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biogas HANDBOOK During fall and winter the digester is filled up. In this stage, the system works with a continuous flow and has a high retention time and good gas yields. Digestate flows into the storage tank which works also as post digester. Horizontal digesters Horizontal digesters (Figure 4.3, page 33 and Figure 7.21) have a horizontal axis and a cylindrical shape. This type of digesters are usually manufactured and transported to the biogas plant site in one piece, so they are limited in size and volume. The standard type for 3 small scale solutions is a horizontal steel tank of 50-150 m , which is used as the main digester for smaller biogas plants or as pre-digesters for larger plants. There is also an alternative of concrete, the channel type digester, which allows a larger digester volume of up 3 to1000m . Horizontal digesters can also run in parallel, in order to achieve larger throughput quantities. Because of their shape, the plug-flow stream is automatically used. The feedstock flows slowly from the entry side to the discharge side, forming a plug-flow, streaming through the digester. The risk of discharging un-decomposed substrate is minimised through a minimum guaranteed retention time (MGRT) of the substrate inside the digester. Horizontal continuous flow digesters are usually used for feedstock like chicken manure, grass, maize silage or manure with a high straw content. The insulated digester is equipped with a heating system, gas dome, manure pipes and stirrer. The heating system consists of stirring heat pipes, with a warm water supply-drain or of diagonally built-in radiators. The arms of the slow mowing paddle stirrer are spirally arranged on the stirring axle, in order to insure an equal distribution of the torque. The big number of paddles is able to transport sand fallout to the drain tanks. By ensuring a continuous in and out flow of the feedstock, an HRT of 15-30 days can be obtained. The filling level of the digester always reaches the same height and will fluctuate within the gas dome during filling and stirring. The level is regulated by a siphon at the outflow. The digester is fitted out with a weatherproof cover or placed under a roof. It can be built up either on site or manufactured as a small series product. Digesters of steel and stainless steel are always manufactured above ground and placed on and fastened to a concrete foundation. The screw connections have to be sealed. Multiple tank systems Large farm scale co-digestion plants usually consist of several digester tanks. They are normally operated as continuous flow system, including one or several main digesters and post digesters. Like in the case of single digesters, the multiple tank system can consists of vertical digesters only or a combination between vertical and horizontal digesters. The storage tanks for digestate serve also as post-digesters and should always be covered with gas tight membrane. 78PDF Image | biogas HANDBOOK
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