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biogas HANDBOOK corrosive coatings, which are cooled by the surrounding medium. They are completely immersed in the feedstock and usually have two or three winged, geometrically optimised propellers. Due to their guiding tubing system, consisting of gibbet, cable winch and lead profile, the stirrers can usually be adjusted to height, tilt and to the side. Paddle stirrers have a horizontal, vertical or diagonal axis (Figure 7.23 and 7.24). The motor is positioned outside the digester. Junctions, where the shaft passes the digester ceiling, membrane roof or the digester wall, have to be tight. Another possibility for mechanical mixing is axial stirrers. They are often operated continuously. Axial stirrers are usually mounted on shafts that are centrally installed on the digester ceiling. The speed of the engine, which is placed outside of the digester, is reduced to several revolutions per minute, using a transmission. They should create a steady stream in the digester that flows from the bottom, up to the walls. In horizontal digesters, the slow running paddle-reel stirrers are usually used, but they can also be installed in vertical digesters. Paddles are fixed on the horizontal stirring axis, which is mixing and pressing forward (plug-flow) the feedstock. The stirring effect should only provide vertical mixing of the feedstock. The horizontal plug-flow stream is assured by the insertion of fresh feedstock into the digester. Heating tubes for heating the feedstock are often integrated in the drive shaft and in the stirrer arms. Paddle- or reel stirrers run several times per day, with short sequences and low speed. 7.7.2 Pneumaticstirring Pneumatic stirring uses the produced biogas, which is blown from the bottom of the digester through the mass of the feedstock. The bubbles of rising gas cause a vertical movement and stir the feedstock. This system has the advantage that the necessary equipment is placed outside the digester (pumps and compressors), so the wear is lower. Pneumatic stirring not frequently used in agricultural biogas plants, as the technology is not appropriate for destruction of floating layers. Pneumatic stirring can only be used for thin liquid feedstock, with low tendency of forming floating layers. 7.7.3 Hydraulicstirring If stirred hydraulically, the feedstock is pressed by pumps and, horizontal or additional vertical pivoted vents, in the digester. The suction and discharging of the feedstock must be designed in such a way that the digester content is stirred as thoroughly as possible. Hydraulically stirred systems have the advantage that the mechanical parts of the stirrers are placed outside the digester, subject to lower wear and can be easily maintained. Hydraulic mixing is only occasionally appropriate for destruction of floating layers and, like the pneumatic stirring, only used for thin liquid feedstock, with low tendency of forming floating layers. 7.8 Biogasstorage Biogas production must be maintained as stable and constant as possible. Inside the digester, biogas is formed in fluctuating quantities and with performance peaks. When biogas is utilised in e.g. a CHP unit, the demand for biogas can vary during the day. To compensate for 82PDF Image | biogas HANDBOOK
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