Biological Processes of Biogas Purification

Biogas is considered an essential source of secondary energy derived from the anaerobic digestion of biodegradable organic compounds and is widely used for fuel or as a reactant for producing chemicals or synthesis gas. Significant components of biogas are mainly methane (CH4), having the composition of 50–70% of total production, and the remaining are considered contaminants like carbon dioxide (CO2), 30–45%, methyl siloxanes, water vapor (H2O), ammonia (NH3), hydrogen sulfide (H2S), nitrogen (N2), oxygen (O2), halogenated volatile organic compounds (VOCs), carbon monoxide (CO), and hydrocarbons in minute amount. The presence of these contaminants in the biogas poses particular problems with its calorific value. Hence, removing contaminants can increase the calorific value of the product obtained from the anaerobic digestion of biodegradable biological reactants. Removal of only hydrogen sulfide and carbon dioxide will significantly increase the quality and calorific value of the biogas. Various technologies were developed to purify biogas produced from the anaerobic digestion of biodegradable organic compounds. These include absorption by chemical solvents, physical absorption, cryogenic separation, membrane separation, and chemical and biological methods. Compared to physiochemical and biological methods, the physiochemical method is quite expensive and hazardous while, on the other hand, the biological process is eco-friendly and feasible.