LONG-TIME farm renewables innovators John Rennie and Sons, of Gask Farm in Turriff, have installed a NEW combined heat and power plant to generate renewable electricity from the gas produced in their well-established anaerobic digester.

Rennie and Sons run an arable and pig unit, and led the way with the addition of an AD plant 10 years ago, which now uses about 15,000 tons of feedstock – mostly food waste and abattoir material, with some of the farm's own pig slurry used as well, to produce biogas.

This biogas will run the new CHP plant, built by Wolf Power Systems, with an expected output is 550kW, of which 500kW will be exported to the national grid, and the rest is used to run the facility.

Anaerobic digestion is a great way for managing local waste streams and reducing the emission of methane into the atmosphere. The CHP is a very important addition to this process, as it ensures the effective use of AD biogas. Via burning to drive a turbine, the methane is converted into electricity, but the CHP also produces a lot of hot water, which is used to keep the digester at a constant 40°C.

Some of the hot water is also used by an on-site pasteurisation plant, which heats up the digestate to over 70°C and thereby kills any pathogens which may have been in the animal by-products or food waste. The digestate can then replace mineral fertilisers – another win for the environment.

Managing director Andrew Rennie said: "I think it is great that we can take the energy out of the waste streams, put it through the CHP and clean the digestate up again with its own energy and still be left with a good, clean and nutritious fertilizer to grow next year's crops. By doing this we have managed to reduce our fertiliser bill by 90% and have gone carbon neutral."