Containerised anaerobic digestion plants offer an affordable option for producers looking to generate biogas from herd waste and cut energy costs and environmental emissions.
TEXT ANGELA BYWATER
Biogas production can boost fossil fuel replacement on dairy units and, based on its ability to curb emissions, anaerobic digestion (AD) should be adopted across the sector. So says Richard Gueterbock from Foodchains, a consultancy that focuses on the decarbonisation of agricultural supply chains, with a particular expertise in anaerobic digestion.
“In addition to creating value from manure and slurry, on-farm AD plants can also enhance nutrient management,” he says.
Benefits of on-farm AD plants and biogas production were highlighted at a recent event run in June by the Royal Agricultural Society of England (RASE), at Wyke Farms. The Somerset-based cheesemaker invested in an AD plant, to treat slurry, crop material and whey from the creamery, more than 10 years ago.
The creamery’s large-scale system contrasts with modular biogas systems, which offer a viable solution for smaller businesses, and the open day also include a visit to see one of these modular systems, built by local company Biofactory, in action.
The company is set to install its first ‘modular’ AD plant on a dairy unit close to Wyke Farms this year and estimates a typical return on investment for the business of between three and seven years.
Modular system: units are designed to require minimal on-site installation work
Containerised units
These smaller-scale AD plants can handle slurry from between 100 and 500 cows, and multiple units can be installed to process waste from larger herds. The AD plant is based on standard containerised units and payback is achieved through offsetting existing energy costs.
The system uses slurry from the herd and produces energy for use on-site, helping to reduce the operating costs of the dairy business. The company estimates that a typical farm can save 100+ tonnes of CO2 equivalent yearly, but this is installation dependent.
“This and other smaller ‘plug and play’ AD plant systems are designed to require minimal on-site work, as well as to be relatively easy to secure planning permission for,” explains Mr Gueterbock.
The biogas captured via fugitive slurry-store emissions or from the small, modular AD of slurries and manures can be used directly to create heat through a boiler or, more commonly, electricity and heat via a combined heat and power (CHP) plant. Small-scale, off gas-grid upgrading systems extract the methane and are starting to be cost-effectively deployed on farms. The methane can be used as vehicle fuel (in milk tankers or some tractors), or in generators or CHP plants.
On-site generation
“With the price of battery technology falling, the economics of storing some of the electricity from CHP, solar and wind are also starting to become increasingly positive, allowing flexible on-site electricity use, particularly when grid electricity is expensive,” says Mr Gueterbock. “On-site generation of biogas/biomethane can shield farm businesses from volatile electricity prices and help replace diesel and fuel oil.”
He adds that, as well as generating energy from manure and slurry, on-farm AD also has agronomic and environmental benefits. “The digestate produced by the plant has increased nitrogen availability, helping to reduce fertiliser costs, as well as pathogen, weed seed and odour reduction. It also offers a lower pollution risk, and fields spread with digestate can be grazed sooner than those spread with ‘untreated’ slurry.”
Digesters differ in both their size and the ‘feedstocks’ they treat. On dairy units, the choice of AD system should reflect nutrient management needs and herd size. For small-scale units, with fewer than 500 cows, options include modular container-based AD plants or covered ‘active’ slurry stores with fugitive biogas recovery.
“These offer a flexible, affordable solution and can ensure that the AD plant is integrated with farming operations,” says Mr Gueterbock.
Medium-scale units, with more than 500 cows, can invest in modular systems to suit herd size, as well as covered lagoons, to generate biogas. “But as herd size gets closer to 1,000 cows, larger tank-based systems, or large covered-lagoon systems with a gas dome, may offer a more robust and cost-effective option.”
At more than 1,000 cows, it is possible to include mixed feedstocks, as is the case at Wyke Farms, but comparable multi-tank systems with mixed feedstocks will require feedstock-processing equipment. “Centralised sites are an option, but transport of slurry is only viable for a few miles,” stresses Mr Gueterbock.
Investment in AD plants, particularly on dairy units, has stalled during the past five years, predominantly due to the fact that the Green Gas Support Scheme excludes smaller farm AD plants in favour of large biomethane-to-grid systems.
But small-scale on-site AD can be better supported, according to Mr Gueterbock.
Government support
“Under the ‘Dairy Sector Roadmap’, UK dairy is reducing its external impacts, but urgent action is still required, particularly at farm level, but also through processors, including in milk transport.
“The next government needs to support rural decarbonisation and off-grid bioenergy through a consistent, long-term support framework and streamlined planning to encourage producers and suppliers to invest. This would reduce the impact of emissions from agriculture, as well as helping to replace fossil energy in a more circular economy.
“Because AD is key to managing dairy-sector emissions, the next government should be urged to support on-farm biogas generation and integrate this with slurry management regulations,” adds Mr Gueterbock.
“Well-managed biogas plants can improve nutrient management and boost dairy business efficiency.”
Comments