Biogas & Bio-methane > What is Biogas?

What is Biogas?

Biogas occurs widely in nature. Biogas forms wherever organic material accrues under exclusion of oxygen (called anaerobic digestion), e.g. in bogs, on the bottom of lakes or in ruminants’ stomachs.

The organic matter is almost entirely converted into biogas in these conditions. The actual process by which biogas forms involves the complex interac tion of various microorganisms. The solid material that remains in the digester after fermentation can be used as an organic fertilizer. 
The end product of anerobic digestion is the combustible biogas that is mainly composed as follows:

50 – 75 % methane (CH4)
25 – 45 % carbon dioxide (CO2)
  2 – 7 % water (H20)
     < 2 % oxygen (O2)
     < 2 % nitrogen (N2)
     < 1 % ammonia (NH3)
     < 1 % hydrogen sulphide (H2S)

How does biogas develop?

The actual process by which biogas forms involves the complex interac tion of various microorganisms and takes place in basiclly four separate phases.

The first stage of decomposition in methane producing fermentation is the liquefaction phase, which splits longchain organic compounds (e.g. fats, carbohydrates) into simpler organic compounds (e.g. amino acids, fatty acids, sugars) through bacterial action.

The products of hydrolysis are subsequently metabolised in the acidification phase (acidogenesis) by acidogenic bacteria and broken down into short-chain fatty acids (e.g. acetic, propionic and butyric acid). Acetate, hydrogen and carbon dioxide are also created and act as initial products for methane formation.

In the acetic acid phase (acetogenesis), the organic acids and alcohols are broken down into acetic acid, hydrogen and carbon dioxide. These products act as a substrate for methanogenic microorganisms.

In the fourth and finale phase, during which methane is formed (methanogenesis), the products from the previous phases are converted into methane by methanogenic microorganisms (archaea). The end product of fermentation is the combustible biogas.

Which materials can biogas be made from?

Biogas is gained from organic materials. The origin of the substrates can vary, ranging from livestock waste, harvest surplus, vegetable oil remains, to materials from household organic waste collection containers. The evaluation of materials for implementation in the biogas process depends on their potential attainable yield (see following Figure on Biogas yield and methane content of various substrates).

Along with renewable raw materials, non-agricultural substrates are also suitable for producing biogas, such as residues from the food industry (e.g. pomace, distiller’s wash, grease separator residues), vegetable waste from wholesale markets, food waste or grass clippings and organic waste from municipal waste disposal. The fermentation of residues material (called co-fermentation) provides a possibility of closing the cycle and dealing with them in a way that produces few emissions and is hygienic.

Apart from conventional materials, grass can also be fermented due to progress in the fermentation method: Farmers become "energy hosts" due to the fermentation of green material. The clean energy from grasslands can easily be transported and contributes to the reduction of greenhouse gases. Fermentation of grass can provide an ecologically and economically smart contribution to energy production of the future.

The substrate used in biogas plants across Germany is composed of about 48 % animal excrement, 26 % organic waste and industrial and agricultural residues, and 26 % renewable raw materials.


General factors which influence biogas production

The production of biogas is a natural process that functions in suited facilities. The following factors must be considered during biogas production:

  • organic substrates must be able to be biologically decomposed by microorganisms. the substrate should contain only a minimum of microbiological restrictors.
  • the temperature during the decomposing process must be within a certain range (35° to 55°C). The pH-value should range from 6 to 8 the fermentation must take place in an area sealed off from air and light.
  • the fermentation tank must be mixed at regular intervals.

The heart of the system: The fermenter - different types

During the fermentation of grass or other energy crops, the fermenter must meet higher demands in contrast to conventional biogas facilities. Within a short time period, very large quantities of materials are fermented.

Storing system

The fermenting substrate is inoculated with microorganisms and fermented three to four weeks at a constant temperature. Storage and fermentation of the material takes place in the same tank. Today it is rarely in use, though.

Flow system 

In contrast to the storing system, the filling of the fermenter takes place continuously. Storage and fermentation take place in different containers.


The storing-flow-system is a combination of a storing and flow system and in practice frequently implemented.

  • Advantages: Low investment costs, simple construction
  • Disadvantages: Clearing away of inconvenient material (such as stones) can be troublesome. Some materials (e.g. long grasses) can cause motion of swimming layers.

Steel tank

The horizontal stirring system enables thorough mixing of the fermenting substrate. This type of fermenter is frequently found in combination with a gas tight manure pit for after-fermentation.

  • Advantages: The heating system can be integrated either in the double mantel of the tank or in the paddeling system. Inconvenient materials can be easily removed at the bottom of the tank. Due to the small size of the reactor the fermenter can be filled and depleted in a short time.
  • Disadvantages: Limited volume of the tank.

Desulphurisation of the biogas

Cleaning of the biogas is recommended because of the very corrosive effect of hydrogen sulphide (H2S) which biogas contains. In principle, there are two basic procedures:

  • Absorption of hydrogen sulphide by ferric oxide and
  • microbial desulphurisation by the addition of air.

In the later process, about 4% of the surrounding air is injected with an air pump which bacteria use to reduce the hydrogen sulphide to elementary sulphur. The sulphur simply turns into precipitation.

How is the biogas converted into energy?

The part of biogas which can be used for energy production is methane. In combined heat-to-power-couplings it is converted into electricity and heat. The overall efficiency of the energy is about 80 to 90 %.

Heat is used directly for the heating of accommodations or for warm water preparation. In some cases it can also be supplied to a close or a long-distance heating network. The mechanical power generated by the processing of the biogas in the combined heat-to-power-couplings is converted into electricity by generators. Electricity is used for powering the facilities and is fed to the public power supply system. Profits from local power suppliers are an additional source of income for many operators of biogas plants. Biogas can also be supplied directly via pipelines into a biomass heating plant. By doing so, the user can avoid the implementation of an oversized boiler and the installation of an additional heat source.

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