A new option for small-scale biogas - 9/8/2011 - Farmers Weekly was launched by technology provider Biogas Nord at this year's Dairy Event & Livestock show.
(In the video (below) a hydrolysis stage is shown, but the video was produced by another biogas company, and is included here to show the high temperature heat) hydrolysis stage, to explain the use of hydrolysis in anerobic digestion (biogas) processes -Ed.)
The 75kW BiNoMiniMax has been designed for small to medium-sized livestock farms and runs primarily on slurry or manure from cattle or pigs, plus some supplementary silage.
The company's Owen Yeatman said the single 678m3 digester, with 75kW combined heat and power unit, associated control systems and slurry tank, would cost around £500,000 to install, excluding grid connection. As a guide, it could run on the slurry and manure from 160 cows (around 3000-4000t), plus 900t of maize silage per year, or the slurry from 920 sows plus 900t of silage, he said.
Annual electricity savings for such a dairy unit - assuming 95% efficiency of the CHP system - would be around £20,000-30,000 and with heat used to warm cleaning water for the parlour, as well as the farmhouse, another £10,000 saving could possibly be added, he said. Combined with additional Feed-in Tariff income worth 14p/kWh and Renewable Heat Incentive payments of 6.5p/kWh, that would give a payback of around six years, he claimed.
"It's a simple design that doesn't need a large amount of extra land for the feedstock, as bigger systems do. The 900t of silage needed is only about 2.5t per day, much of which could come from waste silage off clamps, or rougher quality second- or third-cut."
Farmers would have the option to expand the system in the future by adding a second digester tank, he noted.
"A system such as this could run mainly on the farm waste with few extra substrates needed. Biogas plants do require more managing than things like solar or wind, but the returns can be potentially higher and aren't reliant on the weather. It could be useful way of getting a son or daughter involved in the business."
Boosting gas yields
The company also launched a new system for improving the biogas yield from woody or high lignin feedstocks, such as straw, grass, hay and manure. The BioNoLiquifeed hydrolysis unit uses a mix of bacteria to pre-treat feedstocks before they are fed into the digester.
The process reduces the dry matter content and converts the fibrous biomass into acetic acid in around three to five hours, depending on substrates. The pre-treated substrates are then pumped into the primary digester.
Mr Yeatman said the system, which costs around £150,000, could shorten retention times in the digester by up to 20 days and improved the biogas yield from fibrous substrates by 15-20%. It could also mean that other previously overlooked feedstocks, such as oilseed rape straw, could be used, he suggested.
Substrates are fed into the hydrolysis unit using a wheel loader, which reduces the need for more expensive and higher-maintenance slurry stores, walking floors and screw conveyors, he added.