Biogas is produced through fermentation. This is the microbiological decomposition of organic substances in a humid environment and in the absence of air (anaerobic environment). The functional principle of a biogas plant is based on the fact that a controlled biological decomposition process (digestion/fermentation) is induced, which converts the organic biomass mainly into the components water, carbon dioxide and methane.
The end product is the combustible biogas, a mixture consisting mainly of methane (50-75 %), carbon dioxide (25-45 %) and small amounts of water and trace gases such as hydrogen sulphide, oxygen, nitrogen, ammonia and hydrogen.
The methane in biogas is chemically equivalent to natural gas and is the main energy-bearing component. The energetic benefit of a cubic metre of biogas is about six kilowatt hours at 60 percent methane content. The average calorific value of a cubic metre of biogas is therefore about 0.6 litres of heating oil. Before being fed into the public grid, the biogas is upgraded, i.e. freed from unwanted components and “methane-optimised”.
The digestate (undegraded biomass and minerals) left over after fermentation has excellent fertilising properties and can subsequently be used in agricultural arable farming.
At temperatures between 0 ºC and 70 ºC, organic substances ferment in the absence of air – assuming a moist environment – and under the influence of methane bacteria (Methanococcus and Methanobacterium species). During fermentation, the carbon present in the substrates is broken down into biogas in four phases. These four phases are basically divided into hydrolysis (first phase), acidification (second phase), acetic acid formation (third phase) and methane formation (fourth phase).
In contrast to composting, which is a rotting process, no heat is generated during fermentation, but combustible methane gas is produced. In addition, neutral carbon dioxide, water and trace gases such as hydrogen sulphide, ammonia, elemental nitrogen, hydrogen and oxygen are produced.
The principle of anaerobic digestion is widely used. It occurs, among other things, in marine mud, in rivers and lakes, in swamps and bogs, in non-aerated soil layers, landfills, slurry and septic tanks, in rice cultivation and so on. Depending on the occurrence, one therefore speaks of sump gas, digester gas, sewage gas, mine gas, landfill gas or, in the agricultural sector, biogas.
The density of biogas is 1.21 kg/m³ with an average content of methane of approx. 60 %, carbon dioxide of approx. 35 %, as well as trace gases (O2, H2S, NH4, etc.).
