Almost two third of electricity requirement of the world is fulfilled by thermal power stations. In these power stations, steam is produced by burning some coal fuel like coal and then used to run a steam turbine. Thus, a thermal power station may sometimes be called as a Steam Power Station. After the steam passes through the steam turbine, it is condensed in a condenser and again fed back into the boiler to become steam. This is known as ranking cycle.
Layout And Working Of Thermal Power Station
A simplified layout of a thermal power station is shown below.
Coal: In a coal-based thermal power plant, coal is transported from coal mines to the generating station. Bituminous coal or brown coal is used as fuel. The coal is stored in either ‘dead storage’ or in ‘live storage’.
Dead storage is generally 40 days of backup coal storage which is used when the coal supply is unavailable. Live storage is a raw coal bunker in the boiler house.
The coal is cleaned in a magnetic cleaner to filter out if any iron particles are present which may cause wear and tear in the equipment. The coal from live storage is first crushed in small particles and then taken into a pulverizer to make it in powdered form.
Finely powdered coal undergoes complete combustion, and thus pulverized coal improves the efficiency of the boiler. The ash produced after the combustion of coal is taken out of the boiler furnace and then properly disposed of. Periodic removal of ash from the boiler furnace is necessary for proper combustion.
Boiler: The mixture of pulverized coal and air (usually preheated air) is taken into the boiler and then burnt in the combustion zone. On ignition of the fuel, a large fireball is formed at the centre of the boiler and a large amount of heat energy is radiated from it. The heat energy is utilized to convert the water into steam at high temperatures and pressure. Steel tubes run along the boiler walls in which water is converted to steam. The flue gases from the boiler make their way through the superheater, economizer, and air preheater and finally get exhausted to the atmosphere from the chimney.
Superheater: The superheater tubes are hung at the hottest part of the boiler. The saturated steam produced in the boiler tubes is superheated to about 540 °C in the superheater. The superheated high-pressure steam is then fed to the steam turbine.
Economizer: An economizer is essentially a feed water heater which heats the water before supplying it to the boiler.
Air pre-heater: The primary air fan takes air from the atmosphere and it is then warmed in the air pre-heater. Pre-heated air is injected with coal in the boiler. The advantage of pre-heating the air is that it improves coal combustion.
Steam turbine: High-pressure superheated steam is fed to the steam turbine which causes turbine blades to rotate. Energy in the steam is converted into mechanical energy in the steam turbine which acts as the prime mover. The pressure and temperature of the steam fall to a lower value and it expands in volume as it passes through the turbine. The expanded low-pressure steam is exhausted in the condenser.
Condenser: The exhaust steam is condensed in the condenser by means of cold water circulation. Here, the steam loses its pressure as well as temperature and it is converted back into the water. Condensing is essential because compressing a fluid which is in the gaseous state requires a huge amount of energy with respect to the energy required in compressing a liquid. Thus, condensing increases the efficiency of the cycle.
Alternator: The steam turbine is coupled to an alternator. When the turbine rotates the alternator, electrical energy is generated. This generated electrical voltage is then stepped up with the help of a transformer and then transmitted where it is to be utilized.
Feed water pump: The condensed water is again fed to the boiler by a feed water pump. Some water may be lost during the cycle, which is suitably supplied from an external water source.
This was the basic working principle of a thermal power station and its typical components.