Combined cycle power plants 14.12.2020

JNTUH Syllabus Power Plant Engg

1. Combined Cycle Power
Plants

3. COMBINED CYCLE POWER PLANTS AND
COMPARISON
•Simplecyclegasturbine(GTs)plantsusingnaturalgasandfueloiloperateataround33percentand25per
centefficiency,respectively,majorpartoftheheatbeingwastedasthermalenergyinthehotexhaustgases.
•Combiningofmultiplethermodynamiccyclestogeneratepower,overallplantefficiencycanbeincreasedup
to60percentbyusingaheatrecoverysteamgenerator(HRSG).
•TheHSRGcapturesheatfromhigh-temperatureexhaustgasestogeneratesteam,whichisthensuppliedtoa
steamturbinetogenerateadditionalelectricpower.
•Thecycleworkingunderthisprincipleisknownascombinedcycle.Inmostoftheinstanceacombinedcycle
powerplantutilizesgasturbinesinconjunctionwithasteamturbineandiscalledacombinedcyclegas
turbine(CCGT)plant.
•DifferentconfigurationsofCCGTpowerplantsareinusewhereeachGThasitsownassociatedHRSG,and
multipleHRSGssupplyingsteamtooneormoresteamturbines.
•Forexample,inaplantwitha31configuration,threeGT/HRSGtrainssupplytoonesteamturbine.
Similarly,therecanbe11,21or41arrangementsalso.Thesteamturbinecapacityisdecidedtomatch
thenumberandcapacityofsupplyingGTs/HRSGs.Thesetypesofpowerplantsarebeinginstalledin
increasingnumbersroundtheworldwherethereisaccesstosubstantialquantitiesofnaturalgas.

4. Combined Cycle Principles of Operation
•TheHRSGisbasicallyaheatexchangerpopularlyknownasaboiler,orcomprisesaseriesofheat
exchangers.
•Itgeneratessteamforthesteamturbineexchangingheatfromthehotexhaustgasflowfromagas
turbinethroughheatexchangertubebanks.
•Thetubesarearrangedinsections,ormodulesalsoknownaseconomizers,evaporators,
superheater/reheaterandpreheaters.
•ThefluidcirculationinHRSGcouldbeeithernaturalorforcedcirculationusingpumps.
•Saturatedsteamfromthesteamdrumsoronce-throughsystemispassedthroughsuperheateror
reheatertubestosuperheatthesteam.
•ThesuperheatedsteamproducedbytheHRSGissuppliedtothesteamturbinewhereitexpands
throughtheturbineblades,impartingrotationtotheturbineshaft.
•Theenergydeliveredtothegeneratorcoupledtothedriveshaftisconvertedintoelectricity.
•Afterexitingthesteamturbine,thesteamcondensesinacondenserandfedbacktotheHRSG.

5. Figure shows a schematic diagram of a combined cycle gas turbine plant

6. •Depending on the exhaust gas characteristics HSRGs are designed and
configurations, steam requirements, etc., are decided later.
•Due to the high temperature of exhaust gases available at the gas turbine exit
(600°C), GTs are designed to produce steam at multiple pressure levels (high-
pressure steam in a large CCGT plant can reach up to 110 bar) to optimize energy
recovery.
•Generally, three sets of heat exchanger modules –one each for high pressure (HP)
steam, intermediate pressure (IP) steam, and low pressure (LP) steam –are used for
this purpose.
•The HRSGs present operational constraints on the CCGT power plant owing to
their location, directly downstream of the gas turbines. Due to the changes in
temperature and pressure of the exhaust gases thermal and mechanical stresses are
set up.
•When CCGT power plants are operated under fluctuating load conditions,
characterized by frequent starts up and shut downs or when operating under part-
load conditions, thermal stresses developed could cause damage to some
components of the HRSG. The HP steam drum and superheater headers are
subjected to the highest exhaust gas temperatures.

7. •WhenCCGTpowerplantsareoperatedunderfluctuatingloadconditions,
characterizedbyfrequentstartsupandshutdownsorwhenoperatingunder
part-loadconditions,thermalstressesdevelopedcouldcausedamagetosome
componentsoftheHRSG.TheHPsteamdrumandsuperheaterheadersare
subjectedtothehighestexhaustgastemperatures,andhencearemoreprone
toreducedmechanicallife.Someimportantdesignandoperating
considerationsarethefollowing:
(i)Temperaturesofthegasandsteamthatthemodulematerialscan
withstand
(ii)Mechanicalstabilityforturbulentexhaustflow
(iii)CorrosionofHRSGtubes
(iv)Steampressurestodecidedrumthickness

8. Coupled Cycle –GT–ST Plant Operation
Inthissystem,anopen-circuitgasturbinehasacompressor,acombustoranda
turbine.Forthistypeofcycle,theinputtemperaturetoturbineandtheoutput
temperatureoffluegasesareveryhigh.Thishigh-temperaturefluegaseshave
heatenergyhighenoughtoprovideheatforasecondcyclethatusessteamas
theworkingmedium,thatis,thermalpowerplant.Figureshowstheworking
principleofcombinedcyclegasturbine(CCGT)plant.Differentcomponents
oftheplantarediscussedfurtherindetail.

10. 1. Air Inlet system
•Airisdrawnthoughthelargeairinletsectionwhereitiscleaned,cooledandcontrolled.
•Majorityofallheavy-dutygasturbinesaredesignedtooperateunderawidevarietyofclimaticand
environmentalconditionsduetoinletairfiltrationsystems.
•Thesefiltrationsystemsarespecificallydesignedtosuittheplantlocation.
•Undernormaloperatingconditions,theinletsystemhasthecapabilitytoprocesstheairbyremoving
contaminantstolevelsbelowthosethatareharmfultothecompressorandturbine.
•Ingeneral,theincomingairmayhavecontaminantsinsolid,liquidandgaseousstatesinadditionto
corrosivecomponents.Gaseouscontaminantsincludeammonia,chlorine,hydrocarbongases,sulfurinthe
formofH
2S,SO
2,dischargefromoilcoolervents,etc.Someoftheliquidcontaminantsincludechloride
saltsdissolvedinwater(sodium,potassium),nitrates,sulfatesandhydrocarbons,etc.
•Solidcontaminantsmayincludesand,aluminaandsilica,rust,dustparticles,aluminaandsilica,calcium
sulfate,ammoniacompoundsfromfertilizerandanimalfeedoperations,airborneseeds,etc.Inadditionto
these,somecorrosiveagentssuchaschlorides,nitratesandsulfatesmayalsodepositoncompressorblades
inducingstresscorrosionattackand/orcausecorrosionpitting.
•Alkalimetalssuchassodiumandpotassiummayalsocombinewithsulfurtoformahighlycorrosiveagent
thatmayattackportionsofthehotgaspath.Thesecontaminantsareremovedbypassingthroughvarious
typesoffilters.Gasphasecontaminantssuchasammoniaorsulfurthatcannotberemovedbyfiltrationare
removedbasedonspecialmethods.

11. 2. Turbine system
•Thepurifiedairisthencompressedandmixedwithnatural
gasandignitedinthecombustionchamberafterinjectingthe
fuel.
•Thehigh-pressuregasstreamgeneratedinthecombustion
chamberexpandsintheturbineandspinstheturbinerotor
andagenerator,producingelectricity.
•Heatofthegasturbine’sexhaustisfurtherpassedthroughthe
heatrecoverysteamgenerator(HRSG),wherelivesteamat
temperaturebetween420°Cand580°Cisgenerated,whichis
usedasaworkingfluidinthesecondarycircuit.

12. 3.Heatrecoverysteamgenerator
•Inheatrecoverysteamgenerator(HRSG),heatexchangetakesplace
betweenthehighlypurifiedwaterflowingintubesandthehotfluegases
surroundingthem,generatingsteam.Asexplainedearlier,steamexpandsin
theturbinetoruntheturbinerotorandhenceacoupledgenerator,toproduce
electricity.ThehotgasesleavetheHRSGataround140°Cbeforebeing
dischargedintotheatmosphere.Thesteamcondensingandwatersystemin
thiscircuitaresimilartothatinatypicalthermalpowerplant.TheHRSG
takesmoretimetowarmupfromcoldconditionscomparedtohot
conditions.

13. Configuration of CCGT Plants
•Thecombinedcyclesystemmayhavesingle-shaftandmulti-shaftconfigurations.
•Asingle-shaftsystemconsistsofagasturbine,asteamturbine,ageneratoranda
heatrecoverysteamgenerator(HRSG).Thegasturbineandsteamturbineare
coupledtothesinglegeneratoronasingleshaft.
•Inamulti-shaftsystem,oneormoregasturbine-generatorsandHRSGsareused
tosupplysteamthroughacommonheadertoaseparatesingle-steamturbine
generator.Overallinvestmentonamulti-shaftsystemisabout5percenthigher
comparedtoasingle-shaftsystem.Themajordisadvantageofmulti-shaftsystem
isthatthenumberofsteamturbines,condensersandcondensatesystems,cooling
towersandcirculatingwatersystemsalsoproportionatelyincreasestomatchthe
numberofgasturbines.

14. Efficiency of CCGT Plant
•Roughly,thesteamturbinecycleproducesone-thirdofthepowerand
gasturbinecycleproducestwo-thirdsofthepoweroutputofthe
CCGT.Bycombiningbothgasandsteamcycles,highinput
temperaturesandlowoutputtemperaturescanbeachieved.The
efficiencyofthecyclesaddsbecausetheyarepoweredbythesame
fuelsource.
•Toincreasethepowersystemefficiency,itisnecessarythattheHRSG
beoptimized,whichservesasthecriticallinkbetweenthegasturbine
cycleandthesteamturbinecyclewiththeobjectiveofincreasingthe
steamturbineoutput.

15. •Overallefficiencyofthecombinedcyclepowerplant
dependsontheperformanceofHRSG.Thereareinstancesof
electricefficiencyofanewcombinedcyclepowerstation
reaching58percentatcontinuousoutput.Inaddition,
combinedcycleunitsmaybeutilizedtodeliverlow
temperatureheatenergyforindustrialprocesses,district
heatingandotheruses.Thisiscalledcogeneration,andsuch
powerplantsareofthenreferredtoasacombinedheatand
power(CHP)plant.

16. •TheefficiencyofCCGTisincreasedbysupplementaryfiringand
bladecooling.Agasturbinecoolingair(TCA)cooler,withaheat
exchangerusingfeedwater,andafuelgasheater(FGH)werealso
usedtofurtherenhancetheefficiencyoftheplantinKawasaki
ThermalPowerStation.Thisminimizedthelossofheatenergy
allowingtheplanttoachievehigherthermalefficiency,outperforming
conventionalcombinedcycleplants.Inthisplant,supplementary
firingwasarrangedatHRSGandingasturbineapartofthe
compressedairflowbypassestocooltheturbineblades.

17. Fuels
•The turbines used in combined cycle plants are
commonly fuelledwith natural gas, and it is more
versatile than coal or oil and can be used in 90 per cent
of energy applications. Combined cycle plants are
usually powered by natural gas, although fuel oil,
synthesis gas or other fuels can be used.

18. Emission Control
•Inordertocontroltheemission,differenttoolsandtechniquesareused,namely,a
selectivecatalyticreduction(SCR)system,aqueousammoniasolution,alowNOx
combustorandadryNOxremovalapparatus,asdiscussedbelow:
•1. Selective catalytic reduction
•To control the emissions in the exhaust gas so that it remains within permitted levels as it
enters the atmosphere, the exhaust gas passes though two catalysts located in the HRSG.
One catalyst controls carbon monoxide (CO) emissions, whereas the other catalyst
controls oxides of nitrogen, (NOx) emissions.
•(i) Aqueous ammonia: Apart from the SCR, a mixture of 22 per cent ammonia and 78 per
cent water, known as aqueous ammonia is also injected into system to further reduce Nox
levels.
•(ii) To reduce NOx emissions that increase with the combustor exit gas temperature
(1500°C), a low NOx combustor can also be used.
•(iii) A dry NOx removal apparatus can be built in the heat recovery steam generator to
operate the plant confirming environmental regulation limits.

19. Advantages and Disadvantages of CCGT Plants
•Fuel efficiency:
1.Asagainsttheconventionalpowerplantsturbinesfuel-conversionefficiencyof33percent,
turbinesincombinedcyclepowerplanthaveafuel-conversionefficiencyof50percentormore.
ThismeanstheCCGTplantuseonly50percentoffuelcomparedtoaconventionalplanttogenerate
sameamountofelectricity.
2.Lowcapitalcosts:Thecapitalcostforbuildingacombinedcycleunitistwo-thirdsthecapitalcost
ofacomparablecoal-basedpowerplant.
3.Abilitytohandlevarietyoffuelsources:Theturbinesusedincombinedcycleplantsarefueled
withnaturalgas,whichismoreversatilethanaconventionalcoaloroil.Tomeettheenergydemand,
theplantcanalsobedesignedtorunonalternativefuelssuchasagriculture-basedbiogas.
4.Loweremissionandfuelconsumption:Thespecificfuelconsumptionofcombinedcycleplantsis
less(fuelperkilowatthour)andproducesfeweremissionsthanconventionalthermalpowerplants.
Thisreducestheenvironmentaldamagecausedbyelectricityproduction.ThefuelusedinCCPTis
muchcleanerwhencomparedtocoal-firedpowerplant.
5.Commercialavailability:Combinedcycleunitsarecommerciallyavailablefromallpartsofthe
world.Theycanbeeasilymanufactured,shipped,transportedandcommissioned.

20. Disadvantages
•1. The gas turbine can only use natural gas or high-grade oils such as
diesel fuel.
•2. Its operation is location specific because the combined cycle can be
operated only in locations where these fuels are available and cost
effective.