Biomass: Case Studies
Sewage treatment works - Minworth, West Midlands
The Severn Trent plc sewage treatment works at Minworth, West Midlands, is 100 per cent self-sufficient in electricity and exports 12 per cent surplus to the national electricity grid. The works is the largest inland treatment works in Europe, serving 1.3 million people as well as businesses in the Midlands. During a £90 million upgrade in 2001, 47 acres of biological filter beds were replaced with an activated sludge process.
Minworth’s digesters use anaerobic digestion to process more than 4,000 litres of sludge a day, producing heat which is used to heat its own digesters and 75,000 cubic metres of sewage gas, mostly methane. The methane produced during the breakdown of the sludge is compressed to remove excess moisture to increase the pressure of the gas, which is then piped to the combined heat and power plant. The methane is then pumped into five gas engines, each capable of producing 1.5 megawatts of energy, where it is burnt to generate electricity. The waste heat from the engines is used to heat the digesters. The system is flexible and when more energy is needed, the amount of sludge going into the system can be increased to meet demand. Severn Trent plc saves money by not buying electricity made from fossil fuels from the national grid and earns money by selling its excess electricity production.
Straw-burning power station - Ely, Cambridgeshire
Ely straw-burning power station in Cambridgeshire is the first modern, and the world’s largest, straw-fired power station. It became fully operational in September 2000. The 36-megawatt facility consumes around 200,000 tonnes of straw and generates sufficient electricity every year to satisfy the needs of 80,000 dwellings. The plant is also capable of burning a range of other baled energy crops, such as miscanthus, and can use mixtures of up to 10 per cent natural gas. The facility has created about 50 long-term jobs. Long-term contracts are in place for the sale of the plant’s entire output. The plant is a partnership formed between Energy Power Resources Ltd and Cinergy Global Power.
Landfill site - Greengairs, Scotland
Opened in 1990, Greengairs landfill site is the largest contained landfill site in Scotland. It currently handles 750,000 tonnes of waste a year. Around 55 per cent of this is domestic waste, 30 per cent is commercial or industrial waste, and the remainder is inert waste.
Methane is produced as the biodegradable waste within the landfill site breaks down. This is collected and used as the fuel source for the site’s power station. The power station also exports 3.8 megawatts of power to Scottish Power’s electricity network. This is due to increase by about 2 megawatts as the plant develops.
The gas collection system is designed to take the maximum amount of gas from the waste, reducing the risk of gas migration from the site and any problems with landfill gas odours in the local village. Three thousand cubic metres of gas per hour is taken from over 60 operational gas collection wells drilled into the waste in fully filled areas of the landfill. These wells are connected to the site’s gas flare compound by over 6,000 metres of underground pipework. The collection system controls the emission of gas from the site, and maximises the quality and volume of gas to be used as fuel for the generators.
The landfill gas system at Greengairs works 24 hours a day, 365 days a year, with projected availability of 90 per cent. About £2.5 million has been invested in the gas collection system and the power station.