The digester is a ring-in-ring system with a 32 metre outer ring and a 22 metre inner ring. These tanks are 6 metres deep with a capacity of 2400 m3 in the outer ring and 2200 m3 in the inner ring. A surface mixer in the outer ring (‘Mississippi paddle wheel’) with a below surface mixer through the side keeps the digestate moving round this outer ring. Slurry is fed in to the outer ring through a 7 metre by 4 metre deep (150 m3) reception tank connected to a central pumping station and silage is fed in through a Fliegl feeder. Digestate flows from the outer to the inner ring as feedstock is added. The floor, wall and lid of the tanks are insulated and it is intended to run the digester at a temperature around 42°C.
Gas is collected from the inner and outer rings, cooled and fed into a 250 kW Deutz CHP engine installed in the CHP house. Electricity produced is fed through a control box to supply own usage and the Grid.
An additional 22 metre diameter by 6 metre deep store has been built for storage and future expansion of the system. An existing above ground slurry store is being connected into the system to facilitate movement of slurry from the farm units to the AD unit and digestate back out unto the land.
Total cost of the unit is around £1.2m
A 62 m X 26 m feedstock store has been built beside the AD unit to conveniently store and supply non-liquid energy materials – mainly grass silage.
Proposed operation of the plant
The AD system is being built on the 300 ha Carrowreagh farm integrating with the existing 400 dairy cow enterprise.
Inputs to the system are estimated at around 5500 tonnes slurry and 6500 tonnes silage per year (around 32 t /day) with an estimated annual output of 1m cubic metres of biogas producing about 2m kWh electricity and 2.25m kWh heat from the CHP.
Electricity produced will attract 4 NI ROCs worth around 18p / kWh and a sale value of about 5.5p / kWh.
About ⅓ of the heat produced will be required to maintain the plant temperature and it is proposed to use surplus heat to dry wood for the estate wood chip enterprise.
Digestate from the system (estimated at 11000 t per annum) will be spread back on the land with a potential saving of fertiliser.