Cogeneration in Action: CHP Installed at John Rennie & Sons

A new combined heat and power (CHP) plant will help John Rennie & Sons (Farmers) Ltd generate renewable electricity. Built by Wolf Power Systems (Germany), a pioneer in combining heat and power generation, the plant’s expected output is 550kW, most of which will be exported to the national grid.

John Rennie & Sons’ Gask Farm in Turriff, Aberdeenshire, is an arable and pig farm. An anaerobic digestion (AD) plant had already been operational at the site for 10 years, using about 15,000 tons of feedstock - mostly food waste and abattoir material, with some of the farm’s own pig slurry used as well. The AD process produces 67% methane and 32% carbon dioxide. Running off the biogas from the AD plant, the new CHP plant will be more efficient and have lower running costs. The expected output is 550kW, of which 500kW is exported to the national grid. 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 (a greenhouse gas) into the atmosphere. The CHP is a very important part in the process - using AD with CHP units ensures the effective use of biogas. 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.

Andrew Rennie, Managing Director at John Rennie & Sons, 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”.

Markus Kruse, Chief Executive at Wolf Power Systems, said: “Anaerobic digestion is a great way for managing local waste streams and reducing the emission of methane (a greenhouse gas) into the atmosphere. CHP is a very important part in the process - using AD with CHP units ensures the effective use of biogas and turns a waste product into a useful resource.”