1.1.3 Biogas Composition

Biogas is formed during AD and has two main constituents: methane (about 55-70% by volume) and carbon dioxide (30-40%). Depending on the source of the biogas, other minor components include nitrogen (<2%), hydrogen, oxygen (<1%), hydro­gen sulfide (0-50 ppm), and other sulfide compounds, volatile organic compounds (VOC) 10-270 mg/m3, and siloxanes with concentration ranging from 80 to

2,500 mg/m3 [98, 99]. The VOC comprise aromatic and halogenated compounds. Large amounts of noxious VOC can be produced during digestion of household wastes [100, 101].

Carbon dioxide is not a harmful inert gas but the presence of carbon dioxide in biogas reduces its calorific value. The removal of carbon dioxide is an expensive process and power generation equipment commonly operates with carbon dioxide concentrations up to 40-50%.

The most abundant sulfur compound in biogas is hydrogen sulfide but other reduced sulfur chemicals (e. g., sulfides, thiols) are present as well. The main source of sulfur in biogas is degradation of sulfur containing amino-acids—cysteine and methionine. Hydrogen sulfide at concentrations higher than 300-500 ppm can form unhealthy and hazardous sulfur dioxide (SO2) and sulfuric acid (H2 SO4) which corrodes pipeline metal parts, storage tanks, compressors, and engines [102]. Frequently, it is necessary to install sulfur removing facilities before the biogas application. Another corrosive contaminant is ammonia (NH.) . The burning of biogas with high ammonia concentration increases the emission of nitrogen oxides to the atmosphere. Both hydrogen sulfide and ammonia are contaminants that pose a health risk.

Other compounds of concern in biogas are siloxanes—the organic polymers of silicon coming from a wide range of industrial, personal care, pharmaceutical, and other products. These organic compounds can be oxidized to silicon dioxide and accumulate on valves, gas turbines, and engines causing erosion and decreasing the operating efficiency [103].