Longannet power station

Longannet power station /lɒŋˈænɨt/ is a large coal-fired power station in Fife capable of co-firing biomass, natural gas and sludge. The station is situated on the north bank of the Firth of Forth, near Kincardine on Forth. Its generating capacity of 2,400 megawatts is the highest of any power station in Scotland. The station began generating electricity in 1970, and when in became fully operational, it was the largest coal-fired station in Europe.^[1] It is now the third largest, after Bełchatów in Poland and Drax in England.

The station was opened in 1973 and operated by the South of Scotland Electricity Board, until 1991 when its operation was handed over to Scottish Power following privatisation. The station is a regional landmark, dominating the Forth skyline with its 183 m (600 ft) chimney stack. Like most power stations in Scotland, Longannet lacks cooling towers. Instead it uses water from the River Forth for cooling.

History

The station was designed by Scottish architects Robert Matthew, Johnson Marshall & Partners.^[2] Construction began in the mid-1960s, 4 km (2.5 mi) downstream of the existing Kincardine power station. The station was constructed on land reclaimed from the Firth of Forth using ash from the Kincardine station. Longannet was in full operation by 1973.^[3] At the time of its completion, the station was the largest in Europe.^[1]

Design and specification

Longannet has an installed capacity of 2,400 MWe (megawatts electrical) which is equivalent to two Advanced gas-cooled reactor nuclear stations. There are four 600 MWe generation units, each featuring a single boiler feeding two 300 MWe GEC turbo-alternators, with the steam spaces cross coupled. The declared net capacity is rated at 2,304 MW due to operational losses. On average Longannet produces 10.4 TW·h (terawatt hours) per year and in 2000-2001 achieved the highest generation output in its history by supplying more than 12.25 TW·h to the National Grid.

Operations

Electricity generation

The station consumes up to 4,500,000 tonnes of coal each year. Coal is delivered either by road or rail to the station's coal store, which has the capacity to hold up to 2,000,000 tonnes. Coal is fed from the coal store to the power station by a conveyor belt which is capable of carrying 3,500 tonnes of coal per hour. Coal is fed into bunkers inside the station's boiler house. These feed the station's pulverisers which supply the boilers' burners with fuel. Each of the pulverisers are capable of pulverising 40 tonnes of coal an hour. There are eight pulverisers per each of the station's four Foster Wheeler boilers. These use low NOx burners made by ABB Combustion Ltd. Each boiler provides steam for two 300 megawatt General Electric Company turbo generators.^[3]

Coal supply

Coal was originally supplied directly by conveyor belt from the neighbouring Longannet Colliery. This was the last deep mine in Scotland. It closed in 2002. The station is still supplied by Scotland's open cast mines.^[1] Much of the station's coal must now be imported, the majority via the former British Steel ore loading facility at Hunterston Terminal in Ayrshire. Onward transport is by rail and the level of traffic required to supply Longannet's fuel demand has caused extreme congestion on the Scottish rail network. An alternative route, the Stirling-Alloa-Kincardine rail link, at the mouth of the river Forth was reopened in 2008.

Cooling system

Water is taken for the station's cooling condensers from the Firth of Forth at a rate of 327,000m³ per hour. The water is passed through coarse screens and then circulated by four electrically driven pumps. Once circulated through the station's condensers (which cool the turbines' exhaust heat), the water is discharged into a mile long cooling channel, where heat is dissipated with no harmful effect before reaching a wide part of the Forth.^[3]

Electrostatic precipitators and sulphur trioxide conditioning

The station is fitted with electrostatic precipitators (ESP) to reduce the stations particulate emissions. In the late 1980s the station's units were fitted with sulphur trioxide (SO₃) conditioning equipment to lower the fly ash's electrical resistivity. This was to ensure the station maintained allowable particulate emissions. Between 1989 and 1994 the station's ESPs were given a major refurbishment. This meant that the SO₃ conditioning equipment didn't need to be operated as frequently to maintain the allowed level of particulate emissions. This is beneficial as SO₃ is seen as a hazardous substance.^[3]

NOx reduction

In 1994, the station was awarded funding from the European Commission under the THERMIE Demonstration Programme. With this money, Unit 2 at the station was retrofitted with Gas-reburn Technology. This is the largest scale application of this technology in the world. In this process, natural gas is injected into the boiler. This cuts NOx emissions from Unit 2 by 50%, as well as giving a reduction in CO₂ and SO₂ emissions. Also in 1996, all of the station's boilers were fitted with Low NOx burners. This reduced the station's NOx emissions by 40%.^[3]

Environmental impact

In 2003, Longannet was named as Scotland's biggest polluter in a report by the Scottish Environment Protection Agency (SEPA). The station produces up to 4,350 tonnes of ash per day. This is piped to ash lagoons on the nearby Preston Island, where it is stored in former salt pans. This is then landscaped and used to reclaim the land from the Firth of Forth.^[1]

To improve environmental emissions, Longannet is now fitted with 'Low-NOx' burners to limit the formation of oxides of nitrogen and a 'Gas Reburn system' that uses natural gas to convert NOx into nitrogen and water vapour. Longannet also burns up to 65,000 tonnes of treated and dried sewage sludge per year, which has a similar calorific value to low-quality brown coal. In 2005, a judge ruled the burning of sludge as illegal, but the SEPA continues to allow Scottish Power to burn the sludge illegally as part of an agreement which requires Scottish Power to construct, and have in operation, a biomass plant in 2010.^[4]

In 2007, the WWF named Europe's 30 most climate polluting power stations in absolute terms; of these, Longannet was the most polluting in the UK (relative to power output).^[5]

The UK's first ever carbon capture and storage (CCS) unit was commissioned at the station in 2009.^[1]

Ownership

This facility is owned and operated by Scottish Power, as a subsidiary of the Spanish Electricity Company, Iberdrola.

Future

The station is expected to continue operating until approximately 2020-2025, because of the technical advancements in place at the station. These include the station's low NOx burners, its NOx reburn system and a refurbishment of the station's electrostatic precipitators.^[3]

References