THERMAL POWER STATION
Thermal power generation in India constitutes to about 65% of the total energy generation, the fuel resource supplied mainly from the Bengal-Bihar Belt. Modern thermal power stations operate on the thermodynamic principle of Modified Rankine Cycle.
Boiler or Steam generator helps to burn the coal and transfer the heat to water, thereby generating steam at the required pressure and temperature. Coal is fed to the Raw Coal Bunkers that is sufficient to store one day requirement. Then, the coal is pulverised into a fine powder in the pulverisers or mills and carried away into the boiler by the hot air at 60 – 70 supplied by Primary Air Fans. The fuel/air mixture is fed in the four corners of the boiler in a corner fired boiler, in four or five elevations. For complete combustion of coal in the furnace, secondary air is supplied by the Forced Draft fans. Feed water is supplied to the boiler drum situated at the top of the boiler from the economiser which make use of the heat in flue gas to preheat the feed water to increase the boiler efficiency. Water converts into steam in the furnace area and the dry and saturated steam is collected in the top portion of the drum. The saturated steam is then superheated superheaters. The superheated steam is called main steam that leaves the boiler at rated pressure and temperature.
Turbine is a high speed rotating machine that converts the kinetic energy and pressure energy of the steam into useful work. A turbine generally has three stages viz. High Pressure, Intermediate Pressure and Low Pressure. The main steam enters into HP turbine and after expansion in the turbine the pressure and temperature fall down. The main steam is returned to the boiler for reheating in the Reheater. The Hot Reheated steam is admitted in the intermediate pressure turbine IPT after expansion in the IPT, steam enters into the Low pressure turbine. When the useful work is extracted from the steam, the pressure falls below the atmospheric pressure. Vacuum is maintained in the condenser to create steam flow by means of vacuum pump or steam jet ejectors. Cooling Water flows in the tubes of condenser to cool the steam. The condensate is pumped to the deaerator by condensate extraction pump (CEP) through Low Pressure Heaters where the temperature gain in the condensate is achieved from the heat of extraction steam from the turbine. The deaerator helps to remove the oxygen in the feed water as dissolved oxygen enhances corrosive action.
The feed water from deaerator is pumped to the boiler drum, by Boiler Feed Pump (BFP) through HP heaters and economiser. Boiler Feed Pump is the heart of a thermal power station as it supplies feed water to the boiler continuously.
Turbine is coupled with the TurboGenerator that normally spins at 3000 rpm in countries with 50 Hz supply frequency or at 3600 rpm in countries with 60 Hz supply frequency. The generated voltage is stepped up in Generator Transformer and the power is evacuated through transmission line feeders.
Handling of Fly Ash
After complete combustion of coal in the furnace, the heat in the flue gas is utilised to preheat the water in the economiser and primary and secondary air in AirPreHeaters. The fly ash laden gas is evacuated through chimney by Induced Draft fans through Electro Static Precipitators (ESP). Electro Static Precipitators are devices that separate fly ash from the flue gas and thus Solid Particulate Matter in the exit gas is controlled. Apart from fly ash, bottom ash is also collected in the bottom of the furnace and is disposed off in the form of ash slurry in ash dyke.
The hot water discharge from condenser is cooled in cooling towers of Natural Draft or Induced Draft type in closed circuit cooling system. In open circuit cooling system, the hot water is discharged into sea or perennial river in such a way that it does not affect the flora / fauna of the ecosystem. Cooling Water Pumps (CW pumps) pump the cold water stream back to the condenser.
Though thermal generation is the back bone of our country, the Carbon-di-oxide emission amounts to Green House Gas (GHG) playing a vital role in Global Warming. United Nations Framework Convention of Climate Change (UNFCCC) is working on emission reduction. New technologies like Oxyfuel combustion, Integrated Gasification Combined Cycle (IGCC), CO2 capture in flue gas etc., are on the way to mitigate CO2 emission related problems.