The biogas plant
Depending on which type of process used the biogas production is often divided in to either landfill gas or digester gas. Landfill gas is formed during the natural decomposition of organic waste at landfills. Digester gas is formed when the organic material is digested in a biogas plant. The anaerobic digestion in a biogas plant is a complex multi-step process but in short i works like this:
- The substrates is mixed in a tank under heavy stirring
- The biomass is then pumped to a hygienization step where the biomass is heated to 70 °C during one hour. This is done to eliminate the risk of contamination.
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After the hygenization the biomass i pumped to a digester where the decomposition process takes place in the following steps:
- Hydrolysis, where macromolecules are broken down by enzymes into smaller molecules. These enzymes ar excreated by hydrolytic bacteria
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Acid formation, where the organic compounds are converted into different fatty acids and hydrogen and carbon dioxide. It is importand to maintain a low hydrogen level for an optimal functionality in this step.
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Methane formation, where methane gas is formed from acetic acid, hydrogen, carbon dioxide, methanol and ethanol.
- After this digestion process, that takes about 20-30 days, is the the main quantity of the organic waste degraded. The produced digestion gas is thereafter removed from the biomass and upgraded to natural gas quality or for final use.
- When converting organic waste to biogas a usefull byproduct is also formed. This byproduct is usually called digestate or biofertiliser and after quality assurance according to SPCR 120 (swedish certification rules for biofertilisers) it can be used as fertiliser on arable land