Farmer opts for heat exchanger to improve digestate

A major business embracing heat transfer and its application in thousands of processes as well as anaerobic digestion (AD) has received an order for its first twin-tank continuous batch pasteurisation system which will process energy crops to help produce electricity for 600 Essex homes and the University of Essex.

The system is being installed for HRS Heat Exchangers by Fullcircle Energy Ltd at Allen’s Farm near Colchester, Essex, as part of plans to expand its existing 499kW anaerobic digestion (AD) plant.

The plant is fuelled with energy crops, such as maize, and some waste and by-products such as brewer’s grains. Hailed as an ideal choice for farms with space constraints, it uses the same technology as the popular HRS Three-Tank technique but its compact design reduces the physical footprint of the system by a third.

The biogas produced during the process has high methane content and is burnt in a combined heat and power (CHP) engine to produce electricity for the grid. The heat produced by the engine will be used to pasteurise the digestate to improve the quality of the biofertiliser, by removing potential crop diseases and weeds, before it is spread on the farm.

“The idea of pasteurising is that we can then feed more by-products and waste from our potato and onion crops into the digester and can put the digestate onto our land without worrying about potential issues with weed seeds or spreading crop diseases. It will also remove the need to obtain a permit to spread the digestate, which can be very onerous,” explained David Hunter of Allen’s Farm.

“In turn this will allow us to double the capacity of our plant and run two digesters rather than one, and we will then be able to export the electricity directly to the University of Essex whose campus is just down the road. They are quite excited about the idea which fits in well with both of us,” he added.

The new twin-tank pasteurisation system was selected as a batch system was required, rather than the standard continuous 3-tank system. It includes a primary storage tank before the pasteuriser and post-treatment storage tank which feeds the treated digestate into a lagoon for longer term storage.

When first commissioned at Christmas the unit, which is fully Renewable Heat Incentive (RHI) complaint, is expected to treat around 30 cubic meters of digestate sludge each day, but it has been designed to cope with twice this amount once the plant is increased in size.

The HRS pasteuriser uses a double tube heat exchanger to heat the digestate, which contains up to 14% dry solids, to 75C above the required pasteurisation temperature. This allows for variation in the sludge consistency and its incoming temperature, making sure that the digestate is always properly pasteurised.

Matt Hale, of HRS, said: “This twin-tank version of our HRS 3-tank pasteuriser brings all the benefits of the established system, but in a more compact package and we can see it being of particular interest to smaller plants, farmers and those who want to increase the efficiency of an existing plant where there is limited room to expand.

“This particular pasteuriser has been carefully designed to allow Allen’s Farm to expand their biogas operations, not only by providing sufficient treatment capacity for the enlarged facility in the future, but more importantly, by allowing a wider range of waste feedstock to be used to fuel the planned expansion in the first place.”

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