Biogas is a mixture of gases, according to the fermentation raw materials and fermentation time, the composition of the gas is different, but its main components are methane and carbon dioxide. In the normal stable gas production stage, methane accounts for 45% to 60% of the total gas volume, carbon dioxide accounts for 35% to 45%, and also contains a small amount of water vapor, hydrogen sulfide, hydrogen, carbon monoxide, ammonia, hydrocarbons, nitrogen, oxygen and methylmercaptan, in addition to containing a small amount of granular impurities.