For biogas plants we basically distinguish between the fermentation of solids and liquid mixtures.
The fermentation of free flowing, organic solid matters up to 60 % of dry substance in an airtight fermenter means dry fermentation.
During this fermentation a high quality biogas is produced with a low sulfide content and high methane content of over 60% depending on the feedstock/starting material. With this method, the requirements for the added material are very low. As already mentioned, free flowing organic solids are being considered such as organic waste, grass clippings or manure.
Advantage of fermentation with solid matters
One considerable advantage is that contraries do not have to be sorted out. So for example hop bushes can be used with the wire. The vegetable material will ferment to biogas and non organic material such as the wire from the hop will be left over.
By this method, one minimizes the effort and biomass is converted very efficiently into biogas. The process is divided into separate fermenters, in order to ensure a continuous process and thus a constant gas production. Here, the biomass is fermented hermetically sealed. The fermentation residue can be composted as a high quality organic fertilizer or be applied in agriculture.
Biogas is produced by fermentation of organic substances in a liquid mixture. Biogas is by fermentation of organic substances in a liquid mixture. The resulting gas is a gas mixture with the main components of methane CH4 and carbon dioxide CO2.
As a starting material in a biogas plant all types of organic biomass are suitable. Mainly corn, grass and whole crop silage is used.The plants mentioned are dedicated energy crops which are specially grown for that purpose. Residues from livestock manure or indigestible residues of animal feed are also used.
The biogas production takes place in an airtight container in anaerobic condition at a temperature between 43° C and 55° C. Here, the mentioned energy crops are used from bacteria as nutrients and as an energy source and converted into biogas. The resulting biogas is withdrawn and burnt in a cogeneration plant. In this process heat and electricity are generated. The waste heat from the engine is used to a small proportion of 15 - 20% for heating the vessel *. The majority of the heat produced is used for district heating of the surrounding residential and industrial buildings.The remaining residue after fermentation is known as digestate and is spread on the fields as a good and nutritious organic fertilizer. Thus the soil is given back the nutrients which have been previously withdrawn by the growth of plants.
Biogas plants are paid according to the Renewable Energies Act
Depending on the size of the plant they receive a staggered payment. A biogas plant makes a positive contribution to reducing CO2 emissions and thus saving the environment.* Source: Prof. Dr.-Ing. F. Bischof, Lecture Notes Biogas Technology, Amberg-Weiden University