The Anaerobic Digestion (AD) plant is designed to produce and store both biogas and heat. The biogas is used to product power which will be sold onto the electricity grid. The heat produced in Blakiston will be used to partially supply heat energy to an adjoining residential development. This heat supply can be used as a supplement to individual housing heating systems and should produce a significant reduction in the requirement from the grid to these residences.
The AD system used in Blakiston is produced in Germany and is a 'ring-in-ring' system. There is a 32 metre outer ring made of concrete with a 22 metre inner ring forming two tanks, with one inside the other. The tanks are 6 metres deep providing 2400 cubic metres of capacity in the outer ring and 2200 cubic metres in the inner ring. Livestock waste (in the form of slurry) is pumped into the outer ring through a 140 cubic metre reception tank via a pumping system. Silage is also fed into this outer ring using a conveyor belt. Two types of mixing mechanisms keep the contents of the outer tank moving at all times to ensure that no stationary material blocks the system. These mechanisms operate at the surface and below the surface of the tank contents.
Once full, the digestate overflows from the outer tank to the inner tank. Gas emitted from the inner and outer tank is collected in a 'gas bag' which is installed above the surface of the two tanks. This gas is then piped from the gas bag to a 250kW CHP (combined heat and power) engine. This engine is housed in an adjoining building. Power from the engine is passed to a control panel where electricity is passed to the farm itself and out to the electricity grid.
The heat energy is roughly half of the energy output from the system so it is a significant bi-product of the system from a farm electricity supply perspective. In the case of Blakiston, its proximity to a residential delelopment means that the heat generated by the AD system can be passed to the residences to supplement their heat requirements for water and room heating. Some farms could also use this heat energy for other farm systems such as grain drying, which is an importent part of the grain production process.