Biogas production in agriculture: safety guidelines, cofermentation and emissions from combined heat and power couplings

Biogas production is an effective means for improving the efficiency of slurry utilisation. It has several positive effects : energy gaining from a renewable source, reducing of methane emissions during slurry storage, improving the nitrogen availability after spreading of the slurry. The number of agricultural biogas plants increases continuously. There is a shortage of documets for constructing biogas plants and of saftey guidelines. At the moment there are no standardized guidelines available for planning and building of biogas plants. Those guidelines would be necessary to guarantee a saftey operation and cost effective building of biogas plants as well as for authorizing procedures. The ILUET is developing technical standards for the different components of biogas plants. Those standards are currently realized on several farms. The cofermentation of agricultural wastes together with non agricultural wastes is of growing interest, because the conditions for biogas production and cofermentation have recently improved. There is little knowledge on the influence of composition of the organic wastes, of the relation from organic wastes to agricultural wastes on the biogas yield, biogas quality and on the stability of the digester operation. Investigations deal with the course of biogas yield and composition depending on different cofermentation substrates. Results show that the biogas yield doubles, when 10% of fat are added to slurry from milking cows. From 30% of cofermentation substrates onwards the biogas yield does not increase anymore but the stability of the digester decreases. Therefore the percentage of organic wastes should not exceed 30%. All positive effects of cofermentation can be used. The cycle of the organic wastes is of manageable size which helps to control the quality. Many agricultural farms can use the positive effects of cofermentation. Biogas is converted to electricity and heat by combined heat and power coupling. Exhaust fumes from combustion engines, that are run with biogas, contain air polluting substances : HC, SO2, NO, NO2 and CO e.g. The emissions depend on the composition of the biogas, which is not constant. There is only knowledge on the emissions from combustion engines used in agricultural biogas plants. Influences on the amount of emissions of air polluting substances from biogas combustion engines and on the efficiency factor of the engine are to be found : quality of biogas, exhaust fume cooling, biogas to air relation, lean concept. Combustion engines of different power levels and design are investigated. From the results proposals for means to reduce the emissions are derived. Biogas combustion with a little oxygen surplus (l > 1,3) is at the moment the most cost-effective means to reduce NOx and CO emissions. Construction and operation technique means should be aimed to enable the engine to work in the lean range (l > 1,3). / La production de biogaz est une mesure efficace pour améliorer l'utilisation des lisiers. Cette technique présente de nombreux avantages tels que l'économie d'énergie à partir d'une ressource renouvelable, la réduction des émissions de méthane au cours du stockage et l'amélioration de la disponibilité en azote lors de l'épandage des lisiers. Le nombre d'unités de méthanisation en milieu agricole augmente régulièrement sans qu'il existe de document décrivant les recommandations liées à la construction et à la sécurité de ces installations. La cofermentation de déchets agricoles avec des déchets non agricoles est une option qui se développe bien qu'il reste à améliorer les connaissances sur la composition de ces déchets organiques et leur lien avec le rendement en biogaz et la qualité du biogaz produit ainsi que sur le fonctionnement des unités. Nos résultats ont par exemple démontré que le rendement en biogaz était doublé lorsque l'on ajoute 10% de graisses à du lisier vaches laitières. Le biogaz est converti en électricité et chaleur. Les fumées qui s'échappent des appareils de combustion contiennent des polluants atmosphériques tels que SO2, NO, NO2 et CO. Ces émissions dépendent de la composition du biogaz brûlé qui n'est pas constante. La combustion du biogaz avec un léger excès d'oxygène (l > 1,3) est à présent la méthode la plus efficace et la moins coûteuse pour réduire les émissions de NOx et CO.