Thermal Power Station
2Thermal Power Station2.1 INTRODUCTIONEnergy is an important input in all the sectors of any country’s economy. The standard of livingof a given country can be directly related to per capita energy consumption. Energy crisis is dueto two reasons : firstly, that the population of the world has increased rapidly and secondly thestandard of living of human beings has improved.In this modern world, the dependence on electricity is so much that it has become a partand parcel of our life. The ever-increasing use of electric power for domestic, commercial andindustrial purpose necessitates to provide bulk electric power economically. This is achievedwith the help of suitable power producing units, known as power plants or electric power generating stations. The design of a power plant should incorporate two important aspects. Firstly,the selection and placing of necessary power generating equipment should be such that a maximum of return is obtained from a minimum of expenditure over the working life of the plant.Secondly, the operation of the plant should be such as to provide cheap, reliable and continuousservice. In India, there is still a large scope of development of industry and hence large scope ofincrease of electric power too. Thermal electric power generation is one of the major methods.The design of steam power station requires wide experience as the subsequent operation andmaintenance are greatly affected by its design. The satisfactory design consists of the followingsteps : (i) selection of the site, (ii) capacity of the power station, (iii) selection of boilers andits auxiliaries, (iv) selection of turbine, (v) selection of condensing units, (vi) design of coolingsystem, (vii) selection of electric generator, and (viii) design of control and instrumentation. Inthis chapter concise and relevant points are undertaken.2.2 CONCEPT OF HEAT AND WORKThe fundamental forms of energy with which thermal power stations are mainly concernedare heat and work. Heat produces work and then work is further converted into electrical energythrough some form or energy conversion cycle.The term heat and work designates two fundamental forms of energy flow. Heat flow involvesa quantity aspect and the temperature available. A heat cycle receives a heat flow at a giventemperature pattern, to convert part to work and discard the remainder to the receiver. A cyclewhich uses heat in an ideal or reversible manner to produce work from the available energy, iscalled an ideal cycle.Chap-2.indd 396/26/2017 5:22:03 PM
41THERMAL POWER STATIONThe First Law and the Closed SystemThe open system is one in which mass crosses the boundaries. A closed system is one inwhich only energy and not mass crosses the boundaries. A third system of some interest is theisolated system, a special instance of the closed system. It is one in which neither mass nor energy crosses the boundaries but energy transformations may take place within the boundaries.Because mass does not cross the boundaries in a closed system, the potential, kinetic, andflow energy terms drop out in the first law equation.The CycleIn order to convert forms of energy, particularly heat is to work on an extended or continuous basis, one needs to operate on a cycle. The process begins at one state of the working fluidand ends at another. A cycle, on the other hand, is a series of processes that begins and ends atthe same state and thus can repeat indefinitely, or as long as needed. An example is the idealdiesel cycle, shown as the P-T and T-s in Fig. 2.2.3TPPressure2342411VolumeFig. 2.2sPressure-volume and temperature enthalpy diagram of an ideal diesel cycleIt is composed of an ideal and adiabatic compression process 1-2, a constant-pressure heataddition process 2-3, an ideal and adiabatic expansion process 3-4 and a constant volume heatrejection process 4-1 which returns the cycle back to 1. Because the beginning and the end ofthe cycle is 1, thermodynamic cycle is a closed loop system where, DU U1 – U1 0 and firstlaw for this and all other cycles becomesDQnet QA – QR DWnet (for a cycle)The Second Law of ThermodynamicsThe first law of thermodynamics was one of conservation of energy, declaring that all formsof energy are convertible to another, the second law puts a limitation on the conservation ofsame forms of energy to others. Second law does not negate the equivalence of conversion ofthese two, only the extent. Work is a more valuable commodity. It can be completely and continuously converted to heat. The opposite is not true. Heat cannot be completely and continuouslyconverted to work. The portion of heat that cannot thus be converted to work, called unavailableenergy, has to be rejected as low grade heat after the work has been done. Thus, while energyis conserved, availability is not.Chap-2.indd 416/26/2017 5:22:06 PM
42POWER PLANT ENGINEERING2.4 POWER PLANT CYCLESA thermal power station works on the principle that heat is released by burning fuel whichproduces (working fluid) (steam) from water. The steam so produced runs the turbine coupledto generator which produces electrical energy.A working fluid goes through a repetitive cycle change and this cyclic change involvingheat and work is known as thermodynamic cycle. Thus, a thermodynamic cycle is a series ofoperations, involving a heat source, a heat receiver, a machine and working substance.Types of Power Plant CyclesThermal power plants, in general, may work on(a) Vapour power cycles (b) Gas power cyclesVapour Power cycles can be classified as:(i)(ii)(iii)(iv)Rankine cycleReheat cycleRegenerative cycleBinary vapour cycleGas Power Cycle can be classified as follows:(i)(ii)(iii)(iv)Otto cycleDiesel cycleDual combustion cycleGas turbine cycleRankine CycleRankine cycle is the theoretical cycle on which steam power plant works. Rankine cycle isa vapour-liquid cycle, it is most convenient to draw it on both the P-V and T-s diagrams withrespect to the saturated-liquid and vapour lines of the working fluid, which usually but notalway, is H2O.Figure 2.3 shows a simplified flow diagram of a Rankine kFig. 2.3 Rankine cycleChap-2.indd 426/26/2017 5:22:06 PM
43THERMAL POWER STATIONP-V diagram is shown in Fig. 2.4.Pp1lT1mticbaiaAdnionspaExp2 kT2nVFig. 2.4 P-V diagram of Rankine cycle1. Operation (k-l): Condensed steam is at pressure p2 and temperature T2 which ispumped into the boiler by means of feed pump at pressure p1 and there it is calledtemperature T1.2. Operation (l-m): The hot water at a saturation temperature T1 is evaporated tosteam at pressure p1.3. Operation (m-n): After coming out of the boiler, the steam enters the turbineand expands adiabatically to a pressure p2.4. Operation (n-k): The steam expands and condenses to water in the condenser atthe same temperature and is pressure at point k. Thus, the cycle is completed. Thiscondensed water is again pumped to the boiler and then the next cycle starts.Reheat cycle: An additional improvement in cycle efficiency with gaseous primary fluids asin fossil fuel and gas-cooled power plants is achieved.The improvement in thermal efficiency due to reheat greatly depends on the reheat pressurewith respect to the original pressure of steam.Figure 2.5 shows a schematic diagram of a theoretical single stage reheat cycle. Thecorresponding representation of ideal reheating process on T-s is shown in Fig. 2.6.Chap-2.indd 436/26/2017 5:22:06 PM
44POWER PLANT Fig. 2.5Reheat cycle.Figure 2.6. 5-1 shows the formation of steam in the boiler. The steam at state point 1 (i.e.,pressure p1 and temperature T1) enters the turbine an expands isentropically to a certainpressure p2 and temperature T2. From this state point 2 the whole of steam is drawn out of theturbine and is reheated in a reheater to a temperature T3.T1 13 T3p16p2Increasein Workdone dueto Reheating2p357Fig. 2.64Ideal reheating process onThis reheated steam is then readmitted to the turbine where it is expanded to condenserpressure isentropically. Reheat allows heat addition twice. It results in increasing the averagetemperature at which heat is added and keeps the boiler hot, which results in improvement incycle efficiency. Reheating also results in drier steam at turbine exhaust which is beneficial forreal cycles.Advantages of Reheating1. There is an increased output of the turbine.2. The thermal efficiency of the turbines increases.Chap-2.indd 446/26/2017 5:22:07 PM
45THERMAL POWER STATION3. Efficiencies of nozzle and blade increase.4. Corrosion problems are minimised in steam turbines.5. Dryness factor of steam improved.Disadvantages of Reheating1. Reheating requires maintenance.2. Reheating increases the expenditure.2.5 REGENERATIVE CYCLEIn the Rankine cycle it is observed that the condensate which is fairly at low temperature hasan irreversible mixing with hot boiler water and this results in the decrease of cycle efficiency.Methods are, therefore, adopted to heat the feed water from the hot well of condenser irreversiblyby interchange of heat within the system and thus improving the cycle efficiency. This heatingmethod is called regenerative feed heat and the cycle is called regenerative cycle.The principle of generation can be practically utilised by extracting steam from the turbineat several locations and supplying it to the regenerative heaters. The resulting cycle is knownas regenerative or bleeding cycle. The heating arrangement comprises; (i) for medium capacityturbine — not more than 3 heaters; (ii) for high pressure high capacity turbines — not morethan 5 to 7 heaters; and (iii) for turbines of supercritical parameters—8 to 9 heaters. The mostadvantageous condensate heating temperature is selected depending on the turbine throttleconditions and this determines the number of heaters to be used.Figure 2.7 shows a diagrammatic layout of a condensing steam power plant in which a surfacecondenser is used to condense all the steam that is not extracted by feed water heating. Theturbine is double extracting and the boiler is equipped with a superheater. This arrangementconstitutes a regenerative cycle.SuperheaterBoilerFig. 2.7Regenerator cycle.The condensate from the bled steam is added to feed water.Chap-2.indd 456/26/2017 5:22:07 PM
46POWER PLANT ENGINEERINGAdvantages of Regenerative Cycle Over Simple Rankine Cycle1. The heating process in the boiler tends to become reversible.2. The thermal stresses set up in the boiler are minimised. This is due to the fact thattemperature ranges in the boiler are reduced.3. The thermal efficiency is improved because the average temperature of heat additionto the cycle is increased.4. Heat rate is reduced.5. The blade height is less due to the reduced amount of steam passed through the lowpressure stages.6. Due to many extractions there is an improvement in the turbine drainage and itreduces erosion due to moisture.7. A small size condenser is required.Disadvantages of Regenerative Cycle Over Simple Rankine Cycle1.2.3.4.The plant becomes more complicated.Maintenance cost is more.A large capacity boiler is needed for a given power rating.The heaters are costly and the gain in thermal efficiency is not much in comparisonto the costs.2.6 BINARY VAPOUR CYCLECarnot cycle gives the highest thermal efficiency which is given byÊ T1 - T2 ˆ. To approachÁË T 1this cycle in an actual engine it is necessary that whole of heat must be supplied at constanttemperature T1 and rejected at T2. This can be achieved only by using a vapour in the wet fieldbut not in the superheated field. The efficiency depends on temperature T1 since T2 is fixed bythe natural sink to which heat is rejected. This means that T1 should be as large as possible,consistent with the vapour being saturated.If we use steam as the working medium the temperature rise is accompanied by rise inpressure and at critical temperature of 374.15 C the pressure is as high as 225.65 kgf/cm2which will create many difficulties in design, operation and control. It would be desirable to usesome fluid other than steam which has more desirable thermodynamic properties than water.An ideal fluid for this purpose should have a very high critical temperature combined with lowpressure. Mercury, diphenyloxide and similar compound, aluminium bromide and zinc ammonium chloride are fluids which have the required properties in varying degrees. Mercury is theonly working fluid which has been successfully used. It has high critical temperature (588.4 C)and correspondingly low critical pressure (21 kgf/cm2 abs.). The mercury alone cannot be usedas its saturation temperature at atmospheric pressure is high (357 C). Hence, binary vapourcycle is generally used to increase the overall efficiency of the plant. Two fluids (mercury andwater) are used in cascade in the binary cycle for production of power.Characteristics of ideal working fluid for vapour power cycle are:Chap-2.indd 466/26/2017 5:22:07 PM
47THERMAL POWER STATION(i) It should have high critical temperature at reasonably low pressure.(ii) It should have high heat of vaporisation to keep the weight of fluid in the cycleminimum.(iii) Freezing temperature should be below the room temperature.(iv) It should have chemical stability through the working cycle.(v) It must be non-corrosive to the metals normally used in power plants.(vi) It must have the ability to wet the metal surfaces to promote the heat transfer.(vii) The vapour pressure at a desirable condensation temperature should be nearly atmoshpheric which will eliminate requirement of power for maintenance of vacuumin the condesner.(viii) After expansion through the prime mover the vapour should be nearly saturated sothat a desirable heat transfer coefficient can be obtained which will reduce the sizeof the condenser required.(ix) It must be available in large quantities at reasonable cost.(x) It should not be toxic and, therefore, dangerous to human life.Figure 2.8 shows the schematic line diagram of mercury vapour use mercury and water asworking fluids.MercuryTurbineMercury y Condensor orSteam GeneratorMercuryFeed PumpSteam ElectricGeneratorSteamTubeSteamCondenserWater PumpFig. 2.8 Diagram of binary vapour cycle.2.7 THERMAL STATIONA generating station which converts heat energy of coal combustion into electrical energy isknown as a steam power station.A thermal power station basically works on the Rankine cycle. Steam is produced in theboiler 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 steamturbine drives the alternator which converts mechanical energy of the turbine into electricalenergy. This type of power station is suitable where coal and water are available in abundanceand a large amount of electric power is to be generated.Chap-2.indd 476/26/2017 5:22:07 PM
48POWER PLANT ENGINEERINGAdvantages(i)(ii)(iii)(iv)Less initial cost as compared to other generating stations.The fuel, which is generally coal, is quite cheap.It requires less space as compared to the hydroelectric power station.It can be installed at any place irrespective of the existence of coal. The coal can betransported to the site of the plant by rail or road.(v) The cost of generation is lesser than that of the diesel power station.Disadvantages(i) Running cost is more as compared to hydroelectric plant.(ii) It pollutes the atmosphere due to the production of large amount of smoke and fumes.The schematic line diagram of a modern steam power station is shown in Fig. 2.9. The wholearrangement can be divided into the following stages for the sake of simplicity :(i)(ii)(iii)(iv)(v)(vi)Coal and Dust Handling ArrangementSteam Generating PlantSteam TurbineAlternator GeneratorFeed WaterCooling ArrangementCoal and Dust Handling Arrangement : The coal is transported to the power station byroad or rail and is stored in the coal storage plant. From the coal storage plant, coal is deliveredto the coal handling plant where it is crushed into small pieces in order to increase its surfaceexposure, thus promoting rapid combustion without using large quantity of excess air. Thecrushed coal is fed to the boiler by belt conveyors. The coal is burnt in the boiler and the ashproduced after the complete combustion of coal is removed to the ash handling plant and thendelivered to the ash storage plant for disposal. The removal of the ash from the boiler furnaceis necessary for proper burning of coal.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.Boiler : The heat of combustion of coal in the boiler is utilised to convert water into steamat high temperature and pressure. The flue gases from the boiler make their path throughsuperheater, economiser, air preheater and are finally exhausted to atmosphere through thechimney.Superheater : The steam produced in the boiler is generally wet and is passed through asuperheater where it is dried and superheated. Superheating provides two principal benefits.Firstly, the overall efficiency is increased. Secondly, too much condensation in the last stagesof turbine, which would cause blade corrosion, is avoided. The superheated steam from thesuperheater is fed to steam turbine through the main valve.Chap-2.indd 486/26/2017 5:22:07 PM
49THERMAL POWER STATIONBus-barsChimneyRYBInduceddraught fanIsolatorsCBHat airAirPre-heaterIsolatorsForceddraught fanFlue gasesTransformerEconomiserFlue hauststeamFlue gasesFeed waterheaterFeed irculatingwater pumpWatertreatmentchamberCoolingtowerRiverFig. 2.9. Schematic arrangement of Steam Power StationEconomiser : An economiser is essentially a feed water heater and derives heat from fluegases for this purpose. The feed water is fed to the economiser before supplying to the boiler.The economiser extracts a part of heat of flue gases to increase the feed water temperature.Chap-2.indd 496/26/2017 5:22:08 PM
50POWER PLANT ENGINEERINGAir Preheater : An air preheater increases the temperature of the air supplied for coalburning by deriving heat from flue gases. Air is drawn from the atmosphere by a forced draughtfan and is passed through air preheater before supplying to the boiler furnace. The air preheaterextracts heat from flue gases and increases the temperature of air used for coal combustion. Theprincipal benefits of preheating the air are : increased thermal efficiency and increased steamcapacity per square metre of boiler surface.Steam Turbine : The dry and superheated steam from the superheater is fed to the steamturbine through main valve. The heat energy of steam when passing over the blades of turbine is converted into mechanical energy. After giving heat energy to the turbine, the steamis exhausted to the condenser which condenses the exhausted steam by means of cold watercirculation.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 isdelivered to the bus bars through transformer, circuit breakers and isolators.Feed Water : The condensate from the condenser is used as feed water to the boiler. Somewater may be lost in the cycle which is suitably made up from external source. The feed wateron its way to the boiler is heated by water heaters and economiser. This helps in raising theoverall efficiency of the plant.Cooling Arrangement : To improve the efficiency of the plant, the steam exhausted fromthe turbine is condensed by means of a condenser. Water is drawn from a natural source ofsupply such as a river, canal or
The ever-increasing use of electric power for domestic, commercial and industrial purpose necessitates to provide bulk electric power economically. This is achieved with the help of suitable power producing units, known as power plants or electric power gene- rating stations. The design of a power
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Energies 2018, 11, 1879 3 of 14 R3 Thermal resistance of the air space between a panel and the roof surface. R4 Thermal resistance of roof material (tiles or metal sheet). R5 Thermal resistance of the air gap between the roof material and a sarking sheet. R6 Thermal resistance of a gabled roof space. R7 Thermal resistance of the insulation above the ceiling. R8 Thermal resistance of ceiling .
Thermal Control System for High Watt Density - Low thermal resistance is needed to minimize temperature rise in die-level testing Rapid Setting Temperature Change - High response thermal control for high power die - Reducing die-level test time Thermal Model for New Thermal Control System - Predict thermal performance for variety die conditions
thermal models is presented for electronic parts. The thermal model of an electronic part is extracted from its detailed geometry configuration and material properties, so multiple thermal models can form a thermal network for complex steady-state and transient analyses of a system design. The extracted thermal model has the following .
