Members’ News & Views
SMALL BUT PERFECTLY FORMED Although many on-farm AD plants are around 500 kW or larger, in many circumstances smaller units can be both efficient and profitable. As our reporter Richard Crowhurst found out, the 165 kWe facility developed by AD4Energy at Barr Farm, near Spalding in Lincolnshire, allows arable farmer Jonathan Turner to sell clean electricity to nearby poultry meat producer Moy Park, as well as claim FIT and RHI payments for drying the digestate (an activity which is no longer supported following recent changes to the RHI).
While the plant at Barr Farm was designed to handle a proportion of poultry manure from the adjacent 400,000 bird broiler unit, the decision was taken to stick to energy crop feedstock for the first couple of years to make commissioning and plant management easier. Consequently, 220 hectares of maize have been grown on the farm this season, although whole crop rye is also being considered as an option for the future. Around eight tonnes of the crop is placed into the Trioliet feeder each morning, which keeps the twin digesters operating for the rest of the day. Feedstock is macerated and mixed with liquid digestate by a Vogelsang unit before it passes into one of the two rectangular, semi-submerged concrete tanks.
The AD4Energy plant at Barr Farm
The first part of each tank consists of a membrane-covered chamber separated from the rest of the tank by a retaining wall with a weir system. This allows digestate to pass across while heavier particles such as grit fall to the bottom. It is anticipated that each trap will need cleaning every five or six years, a simple operation which is carried out by removing the cover to access the tank. Because of the twin tank system, the unit can continue to operate while one tank is being cleaned or serviced.
When using maize feedstock, the typical retention time is between 35 and 40 days, although Georgina Hyde of AD4Energy said that some farmers leave the crop for longer. “Some farmers will keep the digestate in the tank for up to 50 days,” she explained. “We’ve also changed the design of the tank to a circular unit since the Barr Farm ones were installed, due to changes in Feed-in Tariffs and qualifying feedstocks. However, we’ve stuck with the concrete design as they can be partially submerged, making them more acceptable to some planners.”
There are no moving parts in the tanks. Mixing is carried out by injecting gas through a series of pipes which run along the top of the tank. These work in sequence to mix material in the digester and ensure that it has a homogenous
Eight tonnes of maize per
day are fed to the digesters
consistency in order to maximise gas production. The gas is collected in a circular gas holder before being used as fuel in the Tedom CHP engine, which can be monitored remotely by the farmer and even operated from a distance by the supplier; shentongroup. An additional boiler uses a small amount of gas to maintain the digester temperature at around 37-38°C.
One of the other notable features of the installation at Barr Farm is the digestate dryer and emissions cleaning unit, both supplied by Dorset Green Machines. Digested material from the plant is macerated and passed through the dryer to produce a very dry digestate, which is much easier to store, transport and spread. The Environment Agency insisted on the addition of the ‘air washing’ cleaning unit, which removes any additional particulate and gas emissions from the exhaust gases from the dryer.
A representative from shentongroup in the control room at the 165 kWe plant 26 AD & BIORESOURCES NEWS | SPRING 2017
“We expect the plant to pay back the capital costs after around five to seven years,” explains Georgina. “A unit like this would typically cost around £1m, but in this case there were other costs associated with the construction of the clamps and lagoon. The funding came from NatWest. We received pre-accreditation from Ofgem in 2014 and the plant was commissioned by the end of February 2016.”
www.ad4energy.com www.vogelsang.info www.dorset.nu/en
www.adbioresources.org adbioresources.org
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