The chemistry of biogas formation process
Formation of biogas from a chemical perspective is a complex multi-stage process that involves several consecutive steps, including hydrolysis, glycolysis, hydrogen oxidation, and other processes.
Under the influence of bacteria in the first stage of anaerobic fermentation, substrate decomposition occurs, resulting in the formation of organic acids, alcohols, and simple carbohydrates. In the second stage, acetic acid, hydrogen, and carbon dioxide are produced. These stages of anaerobic fermentation can be summarized by the following equations:
С6H12O6+2H2O → 2CH3COOH+4H2↑+2CO2↑
СН3СН2ОН+Н2O→СН3СООН+2Н2↑
СН3СНОНСООН+Н2O→СН3СООН+2Н2↑+СО2↑
СН3СН2СН2СООН+2Н2O→2СН3СООН+2Н2↑
СН3СН2СООН+2Н2O→СН3СООН+3Н2↑+СО2↑
СН3СН2СН2СН2СООН+4Н2O→2СН3СООН+5Н2↑+СО2↑
where C6H12O6 is glucose; C2H5OH is ethanol; C3H6O3 is lactate; C4H8O2 is butyric acid; C3H6O2 is propionic acid; C5H10O2 is valeric acid; H2O is water; CH3COOH is acetic acid; H2 is hydrogen; CO2 is carbon dioxide.
Furthermore, methanogenesis occurs through the activity of microorganisms that reduce carbon dioxide to methane and those that ultimately break down acetic acid into methane and carbon dioxide. The equations for the biochemical reactions of the methanogenesis stage of anaerobic fermentation are as follows:
4H2↑ + CO2↑ → CH4↑ + 2H2O CH3COOH → CH4↑ + CO2↑
where CH4 ↑ – methane.
By combining the equations for the acetic acid formation stage with the methanogenesis equations, we obtain the following:
С6Н12О6→3СН4↑+3СО2↑
СН3СН2ОН→1,5СН4↑+0,5СО2↑
СН3СНОНСООН→1,5СН4↑+1,5СО2↑
СН3СН2СН2СООН+Н2O→ 2,5СН4↑+1,5СО2↑
СН3СН2СООН+0,5Н2O→1,75СН4↑+1, 25СО2↑
СН3СН2СН2СН2СООН+1,5Н2O→3,25СН4↑+1,75СО2↑
The material balance calculation for the anaerobic fermentation of the organic substances from the biomass mentioned above, following the equations, will take the following form:
180 kg (С6Н12О6) = 48 kg (СН4)↑ + 132 kg (СО2)↑
46 kg (СН3СН2ОН) = 24 kg (СН4)↑ + 22 kg (СО2)↑
90 kg (СН3СНОНСООН) = 24 kg (СН4)↑ + 66 kg (СО2)↑
88 kg (СН3СН2СН2СООН) + 18 kg (Н2O) = 40 kg (СН4)↑ + 66 kg (СО2)↑
74 kg (СН3СН2СООН) + 9 kg (Н2O) = 28 (СН4)↑ + 55 kg (СО2)↑
102 kg (СН3СН2СН2СН2СООН) + 27 kg (Н2O) = 52 kg (СН4)↑ + 77 kg (СО2)↑
Thus, during anaerobic fermentation, the microbiological breakdown of 1 kg of organic matter is typically accompanied by the release of approximately 0.4 kg of methane and 0.7 kg of carbon dioxide.
For reference, from 1 kg of dry organic matter processed in a biogas plant, you can obtain approximately:
– About 0.9 m³ of biogas from carbohydrates.
– Up to 1.5 m³ of biogas from fats.
– Up to 0.7 m³ of biogas from proteins.
