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biogas HANDBOOK Measures against foam layers Forming of foam and swimming layers can be a sign of process imbalance and their formation is often caused by the types of feedstock supplied. The presence of foam and swimming layers on the surface of biomass, inside the digester, can cause clogging of gas lines. To prevent this, gas lines should be installed as high as possible inside the digester. Foam traps can prevent penetration of foam in the feedstock pipes and to the post digester or storage basins. A foam sensor can be installed in the gas area of the digester, to start automatically spraying foam retardant inside the digester, if there is too much foam on the surface of the substrate. The foam retardants must be used only in emergency situations, as they usually consist of silicate binders which can damage the CHP plant. 7.7 Stirringtechnologies A minimum stirring of biomass inside the digester takes place by passive stirring. This occurs by insertion of fresh feedstock and the subsequent thermal convection streams as well as by the up-flow of gas bubbles. As passive stirring is not sufficient for optimal operation of the digester, active stirring must be implemented, using mechanical, hydraulic or pneumatic equipment. Up to 90% of biogas plants use mechanical stirring equipment. The digester content must be stirred several times per day with the aim of mixing the new feedstock with the existing substrate, inside the digester. Stirring prevents formation of swimming layers and of sediments, brings the micro-organisms in contact with the new feedstock particles, facilitates the up-flow of gas bubbles and homogenises distribution of heat and nutrients through the whole mass of substrate. Stirrers can run continuously or in sequences. Experience shows that stirring sequences can be empirically optimised and adapted to a specific biogas plant (tank size, feedstock quality, tendency to form floating layers etc.). After the supply of the first feedstock load and the start-up of the plant, the optimum duration and frequency of stirring sequences and adjustment of stirrers will be determined by experience, through continuous monitoring of digester performance. Experience from Denmark shows that submerged, electrically driven, medium speed stirrers, largely utilised in the past, proved relatively expensive in operation and difficult to access for service and inspection. A better alternative proved to be the continuously, slow rotating stirrers, installed centrally, in the top of the digesters, although their utilisation requires a precise adjustment of the level of biomass inside the digester, in order to avoid formation of floating layers. 7.7.1 Mechanicalstirring According to their rotation speed, mechanical stirrers can be intensive fast running stirrers, medium running stirrers and slow running stirrers. 80PDF Image | biogas HANDBOOK
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