Adding gas from biomass to the gas grid - SGC

the type and volume of the respective farm waste. Hence, every installation was unique and pricesremained rather high with the consequence that with the decreasing prices of the oil theconstruction of new installations came to a halt.Since 1990, when Switzerland started to promote renewable energy again within the program‘Energy 2000’, a few new plants were erected digesting bio- and food wastes together with manure.Agricultural biogas installationsThe first agricultural biogas plants in Swiss agriculture were constructed right after the first energycrisis in the year 1973. The second crisis boosted the number of plants from 15 in 1980 up toapprox. 130 in 1983. The maximum number was reached in 1985 with 140 plants. Some of thepioneer plants were closed down and a few new digesters have been built since then.Today less than 70 farm-scale biogas plants are in operation in Switzerland. Three installations aretreating solid waste, all the others are running on liquid manure with addition of chopped straw (1.5to 3 kg per animal and day). One of the solid waste digesters is a four vessel batch system made ofprefabricated, concrete side walls with a sandwich insulation and a floating plastic cover. A secondone is an upright cylindrically shaped, continuous flow reactor where the waste is pumped upwardsthrough an inner cylinder and flowing down by gravity through an outer cylinder. The third and lastsolid waste digester is a down-flow pilot plant of 10 m 3 operated by Nova Energie at the SwissFederal Research Station, FAT in Tänikon.Except for five, all of the liquid digesters are operated in a continuous flow mode. The predominantconstructions are either upright cylinders made of concrete or glass fiber reinforced plastic, or sunkin ground concrete digesters of either rectangular or cylindrical shape. The digester volumes rangefrom 30 m 3 to over 600 m 3 . All of the digesters are stirred mechanically. Some of them areconnected to storage tanks covered with gas tight plastic membranes storing at the same time thegas from the reactor and the gas produced during post fermentation in the storage tank.The five systems not fed continuously as mentioned above are so-called accumulation systemswhere the gas-tight and insulated storage tank is heated, thus serving at the same time as reactor.All but one biogas system are operated on individual farms. Centralized anaerobic digestion is notcommon at all. The only two central biogas plants serving two respectively three farmers are codigestingslurry with vegetable and source separated organic waste and manure.Originally, the major motivation for the construction of the plants was ‘energetical independence’followed by ‘improvement of the fertilizer quality’. In recent years however, the two priorities wereexchanged. The production of electricity became important when the price was fixed to CHF 0.16per kWh for renewable energy.page: 90