Biogas belongs to the group of renewable energy sources and consists mainly of methane and carbon dioxide. It is produced in biogas plants using a biotechnological, that is to say, fermentative method. The main components of a biogas plant are: main digester, secondary digester with gas holder, storage tank for the fully fermented residues (for later use as fertiliser) and a combined heat and power plant (CHP plant) for converting the methane gas to electricity. Gas refinement and gas feed-in are also carried out in some cases.
Biogas is formed when the organic material (semi-liquid manure, or maize silage, etc.) that is fed into the biogas plant is metabolised (broken down), via a series of biochemical stages, by a complex population of micro-organisms and their enzymes to produce methane. The methane bacteria only become active at the end of a series of fermentation processes that involve many other fermentation organisms.
The process stages can be described in very simplified terms as follows:
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Hydrolysis
The biomass, consisting of the polymers cellulose, xylan, starch, pectin, lipids (fats), and proteins, for example, is broken down to form the corresponding monomers: xylose, glucose, galacturonic acid, glycerine and fatty acids such as amino acids. -
Acidogenesis
The monomers are then converted by means of acidogenesis into various organic acids, including acetic acid and formic acid, but also into methanol. -
Methanogenesis
In the methane production stage (methanogenesis), methane is produced from the above-mentioned decomposition products (metabolites).
Hence, the microbiological aspect of the biogas production process can be controlled by optimising the fermentation biology with the addition of trace element mixtures, micronutrient preparations and other additives.