In this article we will discuss about:- 1. Introduction to Thermal Power Plant 2. Working of a Thermal Power Plant 3. Advantages 4. Disadvantages.
Introduction to Thermal Power Plant:
Coal is the main fuel used in a thermal power plant. Coal contains combustible elements such as carbon and hydrogen. The percentage composition of carbon in the best quality coal is about 95%. The combustible element in coal combines with oxygen present in air during combustion. For better and effective combustion, excess quantity of air is supplied.
A large quantity of heat is produced after combustion, which is utilized to convert water into high-pressure and high-temperature steam. The high-pressure steam is passed through the steam turbine where its expansion takes place and the heat energy is converted into mechanical energy in the form of rotation of shaft. The shaft of the turbine is coupled with the generator and thus electrical energy is generated.
To improve the thermal efficiency and the overall performance of the plant, various auxiliary equipments are installed. The reheating of steam and regenerative feed heating are some of the steps. To run the boiler safely and effectively, large numbers of boiler mountings are fitted on boiler drum and shell. To run the boilers efficiently, large number of sub-equipments known as accessories, such as superheaters, economizer, and airpreheater, are installed.
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The installation of such units involves high cost and space and as such, except the installation of superheater, the economizer and air pre-heater are applicable only for large capacity plants (where the steam generating pressure is more than 70 bar). There is also an arrangement for the draft fan to have a forced circulation of air and flue gas through the furnace of the boiler.
Working of a Thermal Power Plant:
The complete layout of a thermal power plant is presented in Fig. 5.1. Coal from the mines is transported to the actual site of the thermal power plant by means of rail, boat, ship, or truck depending upon the location of the plant. The coal is stored in storage yard. It is then carried by conveyers for processing; depending upon the size of the plant and availability of the arrangements such as belt conveyers, bucket elevators, etc.
The lump size coal is crushed and passed through magnetic separator to detain any iron impurities before it is carried by conveyers to the pulverized coal mill for pulverization (made into powder).
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The coal is pulverized to increase the surface area to have proper mixing of air for the effective combustion. The pulverized coal along with air is supplied to the furnace of the boiler. Air and fuel must be thoroughly mixed to burn the fuel completely. The furnace flame temperature must be very high to ignite the incoming raw fuel and air.
The product (flue gases) formed after combustion transfers the heat to the water in boiler and is finally discharged to the atmosphere through chimney. On its way, several equipments such as superheaters, economizer, and then air preheaters are placed to transfer the heat. The flow of flue gas is established by the combination of forced and induced draft fan.
Induced draft fan is placed just before the bottom of the chimney as shown in Fig. 5.2. The flue gas is also passed through electrostatic precipitator (ESP) to separate out all the fly ash and cinders and the cleaned gas is discharged through the chimney comparatively at lower temperature.
The feed water in purest form is supplied to the boiler by a feed pump. It first enters into the economizer where it is preheated by the heat of flue gases from the boiler. The preheating of feed water results in the increase in boiler efficiency because there is a recovery of heat from the waste heat through flue gas and it also saves fuel consumptions.
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The feed water gets converted into high-pressure and high-temperature steam. The steam generated inside the boiler is passed through a superheater to get the steam superheated. The superheated steam is supplied to the steam turbine which drives the generator to produce electricity.
The exhaust steam from turbine at low pressure is discharged into a condenser. The condensate is then collected into a hot well or water storage tank from where it is again pumped into the boiler as feed water. The condenser is a closed vessel which is maintained to be at low pressure. The condenser acts as a heat exchanger. The ash formed after the combustion of fuel is disposed off by ash-handling equipments.
The handling of ash is difficult than coal handling due to the following reasons:
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(a) Ash is hot when it comes out from furnace.
(b) It is abrasive and corrosive.
The belt conveyors cannot be used for this purpose. In modern boilers, as it is known, the combustion of pulverized fuel results in the formation of cinders and fly ash which is almost in powder form. In most of the modern boiler plants, the hydraulic system of the disposal of ash can is implemented. In this case, water jet pumps are used.
Advantages of Thermal Power Plant:
(a) Initial cost is low compared to hydro-electric power plant.
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(b) It can be established near the load center, so that transmission losses and cost involved therein are low.
(c) The erection of the plant takes less time as compared to a hydro-electric plant.
Disadvantages of Thermal Power Plant:
Fossil fuel is the main fuel for any thermal power plant. Since fossil fuels are nonrenewable source of energy, they are feared to be exhausted in future.
(a) If thermal power plant location is far away from coal mines, then transportation becomes a costly affair.
(b) Smoke produced after the combustion of fuel causes pollution.
(c) The generation cost is high as compared to a hydro-electric plant.
(d) The life of a thermal power plant is less as compared to a hydro-electric power plant.
Layout of a Hydro Power Plant:
Figure 5.3 shows the layout of a hydro power plant. Water from the bottom of dam is carried to turbine by a large diameter tube known as penstock. This is generally made of reinforced concrete. To avoid the loss of head due to friction, sharp bends are eliminated. According to the need of water to be supplied to the turbine, the amount of water is controlled by a control valve by means of generator.
When water is running inside the penstock, there is a large velocity-producing pulsating vibration which creates water hammer, which damages the penstock. To prevent this, a surge tank is installed in penstock. The water is allowed to pass through draft tube into the tail race.