A new report by the UK’s National Centre for Biorenewable Energy, Fuels and Materials, reveals that using crops in on-farm anaerobic digestion (AD) is vital for the technology to make a meaningful contribution to UK renewable energy targets.
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.
“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.
“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, 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.