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biogas HANDBOOK The type of storage facilities depends on the feedstock used. Storage facilities can be mainly classified into bunker silos for solid feedstock (e.g. maize silage) and storage tanks for liquid feedstock (e.g. liquid manure and slurries). Usually, bunker silos have the capacity to store feedstock more than one year and storage tanks for manure have the capacity to store feedstock several days. In some cases, also vertical cylinder silos can be used as well. The dimensioning of the storage facilities is determined by the quantities to be stored, delivery intervals and the daily amounts fed into the digester. Bunker silos for energy crops Bunker silos were originally developed to store silage as animal fodder and thus to balance its seasonal availability. Nowadays this type of storage is frequently used for storing the energy crops used as feedstock for biogas production. Silage must be made from plant material with suitable moisture content (55-70%, depending on the means of storage, degree of compression and water content that will be lost during storage). Silage undergoes a fermentation process where fermentative bacteria use energy to produce VFA such as acetate, propionate, lactate, and butyrate, which preserve the silage. The result is that silage has lower energy content than the original plant material, as fermentative bacteria use some of the carbohydrates to produce VFA. In countries like Germany, silage is stored in bunker silos, made of concrete (Figure 7.5) or in large heaps on the ground (Figure 7.6). The silage is rolled by tractor in order to pack it as firmly as possible and by this to press out the air. Minimising the oxygen content is necessary in order to avoid aerobic processes. For the same reason, the silage is usually covered by plastic foils, held tight by tyres or sand bags. Alternatively, natural covers can be established, such as a layer of grass silage, which can also tighten the bunker silo. On some silos also wheat is planted and some silos are not covered at all. This reduces costs for the cover, but increases energy losses from the silage. In the case of bunker silos, it must always be considered that the fermentation process of the silage releases liquids which can contaminate water courses, unless precautions are taken. The high nutrient content can lead to eutrophication of surface waters (growth of algae blooms). Silo effluent contains nitric acid (HNO3), which is a corrosive compound. Figure 7.5 Bunker silo (WIKIPEDIA 2008) 64PDF Image | biogas HANDBOOK
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