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On-farm AD

David Finlay runs Rainton Farm near Gatehouse of Fleet, a 340ha mixed livestock farm with the dairy herd supplying the Findlay's Cream O'Galloway ice cream company. A 25 kW AgriDigestore system from Marches Biogas has been fitted to the farm’s slurry tower to capture biogas from dairy slurry and grass silage. “For less capital cost than our 50 kW wind turbine, and delivering three times the electrical power, we’ve converted our slurry tower into an anaerobic digester (with no planning issues), generating enough electricity and hot water to run the dairy,” explains David. “We’ve also enhanced the fertiliser value of our slurry significantly and reduced its pollution potential, while cutting our greenhouse gas emissions and energy use substantially.”

Maximising your return However, there can be no denying that tariff degression is denting investor confidence in the viability of small scale AD operations. With this in mind, it’s now more crucial than ever that on-farm plants are run safely and efficiently. “The way to make small scale, on-farm AD work is to make full use of the heat; for example in greenhouses,” believes Ian Watt of Forum for the Future.

Biogen’s Bygrave Lodge plant is situated on Wallington Farms in Hertfordshire and provides Farm Manager Andrew Watts with nitrogen-rich digestate for his crops

having an on-site power plant makes us more robust and gives us a better platform for growth. With rising energy costs, our AD plant has made our site 90-95 per cent energy self-sufficient in electricity, plus we are now keeping around 10,000 tonnes of potato outgrades and peel waste on site, reducing waste disposal and transport costs. And the digestate we produce saves us around £100,000 a year before application costs.”

For some farmers, digestate production is the primary driver for getting involved in AD. Andrew Watts is Farm Manager of Wallington Farms in Hertfordshire where Biogen’s latest AD plant (Bygrave Lodge, a 2.1 MW food waste facility) stands. “It’s early days for us but the digestate we’ve had from the plant so far is higher in nitrogen than we were expecting – between 6.8-7 kilos per m3

– which is great news. The AD plant should provide enough nitrogen

for two-thirds of our crops and to have it produced on site, as a renewable source, is a good thing. Plus, we’re adding organic matter to the soil. In effect, we’ve made this a mixed farm – we have an organic source of nitrogen, created through a digestion process, just the same as if we had pigs, poultry or cows on the farm.”

Small scale, big benefits While some farmers are choosing to ‘go large’, the smaller scale on-farm AD market is developing too. Indeed, the benefits farmers can gain from developing an AD plant at this size should not be underestimated. “It’s important not to approach AD with a preconceived notion about the size of the plant you want – bigger is not always better,” warns Alistair Wannop, Managing Director of Linstock Castle AD in Cumbria. Alistair runs a 1,200 acre mixed diary and arable farm and operates a 1.1 MW AD plant, which processes muck and slurry from the farm’s dairy heifer rearing operation; grass silage; maize silage; and wholecrop hybrid rye. “Think instead about what you can offer: what feedstock you produce; what heat you could use; and the practicalities of any grid connections.” Although the Linstock Castle AD plant is relatively large, Alistair adds: “Putting a 100 kW or 250 kW plant alongside an existing livestock business is now a real opportunity.”

Matt Hale of HRS Heat Exchangers agrees that maximising the heat generated by an AD plant will pay dividends: “It’s worth remembering what the FIT scheme was designed to do: increase the uptake of renewable energy technologies to the point where they are sufficiently widespread that development costs come down, so that financial support is no longer necessary. They were not intended to be a long-term subsidy on energy generation. Using the latest heat exchanger technology, which harvests and uses heat wasted at an AD plant, will become one of the easiest ways of attracting investment and continuing to operate profitably, both in new and existing projects.”

There are other ways to help maximise returns from on-farm AD; Dr Johannes Thaysen, a German silage researcher working with Bock UK, believes that crop operators could be losing up to 20 per cent of the energy from their maize and grass silage clamps. This can mean the difference between the success and failure of the whole project. “Harvesting the right dry matter, having the correct chop length, even distribution in layers and good compaction when filling the clamps, combined with high quality double layer covers, sidewall sheets and weighted bags, are key to the success of a good clamp,” he reveals. “Growers should be aiming for waste-free silage, with no dark patches under the covers, no losses at the edges or hot spots in the lower levels. It is possible to eliminate these problems by paying attention to detail, particularly regarding clamp design, filling and covering.”


Heat exchanger technology from HRS Heat Exchangers is helping on-farm AD to remain profitable in the face of tariff degression JUNE 2015 | AD & BIORESOURCES NEWS 9

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