Adding gas from biomass to the gas grid - SGC

3.1.2 Survey of technologiesThis paragraph gives details on centralised biogas plants, farm biogas plants, sewage plants andlandfills.Centralised biogas plantsDenmark has a large experience with centralised biogas plants. The major biomass resource foranaerobic digestion in Denmark is animal manure. Approximately 75% of the biomass treated inDanish plants is manure, and around 25% of the biomass is waste that mainly originates from thefood industries. A few plants treat sewage sludge as a supplement to animal manure. A few plantsare capable of treating source separated household waste.Animal manure, mostly slurry, is transported from farms to the biogas plant in vehicles owned by thebiogas plant. The normal situation is that food processing industries and municipalities take care ofthe transportation of waste to the biogas plant.In the biogas plant manure and organic waste are mixed and digested in anaerobic tanks for 12 to25 days. During this period effective sanitation takes place and weeds and pathogens are killed ona satisfactory scale.From the digestion process biogas emerges which is cleaned and normally utilised in combinedheat and power production plants. Heat is usually distributed in district heating systems, andelectricity sold to the power grid.The biomass leaving the digestion tanks is called digested manure. It is returned by vehicle to slurrystorage tanks, until it is end-used as fertiliser.The biogas plant technology is simple, but many practical details have to be considered in order toavoid interruptions and optimise production. Manure and organic waste are mixed in mixing tanksand from here pumped into the closed and stirred reactor tanks were it is heated and anaerobicallydigested. The digestion culture is self-sustaining after a start-up phase. The process is continuous,as a daily fractional extraction is replaced by new feed. The reactor productivity is significantlyimproved by the addition of organic waste.The average residence time is between 12 and 25 days depending on temperature, shortest at hightemperature. Thermofil digestion takes place at 50 - 55 °C and mesofil at 30 - 40 °C.In order to increase production stability and capacity and to minimise power consumption, thefollowing technical details have been studied and further developed based on practical experience:• Preference of thermofil digestion depending on manure composition• Addition of organic waste• Buffer tanks for extracted slurry contributing to gas production• Mixing and stirring equipment• Pump design• Heat exchangers between incoming and extracted slurrypage: 16