Steam Power Station : Part 2

1.3 Constituents of Steam Power Station
       The various constituents of steam power station can be divided into the following stages for the ease of understanding the working of the power plant.
1. Fuel and ash circuit          2. Steam generating circuit
3. Steam turbine                  4. Alternator
5. Feed water circuit            6. Cooing water circuit
1.3.1 Fuel and Ash Circuit
       In steam power plant, the coal is used as a fuel. The coal is stored in a coal storage plant where coal is transferred from all the parts of the country by the rail or the road. The storage helps to supply the coal continuously, in case of situations like strikes, failure of transportation system etc. Then the coal is transferred to the coal handling plant where the coal is pulverized i.e. crushed into small pieces. The pulverization increases the surface exposure of the coal and this helps for rapid combustion of coal without using large quantity of air. Such a crushed coal is transferred to the boiler from the local handling plant.
       As a result of combustion of the coal. large quantity of ash is produced in the boiler. For the proper combustion of the coal, ash is removed to the ash handling plant. Then it is delivered to the ash storage plant, from where it is disposed off.
1.3.2 Steam Generating Circuit
       The main component of steam generating circuit is the boiler. But many other auxiliary equipments are used so as to completely utilize the heat of flue gases.
1. Boiler : The boiler is a closed vessel where water is converted to the steam using the heat of the local combustion. Hence the boiler is called steam generator. In the boilers, the grate is provided for the combustion of coal. The steam produced in the boiler contains suspended water particles and hence called wet steam.
2. Superheater : It is an accessory attached to the boiler and located in the path of flue gases leaving the boiler and flowing towards chimney. By using the heat of the flue gases, the superheater converts the wet steam into superheated dry steam. There are two advantages of superheating that it increases the overall efficiency and it avoids the corrosion of the turbine blades due to wet steam. The superheated steam is then passed to the turbine through a main valve between the two. The two types of superheaters used are radiant type and convection type.
3. Economizer : It is another accessory attached to the boiler and located in the path of flue gases. Thus it utilizes the heat of flue gases which would otherwise wasted to the atmosphere. The water from the feed pump is passed through the economizer to the boiler drum so that before entering the boiler, it is heated and hence less efforts are required to convert it into steam. This increases the overall boiler efficiency, saves the fuel and reduces the stress on the boiler.
4. Air preheater : This is also an accessory attached to the boiler and located in the path of flue gases. The air is required for the local combustion. Air is drawn from the atmosphere by a forced draught fan and is supplied to the air preheater. The air preheater extracts the heat from the flue gases and makes the air hot before supplying to the boiler. This increases the temperature of the furnace and helps in the production of the steam. This increases the thermal efficiency and the steam capacity per square meter of the boiler surface.
       The two types of air preheaters used are recuperative type and the other is regenerative type.
1.3.3 Steam Turbines
       The dry and superheated steam from the supeheater is supplied to the turbine. The hat energy of the steam is converted to the mechanically energy as steam passes over the turbine blades. There are two types of steam prime movers available, steam engine and steam turbine. The steam turbine is practically used because of the following advantages,
i) High efficiency       ii) Simple construction        iii) Low maintenance
iv) High speed          v) Less floor area                vi) No flywheel required
vii) Less problems of vibrations
       The steam turbines are classified into two types as impulse turbine and reaction turbine.
       In the impulse turbine, the steams expands completely in the nozzle and pressure over the moving blades remaining constant. While doing so, the steam attains very high velocity and impacts on moving blades giving rise to an impulsive force on them. Thus the turbines starts rotating.
        In the reaction turbine, steam is partially expanded in the stationary nozzle and remaining expansion takes place on the moving blades. This causes reaction force on the moving blades and the turbine stats rotating.
        The commercial turbines nowadays use series combination of impulse and reaction turbines, due to which steam can be used more efficiently.
1.3.4 Alternator 
       The alternator shaft  is coupled to the turbine. When the turbine shaft rotates, the alternator shaft rotates and converts the mechanical energy into an electrical energy. The electrical energy from the alternator is given to the busbar through transformer, circuit breakers and isolators.
1.3.5 Feed Water Circuit
       The condensate leaving the condenser is used as the feed water. Because it goes to the boiler, It is first heated in a closed feed water heater. Then it is passed to economizer where it is further heated and then passed to the boiler. This increases the overall efficiency of the plant.
        The feed water source is generally river or a canal. It contains suspended and dissolved impurities. The boilers needs clean and soft water for loner life and better efficiency. Hence the feed water is purified. It is stored in the tanks and by the different actions like sedimentation, filtration etc., it is made soft and pure. Such a pure feed water is used for the steam generation in the boiler.
1.3.6 Cooling Water Circuit
        For improving the plant efficiency, the expanded steam coming out of the turbine, passes through the condenser where it is condensed into water. The condenser is very important as it creases a very low pressure at the exhaust of the turbine thus helps in the expansion of steam in the turbine at low pressure.
        For condensation of steam, a flow of natural cold water is circulated through the condenser. This takes the heat from the exhaust steam and gets heated. This hot water is discharged at a suitable location or is passed through a cooling tower so that it is again converted to cold water. Then it is recirculated through the condenser by a pump. The condensed steam can be used as a feed water to the boiler.
       The two types of condensers used are jet condenser and surface condenser.
1.4 Advantages 
1. The fuel used is a coal, which is cheap.
2. The initial cost is less compared to other power station.
3. It requires less floor space area compared to hydro-electric power station.
4. The fuel is easily available.
5. The fuel can be easily transported top the site hence site can be anywhere ad not always near the coal mines.
6. The cost of the generation is less than the diesel.
1.5 Disadvantages 
1. Due to the smoke and fume, pollutes the surrounding atmosphere.
2. Running cost is higher than hydro-electric power palnt
1.6 Efficiency
       For a steam power station, two efficiencies are defined which are thermal efficiency and the overall efficiency.
       The thermal efficiency is the ratio of heat equivalent of the mechanical energy transmitted to the turbine shaft to the heat of the combustion of coal.

       The overall efficiency is the ratio of heat equivalent of electrical output from alternator to the heat of coal combustion. The overall efficiency pf steam power station is very low about 20 to 25%.

       The overall efficiency depends on number of factors and hence can be expressed as,

ηelectrical = Electrical efficiency of an alternator which is practically high, above 90%.
ηboiler = Boiler efficiency considering the effect of economizer and air preheater, which is about 85%.

See Steam Power station part 1

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1 comment:

  1. If you want more to understand about steam power plant(thermal power plant) please visit the below link

    Steam power station or Steam power generation plant