Thermal Power Plant

Thermal power plant also called thermal power station or steam power plant is used to generate electricity.

How electricity is generated in thermal power plant?

Thermal Power Plant

In thermal power plants, the heat energy obtained from the combustion of fuel (coal/natural gas/other carbon base fuel) is used to convert water into steam. The high temperature and pressure steam are used to rotate the turbine blade, turbine shaft is connected to generator. The generator converts the kinetic energy into electric energy.

Thermal Power Plant Layout

Thermal power plant layout

Important Terms Used In Thermal Power Plant Or Steam Power Plant

  1. Saturation Temperature – It is the temperature for a corresponding saturation pressure at which a liquid boil into its vapor phase.
  2. Wet Steam or Unsaturated Steam – It is a mixture which contain both water vapor and liquid water droplets. Most of the best boiler discharge steam containing 3% to 5% wetness. If the mass of steam contains 5 % water it’s said to be 5% wet & 95 % dry.
  3. Dry Steam or Saturated Steam – Dry steam is steam that is at the temperature of saturation, but does not contain water particles in suspension.
  4. Superheated Steam – It is an extremely high temperature vapour generated by heating the saturated steam or wet steam beyond the saturation point.
  5. Condensate – A liquid phase produced by the condensation of steam.

How does a coal power plant work?

The coal based thermal power plant comprises of four main working circuits:

  1. Coal and ash circuit
  2. Air & gas circuit
  3. Feed water and steam flow circuit
  4. Cooling water circuit

Coal and Ash Circuit

Coal and ash circuit in a thermal power plant

Coal arrives at the coal handling plant and after necessary handling, passes on the furnace through pulveriser & fuel feeding device. Ash resulting from the combustion of coal collects at the back of the boiler and is removed to the ash storage through ash handling plant.

Air and Gas Circuit

Air and gas circuit in steam power station

Air is taken from atmosphere through the action of forced draught (FD) fan and passes on the furnace through air preheater. Flue gases are produced when coal is combusted in the boiler. The flue gases after passing around boiler tubes and superheater tubes in the furnace pass through an economiser and finally through the air preheater before being exhausted to the atmosphere via electro static precipitator (ESP) & chimney.

Feed Water and Steam Flow Circuit

feed water and steam circuit in steam power station

In the water and steam circuit condensate leaving the condenser is passes through the deaerator and water heaters before going the boiler through economiser.

Water and steam circuit in thermal power station

In the boiler drum and tubes, water circulates due to the difference between the density of water in the lower temperature and the higher temperature section of the boiler. Wet steam from the drum is further heated up in the superheater before being supplied to the turbine. After expanding in turbine steam is condensed in the condenser. The condensate leaving the condenser is circulated to boiler. A part of steam and water is lost while passing through different components and this compensated by supplying additional feed water. This feed water is purified & softened before going the boiler, to avoid the scaling of the tubes of the boiler. The purification and softening of water are done in water treatment plant.

Cooling Water CircuitCooling water circuit in thermal power station

The cooling water supply to the condenser helps in maintaining a low pressure in it. The cooled water extract heats from turbine exhaust steam in the condenser and become hot. Hot water gets cooled after passing through cooling tower and again circulated to condenser through circulating water pump.

A part of water is lost in the cooling tower and this compensated by supplying the water from a natural source such as river, lake or sea.


Steam boiler