A report by the UK’s National Centre for Biorenewable Energy, Fuels and Materials - the NNFCC - reveals that using crops in farm-scale anaerobic digestion will be important if the technology is to become a major source of renewable energy.
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.
“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,” says the report’s author Lucy Hopwood.
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. Slurry-only systems are generally cheaper to build and operate but 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.