbon dioxide in itself does not have any harmful effects and is not removed from the biogas unless we wish to upgrade it into biomethane. In conventional biogas CHP (Combined Heat and Power) plants the equipment for capturing CO2 is not required.
Nitrogen and oxygen are also present in the biogas mixture; their presence is mainly due to the air introduced in biogas mixture after the anaerobic digestion so as to remove the excess hydrogen sulfide amounts.
Ammonia (NH3) concentration in biogas does not exceed 0.1 mg/m3. The presence of ammonia in higher concentration is attributed to the increased nitrogen content of the substrate used (e.g. poultry manure).
Hydrogen sulphide (H2S) content in biogas determines its quality. Hydrogen sulfide concentration may exceed 0.2 % by volume in raw biogas, capable of causing damages ...

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...ed for building of growth of bacteria. Assuming all other conditions are favourable for biogas production, a carbon-nitrogen ration of about 30:1 is ideal for the raw materials fed into a biogas plant with 2% phosphorous for maximum biological activity. A higher ratio will leave carbon still available after the nitrogen has been consumed, starving some of the bacteria of this. These will in turn die, returning nitrogen to the mixture, but slowing the process. Too much nitrogen will cause this to be left over at the end of digestion (which stops when the carbon has been consumed) and reduce the quality of the fertilizer produced by the biogas plant. The correct ratio of carbon to nitrogen will prevent loss of either fertilizer quality or methane content. Oil cakes and animal urine are found to be suitable nutrients for maintaining C, P and N within the optimum range.

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