2 Thermal Power Stations Coal FiredTurbo alternators driven by steam turbineOil FiredCrude oil OR Residual oilGas FiredFastest growing primary fuel, worldwideCombined cycleFirst stage - Gas turbine & Second stage - Steam TurbineDiesel FiredIC Engine as prime moverStandby power plants
3 Selection of site Nearness to load center Supply of water Availability of coalLand requirementType of landLabour suppliesAsh disposalDistance from populated area
4 thermodynamic cyclea type of steam engine involving a continuous cycle of vaporization of liquid and condensation back to liquid in a sealed system
5 merits & demerits Merits: Fuel used is cheaper Cheaper installation cost comparativelyCheaper production cost in comparison with diesel power plantCan be installed at ay place irrespective of the existence of fuels unlike HEPPCan be located near to load center unlike HEPPAble to respond rapidly changing loads without difficultySteam engines and turbine can work under under 25% of over load continuouslyPortion of steam raised can be used as process steam in various industries (paper mills, textile mills etc)
6 Merits & Demerits Demerits: High maintenance and operating cost PollutionRequirement of water in huge quantityHandling of coal and ash is quite difficultThe plant cost increases with the increases in the operating temperature and pressureRequire long time for erection and put into actionEfficiency fall rapidly below 75% of full load
8 Components1. Cooling tower L.P turbine H.P turbine Coal pulverizer Re-heater2. Cooling water pump 7. Condensate pump De-aerator Boiler steam drum Combustion3. Transmission line Surface condenser Feed water heater Bottom ash Economizer4. Transformer I.P turbine Coal conveyor Super-heater Air pre-heater5. Generator steam control valve 15. Coal hopper Forced draught Precipitator26. Induced draught27. Flue gas stack
9 Fuel & Ash Handling Coal delivery: by trucks, wagons etc Coal storage: Dead & Live storageCrusher: coal is crushed to increase the areaMagnetic separator: removes impuritiesPulverizer: device to transform coal into fine powderAsh pit: when fuel gets burnt, ash is collected in ash pitAsh quenching: Ash is hot & dusty, hence needs to be quenchedAsh delivery: Quenched ash is taken using conveyors to the site
10 Furnace A place where fuel is burnt Encloses burning equipments Types Grate fired: uses stationary or movable grates, suited for solid fuels.Hand fired – uses stationary grate, small size plants, discontinuous processStoker fired – uses movable grate, medium & large size plants, continuous process. Over feed stoker: the fuel & air move are not in the same direction. Under feed stoker: the fuel & air move in the same directionChamber fired: suited for pulverized coal, liquid or gaseous fuels.
11 BoilerWater tube boiler: DM water circulates through tubes & hot flue gases flow over them.Less liable to explosion, produce high pressure steam, high efficiency, heating surface is largeFire tube boiler: hot flue gases pass through the tubes which are surrounded by water.Low cost, compact in size, heating surface is small, cannot produce high pressure steam, liable to explode, low efficiencyWater tube boiler Fire tube boiler
12 Methods to improve thermal efficiency Air pre-heater :hot flue gases are used to pre-heat the airpre-heats the air to be supplied to the furnaceAccelerates combustionEconomizer :hot flue gases are used to heat the feed waterImproves efficiencyReduces heat losses of flue gasReduces fuel consumptionSuper heater :Increases temperature of generated steamHigh thermal efficiencyAvoids corrosion of turbine bladesRe-heaters :Exhaust Steam from hp turbine is expandedHigh turbine efficiency as the stages increase
13 CONDENSER Types Surface condenser Jet Condenser No direct contact between steam & cooling waterImpure water can be used for coolingLow running cost , high vacuum is obtainedIncreased efficiencyRequires large spaceHigh initial costJet CondenserDirect contact between steam & cooling waterLow initial costCompactLow efficiency
16 Cooling towers To reduce thermal pollution which occurs in open system To cool water coming out of the condenser in a closed systemWater cooled, Air cooled cooling towersUses either natural draft or mechanical draftNatural draft – utilizes buoyancy large space, high cost, less efficientMechanical draft uses fan, requires less space, improves efficiencyForced draft tower: blower type fan is located at the base of the tower & forces air into the towerInduced draft tower: fan is located at the top of the tower & pulls air through the tower. The fan induces hot moist air out the
17 Contd..Different types of cooling towers Induced draft
18 DraughtDraught : difference between absolute gas pressure & the ambient atmosphere pressure.If Patm< Pg as draught is positive & Patm>Pgas draught is negativeTypes of Draught: Natural & MechanicalNatural draft is subjected to outside air conditions and temperature of flue gases leaving the furnace, as well as the chimney height. All these factors make proper draft hard to attain and therefore make mechanical draft equipment much more economical.Mechanical draft: reduces chimney height, independent of weather condition, easy controlInduced draft: This is obtained by simply using an induced draft fan (ID fan) which removes flue gases from the furnace and forces the exhaust gas up the stack. Almost all induced draft furnaces operate with a slightly negative pressure.Forced draft: Draft is obtained by forcing air into the furnace by means of a fan (FD fan) and ductwork. Forced draft furnaces usually have a positive pressure & use under fed stoker firing.Balanced draft: Balanced draft is obtained through use of both induced and forced draft. This is more common with larger boilers . The induced draft fan works in conjunction with the forced draft fan allowing the furnace pressure to be maintained slightly below atmospheric.
21 Governing system Functions: To maintain constant shaft speed at all loadsTo maintain constant steam flow through turbineTo maintain constant pass out and inlet/outlet steam pressures at all flowsMethods of governing:Throttle governingNozzle control governingBy-pass governingFly ball speed governing etc
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