Single generator sets of over 600 MW are now used in the UK, though there are many smaller generators in use. A 600 MW generator can supplythe average needs of over 1 million UK households. Three or four such generators are typically installed in a single large coal-fired station which isoften sited close to a coal mine, away from the city dwellers who consume the electricity.
Such generators are usually driven by a compound arrangement of highpressure, intermediate-pressure and low-pressure turbines, increasing in size as the pressure decreases. Modern turbines rotate in a speed range from 1500 to 3500 r.p.m., usually 3000 r.p.m. for the UK’s 50 Hz system. For large coal-fired plant the steam pressure could be 25 megapascals (MPa) with steam temperatures of 500–600 °C to improve the thermodynamic efficiency. In nuclear reactors, which operate under less demanding conditions, the steam is superheated to about 5 MPa and 300 °C. Modern water tube boilers are complex and have fuel-to-steam efficiencies of about 90 per cent which they achieve by several sets of heat exchangers. The rate of supply of steam is critical and must be carefully controlled to match exactly the often rapid variation in demand for electricity. Too much steam and the turbine speeds up, too little and the turbine slows down, consequently affecting the frequency of the electricity generated. The rate of rotation must remain constant so that the frequency is maintained with less than about 0.5 per cent variation.
In order to achieve the best thermodynamic efficiency, the exhaust temperature of the steam is lowered by a condenser through which cooling water is pumped. When a plant is close to a river or the sea, such cooling water is returned over 10 °C warmer. Otherwise the warm water is sprayed down a cooling tower through a rising air current to cool it. Such cooling methods are very wasteful, the wasted thermal energy being the main reason for the low overall energy conversion efficiency. Utilizing this warm water for local space heating in combined heat and power (CHP) schemes brings higher useful overall energy conversion efficiencies.
Modern power station boilers are designed to maximize the energy transfer from the burning fuel. Nonetheless, waste products – ash; oxides of sulphur (SOx) and nitrogen (NOx); and carbon dioxide (about 1 kg per kW h of electricity produced from coal) – present major pollution problems. Many existing stations burn pulverized fuel, particles of which are less than about 0.1 mm, which are blown in a carefully controlled flow of air to the burner jets, ensuring complete combustion of fuel. Energy transfer efficiencies tothe boiler can exceed 90 per cent in good conditions. Rapid fuel combustion reduces the production of NOx. SOx are increasingly removed from the ‘exhaust’ gases by flue-gas desulphurization (FGD) plant in which SOx are combined with limestone, typically in a spray. A disadvantage of pulverized fuel boilers is the fine ash, fly-ash, which has to be removed from the flue-gas exhaust to the atmosphere. In an electrostatic precipitator, the ash particles are charged as they pass through a grid of high-voltage wires and deposited on earthed collectors. FGD and precipitation plant can add 20 per cent to the capital cost of a power station.
Modern stations use fluidized-bed combustion to solve some of the problems of NOx, SOx and ash emissions but not of carbon dioxide. In a fluidized bed boiler, air is blown through fine sand creating a ‘floating bed’ into which the fuel particles are fed. Rapid and efficient combustion ensues, heat being carried away in the water tubes buried in the bed. Limestone particles are also fed into the bed to remove SOx at source. The bed temperature is lower so NOx production is reduced. Ash production is also lessened.
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