Thermal power plant uses steam energy for the generation of the electricity. Fossil fuels like coal, oil, natural gases are burnt in the boiler, a hot flue gases thus generated are used to heat the feed water. A superheated steam is generated. This steam under high pressure then expanded in stem turbine to rotate the turbine. Turbine is coupled with the generator. When turbine rotates, generator also rotates and produces electricity. This arrangement of coupling turbine and generator is called turbo-alternator.
The capacity of the thermal power plant is generally few MW to 1000MW. But now a day it becomes a trend to build thermal power plants of capacity higher than 1000MW.
Following figure shows the general block diagram of the thermal power plant.
Site Selection for Thermal Power Plant:
- Land requirement: The land for the thermal power plant should be large enough so that the present installation and future expansion of the plant can be easily done. Cost and bearing capacity of the land also plays important role while selecting a site for thermal power plant.
- Water supply: Site should be near to the river so that water required for the ash disposal, boiler feed water, cooling and circulating water for condensers should be available easily.
- Fuel supply: Plant should be near to the coal mines, because cost of transmission of electricity is less than cost of transportation of fuel.
- Ash disposal: Making ash ponds is a part of installation of the plant. Enough space should be there for ash ponds and water supply should available for it.
- Transport facilities: For fuel supply – road and railway links. Located near coal mines because if quality of coal is poor transportation of coal is costly.
- Environment requirement: Thermal power plant produces lot of pollution. Ash ponds may produce water and air pollution, smoke from chimneys produces air pollution. To avoid affects of pollution site should be far from populated area.
Main power plant can be subdivided n to several units as follows:
- Feed water unit
- Boiler unit
- Turbine unit
- Generator unit
- Cooling water unit
- Fuel handling unit
- Ash handling unit
Following are main elements of thermal power plant:
It is a steam generation unit in thermal power plant. There are two types of boilers: a) fire tube, b) water tube. In fire tube boilers, hot combustion gas flows through tubes which are surrounded by water. In case of water tube boilers, feed water tube pass through combustion chamber where hot flue gases flow over them.
- Boiler feed pump: function is to feed the water to be steamed in the boiler of thermal power plant. Condensate water to be coming out from condenser and make up water coming from water treatment plant ad together forming total feed water.
- Economizer: economizer is used as a heat exchanger. The remaining heat energy of the flue gases is passed through economizer so as to increase the efficiency of the plant.
- Superheater: In thermal power plant a saturated steam is first generated in the boiler which contains some amount of water droplets (or moisture). So to make the saturated steam from superheated stem a superheater is used.
- Coal mills: in modern power plant pulverized coal is used. Once pulverized, coal mills are employed for crushing the coal into a powder form. This increases the efficiency of boiler hence the plant.
- Air preheater: the remaining heat energy of the flue gases after passing through economizer is utilized for air preheating. The air required for completer combustion of the coal is first passed through air preheater so as to increase the efficiency of the boiler.
- Draught system: to overcome the resistance offered by the pipelines, air ducts, fuel beds, dampers, chimney, etc. for the flow of air, flue gases draught system is required in thermal power plant. Normally FD (forced draft) fan, ID (induced draft) fan and chimney facilitate this function. FD fan provides air from atmosphere to the boiler with the positive pressure, where as ID fan along with chimney sucks an air and waste gas from boiler to the atmosphere.
In thermal power plant, turbo alternator is usually coupled unit of steam turbine and electrical generator. Steam turbine rotates with the help of superheated steam expanded from high pressure and high temperature. As the turbine rotates, alternator also rotates gives rise to generation of electricity.
- Steam turbine: there are two types of steam turbine: impulse turbine and reaction turbine. In impulse turbine the heat energy of the steam is first converted in to kinetic energy by first passing through fixed nozzles. The steam coming out of fixed nozzles at high velocity impinges on the blades of the rotor. In reaction turbine the steam expands partially while passing through fixed nozzles and partially over the moving blades.
- Condenser: exhaust gases from the turbine are passed through condensers fro two purposes. To condensate steam to water to recirculation as a feed water and to remove the incondensable gases through water circuit.
- Cooling tower: In thermal power plant, ordinary water pumped from river, canal or pond is used to cool the exhaust steam from the turbine in the condenser. This process requires large amount of water. So to reuse that water, cooling towers are used to cool the heated water coming out from condenser.
- Generator: this is the main unit of any power plant. In this unit the generation of electricity is done. It is mechanically coupled with the turbine, so that when turbine rotates, it also rotates. The speed of the generator is given by,
Where N = speed of generator,
f = frequency of the system (50Hz in India)
P = number of poles.
5. Governors (Speed controllers): In thermal power plant, the speed of the governor should be kept at the constant rate (50Hz in India) ± some tolerance. So to keep the speed constant the flow of stem in the turbine has to be controlled. So to overcome this problem governors or electrical speed controllers are used.
6. Alternator cooling system: In thermal power plant, large amount of heat is generated in the windings due to copper and Eddy current losses. This heat should be removed continuously so as to keep the insulating material from damaging. There are two methods of cooling systems, open-circuit and closed-circuit method.
For low capacity generators open-circuit cooling system is employed in which atmospheric air is passed through one side and sucks out from the another side.
For medium capacity generators closed circuit cooling system is employed in which same air passed again and again after cooling it. In case of high capacity generators same closed circuit cooling system is employed only with difference of air is replaced with oxygen.
Finally the overall efficiency of the thermal power plant is given by,
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