GER-3567H - GE Gas Turbine Performance

GE Gas Turbine Performance Characteristicsthis pressure ratio is also equal to the pressureat point 3 divided by the pressure at point 4.However, in an actual cycle there is some slightpressure loss in the combustion system and,hence, the pressure at point 3 is slightly lessthan at point 2.The other significant parameter, firing temperature, is thought to be the highest temperaturereached in the cycle. GE defines firing temperature as the mass-flow mean total temperatureOPEN LOOPAIR-COOLED NOZZLEsented as firing temperature by point 3 in Figure4.Steam-cooled first stage nozzles do not reducethe temperature of the gas directly throughmixing because the steam is in a closed loop.As shown in Figure 5, the firing temperature ona turbine with steam-cooled nozzles (GE’s current “H” design) has an increase of 200degrees without increasing the combustionexit temperature.ADVANCED CLOSED LOOPSTEAM-COOLED NOZZLE200F More Firing Temp. at Same NOx Production PossibleGT25134Figure 5. Comparison of air-cooled vs. steam-cooled first stage nozzleat the stage 1 nozzle trailing edge plane.Currently all first stage nozzles are cooled tokeep the temperatures within the operating limits of the materials being used. The two types ofcooling currently employed by GE are air andsteam.Air cooling has been used for more than 30years and has been extensively developed in aircraft engine technology, as well as the latest family of large power generation machines. Airused for cooling the first stage nozzle enters thehot gas stream after cooling down the nozzleand reduces the total temperature immediatelydownstream. GE uses this temperature since it ismore indicative of the cycle temperature repreGE Power Systems GER-3567H (10/00) An alternate method of determining firing temperature is defined in ISO document 2314, “GasTurbines – Acceptance Tests.” The firing temperature here is a reference turbine inlet temperature and is not generally a temperature thatexists in a gas turbine cycle; it is calculated froma heat balance on the combustion system, usingparameters obtained in a field test. This ISOreference temperature will always be less thanthe true firing temperature as defined by GE, inmany cases by 100 F/38 C or more for machinesusing air extracted from the compressor forinternal cooling, which bypasses the combustor.Figure 6 shows how these various temperaturesare defined.5

GE Gas Turbine Performance CharacteristicsTurbine InletTemperature- Average Gas Tempin Plane A. (TA)Firing Temperature- Average Gas Tempin Plane B. (TB)CLISO Firing Temperature- Calculated Tempin Plane C.TC f(Ma , Mf)GE Uses Firing Temperature TB Highest Temperature at Which Work Is ExtractedGT23056Figure 6. Definition of firing temperatureThermodynamic AnalysisClassical thermodynamics permit evaluation ofthe Brayton cycle using such parameters as pressure, temperature, specific heat, efficiency factors and the adiabatic compression exponent. Ifsuch an analysis is applied to the Brayton cycle,the results can be displayed as a plot of cycleefficiency vs. specific output of the cycle.Figure 7 shows such a plot of output andefficiency for different firing temperatures andvarious pressure ratios. Output per pound ofairflow is important since the higher this value,the smaller the gas turbine required for the sameoutput power. Thermal efficiency is importantbecause it directly affects the operating fuel costs.Figure 7 illustrates a number of significantpoints. In simple-cycle applications (the topcurve), pressure ratio increases translate intoefficiency gains at a given firing temperature.GT17983AFigure 7. Gas turbine thermodynamicsGE Power Systems GER-3567H (10/00) 6

GE Gas Turbine Performance CharacteristicsThe pressure ratio resulting in maximum output and maximum efficiency change with firingtemperature, and the higher the pressure ratio,the greater the benefits from increased firingtemperature. Increases in firing temperatureprovide power increases at a given pressureratio, although there is a sacrifice of efficiencydue to the increase in cooling air lossesrequired to maintain parts lives.In combined-cycle applications (as shown in thebottom graph in Figure 7 ), pressure ratioincreases have a less pronounced effect on efficiency. Note also that as pressure ratio increases, specific power decreases. Increases in firingtemperature result in increased thermal efficiency. The significant differences in the slopeof the two curves indicate that the optimumcycle parameters are not the same for simpleand combined cycles.Simple-cycle efficiency is achieved with highpressure ratios. Combined-cycle efficiency isobtained with more modest pressure ratios andgreater firing temperatures. For example, theMS7001FA design parameters are 2420 F/1316 Cfiring temperature and 15.7:1 pressure ratio;while simple-cycle efficiency is not maximized,combined-cycle efficiency is at its peak.Combined cycle is the expected application forthe MS7001FA.Combined CycleA typical simple-cycle gas turbine will convert30% to 40% of the fuel input into shaft output.All but 1% to 2% of the remainder is in theform of exhaust heat. The combined cycle isgenerally defined as one or more gas turbineswith heat-recovery steam generators in theexhaust, producing steam for a steam turbinegenerator, heat-to-process, or a combinationthereof.Figure 8 shows a combined cycle in its simplestform. High utilization of the fuel input to thegas turbine can be achieved with some of themore complex heat-recovery cycles, involvingmultiple-pressure boilers, extraction or toppingsteam turbines, and avoidance of steam flow toa condenser to preserve the latent heat content.Attaining more than 80% utilization of the fuelinput by a combination of electrical power generation and process heat is not GenGas TurbineGT05363CFigure 8. Combined cycleGE Power Systems GER-3567H (10/00) 7

GE Gas Turbine Performance Characteristicsparameters and component efficiencies as wellas air mass flow.Combined cycles producing only electricalpower are in the 50% to 60% thermal efficiency range using the more advanced gas turbines.Correction for altitude or barometric pressureis more straightforward. The air density reducesas the site elevation increases. While the resulting airflow and output decrease proportionately, the heat rate and other cycle parameters arenot affected. A standard altitude correctioncurve is presented in Figure 10.Papers dealing with combined-cycle applications in the GE Reference Library include:GER-3574F, “GE Combined-Cycle Product Lineand Performance”; GER-3767, “Single-ShaftCombined-Cycle Power Generation Systems”;and GER-3430F, “Cogeneration ApplicationConsiderations.”HumidityFactors Affecting Gas TurbinePerformanceSimilarly, humid air, which is less dense thandry air, also affects output and heat rate, asshown in Figure 11. In the past, this effect wasthought to be too small to be considered.However, with the increasing size of gas turbinesand the utilization of humidity to bias water andsteam injection for NOx control, this effect hasgreater significance.Air Temperature and Site ElevationSince the gas turbine is an air-breathing engine,its performance is changed by anything thataffects the density and/or mass flow of the airintake to the compressor. Ambient weatherconditions are the most obvious changes fromthe reference conditions of 59 F/15 C and 14.7psia/1.013 bar. Figure 9 shows how ambient temperature affects the output, heat rate, heat consumption, and exhaust flow of a single-shaftMS7001. Each turbine model has its own temperature-effect curve, as it depends on the cycleIt should be noted that this humidity effect is aresult of the control system approximation offiring temperature used on GE heavy-duty gasturbines. Single-shaft turbines that use turbineexhaust temperature biased by the compressorpressure ratio to the approximate firing temperature will reduce power as a result of130120110Heat RatePercentDesign10090Exhaust FlowHeat Cons.Output8070CompressorInletTemperature0204060 F80100120-18-7416 C273849GT22045DFigure 9. Effect of ambient temperatureGE Power Systems GER-3567H (10/00) 8

GE Gas Turbine Performance CharacteristicsGT18848BFigure 10. Altitude correction curveGT22046BFigure 11. Humidity effect curveincreased ambient humidity. This occursbecause the density loss to the air from humidity is less than the density loss due to temperature. The control system is set to follow the inletair temperature function.By contrast, the control system on aeroderivatives uses unbiased gas generator discharge temperature to approximate firing temperature.The gas generator can operate at differentspeeds from the power turbine, and the powerwill actually increase as fuel is added to raise theGE Power Systems GER-3567H (10/00) moist air (due to humidity) to the allowabletemperature. This fuel increase will increase thegas generator speed and compensate for theloss in air density.Inlet and Exhaust LossesInserting air filtration, silencing, evaporativecoolers or chillers into the inlet or heat recovery devices in the exhaust causes pressure lossesin the system. The effects of these pressure losses are unique to each design. Figure 12 shows9

GE Gas Turbine Performance Characteristics4 Inches (10 mbar) H2O Inlet Drop Produces:1.42% Power Output Loss0.45% Heat Rate Increase1.9 F (1.1 C) Exhaust Temperature Increase4 Inches (10 mbar) H2O Exhaust Drop Produces:0.42% Power Output Loss0.42% Heat Rate Increase1.9 F (1.1 C) Exhaust Temperature IncreaseGT18238CFigure 12. Pressure drop effects (MS7001EA)the effects on the MS7001EA, which are typicalfor the E technology family of scaled machines(MS6001B, 7001EA, 9001E).FuelsWork from a gas turbine can be defined as theproduct of mass flow, heat energy in the combusted gas (Cp), and temperature differentialacross the turbine. The mass flow in thisequation is the sum of compressor airflowand fuel flow. The heat energy is a functionof the elements in the fuel and the productsof combustion.Tables 1 and 2 show that natural gas (methane)produces nearly 2% more output than does distillate oil. This is due to the higher specific heatin the combustion products of natural gas,resulting from the higher water vapor contentproduced by the higher hydrogen/carbon ratioof methane. This effect is noted even thoughthe mass flow (lb/h) of methane is lower thanthe mass flow of distillate fuel. Here the effectsof specific heat were greater than and in opposition to the effects of mass flow.Figure 13 shows the total effect of various fuelson turbine output. This curve uses methane asthe base fuel.Although there is no clear relationship betweenfuel lower heating value (LHV) and output, it isGE Power Systems GER-3567H (10/00) possible to make some general assumptions. Ifthe fuel consists only of hydrocarbons with noinert gases and no oxygen atoms, outputincreases as LHV increases. Here the effects ofCp are greater than the effects of mass flow.Also, as the amount of inert gases is increased,the decrease in LHV will provide an increase inoutput. This is the major impact of IGCC typefuels that have large amounts of inert gas in thefuel. This mass flow addition, which is not compressed by the gas turbine’s compressor,increases the turbine output. Compressorpower is essentially unchanged. Several sideeffects must be considered when burning thiskind of lower heating value fuels: Increased turbine mass flow drives upcompressor pressure ratio, whicheventually encroaches on thecompressor surge limit The higher turbine power may exceedfault torque limits. In many cases, alarger generator and other accessoryequipment may be needed High fuel volumes increase fuel pipingand valve sizes (and costs). Low- ormedium-Btu coal gases are frequentlysupplied at high temperatures, whichfurther increases their volume flow10

GE Gas Turbine Performance Characteristics60100%H2302010LHV-Btu/lb (Thousands)Kcal/kg (Thousands)504030100%CH420100%CH4H101075% N2 - 25% CH475% CO2 - 25% CH4100% CO0100105110115120125Output - Percent130GT25842Figure 13. Effect of fuel heating value on output Lower-Btu gases are frequentlysaturated with water prior to deliveryto the turbine. This increases thecombustion products heat transfercoefficients and raises the metaltemperatures in the turbine sectionwhich may require lower operatingfiring temperature to preserve partslives As the Btu value drops, more air isrequired to burn the fuel. Machineswith high firing temperatures may notbe able to burn low Btu gases Most air-blown gasifiers use airsupplied from the gas turbinecompressor discharge The ability to extract air must beevaluated and factored into the overallheat and material balancesAs a result of these influences, each turbinemodel will have some application guidelines onflows, temperatures and shaft output to preserveGE Power Systems GER-3567H (10/00) its design life. In most cases of operation withlower heating value fuels, it can be assumed thatoutput and efficiency will be equal to or higherthan that obtained on natural gas. In the case ofhigher heating value fuels, such as refinerygases, output and efficiency may be equal to orlower than that obtained on natural gas.Fuel HeatingMost of the combined cycle turbine installationsare designed for maximum efficiency. Theseplants often utilize integrated fuel gas heaters.Heated fuel results in higher turbine efficiencydue to the reduced fuel flow required to raisethe total gas temperature to firing temperature.Fuel heating will result in slightly lower gas turbine output because of the incremental volumeflow decrease. The source of heat for the fueltypically is the IP feedwater. Since use of thisenergy in the gas turbine fuel heating system isthermodynamically advantageous, the combined cycle efficiency is improved by approximately 0.6%.11

GE Gas Turbine Performance CharacteristicsDiluent InjectionSince the early 1970s, GE has used water orsteam injection for NOx control to meet applicable state and federal regulations. This isaccomplished by admitting water or steam inthe cap area or “head-end” of the combustionliner. Each machine and combustor configuration has limits on water or steam injection levelsto protect the combustion system and turbinesection. Depending on the amount of water orsteam injection needed to achieve the desiredNOx level, output will increase because of the130Generally, up to 5% of the compressor airflowcan be extracted from the compressor discharge casing without modification to casingsor on-base piping. Pressure and air temperaturewill depend on the type of machine and siteconditions. Air extraction between 6% and 20%may be possible, depending on the machineand combustor configuration, with some modifications to the casings, piping and controls.Such applications need to be reviewed on acase-by-case basis. Air extractions above 20%will require extensive modification to the turbine casing and unit configuration. Figure 15With 5%SteamInjection120110Output%10090No ompressor Inlet TemperatureGT18851AFigure 14. Effect of steam injection on output andheat rateadditional mass flow. Figure 14 shows the effectof steam injection on output and heat rate foran MS7001EA. These curves assume that steamis free to the gas turbine cycle, therefore heatrate improves. Since it takes more fuel to raisewater to combustor conditions than steam,water injection does not provide an improvement in heat rate.Air ExtractionIn some gas turbine applications, it may bedesirable to extract air from the compressor.GE Power Systems GER-3567H (10/00) GT22048-1CFigure 15. Effect of air extraction on output and heatrateshows the effect of air extraction on output andheat rate. As a “rule of thumb,” every 1% in airextraction results in a 2% loss in power.Performance EnhancementsGenerally, controlling some of the factors thataffect gas turbine performance is not possible.The planned site location and the plant configuration (such as simple- or combined-cycle)determine most of these factors. In the eventadditional output is needed, several possibilitiesto enhance performance may be considered.12

GE Gas Turbine Performance CharacteristicsInlet CoolingThe ambient effect curve (see Figure 9) clearlyshows that turbine output and heat rate areimproved as compressor inlet temperaturedecreases. Lowering the compressor inlet temperature can be accomplished by installing anevaporative cooler or inlet chiller in the inletducting downstream of the inlet filters. Carefulapplication of these systems is necessary, as condensation or carryover of water can exacerbatecompressor fouling and degrade performance.These systems generally are followed by moisture separators or coalescing pads to reduce thepossibility of moisture carryover.As Figure 16 shows, the biggest gains from evaporative cooling are realized in hot, low-humidity climates. It should be noted that evaporative cooling is limited to ambient temperaturesof 59 F/15 C and above (compressor inlet temperature 45 F/7.2 C) because of the potentialfor icing the compressor. Information contained in Figure 16 is based on an 85% effectiveevaporative cooler. Effectiveness is a measureof how close the cooler exit temperatureapproaches the ambient wet bulb tempera-Figure 16. Effect of evaporative cooling on outputand heat rateture. For most applications, coolers having aneffectiveness of 85% or 90% provide the mosteconomic benefit.Chillers, unlike evaporative coolers, are not limited by the ambient wet bulb temperature. Theachievable temperature is limited only by thecapacity of the chilling device to producecoolant and the ability of the coils to transferheat. Cooling initially follows a line of constant100% % RH35.0153040% RHBtu Per Poundof Dry AirEvaporativeCooling Process25.010SpecificHumidity20% RH20Inlet ChillingProcess15.00510% RHDry BulbTemperature F 406080100120 C 416273849.000GT21141DFigure 17. Inlet chilling processGE Power Systems GER-3567H (10/00) 13

GE Gas Turbine Performance Characteristicsspecific humidity, as shown in Figure 17. As saturation is approached, water begins to condensefrom the air, and mist eliminators are used.Further heat transfer cools the condensate andair, and causes more condensation. Because ofthe relatively high heat of vaporization of water,

Table 2. GE gas turbine performance characteristics - Mechanical drive gas turbine ratings MS7000 PG 12 (EA) Number Model of Shafts Application Series Power Approx Output Power in Hundreds, Thousands, or 10 Thousands of Horsepower R - Regen Blank - SC Frame 1 or 2 3,5,7 6,9 Mech Drive Pkgd Gen M - PG - 7 1 Figure 1.Heavy-duty gas turbine model .