How Electricity is Produced from Coal

A power station has three main types of equipment – (i) a boiler, (ii) a turbine and (iii) a generator. In the boiler the energy in a fuel is used to convert water into steam at high temperatures and pressures. The type of fuel used for producing steam from water defines the type of power stations. For example, a coal-fired plant burns coal for this purpose. The steam is then piped to turn the turbine which drives the generator to produce electricity.

Efficiency

Materials for Coal Fired Power Stations

Recent Developments

Efficiency
For high efficiency conversion of the energy in the fuel to electricity, the steam temperature and pressure must be as high as possible. However, power plants have commercially useful lives of 30 years or more. During this life, their utilisation can vary from non-stop duty at full capacity to part-load and/or part-time operation to suit customers’ demand for energy. Whatever the demand, components must operate safely and reliably throughout the life of the plant.

Materials for Coal Fired Power Stations (PHOTO)

The combination of high temperature, pressure and long-life determine the properties required in the materials of construction.

	High strength at the service temperature, so that part can be sustain the stress throughout the intended life and that the construction can be thin, hence light-weight and economical.
	Thick components in particular must not fail under thermal stresses generated during start-up/shutdown.
	Materials can be joined to itself and to other alloys and the joints possess adequate properties.
	High ductility and toughness so that a failing component deforms slowly rather than fracture in a catastrophic manner.
	Resistant to oxidation, corrosion and erosion so that they do not suffer excessive loss of thickness, leading to failures. Possible effects of stress, temperature localised behaviour (e.g. around welds, contact with more noble materials) have to be considered.
	Ideally, properties should not degrade or at least must not degrade so rapidly as to compromise the component integrity.
	Materials availability in the sizes and quantities required, at economical costs.

Power plants can now be designed and constructed to operate with steam at up to 300 bar and over 600 °C, compared with only 170 bar at 565 °C until 1980’s, increasing the efficiency from around 37% to 47%. This has become possible due to the development of new steels, particularly for thick section components e.g. headers, pipes, turbine rotors etc. These new materials also possess other properties required and listed above. European countries are collaborating to raise the steam temperature further, to over 700 °C and efficiencies to 55% or more.

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