The biogas process
How is biogas produced?
Biogas is produced by means of anaerobic digestion, i.e. the microbiological decomposition of organic materials in a humid environment under exclusion of air (anaerobic milieu). The biogas plant’s principle of operation is based on bringing about a controlled biological breakdown process (digestion/fermentation) which converts the organic biomass into its constituents water, carbon dioxide and methane.
What is biogas composed of?
The end-product is combustible biogas, a mixture that mainly consists of methane (50-75 %), carbon dioxide (25-45 %) and small proportions of water as well as trace gases, such as hydrogen sulphide, oxygen, nitrogen, ammonia and hydrogen.
The biogas process in detail
Organic materials ferment at temperatures between 0 ºC und 70 ºC under the exclusion of air - provided a humid environment- and under the effect of methanogenic bacteria (methanococcus and methanobacterium species). During the digestion process, the carbon in the substrates is converted into biogas in four phases. These four phases are divided into hydrolysis (first phase), acidification (second phase), acetogenesis (third phase) and methanogenesis (fourth phase).
Unlike composting, which represents a rotting process, digestion does not generate heat but combustible methane gas. Neutral carbon dioxide, water and trace gases, such as hydrogen sulphide, ammonia, elementary nitrogen, hydrogen and oxygen, are also generated.
The principle of anaerobic digestion is widespread. Among others, it occurs in sea muds, rivers and lakes, swamps and marshes, unventilated soil layers, landfill sites, slurry and sewage pits or rice farming. Depending on where anaerobic digestion occurs, it is referred to as marsh gas, fermentation 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 methane content of approx. 60 % and carbon dioxide content of approx. 35 %, as well as trace gases (O2, H2S, NH4, etc.).
Biogas as a substitute for natural gas
In principle, the methane in biogas is chemically equivalent to natural gas and it is the main energy-bearing component. One cubic meter of biogas with a methane content of 60 % corresponds to an energy value of approx. six kilowatt hours. The average heating value of one cubic meter of biogas is therefore equivalent to approximately 0.6 liters of heating oil. Prior to being fed into the public network, the biogas is upgraded, i.e. undesirable components are removed and the biogas is “methane-optimized”.
Fertilizer from biogas production
The digestate remaining after digestion (biomass and minerals that have not decomposed) has outstanding fertilizing properties and can be used in arable farming.