FROM COLD WAR TO COLD STORE A former military airbase in Suffolk, UK is the location of what has become one of the larger biogas power plants in the country. Originally commissioned in 2011 as a 500 kW combined heat and power plant, the AgriGen facility has since grown by a factor seven to 3.5 MW electrical output. BENTWATERS PARKS, IN SUFFOLK, about 150 km northeast of London was a former Royal Air Force (RAF) airfield during World War II that subsequently became a US Air Force base during the Cold War. Heralding a new era, the privately held 404 ha (1 000 acre) airfield with ancillary buildings was retired from military service some 20 years ago and repurposed into a commercial enterprise park. Reduce energy costs Founded in 2004 Suffolk Produce Ltd (SPL) is a co-owned farmers cooperative producer organisation (PO) based at Bentwaters Parks. Through SPL eleven farms representing over 13 300 ha (33 000 acres) in and around the airfield manage and market crops, primarily onions and tuber vegetables such potatoes, parsnips and swedes. SPL have converted a number of aircraft hangers into vegetable processing, packaging and cold storage facilities. For SPL the cost of energy is a key competitive concern and as a producer organisation (PO) SPL can invest in capital projects. Furthermore being a EU recognised PO, SPL is able to access applicable matched EU grant funding. In 2009 SPL formed a subsidiary AgriGen Ltd to setup a biogas plant at Bentwaters Parks that could use agricultural feedstock grown or sourced from its members to supply space heating and electricity. – Drying out our onions and keeping them cool in storage uses a lot of energy so that’s why we set up AgriGen, said Graham Thorne, Manager for AgriGen. 42 Bioenergy International No 83, 7-2015 42 Bioenergy International No 83, 7-2015 Maximising power out Rather than build a single large plant in one go, AgriGen opted to build and commission in stages. The Austrian company, Thöni Industriebetriebe GmbH was selected to build the multimillion pound biogas plant. Thöni is an industrial group that has a division specialised in environmental technologies including composting and anaerobic digestion (AD) of energy crops. In 2011 “AgriGen I”, a 500 kW plant consisting of a fermenter, a secondary digester and a MWM combined heat and power unit (CHP), was commissioned supplying a share of heat to a local heating network and power to the grid. In July 2012, “AgriGen II”, was started adding another 2 MW capacity with two larger digesters. This was ready in early 2013. A decision was made to install a 3 MWe GE-Jenbacher biogas engine from Clarke Energy and in September AgriGen ordered two 125 kW GE Clean Cycle Organic Rankine Cycle (ORC) units. – The electrical efficiency of our Jenbacher engine is market-leading. However, we also saw the potential for the utilisation of the waste heat to generate more electricity. We selected ORC technology to boost the plant’s performance further and deliver one of the most electrically efficient biogas facilities in the UK, said Thorne. Plant setup Consisting mainly of maize and triticale silage the feedstock is stored in silage clamps that are compressed and covered to limit degradation that would otherwise result in energy loss and odour emissions. A wheel loader is used to transport feedstock from the silage clamps to the digester in-feed, which consists of a special container module with automated conveyor. A reception pit for liquid feedstock such as manure also doubles as a storage tank for captured surface and drainage water from the silage clamps. Here also other solid feedstock material is received and mixed with the liquid feedstock before pumping into the heated and insulated reinforced concrete digesters with gas storage unit on top. An external desulphurisation unit removes hydrogen sulphide (H2S) from the biogas before it is used in the CHP units. A separator unit separates the final digestate into a solid and a liquid phase with the latter pumped to a storage tank. Both are used as agricultural fertiliser. Proprietary paddle-mixers A key component of the plant is a custom-built and patented Thöni paddle-mixer. Substrate homogenisation in the digesters is achieved by means of slow turning paddle-mixers and by fast turning propeller mixers. The mixers also help to prevent the formation of floating layers and sedimentation while creating optimum conditions for microbial decomposition allowing the methane to escape from the substrate. The paddlemixer is especially suitable for efficient mixing of different feeds with relatively high dry matter content as high mixing power eliminates the need for diluting the fermenter contents with water or recirculates. This means that fewer liquids have to be pumped through the plant resulting in lower energy consumption and maintenance costs. » BIOGAS A paddle-mixer under installation inside a digester.
Bioenergy International no 7 December 2015
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