cutting silage

Green energy supplier Ecotricity has launched a UK-wide scheme to expand the production of biomethane from farm-based anaerobic digestion (AD).

These will feed gas directly into the distribution grid from leased sites on farms.

Through its Green Gas Mills project, the company is looking for farm sites to build and operate its AD plants in return for a long-term rental payment. It also wants to recruit farmers to supply feedstocks, principally grass for silage.

See also: Strong anaerobic digestion growth but uncertain times ahead

Ecotricity will cover feasibility, planning, design, construction, operation and plant maintenance costs, and will negotiate power purchase agreements. The farmer or landowner must remain an active farmer and maintain and improve stewardship management.

The firm will not reveal details of how much rental payments will be worth, the terms and conditions of contracts, or how the project is being financed, but a spokesman insisted agreements will be incentivised to make it worthwhile for farmers.

“Our AD plants will be matched to grid injection capacity and most sites are chosen for maximum capacity of 9.4MW thermal. Any larger and they are logistically demanding and less carbon efficient,” he said.

Ecotricity green gas plans

  • Farm sites must be on Grade 3 land or poorer
  • Crop contracts and digestate spreading subject to controls to minimise carbon footprint
  • Contracts will incentivise grass silage production to make it competitive with growing for stock feed
  • Host farmers must continue as active farmers
  • Each Ecotricity Green Gas Mill will produce enough gas to power almost 6,000 homes

Three sites have been identified and should have planning applications submitted later this year. The first will produce 78.84GWh a year from 75,000t of grass and forage rye silage.

The company has no set target for how many projects it wants to build. “We are at the beginning of a revolution in gas,” said Ecotricity founder Dale Vince. “We can take what we’ve done with green electricity, which last quarter generated 22% of the UK’s electricity needs, and apply it to gas.

“Green Gas Mills will produce gas that is carbon neutral, supports food production and is sustainable, with the process improving the local environment rather than damaging it. It’s the antithesis of fracking.”

Sourcing feedstocks

As with any AD project, securing feedstock supplies is crucial to long-term viability. Ecotricity’s main feedstock will be grass silage to encourage carbon dioxide sequestration and reduce carbon footprint. However, forage rye may also be used, and the firm is looking at supplies from the food industry, such as waste potato peel from chip factories.

Ecotricity says feedstock will be sourced on flexible contracts, which will generally require a minimum area of 20ha incentivised to encourage renewal throughout the life of the plant, expected to be at least 20 years.

It says feedstock will come from marginal land, Grade 3 or poorer, in low grade arable use or under-grazed grassland – within a 15km radius of the AD plant, either from the host farmer or neighbours. All silage will be clamped at the AD plant site.

The firm is keen to minimise the carbon footprint of feedstocks and the entire energy generation process, so will require farmers to follow guidelines for growing crops and spreading digestate.

“To maintain sustainability measures we will rely on accurate, suitable and timely application of digestate to all of the growing areas,” the spokesman says. “This is an expensive part of each project operation but is justified by the saving of chemical fertiliser costs and the improved carbon neutrality of the operation.

“Commissioning each project will be based on its environmental credentials as much as its financial sustainability.”

Returns for farmers supplying feedstock are based on tonnage supplied and would be “sufficient to encourage AD feedstock production as an alternative to feed crop production, with the added benefit of soil improvement,” said the company.


Gas grid constraints

Gas grid connection can potentially be tougher than electrical grid connection and comes at a considerable cost, so projects need to be close to the injection point, said Ecotricity.

“Access to mains gas supply is a priority, as is the availability of summertime low consumption on the grid that can accommodate the output from the AD plant.

“Finding gas grid with available capacity is a limiting factor and unlike electrical grid, the gas grid capacity is very difficult to upgrade and in most cases impossible.”