Neil Gemmell, who farms at Clayton Hall Farm near Wakefield, has installed a 400kW anaerobic digestion plant, to process 6,000t of feedstock a year. Although the plant would be viable with energy crops from the farm, in energy terms, some 80% of the feedstock comes from food waste.
Video shows plant used elsewhere by Biogas Nord
The large digester has a capacity of 2,800cu m, which is required for the long retention times that are a feature of the system supplied by Biogas Nord.
"The system is designed to run on less than 4kg of solid material per cubic metre per day," explains Biogas Nord's UK representative Owen Yeatman.
"Traditional British digesters are smaller, but the economics are driven by gate fees. We want to extract all the energy from whatever comes in. For example, with maize we would expect 90-95% breakdown of the material."
One advantage of the large digester is the amount of gas storage it provides, something that other installations use secondary digesters or separate gas stores to achieve.
To handle food waste, the plant needs to comply with animal by-products (ABP) legislation, which includes a requirement for an enclosed reception area. At Clayton Hall a redundant grain store was used, meaning that extra capital was required to comply with this aspect of the rules.
From the reception building, the solid material is augured into the digester where agitators keep the mixture moving for up to 60 days. A heating system incorporated inside the concrete walls maintains the temperature at 40-42C. Once the gas has been extracted, the digestate is sent to a pasteuriser, which is a key difference from most on-farm plants. The material is pumped through a macerator to ensure it is small enough to comply with the ABP rules and then pasteurised in batches at 72C for one hour.
In terms of cost, Mr Yeatman reckons that the need for pasteurisation increased the overall capital cost of the plant by as much as 10%. "There is also a significant operating cost in terms of monitoring and meeting statutory requirements," he adds.
The pasteurised digestate is currently stored in a lagoon, although the farm plans to install a separator to separate the liquid and solid fractions, which can then be marketed under the PAS110 standard for quality digestate.
Mr Gemmell is very happy with the system, but he warns other farmers thinking about developing a waste-fed biogas plant to think carefully. "It certainly comes with a whole raft of extra things on top," he says.
While the costs of the plant are confidential, Mr Yeatman says that a typical 500kW plant using farm-based inputs would be around £1.6m, excluding digestate storage. He estimates that once adding compliance with ABP regulations has been offset against the need not to construct a silage clamp for feedstock storage, the additional cost for a food waste plant of the same size would be around £200,000. Gate fees and gas output should increase the margins on the plant, but at higher management, regulatory and capital costs compared with plants based on farm inputs.
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