Steam Power Station (Thermal Station) A generating station which converts heat energy into electrical energy through turning water into heated steam is known as a steam power station. A steam power station basically works on the RANKINE cycle.
Steam is produced in the boiler by utilizing the heat of combustion. The steam is then expanded in the prime mover (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. Flue gases from the boiler pass through super-heater, economizer, air pre-heater and are finally exhausted to atmosphere through the chimney.
Hot air Flue gases Chimney induced draught fan Air pre heater Forced draught fan Bus-bars isolators C.B. isolators R Y B Fuel storage and handling Flue gases Economizer Flue gases Super Boiler heater Feed water heater Feed water pump Valve Transformer Turbine Condensate extraction pump Alternator Exhaust steam Condenser Exciter Circulating water pump Water treatment chamber Cooling Tower River Schematic arrangement of Steam Power Station
Equipment of Steam Power Station Water treatment plant Condenser Steam generating equipment Prime mover Electrical & control equipment Boiler Alternator Boiler furnace Exciter Super heater Transformers Switchgear Economizer Pre heater Control room
Water treatment plant Boilers require clean and soft water for longer life and better efficiency. However, the source of boiler feed water is generally a river or lake which may contain suspended and dissolved impurities and dissolved gases. Therefore, it is very important that water is first purified and softened by chemical treatment and then delivered to the boiler. Source water is stored in storage tanks, where suspended impurities are removed through sedimentation, coagulation and filtration. Dissolved gases are removed by aeration and degasification. The water is then softened by removing temporary and permanent hardness through different chemical processes. The resulting pure and soft water is fed to the boiler for steam generation.
Boiler A boiler is a closed vessel where the heat of combustion is utilized to convert water into steam at high temperature and pressure. Types of boilers: Water tube boilers: -Water flows through the tubes and the hot gases of combustion flow over these tubes - Requires less space - High working pressure - Less liable to explosion Fire tube boilers: -The hot gases of combustion pass though the tubes surrounded by water - Not suitable for large capacity plants
Boiler furnace - A chamber in which fuel is burnt to liberate heat energy. - Provides support and enclosure for the combustion equipment - Walls made of refractory materials that resist change of shape or physical properties at high temperatures (fire clay, silica, kaolin). Types of wall construction Plain refractory walls: Suitable for small plants where the furnace temperature is not high. Hollow refractory walls with arrangement for air cooling: Suitable for large plants. Air is circulated through hollow space to keep furnace walls at low temperature. Water walls: Recent development suitable for large plants. Plain tubes arranged side by side on the inner face of refractory walls. Tubes are connected to the upper and lower headers of the boiler. The boiler s water is made to circulate through these tubes. Water walls absorb radiant heat in the furnace.
Super-heater Wet steam produced in the boiler is passed through a super-heater where it is dried and superheated by flue gases on their way to chimney. Superheating provides two principal benefits; the overall efficiency is increased, and too much condensation in the last stages of turbine, liable to cause blade corrosion, is avoided. The superheated steam from the super-heater is fed to steam turbine through the main valve. Radiant Super-heater -Placed in furnace between the water walls. - Receives heat from burning fuel through radiation process - Requires careful design accounting for being superheated - Its temperature falls with increased steam output Convention Super-heater -Placed in the boiler s tube bank - Receives heat from flue gasses through convention process - Its temperature increases with increased steam output
Economizer The economizer is a feed water heater that extracts a part of heat of flue gases to increase the feed water temperature. Air pre-heater The air pre-heater extracts heat from flue gases and consequently increases the temperature of the air supplied for fuel burning. Increased thermal efficiency and increased steam capacity per square meter of boiler surface are attained accordingly. Types depend on heat transfer method Recuperative type: -Consists of a group of steel tubes, through which flue gases are passed. - Heat is transferred from gases to air flowing externally. Regenerative type: -Consists of a slowly moving corrugated metal plates drum - Flue gases flow continuously on one drum side, and air on the other permitting heat transfer.
Condenser - Condenses steam at turbine s exhaust creating low pressure. - Permits steam s expansion in prime-mover at very low pressure. - Accordingly helps energy conversion in prime-mover. - Condensed steam can be reused as feed water in surface type Jet condenser: -Cooling water and exhaust steam are mixed together - Requires low initial cost, less floor area, less cooling water and low maintenance charges. - Limited to industrial sizes; 1000 kw with 50 125 mm.hg vacuum. -Condensate is wasted and High power is required to pump water Surface condenser: - Consists of a bank of horizontal tubes enclosed in cast iron shell. - Cooling water flows through the tubes, while exhaust steam flows over its surface giving up its heat and condensing. - Prevail in Large installations; (best vacuum: 12 50 mm.hg) - Condensate can be used as feed water, Less pumping power is required and Better vacuum is created at turbine exhaust. - Requires high initial cost, large floor area, high maintenance
Prime-movers Steam Engines Steam Turbines: - High efficiency, Simple construction, less floor area, low maintenance requirements Impulse Turbines: -Steam attains high velocity as it completely expands in stationary nozzles or fixed blades, while pressure over moving ones remains constant. - Rotation results by impulse force of steam impinging the moving blades. Reaction Turbines: -Steam partially expands in stationary nozzles, while remaining expansion occurs during flow over moving blades. - Rotation results by reaction force originated from steam s momentum.
Generator Alternator: - Cylindrical rotor type - Nitrogen or air cooled Exciter: - Separate (old) - - Brushless (most used)
Switchyard - Contains transformers and switchgear. - Provided with protective lightning arrestors. -Transformers: - Main step-up transformers - General service transformers - Auxiliary transformers Switchgear: - Circuit breakers - Switches - Relays.
Control room - Contains panels of measuring instruments and communication arrangements. - Contains control equipment needed for alternator, feeder, automatic voltage regulator, synchronizing gear and protective gear. - Separate battery room and either a motor/generator set or a rectifier are installed to supply make and break switchgear circuits.
Steam plant Efficiency: Steam plant Efficiency = Thermal efficiency Electric efficiency Thermal efficiency depends on the following factors: - Pressure of steam entering the turbine (increases noticeably with increased pressure) - Temperature of steam entering the turbine (increases with increased temperature) - Pressure in the condenser (increases with decreased condenser pressure; usually kept at 0.04 kg/cm 2 ) To increase thermal efficiency beside high steam pressure and temperature, and low condenser pressure, reheating of steam between turbine stages, and bleeding steam for heating feed water may be adopted
η thermal = Heat equivalent of mech enery transmitted to turbine shaft Heat of feul cobustion Huge amount of heat is lost in condenser ( 50%). Unavoidable as energy conversion requires temperature difference, necessitating low steam temperature in condenser, while the greater the temperature difference, the greater the heat lost is. Low Efficiency ( 30%). Heat is losses at other various stages of plant ( 20%). η overall = Heat equivalent of elctric enery output Heat of feul cobustion Low Efficiency ( 29%).
Advantages (i) Less initial cost as compared to other generating stations. (ii) It can be installed at any place (iii) It requires less space as compared to the hydroelectric power station. (iv) The cost of generation is less than that of the diesel power station. Disadvantages (i) Pollution due to the production of large amount of smoke and fumes. (ii) Higher running cost as compared to hydroelectric plant.