Drax Power Station, Selby, United Kingdom

Claiming to be the largest, cleanest and most efficient coal-fired power station in the UK, the 4,000MW Drax plant supplies around 7% of the UK's electricity needs. Drax is located in Selby, Yorkshire (UK), and is owned by Drax Power Limited (the operating subsidiary of Drax Group plc).

The company has recently contracted with Sempra Energy Europe for the supply of 150MW of power and one million tonnes of coal over two years, starting April 2006. Drax has also commissioned a 'direct injection' system on one of its six generating units, burning coppiced willow.

REDUCED EMISSIONS

Emissions from the plant are controlled. The generators are fitted with Flue Gas Desulphurisation (FGD) equipment that removes 90% of the sulphur dioxide (SO2) from emissions, and work is underway to further reduce emissions of oxides of nitrogen (NOX).

Byproducts of the coal combustion and FGD processes are recycled for the construction industry. Any ash from the combustion process that is not sold is deposited in an ash disposal site. Previously derelict land, the disposal site has been reclaimed and restored for agricultural use and woodland planting.

Drax Power employs 560 people and supports many other local jobs indirectly. It is also a major customer for the UK's remaining coalmines.

SIX GENERATORS

Coal is transported to the station by 1,000t coal trains, with shuttle conveyors distributing the coal between the bunkers. These store the coal before it goes to the pulverising mills. These crush the coal into a fine powder using ten large metal balls.

Each boiler has two forced draught fans that draw warm air from the top of the boiler house through large air heaters. The 'primary' air used to blow the coal from the mill to the boiler is supplied by fan.

Each boiler has 48 or 60 propane and fuel oil burners that start the boiler up. Once a stable flame is established the coal / air mix is blown through the burner where it lights spontaneously. The propane and oil are then shut off.

To produce steam Drax has six boilers, each weighing 4,000t. They convert energy from coal into steam at a rate of 563kg/s (4,468,243lb/hr). Pure boiler feedwater is turned to steam inside 480km of steel tubing by the heat of the boiler or furnace.

The steam temperature is raised to 568°C and the pressure to 166bar (2,400lb/in²). The boilers operate either continuously or on a daily cycle of morning start-up and night shutdown as required by demand ('two-shifting').

Each of the six turbines consists of five separate turbines – one High Pressure (HP), one Intermediate Pressure (IP) and three Low Pressure (LP).

ELECTRICITY GENERATION

Exhaust steam from the 140MW HP turbine is returned to the boiler for reheating before being used in the 250MW IP turbine at 565°C and 40.2bar. It then passes to the three 90MW LP turbines at 308°C and 6.32bar.

The steam strikes and lifts a series of angled blades mounted on the turbine shaft making it rotate at 3,000 revolutions per minute (50 cycles per second). The steam then passes to two condensers and is taken to the cooling towers. There are 12 cooling towers at Drax, two for each generating unit.

To generate electricity, the electromagnet on the rotor rotates at 3,000rpm inside a stator of copper windings.

These generate 19,000A at 23,500V. A transformer increases the voltage to 400,000V before sending it via cables to the National Grid sub-station for distribution.

WILLOW COPPICING TRIALS

In September 2005, Drax's short rotation coppice willow processing trial proceeded to its second phase. Phase one successfully identified the issues associated with the processing of wet wood, including the transport and handling of processed fuel.

Wood-derived biomass fuel, including Short Rotation Coppice (SRC) willow was suplied to Drax by Renewable Fuels Ltd. The SRC used in the trial originates from 1,100ha of willow grown locally by the producer group, Renewable Energy Growers.

Drax Power commissioned a 'direct injection' system on one of its six generating units. 'Direct injection' allows biomass materials to be injected either directly into the boiler or into the fuel line, which delivers a mixture of pulverised coal and hot air into the boiler. This bypasses the pulverising fuel mills, potentially allowing greater throughput of biomass.

The second phase of the processing trial involves linking up the processing equipment with the 'direct injection' system to provide an online supply of wood-derived biomass.

The biomass trials at Drax have successfully demonstrated the potential to handle and co-fire significant quantities of biomass fuels, including SRC willow and round wood. The company has successfully established SRC in Yorkshire with a further planting programme for local growers in 2006.

THE FUTURE OF COAL

Drax Power has published a report Coal: Fuelling Our Future Generation. It claims to identify several ways to address the carbon challenge through investment in both existing coal-fired power stations and new build clean coal technology plants.

The paper states that a combination of methods such as co-firing with renewable biomass materials, and deploying new technology could enable coal-fired power stations to rival the environmental performance of gas-fired plant.

The cumulative effect of current technology is capable of cutting emissions of carbon dioxide by over 40% while co-firing alone can reduce CO2 emissions by 20% in the short-term for relatively low cost, says the paper.

Coal reserves are claimed to last at least 200 years. With a third of the existing coal-fired capacity due to retire over the next ten years, the current projections for electricity from coal in 2020 would mean that without new build the remaining plant would be operating at similar load factors as they are today.

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