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.
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.
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.