Steam Power Station

A generating station which converts the heat energy of local combustion into an electrical energy is called steam or thermal power station.

       In this power station, the steam is produced in the boiler by using the heat of the cool combustion. The steam is then expanded in steam turbine which drives the alternator which converts the mechanical energy of the turbine into an electrical energy. The exhaust steam gets condensed in the condenser and fed back into the boiler again, completing the cycle of the power station. This principle is called Rankine cycle.

        The energy conversion involved in steam power station is shown in the Fig. 1.

Fig. 1  Energy conversion

1.1 Factors of Selection of Site

       The following factors are to be considered for the selection of site for the steam power station, in order to achieve the economical and successful operation of the plant.

1. Supply of fuel : The main fuel for the steam power plant is coal. Thus the power station should be located near the coal mine so that the fuel supply is continuous and adequate. If the plant is located away from the coal mine then sufficient transportation facility must be available.

2. Availability of Water : For the condenser, huge amount of water is required. Hence site must near be the river so that abundant quantity of cooling water is available.

3. Transportation facilities : For transporting the equipments and the machinery required by a modern steam power plant, he site selected must be easily accessible by rail and road.

4. Cost and type of land : The land must be available at a reasonable price to keep the initial cost low. There must be provision for the extension of the plant. The type of the land must be such that it should be able to withstand the weight of the heavy equipments to be installed.

5. Distance from load centers : To keep the cost of the transmission and transmission losses to minimum, the site must be nearer to the load centers. For d.c. system, transmission loss plays an important role but a.c. power can be transmitted at high voltage with reduced transmission cost. Thus this factor is more important for d.c. supply system.

6. Distance from populated area : The continuous burning of coal at the power station produces smoke, Fumes and ash, which pollutes the surrounding area. Such a pollution due to smoke  is dangerous for the people living around. Hence the site of the plant must be at a considerable distance from the populated area.

        All these factories affect the selection of site for the steam power station.

1.2 General Arrangement of Steam Power Plant

       Though steam power plant simply involves the conversion of heat energy to the mechanical energy, it requires many types of supporting arrangements. The Fig. 2 shows the schematic arrangement of steam power station.

       The coal is burnt in a place called grate in a boiler. The flue gases are evolved which heats the water in a boiler. The water is converted to a steam by absorbing heat from the flue gases. This steam is called wet steam as it contains suspended water particles. This steam is passed to the superheater where it is converted to superheated steam from the wet steam. This superheated steam is then expanded in the turbine which rotates the turbine. Thus the heat energy is converted to a mechanical energy. The turbine shaft is coupled to an alternator which converts the mechanical energy into an electrical energy. This is then given to the busbar through a transformer and proper switchgear arrangement.

        After expanding in the turbine, the exhaust steam is passed through the condenser. In the condenser, the steam is converted into liquid condensate. Using the condensate extraction pump, the condensate is taken to economizer. The economiser again transfer the heat from flue gases to the condensate and then transfer the heated water to the boiler. Thus the cycle is completed. The exhaust flue gases are released to the atmosphere through the chimney.

Fig 2   Schematic arrangement of steam power station

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,

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