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Steven's Croft Biomass Power Station
Johnstonebridge Road, northwest of Lockerbie, Scotland, UK
associated engineer
Mott MacDonald
date  October 2005 - December 2007
UK era  Modern  |  category  Power Generation  |  reference  NY119858
Stevenís Croft burns a variety of wood biomass products, and is the largest dedicated wood-fuelled power station in Britain. It is actually two plants side-by-side ó one for fuel processing and one for electricity generation.
The power station has a capacity of 44MW and generates enough electricity to power 70,000 homes annually. It uses more than 480,000 tonnes of biomass every year, sourced mainly within a 96km radius of the station.
The sustainable timber fuel comprises 60 percent sawmill co-products (such as sawdust) and small round wood, 20 percent short rotation coppice (willow) and 20 percent recycled fibre from wood product manufacture. Freshly cut wood can be moist, and the power station can burn timber products with a range of moisture contents (46-58 percent).
The fuel processing facility has storage for up to 14 days supply of round wood and up to 6,000 cubic metres of chipped fuel.
Dried wood chips are stored in a concrete hopper 60m long. Along each side of the hopper is a 1.2m deep and 48m long slot, which connects to a preformed concrete tunnel containing an Archimedes screw. The chips are pushed through the slots and conveyed into the furnace at 235 cubic metres per hour.
To ensure column-free slots, the hopper walls are suspended from heavily reinforced concrete columns constructed at 6m intervals within the outer walls of the screw feed tunnels.
The boiler house, turbine hall and other buildings are of steel frame construction founded on concrete slabs. The boiler house stands 46m above its surroundings, and the chimney is 85m tall.
The bubbling fluidised bed combuster and boiler produces steam at 537 degrees Celsius and 13,700kN per square metre, which drives a turbine. The SST-800 turbine has a single-casing direct drive with reverse flow design, and can be used for both condensing and back pressure applications. It runs at 3,000rpm or 3,600rpm for generator drives and at up to 5,000rpm for compressor drives.
Electricity is supplied to the UK's National Grid via a 33kV insulated cable with two, three-phase, medium-voltage circuits. This runs underground for 26km between Stevenís Croft and Chapel Cross substation, and included directional drilling for the link beneath the M74 motorway.
Steven's Croft was opened on 27th March 2008 by First Minister for Scotland Alex Salmond. It won the Best Renewable Project Award at the Scottish Green Energy Awards 2007, and participated in the Scottish Renewables Festival 2009. It is operated by E.ON UK.
The foundations were designed to limit settlement to just 20mm, and rest on 2,000 vibrated stone columns. Foundation bearing pressure of 250kN per square metre was required.
The scheme cost £90m to build and included an £18m Bioenergy Capital Grant from the National Lottery. The station is carbon neutral (the quantity of carbon dioxide emitted through combustion is the same as that absorbed by the growing wood), and displaces 140,000 tonnes of greenhouse gases annually.
The site was chosen for its proximity to forested areas ó and existing sawmills ó as well as good transport links. Forestry is a burgeoning industry in Scotland, expected to reach 10 million cubic metres per year by 2017.
Main contractors: Siemens Power Generation, and Kvaerner Construction
Grid connection cable: Nexan
Concrete hopper engineer: Envi Con
Concrete hopper construction: Alfred McAlpine
Hopper formwork: SGB
Ground investigation: Fugro
Vibro stone columns: Keller Ground Engineering
Research: ECPK

Steven's Croft Biomass Power Station