Steam Power Plant:
Here now we going to discuss only steam power station or steam power generation plant and all other power station in next coming articles.We have the advantages, disadvantage, layout, working principle of steam power station or steam power plant in this article.
A generating station which converts heat energy of coal combustion into electrical energy is known as a steam power station.It can also be called as the thermal power station or thermal power generating plant.
A steam power station basically works on the Rankine cycle.Steam is produced in the boiler by utilising the heat of coal combustion. The steam is then expanded in the prime mover (i.e.., steam turbine) and is condensed in a condenser to be fed into the boiler again.The steam turbine drives the alternator which converts mechanical energy of the turbine into electrical energy.This type of power station is suitable where coal and water are available in abundance and a large amount of electric power is to be generated.
Steam Power Plant Schematic Diagram:
Although steam power station simply involves the conversion of the heat of coal combustion into electrical energy, yet it embraces many arrangements for proper working and efficiency.The schematic diagram of steam power station is shown in the figure below.The whole arrangement can be divided into the following stages for the sake of simplicity :
1. Coal and ash handling arrangement
2. Steam generating plant
3. Steam turbine
4. Alternator
5. Feed water
6. Cooling arrangement
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| Layout of steam power station or steam power generation plant |
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1.Coal and ash handling plant: The coal is transported to the steam power station by road or rail and is stored in the coal storage plant.Storage of coal is primarily a matter of protection against coal strikes, failure of the transportation system and general coal shortages.From the coal storage plant, coal is delivered to the coal handling plant where it is pulverised (i.e., crushed into small pieces) in order to increase its surface exposure, thus promoting rapid combustion without using large quantity of excess air.
The pulverised coal is fed to the boiler by belt conveyors.The coal is burnt in the boiler and the ash produced after the complete combustion of coal is removed to the ash handling plant and then delivered to the ash storage plant for disposal.The removal of the ash from the boiler furnace is necessary for proper burning of coal.
It is worthwhile to give a passing reference to the amount of coal burnt and ash produced in a modern steam power station.A 100 MW station operating at 50% load factor may burn about 20,000 tons of coal per month and ash produced may be to the tune of 10% to 15% of coal fired i.e., 2,000 to 3,000 tons.In fact, in a thermal station, about 50% to 60% of the total operating cost consists of fuel purchasing and its handling.
2. Steam generating plant: The steam generating plant consists of a boiler for the production of steam and other auxiliary equipment for the utilisation of flue gases.
(i) Boiler: The heat of combustion of coal in the boiler is utilised to convert water into steam at high temperature and pressure.The flue gases from the boiler make their journey through super heater economiser, air pre-heater and are finally exhausted to atmosphere through the chimney.
(ii) Super heater: The steam produced in the boiler is wet and is passed through a super heater where it is dried and superheated (i.e.., the steam temperature increased above that of the boiling point of water) by the flue gases on their way to the chimney.Super heating provides two principal benefits.Firstly, the overall efficiency is increased.Secondly, too much condensation in the last stages of the turbine (which would cause blade corrosion) is avoided.The superheated steam from the super heater is fed to the steam turbine through the main valve.
(iii) Economiser: An economiser is essentially a feed water heater and derives heat from the flue gases for this purpose.The feed water is fed to the economiser before supplying to the boiler.The economiser extracts a part of the heat of flue gases to increase the feed water temperature.
(iv) Air pre-heater: An air pre-heater increases the temperature of the air supplied for coal burning by deriving heat from flue gases.Air is drawn from the atmosphere by a forced draught fan and is passed through air pre-heater before supplying to the boiler furnace.The air pre-heater extracts heat from flue gases and increases the temperature of air used for coal combustion.The principal benefits of preheating the air are increased thermal efficiency and increased steam capacity per square metre of boiler surface.
3. Steam turbine: The dry and superheated steam from the super heater is fed to the steam turbine through the main valve.The heat energy of steam when passing over the blades of the turbine is converted into mechanical energy.After giving heat energy to the turbine, the steam is exhausted to the condenser which condenses the exhausted steam by means of cold water circulation.
4.Alternator: The steam turbine is coupled to an alternator.The alternator converts mechanical energy of turbine into electrical energy.The electrical output from the alternator is delivered to the bus bars through transformer, circuit breakers and isolators.
5. Feed water: The condensate from the condenser is used as feed water to the boiler.Some water may be lost in the cycle which is suitably made up from external source.The feed water on its way to the boiler is heated by water heaters and economiser.This helps in raising the overall efficiency of the plant.
6. Cooling arrangement: In order to improve the efficiency of the steam power plant, the steam exhausted from the turbine is condensed by means of a condenser. Water is drawn from a natural source of supply such as a river, canal or lake and is circulated through the condenser.The circulating water takes up the heat of the exhausted steam and itself becomes hot.
This hot water coming out from the condenser is discharged at a suitable location down the river.In case the availability of water from the source of supply is not assured throughout the year, cooling towers are used.During the scarcity of water in the river, hot water from the condenser is passed on to the cooling towers where it is cooled.The cold water from the cooling tower is reused in the condenser.
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Steam Power Plant Advantages:
The following are the advantages of steam power plant:
(i) The fuel (i.e., coal) used is quite cheap.
(ii) Less initial cost as compared to other generating stations.
(iii) It can be installed at any place irrespective of the existence of coal.The coal can be transported to the site of the plant by rail or road.
(iv) It requires less space as compared to the hydro electric power station.
(v) The cost of generation is lesser than that of the diesel power station.
Steam Power Plant Disadvantages :
The following are the disadvantages of steam power plant:
(i) It pollutes the atmosphere due to the production of a large amount of smoke and fumes.
(ii) It is costlier in running cost as compared to hydro electric plant.
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