Cockenzie power station

Cockenzie power station
Cockenzie Power Station Viewed from the south in March 2003
Location of Cockenzie power station
Country	Scotland
Location	Cockenzie
Coordinates	55.968263°N 2.971544°W / 55.968263; -2.971544
Status	Operational
Commission date	1967
Operator(s)	South of Scotland Electricity Board (1967-1991) Scottish Power (1991-present)
Power station information
Primary fuel	Coal
Secondary fuel	Biomass
Power generation information
Installed capacity	1,200 MW
Website www.cockenziepowerstation.com
grid reference NT394754

Cockenzie power station is a coal-fired power station in East Lothian, Scotland, capable of co-firing biomass. It is situated on the south shore of the Firth of Forth, near the town of Cockenzie and Port Seton, 8 mi (13 km) east of the Scottish capital of Edinburgh. The station has dominated the local coastline with its distinctive twin chimneys, since it opened in 1967. Initially operated by the nationalised South of Scotland Electricity Board, it has been operated by Scottish Power since the privatisation of the industry in 1991. In 2005 a WWF report named Cockenzie as the UK's least carbon-efficient power station, in terms of carbon dioxide released per unit of energy generated. The 1,200 megawatt power station is set to close by 2016, but there are plans to replace the current station with a Combined Cycle Gas Turbine (CCGT) power station.

History

Under a design by the firm of celebrated architect Sir Robert Matthew, construction of Cockenzie power station began in 1959 on the site of the former Preston Links Colliery. The site is also thought to have been the hiding place of General John Cope after the defeat of his army at the Battle of Prestonpans on 21 September 1745. Much of the electrical equipment for the station was manufactured by Bruce Peebles & Co., Edinburgh. The station began generating electricity in 1968 for the then South of Scotland Electricity Board (SSEB). In the year 2000-01, Cockenzie generated a record load factor, supplying 3,563 GWh of electricity and burning 1,500,000 tonnes of coal.

Specification

The power station occupies a 24 hectare site. It generates electricity using four identical 300 megawatt (MW) generating units, for a peak supply of 1200 MW.

Operations

Coal deliveries

Coal was originally supplied to the station directly from the deep mines of the neighbouring Lothian coalfield, but these have since been exhausted or closed. Subsequently coal has been supplied from open cast mines in the Lothians, Fife, Ayrshire and Lanarkshire. Russian coal is now used as it has a low sulphur content which helps reduce Sulphur Oxide (SOx) emissions to the atmosphere. The power station was the first to use the new "merry-go-round" system of coal deliveries by rail. This system uses hopper wagons which carry around 914 tonnes of coal each. Coal is also delivered by lorries and is known as 'road borne' coal.

Coal handling plant

The coal is delivered to the station's coal plant, which has the capacity to hold up to 900,000 tonnes of coal on a storage bing. The coal plant and storage bing is situated on the opposite side of B1348 road between Prestonpans and Cockenzie and Port Seton, and the main station. Coal is transported from the coal plant to storage bunkers in the main station by a conveyor belt. The coal, known as 'raw coal' at this stage is weighed, sampled and screened for metal and stones before being transported to the main station and stored in bunkers.

Milling plant

There are 6 pulverising mills per unit which grind down the raw coal until it has the consistency of sand. The ground coal is called 'pulverised fuel' (PF). PF burns more efficiently than large lumps of coal, which reduces waste. The PF is then blown into the furnace along with preheated air by 6 large mechanical fans called Primary Air (PA) Fans. At full load each unit will burn around 100 tonnes of coal per hour.

Draught plant

Each unit has 2 large Forced Draught (FD) Fans. For efficiency these fans draw warm air from an intake at the top of the station. This combustion air is passed through the Air Heater which increases its temperature. Dampers are used to control the quantity of air admitted to the furnace and direct some of the air to the Primary Air (PA) Fans. There are also 2 large Induced Air (ID) Fans on each unit. These draw the hot gasses from the Boiler, through the Air Heater where the heat is transferred to the incoming combustion air. The gasses are then exhausted up the chimney. The exhaust flow of these fans is also controlled by dampers in the ducting. Working in tandem the Draught plant ensures the boiler is always under a slight vacuum. This creates a draw of combustion air into the boiler and an exhaust to the chimney. The exhaust gasses must not be allowed to cool below a certain temperature. There is a risk of the Sulphur and Nitrogen Oxides in the flue gasses condensing and forming acids which will damage the ID Fans.

Water use

The water used in the station's boilers is taken from the local water supply and is known as 'towns water'. This is the same as the drinking water used to supply households. This water is used as it has already been screened and purified by Scottish Water.

Demineralisation plant

The station's water treatment plant further demineralises the towns water and removes any impurities. Although safe to drink the water still contains dissolved Silica (sand) and Sodium (salt) particles. Silica particles leave scale deposits on the boiler pipes which acts as an insulator, reducing heat transfer from the furnace to the water inside. This reduces the efficiency of the station and leads to increased running costs. Sodium encourages rust which weakens the walls of the boiler pipes and can cause them to fracture and burst, known as a boiler tube leak. A serious tube leak can lead to reduced generation or loss of the unit until repairs are carried out.

Hydrazine is added and used as a reducing agent to remove excess oxygen from the water. Free oxygen atoms in the water also encourage rust to form inside the boiler pipes. Caustic is also added to 'scour' the inside of the boiler tubes and remove any Silica particles.

The demineralised water is then stored in large tanks inside the main station, ready to be used in the boilers.

Feedheating plant

Before the feedwater is introduced to the boiler it is heated up in stages. There are 7 feedwater heaters on each unit which gradually increase the pressure and temperature of the water, until it reaches a final feed temperature of around 210°C. A steam driven Main Boiler Feed Pump (MBFP) pumps the water through the boiler Economiser and into the boiler Drum.

Boiler

The boiler Drum is a pressurised high tensile steel chamber where the water changes state into steam. The water here is pressurised to 170 bar and heated further to 360°C. On the bottom of each Drum are 6 large bore pipes known as Downcomers. These direct the water into the boiler water pipes where it is heated by the furnace. The water is then directed back into the Drum where it flashes off into saturated steam. The steam is then further heated by passing through Primary and Secondary Superheaters until it reaches 565°C. The superheated steam is then piped to the unit turbine.

Turbine

There is 1 turbine and alternator per unit. The turbine has 1 High Pressure (HP) stage, 1 Intermediate Pressure (IP) stage and 2 Low Pressure (LP) stages all connected in tandem to the same shaft. The superheated steam enters the HP turbine at a temperature of 566°C and a pressure of 162 bar. The exhaust steam from the HP turbine travels back through the boiler Reheater and enters the IP turbine at the same temperature but at a lower pressure of 43.5 bar. The exhaust steam from the IP turbine then enters the LP turbines. The steam is used to drive the turbines causing the shaft to rotate at 3,000 rmp. This speed drives the Alternator and gives a frequency of 50 cycles per second (Hz) and allows connection to the National Grid. Electricity is generated at 17 kilovolts (kV).

Condenser

After use, the steam is then condensed back into water, by passing it through the Condenser. Seawater from the Firth of Forth is used as a cooling medium. Over 500,000 litres per minute of water are used for cooling. The seawater is then discharged back into the Firth of Forth. Controls ensure the discharged seawater is kept close the temperature of the sea, to avoid creating a 'tropical' environment and upsetting the local ecosystem.

Ash use and removal

The ash lagoon at Musselburgh

The burning of coal in power stations generates a lot of ash and dust. The station's electrostatic precipitators capture fly ash from the flue gases, preventing them from entering the atmosphere. Bottom ash is also produced by the station. Ash from the station is sold through the ScotAsh company, a joint venture between Scottish Power and Blue Circle. It is used in the construction industry and in products such as grout and cement. Any remaining ash is piped to the large lagoons in the nearby town of Musselburgh, where it is capped and planted, and used as a nature reserve.

Electricity distribution

The electricity is initially generated at 17 kV. This is stepped up via a transformer to 275 kV for distribution on the National Grid. The electricity is not just distributed to Scotland, but England too, which it is connected to via a double circuit overhead line, operating at 275 kV and 400 kV, to Stella near Newcastle upon Tyne.

Post-privatisation and future

Since 1991, the station has been owned by the privatised Scottish Power utility group. It recently surpassed its originally intended lifespan. It is now run as a 'marginal station', guaranteeing seasonal and peak supply and non-availability of other power stations. For this reason considerable investment has been made to improve start-up times to maximise generating opportunities in the deregulated electricity generation market. Since 2001, the station has exported electricity to Northern Ireland via an undersea power link.

CCGT replacement

The existing coal fired power station must close by 31 December 2015, so Scottish Power are currently considering replacing it with a Combined Cycle Gas Turbine (CCGT) power station. Natural gas is a much more efficient fuel than coal and will more than halve carbon and nitrogen dioxide emissions compared to the existing power station. If the station is built, it will require a 17 km (11 mi) gas pipeline from East Fortune, to supply it with fuel.

In 2011 planning permission to replace the coal fired power station was approved by the Scottish Government. The new 1,000 megawatt (MW) CCGT power station will create up to 1,000 jobs in demolition and construction and 50 full time positions when completed. The approval is in line with the recommendations of the report of the public inquiry. Conditions imposed on the consent will minimise disturbance in the area during construction and minimise impacts on the environment and on protected species. The development will be carbon capture ready and will be required to fit full carbon capture and storage technology once it is commercially and technically proven. A separate application for a 17 km (11 mi) pipeline from the existing gas network at East Fortune to the new power station has also been approved.

External links

• YouTube - Lighting of the Chimneys during the Three Harbour Arts Festival
• YouTube - Steeplejacks on the station's chimneys in July 1997