Drax Power Station, 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).
In 2008 Drax Power began an £100m co-firing upgrade project that reduces the company's CO2 usage by one million tonnes a year. The existing co-firing facility of the plant provides 500MW of renewable electricity by operating alongside the new 400MW facility.
The upgrade project was completed in June 2012 and increased the plant's efficiency by 40%.
Emissions from the plant are controlled. The generators are fitted with flue gas desulphurisation (FGD) equipment that removes 90% of sulphur dioxide (SO2) from emissions. Drax also uses retrofitting boosted over fire air (BOFA) technology for its boilers to 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 employs 560 people and supports many other local jobs indirectly. It is also a major customer for the UK's remaining coalmines.
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 of the six turbines consists of five separate turbines - one high pressure (HP), one intermediate pressure (IP) and three low pressure (LP).
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).
Sempra Energy Europe has supplied 150MW of power and one million tons of coal to the plant over two years, starting in April 2006.
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 a minute (50 cycles a 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 supplied to Drax by Renewable Fuels. 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
In April 2009 a design build contract was signed between Drax Power and C Spencer. Under the £18m contract, Spencer supplied rail unloading equipment and biomass bulk storage and handling systems.
Drax uses this system to feed the direct injection biomass co-firing systems installed at the station.
In this issue we explore the industry's efforts to reduce emissions from power generation, looking at the latest developments in cleaner coal generation and carbon capture and storage, as well as new tools for accurate measuring of carbon emissions and the push for investment in cleaner energy technologies.