Chemistry of the Biogas Formation Process

From a chemical perspective, biogas formation is a complex multistage process involving several sequential phases, including hydrolysis, acidogenesis, acetogenesis, and methanogenesis.

During the first stage of anaerobic digestion, microorganisms decompose the substrate into organic acids, alcohols, and simple carbohydrates.

In the second stage, these intermediate products are converted into acetic acid, hydrogen, and carbon dioxide.

These stages of anaerobic digestion can be summarised by the following reaction 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 – glucose
CH₃CH₂OH – ethanol
CH₃CHOHCOOH – lactate
CH₃CH₂CH₂COOH – butyric acid
CH₃CH₂COOH – propionic acid
CH₃CH₂CH₂CH₂COOH – valeric acid
H₂O – water
CH₃COOH – acetic acid
H₂ – hydrogen
CO₂ – carbon dioxide

In the subsequent stage, methanogenesis is carried out by methanogenic microorganisms that reduce carbon dioxide to methane, as well as by microorganisms that convert acetic acid into methane and carbon dioxide. The biochemical reactions of the methanogenesis stage of anaerobic digestion are represented by the following equations:

4Н₂↑+ СО₂↑→СН₄↑+2Н₂О
СН₃СООН → СН₄↑+ СО₂↑
where CH₄ is methane.

By combining the equations describing the formation of acetic acid, hydrogen, and carbon dioxide with the methanogenesis equations, the overall reaction equations for anaerobic digestion can be derived.

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 calculated material balance for the anaerobic digestion of the above-mentioned organic compounds of biomass, based on the reaction equations presented above, is as follows:

180 кг (С₆Н₁₂О₆) = 48 кг (СН₄)↑ + 132 кг (СО₂)↑
46 кг (СН₃СН₂ОН) = 24 кг (СН₄)↑ + 22 кг (СО₂)↑
90 кг (СН₃СНОНСООН) = 24 кг (СН₄)↑ + 66 кг (СО₂)↑
88 кг (СН₃СН₂СН₂СООН) + 18 кг (Н₂O) = 40 кг (СН₄)↑ + 66 кг (СО₂)↑
74 кг (СН₃СН₂СООН) + 9 кг (Н₂O) = 28 (СН₄)↑ + 55 кг (СО₂)↑
102 кг (СН₃СН₂СН₂СН₂СООН) + 27 кг (Н₂O) = 52 кг (СН₄)↑ + 77 кг (СО₂)↑

Thus, the microbial degradation of 1 kg of organic matter during anaerobic digestion results in the production of approximately 0.4 kg of methane and 0.7 kg of carbon dioxide.

As a general reference, the anaerobic digestion of 1 kg of dry organic matter in a biogas plant produces approximately:

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.