The biogas is generated by the bacterial activity without access to air. The Fermentation involves different types of microorganisms.
We can divide whol process into 4 phases:
1. Hydrolysis - in the environment is still oxygen in the air. Polymeric organic compounds (polysaccharides, fats, proteins) are broken down into simpler monomers - alcohols and fatty acids, releasing hydrogen (H2) and carbon dioxide (CO2).
2. Acidogenesis - consumes the remaining oxygen in the air and creates the anaerobic environment (without oxygen). This phase is made by viable microorganisms in the environment with or without oxygen. This creates higher organic acids.
3. Acetogenesis - with help of bacteria are more organic acids and alcohols transformed to acetic acid, H2 and CO2.
4. Methanogenesis - final stage of degradation process. Bacteria are strictly capable of life only in an environment without access to air, acetic acid decomposes into methane (CH4) and CO2. Some strains produce methane from CO2 and H2. This final phase is about five times slower than the previous three phases, so the size of the fermenter must be adapted to dosage of starting materials.
The resulting biogas is essentially a mixture of methane and CO2, which contains impurities such as N2, H2S, NH3, H2O:
The most problematic chemical is hydrogen sulfide (H2S), because if it is in a quantity over 0.1% it is corrosive to engines and technological equipment. Ammonia (NH3) is a source of odors. The presence of CO2 in the biogas is a benefit if the biogas is burned in cogeneration, CO2 acts as if the anti-knock in internal combustion engines.