How is biogas produced?

A biogas plant is a set of devices that simultaneously produces electricity, heat, and environmentally friendly fertilisers. Its main task is the production of biogas, which takes place during the anaerobic methane fermentation process of various organic substances or waste.

How is biogas produced?

The organic substances in the substrates are broken down with the help of methane bacteria. It should be noted that fermentation is a multi-stage process, the first step being hydrolysis, which leads to the breakdown of organic polymers into structurally simpler compounds. This includes the breakdown of proteins into amino acids, lipids into alcohols as well as higher fatty acids and carbohydrates into monosaccharides. The second step is acidogenesis, in which carboxylic acids are produced from the products of hydrolysis. The step after is acetogenesis, in which acetate is produced by heterotrophs from glucose and by autotrophs from carbon dioxide and hydrogen. The fourth stage is methanogenesis, i.e. the production of methane by methanogens from acetate or by reduction of carbon dioxide by hydrogen. The first three steps take a few minutes, while the genesis of methanogenic bacteria takes several tens to hundreds of hours.

The basic component of biogas

The basic component of biogas is methane, which makes up 52-85% of the total mixture. Indeed, the methane content is the most determining factor in using biogas for energy purposes. The next most important component is carbon dioxide, whose share ranges from 14 to 48%. In addition to these two components, hydrogen sulphide (0.08-5.5%), hydrogen (0-5.5%), carbon monoxide (0-2.1%), nitrogen (0.6-7.5%) and oxygen (0-1%) as well as moisture and solid particles can also be found in trace amounts.

The substrates used in biogas

The list of substrates that can be used in a biogas plant is very long and it includes cereals, grasses, domestic waste, sewage and landfill sludge, manure, tubers, whey, silage, slaughterhouse waste, and many other components, the most productive being maize silage, some types of sewage waste and grasses. It is now common for a biogas plant to process a mixture of several substrates. This technique is known as co-digestion. The diversification of substrates helps improve process parameters and increases security when it comes to feedstock supply. The inputs to a biogas plant must guarantee a high biogas yield, a stable fermentation process, and the possibility of using the resulting digestate in accordance with current legislation. In large biogas plants, the fermentation of a mixture of animal manure with waste from the food industry or energy crops is used. The appropriate mixture composition at the design stage should be picked by an experienced technologist. Finally, for the safety of the fermentation process, it is advisable to systematically test the substrates in a laboratory.
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