AD is the process where microorganisms break down organic material to produce a gas, which can be used to generate electricity or heat buildings. On-farm AD plants can operate using just slurry and manure, but this research shows digesters operate more efficiently when crops, like grass and maize, are also added.
Every year the UK produces over 100 million tonnes of organic waste that is suitable for AD. If used to produce energy, this waste could generate up to 7 per cent of the UK’s anticipated renewable energy by 2020, or 10-20 TWh each year. In addition, crops considered most appropriate for production in the UK as a supplementary feedstock for AD are maize silage, grass silage and whole crop cereals.
“In this report we show that crops have a valuable role to play in producing energy through AD,” says the report’s author Lucy Hopwood, who is also the NNFCC Head of Biomass & Biogas.
“If energy output is the main policy driver for AD then higher incentives are needed to optimise economic returns and maximise energy output; otherwise on-farm AD can only be viewed as a slurry management tool.”
The modelling was carried out using the NNFCC’s AD calculator, and examined a range of farm sizes, slurry to crop ratios and crop types.
At the medium-scale (250 dairy cows with followers), the most financially attractive option was the slurry-only model, followed by a slurry to crop ratio of 70 to 30. However, slurry-only systems generate far less energy; by using a modest amount of crop material (30 per cent), an AD plant can increase energy output tenfold for only three times the capital cost.
Using a larger proportion of crop material in a digester will increase gas production, but such systems may require more expensive equipment and high amounts of water to allow the microorganisms to break down the drier feedstock; this should be taken into account in economic forecasting, concludes the report.
The research is now being fed back to the UK Department of Energy and Climate Change to support policy development on AD.
The NNFCC is committed to the sustainable development of markets for biorenewable products. It also operate the UK‟s official information portal for anaerobic digestion; www.biogas-info.co.uk. This site gives access to all the information you need on AD, from how to navigate through the planning process to making the best use of biogas.
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