Advanced Anaerobics Limited (AAL) has announced the launch of SlurryGen, a range of farm-scale AD plants that use cow slurry only.
At last, an anaerobic digester that is the right size for Britain’s dairy farms. Extract energy from fresh cow slurry only. No energy crops, no imported waste streams, just fresh cow slurry. SlurryGen from AAL generates electricity and heat to power the farm. Based on a German technology that has been in commercial operation for over three years, re-engineered by AAL for the specific requirements of Britain’s farms, SlurryGen is making its debut this spring.
Two models are available. The SlurryGen-30 has a 30kW combined heat and power generator (CHP) and produces 720 kWh of electricity per day, using only the slurry from 200-220 cows. SlurryGen-50 has a 50kW CHP and produces 1100 kWh of electricity per day, using the slurry from 300-330 cows.
SlurryGen avoids complex planning and permitting requirements in most locations (subject to site review). Due to its small size, the unit should be “permitted development” for planning purposes. It qualifies for exemptions from environmental and digestate use regulations. As only slurry is treated, the farm will not need a waste management permit.
Both models are small. They look like agricultural buildings and will fit well with existing structures. They have a limited “footprint” of 25m x 12m. Most farms will have a suitable site available.
SlurryGen consists of a slurry reception tank, up to four fermenter tanks, a gas cleanup unit and a containerized plant room. The plant room houses the CHP, and plant controls. A heat system collects surplus heat from the cooling system of the CHP. It includes a thermal store buffer tank which provides heat for a tube-in-tube heat exchanger.
Each day, fresh slurry is transferred from a sump on the farm slurry system into SlurryGen’s reception tank. From there, it is pumped through the heat exchanger, to warm it to the preferred temperature for digestion (38-40oC), before it enters the first fermenter tank. Trace heating in the tanks keeps the fermenting material at the right temperature, even in severe winter conditions.
Bacteria in the fermenters break down organic materials in the slurry, in the absence of air, to produce biogas. The biogas is collected and passed through the gas cleanup system, removing corrosive hydrogen sulphide (H2S), before being used to fuel the CHP.
In SlurryGen, the slurry stays in the fermenters for an average residence time of 30 days, to allow optimum gas yields. At the end of this time, the depleted material is pumped to the existing farm slurry store, for use as fertiliser, exactly as untreated slurry would have been.
The CHP is connected to both the farm electrical system and the grid. The electrical power produced is used to displace current power purchases. Surplus power is exported to the grid. All of the power produced attracts a Feed-in Tariff (FiT).
Around half of the heat produced by the CHP is used by the plant. The balance may be used to heat wash-down water and other applications on-farm.
The combination of energy purchases avoided and the FiT on all the electricity generated make a strong economic argument and a positive financial contribution over the 20 year life of the FiT contract. On-farm heat uses may qualify for the Renewable Heat Incentive (RHI). In this case, the economics of SlurryGen become even more compelling, with a short payback period and a high internal rate of return.
David Kaner, CEO of AAL says, “The dairy industry has been waiting for an anaerobic digester designed specifically for its needs. Our vision is of SlurryGen working quietly and unobtrusively in the corner of every farmyard, making money for Britain’s food producers”.
AAL is about to start the installation process for its first two plants, in South West Scotland. Both are SlurryGen-50s. They should be up and running by late-summer 2014.
