Power Plant Boilers.pdf

What is a boiler, Characteristics of a Good Boiler, Classification of Boilers, Subcritical and Supercritical Boilers, FBC and its advantages

1. Power Plant Boilers
Prepared by
Ankur Sachdeva
Assistant Professor, ME

2. What is a Boiler?
•Aboilerisaclosedvesselthatproducessteamorvaporsinwhichwaterisconvertedinto
steambyapplyingheat.
•Thermalenergyreleasedbythecombustionoffuelinthepresenceofairistransferredto
water,whichvaporizesandgetsconvertedintosteamatthedesiredtemperatureandpressure.
•Usuallyboilersarecoaloroil-fired.
Thesteamapplication:
1.Producingmechanicalworkbyexpandingitinasteamengineorsteamturbine.
2.Heatingtheresidentialandindustrialbuildings.
3.Performingcertainprocessesinthesugarmills,chemical,andtextileindustries.

3. Characteristics of a Good Boiler
1.Safety:Theboilershouldbesafeunderoperatingconditions.
2.Accessibility:Thevariouspartsoftheboilershouldbeaccessibleforrepairand
maintenance.
3.Capacity:Theboilershouldbecapableofsupplyingsteamaccordingtothe
requirements.
4.Efficiency:Topermitefficientoperation,theboilershouldbeabletoabsorba
maximumamountofheatproducedduetotheburningoffuelinthefurnace.
5.Itshouldbesimpleinconstructionanditsmaintenancecostshouldbelow.
6.Itsinitialcostshouldbelow.
7.Theboilershouldhavenojointsexposedtoflames.
8.Theboilershouldbecapableofquickstartingandloading.

4. Critical Point of Water
•Thecriticalpointofasubstanceisthe
temperatureandpressureatwhichthat
substancecanbehavelikeagasanda
liquidatthesametime,hence
indistinguishablegasandliquidphases
occur.
•Thatisbecausethedensityofthegasphase
andtheliquidphaseisequalatthispoint.
•Asubstancethatexistsattemperatureand
pressure,aboveitscriticalpointisknownas
asupercriticalfluid.
•Asubstancethatexitsbelowitscriticalpoint
isknownasasubcriticalfluid.
•Inaphaseequilibriumcurve,thecritical
pointistheendpointofthecurve.

5. Pressure v/s Temperature & Latent Heat
Absolute Pressure
(Bar)
Saturation
Temperature (
o
C)
Latent Heat (K
J/Kg.)
50 264 1640
150 342 1004
200 366 592
221 374 0

6. Classification of Boilers
➢OnthebasisofPressure
Low-PressureBoiler:P<60bar
High-PressureBoiler:P>60bar
CriticalPressureBoiler:P=221.2bar
SuperCriticalPressureBoiler:P>221.2bar
Sub-CriticalBoiler:P<221.2bar(generallyinbetween130to180bar)
➢OnthebasisofSteamflowrate
LowCapacityBoiler:m
s=4,000to6,000kg/hr
HeavyDutyboiler:m
s>100,000kg/hr

7. High Pressure Boilers
•Inallmodernpowerplants,high-pressureboilers(>100bar)operatingatsupercriticalpressuresare
universallyused.
•Inordertoobtainefficientoperationandhighcapacity,forcedcirculationofwaterthroughboilertubesis
foundhelpful.
•Theefficiencyandthecapacityoftheplantcanbeincreasedasreducedquantityofsteamisrequiredforthe
samepowergenerationifhigh-pressuresteamisused.
ADVANTAGES
1.Theforcedcirculationofwaterthroughboilertubesprovidesfreedominthearrangementoffurnacesand
waterwalls,inadditiontothereductionintheheatexchangearea.
2.Thetendencyofscaleformationisreducedduetothehighvelocityofwater.
3.Thedangerofoverheatingisreducedasallthepartsareuniformlyheated.
4.Thedifferentialexpansionisreducedduetouniformtemperatureandthisreducesthepossibilityofgasand
airleakages.

8. Subcritical and Supercritical Boilers
•SubcriticalBoilers:
•Subcriticalboilersareboilersthatworkattemperaturesupto374°Candatapressureof
3,208psi(thecriticalpointofwater).
•Theseboilerscomposeasystemwithaconstantevaporationendpoint.
•Atypicalexampleofasubcriticalboileristhedrum-typesteamgenerator.
SupercriticalBoilers:
•Supercriticalboiler(supercriticalsteamgenerator)isatypeofboilerthatworksunder
supercriticalpressureconditions.
•Thistypeofboilerisoftenusedingeneratingelectricity.
•Unlikeinsubcriticalboilers,thereisnobubbleformationinsupercriticalboilers,and
liquidwaterimmediatelyconvertsintosteam.
•Supercriticalboilerworksattemperaturesaround538–565°Candpressuresabove3,200
psi

9. LA MONT BOILER
•Itisaforcedcirculation,high-pressurewatertube
boiler.Thecirculationofthewaterismaintainedby
acentrifugalpump.
•Thispumpisdrivenbyasteamturbineusingthe
steamfromtheboiler.InthisLaMontBoiler,the
separatordrumiskeptoutsidetheboiler.
•Fromthehotfluegasesproducedfromthe
combustionofthefuel,theheatissuppliedtothe
waterintheevaporatortubes,superheatertubes,
economizertubes,andtheairintheairheatertubes.
•Thenthewastegasesmovetotheatmosphere
throughthechimney.
•Theseparatordrumseparatesthesteamfromthe
water.Steampassesfromtheevaporatorandfeed
waterisfedfromtheeconomizertothedrum.
•Thesteamisseparatedfromthewater.Thewateris
againpumpedtotheevaporatorbythecentrifugal
pumpviathedistributingheader.

10. LA MONT BOILER
•Theseparatedsteamissenttothesuperheaterwhich
receivestheheatfromthefluegasesflowingfromthe
combustionchamber.Thissuperheatedsteamisthen
deliveredoutthroughastopvalve.
•Aneconomizerisprovidedinthewatercircuittopreheat
thefeedwaterusingthehotgasesleavingtheboiler.
•Thefeedwaterissentinsidetheboilerthroughthe
economizer.Thecentrifugalpumpisusedtocirculatethe
watertotheeconomizer.
•Thecentrifugalpumpdeliversthefeedwatertothe
headersatapressureof2.5atmabovethedrumpressure.
Intheevaporator,waterisdistributedthroughthe
nozzles.
•Thesteamisgoingtothesuperheaterbeforeitgoesto
theprimemover.
•Achokeisusuallyfittedattheentrancetoeachunit,in
ordertogiveasecureuniformflowoffeedwater
throughthepipelines.

11. Disadvantages of LA MONT BOILER
•The major disadvantage is the formation and attachment of bubbles on
the inner surfaces of the heating tubes.
•This reduces the heat flow and steam generation.

12. BENSON BOILER
•In1927,theBensonboilerwasdevelopedbyBensoninWest
Germany.Itwasthefirstsuper-criticaldrumlessboiler.
•Itisahigh-pressure,vertical,firetubeboiler.Thisboilerhas
nodrumandisdesignedtooperateatacriticalpressureof225
bar.
WORKING
•Thefuelisburntonthegrateandthehotfluegasesflowover
theradiantevaporator,convectionsuperheater,convection
evaporator,economizer,andairpreheaterandthenpass
throughthechimney.
•Thefeedwaterispumpedthroughtheeconomizertubesand
receivesheatfromthefluegases.
•Thenthisheatedwaterflowsintotheradiantsuperheater
whereitreceivesfurtherheatfromthefluegasesandgets
evaporated.
•Theremainingwaterisevaporatedintheconvection
superheater.
•Thesteamnowbecomessaturatedsteamintheconvection
superheater.
•Thenthesteamisdeliveredoutthroughthestopvalve.

13. BENSON BOILER
•Thewaterispassedtotheradiantevaporator
throughtheeconomizer.
•Intheeconomizer,themajoramountofwateris
convertedintosteam.
•Theremainingwaterisevaporatedinthefinal
evaporatorabsorbingtheheatfromthehotgases
byconvection.
•Themaindisadvantageissaltdepositioninthis
systeminthetransformationzonewhenall
remainingwaterisconvertedintosteam.
•Toavoidthis,forevery4000hrs,afterworking
periods,theboileriscleanedbyhigh-pressure
water.
•Themaximumpressureobtainedfromthe
Bensonboileris500atm.

14. Advantages of BENSON BOILER
1.Inthissystem,thereisnodrum.SothetotalweightoftheBensonboileris
reducedby20%whencomparedtootherboilers.
2.Theerectionofthisboileriseasierandquicker.
3.Transformationiseasy.
4.Itoccupiesverylittlespace.
5.Itcanbestartedveryquicklysinceithasweldedjoints.
6.Itisaneconomicalone.
7.Suddenfallofdemandcreatescirculationproblemsduetobubbleformation
inthenaturalcirculationboilerwhichneveroccursintheBensonboiler.
8.Aroundonly4%ofblow-downlossesareoccurredintheBensonboiler.
9.Thereisnoexplosionhazards.

15. LOEFFLER BOILER
•ThemajordifficultyexperiencedintheBenson
boileristhedepositionofsaltandsedimentonthe
innersurfacesofthewatertubes.
•Thedepositionreducedtheheattransferand
ultimatelythegeneratingcapacity.Thisfurther
increasedthedangerofoverheatingthetubesdueto
saltdepositionasithashighthermalresistance.
•ThedifficultywassolvedintheLofflerboilerby
preventingtheflowofwaterintotheboilertubes.
•Mostofthesteamisgeneratedoutsidefromthefeed
waterusingpartofthesuperheatedsteamcomingout
fromtheboiler.
•Thepressurefeedpumpdrawsthewaterthroughthe
economizeranddeliversitintotheevaporatordrum.
•About65%ofthesteamcomingoutofthe
superheaterispassedthroughtheevaporatordrumin
ordertoevaporatethefeedwatercomingfromthe
economizer.

16. LOEFFLER BOILER
•Thesteamcirculatingpumpdrawsthesaturatedsteam
fromtheevaporatordrumandispassedthroughthe
radiantsuperheaterandthentheconvectivesuperheater.
•About35%ofthesteamcomingoutfromthesuperheater
issuppliedtotheH.P.steamturbine.
•ThesteamcomingoutfromtheH.P.turbineispassed
throughthereheaterbeforebeingsuppliedtotheL.P.
turbine.
•Theamountofsteamgeneratedintheevaporatordrumis
equaltothesteamtapped(65%)fromthesuperheater.
•Thenozzlesthatdistributethesuperheatedsteamthrough
thewaterintotheevaporatordrumareofspecialdesignto
avoidprimingandnoise.
•Thisboilercancarryhighersaltconcentrationthanany
othertypeandismorecompactthanindirectlyheated
boilershavingnaturalcirculation.Thesequalitiesfititfor
landorseatransportpowergeneration.
•Lofflerboilerswithageneratingcapacityof94.5
tonnes/hrandoperatingat140barhavealreadybeen
commissioned.

17. VELOX BOILER

18. VELOX BOILER
•Whenthegasvelocityexceedsthesoundvelocity,theheatistransferredfromthegasatamuchhigherrate
thanratesachievedwithsub-sonicflow.
•Theadvantagesofthistheoryaretakentoobtainthelargeheattransferfromasmallersurfaceareainthis
boiler.
•Airiscompressedto2.5barswiththehelpofacompressorrunbyagasturbinebeforebeingsuppliedto
thecombustionchambertogetthesupersonicvelocityofthegasespassingthroughthecombustion
chamberandgastubesandhighheatreleaserates.
•Theburnedgasesinthecombustionchamberarepassedthroughtheannulusofthetubes.
•Theheatistransferredfromgasestowaterwhilepassingthroughtheannulustogeneratethesteam.
•Themixtureofwaterandsteamthusformedthenpassesintoaseparatorwhichissodesignedthatthe
mixtureenterswithaspiralflow.
•Thecentrifugalforcethusproducedcausestheheavierwaterparticlestobethrownoutwardonthewalls.
Thiseffectseparatesthesteamfromthewater.

19. VELOX BOILER
•Theseparatedsteamisfurtherpassedtothesuperheaterandthensuppliedtothe
prime-mover.Thewaterremovedfromthesteamintheseparatorisagainpassedinto
thewatertubeswiththehelpofapump.
•Thegasescomingoutfromtheannulusatthetoparefurtherpassedoverthe
superheaterwhereitsheatisusedforsuperheatingthesteam.
•Thegasescomingoutofthesuperheaterareusedtorunagasturbineastheycarry
sufficientkineticenergy.
•Thepoweroutputofthegasturbineisusedtoruntheaircompressor.
•Theexhaustgasescomingoutfromthegasturbinearepassedthroughtheeconomizer
toutilizetheremainingheatofthegases.
•Theextrapowerrequiredtorunthecompressorissuppliedwiththehelpofan
electricmotor.
•Feedwaterof10to20timestheweightofsteamgeneratediscirculatedthroughthe
tubeswiththehelpofawatercirculatingpump.Thispreventstheoverheatingof
metalwalls.

20. Advantages of Supercritical Boilers
•Theefficiencyofthesupercriticalboilerishigherbecausetogeneratethesame
heatenergy,ittakeslessfuelthanthesubcriticalboiler.
•Theexhaustemissions,especiallycarbondioxide,arerelativelylowerthanthose
ofsub-criticalboilers.
•Thesizeoftheboilerisrelativelysmallerthanthesubcriticalboilerwiththe
samegenerationcapacity.
•Thetermofboilerisnotappropriatetouse,sincethewaterneveractuallyboiled
atasupercriticalboiler.
•Itisbettertousethe“SupercriticalSteamGenerators”terminsteadof
supercriticalboilers.

21. Introduction to Fluidized Bed Combustion
•ThemajorportionofthecoalavailableinIndiaisoflowquality,highash
content,andlowcalorificvalue.
•Thetraditionalgratefuelfiringsystemshavegotlimitationsandaretechno-
economicallyunviabletomeetthechallengesofthefuture.
•Fluidizedbedcombustionhasemergedasaviablealternativeandhassignificant
advantagesoverconventionalfiringsystemsandoffersmultiplebenefits–
compactboilerdesign,fuelflexibility,highercombustionefficiency,andreduced
emissionofnoxiouspollutantssuchasSOxandNOx.
•Thefuelsburntintheseboilersincludecoal,washeryrejects,ricehusk,bagasse,
andotheragriculturalwastes.
•Thefluidizedbedboilershaveawidecapacityrange-0.5T/hrtoover100T/hr.

22. Fluidized Bed Combustion
•Fluidizedbedcombustionboiler(FBC)isa
combustiontechnologyusedtoburnsolid
fuels.
•Initsmostbasicform,fuelparticlesare
suspendedinahot,bubblingfluiditybedof
ashandotherparticulatematerialsthrough
whichjetsofairareblowntoprovidethe
oxygenrequiredforcombustionor
gasification.
•Theresultantfastandintimatemixingofgas
andsolidspromotesrapidheattransferand
chemicalreactionswithinthebed.
•FBCplantsarecapableofburningavariety
oflow-gradesolidfuels,includingmost
typesofcoal,coalwaste,andwoody
biomass,athighefficiencyandwithoutthe
necessityforexpensivefuelpreparation
(e.g.,pulverizing).

23. Fluidized Bed Combustion
•Whenthehigh-velocitygasispassedthrougha
packedbedoffinelydividedsolidparticles,the
particlesbecomesuspendedinthegasstream
andthepackedbedbecomesafluidizedbed.
•Whenthegasvelocityisveryhigh,thefluidized
bedbecomesturbulent,andrapidmixingof
particlesoccurs.
•Ultimately,thebehaviorofamixtureofsolid
particlesandgasbecomesafluid.
•Burningofafuelinsuchastateisknown
asFluidizedBedCombustion.
•Theboilerplantusingthisfluidizedbed
combustionisknownasafluidizedbedboiler.

24. Advantages of Fluidized Bed Combustion
•HighEfficiency
•Reduction in BoilerSize
•FuelFlexibility
•Ability to Burn Low-GradeFuel
•Ability to BurnFines
•PollutionControl
•Low Corrosion andErosion
•Easier Ash Removal –No ClinkerFormation
•Less Excess Air –Higher CO
2 in FlueGas
•High Efficiency of PowerGeneration
•Simple Operation, Quick Start-Up