Energy crop use in AD backed by report

Energy crops are vital for maximising the output from on-farmed anaerobic digestion, according to a new report.

The National Non-Food Crops Centre study aims to counter the cautious attitudes towards energy crops highlighted in the recent review of Feed-in Tariffs and introduction of the Renewable Heat Incentive.

In both cases government acknowledged energy crops were required in some cases, but warned it did not want to see their large-scale use and could introduce measures to prevent this.

But the NNFCC report said using crops in on-farm anaerobic digestion (AD) was vital for the technology to make a meaningful contribution to UK renewable energy targets. Even modest amounts of crop material could significantly boost energy output from slurry systems at relatively little extra cost, it said.

“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,” report author Lucy Hopwood said.

The report modelled three farm sizes, plus a range of slurry to crop ratios and three crop types (maize, grass and wholecrop wheat silage) to illustrate the impact on costs and returns of including energy crop material in a dairy slurry-based AD system. While plants worked well using just slurry, the addition of energy crops significantly boosted output, it found.

Small scale

• 130 cows with followers
• Cattle housed 200 days a year
• CHP capacity required: 5-60kWe

For small-scale dairy units with limited land and resources, the most favourable return under the support system came from a slurry-only plant, the report said. This was mainly because the cost of using extra feedstock reduced overall profitability; in many cases supplementary crop would need to be bought-in so as not to compromise summer grazing for the herd.

However, slurry alone has a low energy value and adding grass or maize silage, up to a 50:50 mix, increased biogas output. Adding vegetative material significantly reduced the unit cost of investment, from £25,000 to £8,621 per kW of installed electricity capacity. But, the cost of producing crop material also increased running costs and therefore reduced the Internal Rate of Return.

The report found higher proportions of vegetative material, above 50%, would require different technology due to the high dry matter content of the feedstock mix, or for water to be added or recirculated. This increased the size of the digester tank(s) required and increased capital costs.

Medium scale

• 250 cows with followers
• Cattle housed 200 days a year
• CHP capacity required 10-150kWe

The highest rate of return for this size unit was from a plant that used 70% slurry and 30% crop material as the feedstock. Beyond 30%, feedstock costs increased faster than energy production, which reduced overall returns, the report said. “This optimum ratio will be highly sensitive to costs.”

Again, it could be necessary to import crop material to supplement this system, depending on the scale and productivity of the existing business.

For both the medium and small-scale plants, the report said grass silage, rather than maize, was likely to be the best feedstock type due to its lower cost of production. “However, labour requirements would be increased as grass is harvested three times a year as opposed to maize only once.”

Large scale

• 500 cows with followers
• Zero-grazing – cattle housed all year
• CHP capacity required: 37-550kWe

Returns were generally more attractive for larger scale plants due to the economies of scale and lower unit costs for capital investment.

Feedstock costs accounted for a smaller proportion of total running costs, therefore the extra biogas yield from increasing the proportion of crop material outweighed the additional cost incurred. This type of system was also more suited to arable areas of the UK, where maize could be grown as an annual break crop and used as the main feedstock instead of grass silage.

The most attractive option at the large scale was a higher crop to slurry ratio, while remaining under the 500kWe threshold for the higher Feed-in Tariff payment.

“Above 50:50 ratio there is a need to assess technology suitability, as the volume of water to be added or recycled will become significant. At a higher ratio dry AD may be more appropriate, however this may limit the use of slurry, especially in this model where cattle are housed all year round and thus slurry provision is not seasonal.”