Industrial Boilers and Heat Recovery Steam Generators: Design, Applications, and Calculations

579 Pages·2002·2.9 MB·English

by  V. Ganapathy

Industrial Boilers
and Heat Recovery
Steam Generators
Design, Applications,
and Calculations
V. Ganapathy
ABCO Industries
Abilene, Texas, U.S.A.
Marcel Dekker, Inc. New York Basel
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Current printing(last digit):
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PRINTED INTHE UNITEDSTATES OFAMERICA
To all professionals involved in steam generation
and energy conservation.
Preface
Theroleofboilersandheatrecoverysteamgenerators(HRSGs)intheindustrial
economy has been profound. Boilers form the backbone of power plants,
cogeneration systems, and combined cycle plants. There are few process
plants, refineries, chemical plants, or electric utilities that do not have a steam
plant. Steam is the most convenient working fluid for industrial processing,
heating, chilling,andpowergenerationapplications.Fossilfuelswillcontinueto
be the dominant energy providers for years to come.
This book is about steam generators, HRSGs, and related systems. There
are several excellent books on steam generation and boilers, and each has been
successful in emphasizing certain aspects of boilers and related topics such as
mechanical design details, metallurgy, corrosion, constructional aspects, main-
tenance, or operational issues. This book is aimed at providing a different
perspective on steam generators and is biased toward thermal and process
design aspects of package boilers and HRSGs. (The terms ‘‘waste heat boiler’’
and‘‘HRSG’’areusedinthesamecontext.)Myemphasisonthermalengineering
aspects of steam generators reinforced by hundreds of worked-out real-life
examples pertaining to boilers, HRSGs, and related systems will be of interest
toengineersinvolvedinabroadfieldofsteamgenerator–relatedactivitiessuchas
consulting, design, performance evaluation, and operation.
DuringthelastthreedecadesIhavehadtheopportunitytodesignhundreds
ofpackageboilersandseveralhundredwasteheatboilersthatareinoperationin
theU.S.andabroad.Basedonmyexperienceinreviewingnumerousspecifica-
tionsofboilersandHRSGs,Ifeelthatconsultants,plantengineers,contractors,
anddecisionmakersinvolvedinplanninganddevelopingsteamplantsoftendo
notappreciatesomeoftheimportantandsubtleaspectsofdesignandperfor-
manceofsteamgenerators.
(cid:1)Manyengineersstillfeelthatbyraisingtheexitgastemperatureinboilers
witheconomizers,onecanavoidaciddewpointconcerns.Itisthefeedwater
temperature—notthegastemperature—thatdeterminesthetubewall
temperature(andhencethecorrosionpotential).
(cid:1)Softenedwaterissometimessuggestedforattemperationforsteamtempera-
turecontrol,eventhoughitwilladdsolidstosteamthatcancauseproblems
suchasdepositionofsolidsinsuperheatersandsteamturbines.
(cid:1)Tooperatesteamplantsmoreefficiently,plantengineersshouldbeableto
understandandappreciatethepartloadcharacteristicsofboilersandHRSGs.
HoweverwhilespecifyingboilersandHRSGs,oftenonlytheperformanceat
100%loadisstressed.
(cid:1)HRSGsteamgenerationandtemperatureprofilescannotbearbitrarilyarrived
at,aspinchandapproachpointsdeterminethis.Forexample,Ihaveseen
severalspecificationscallfora300 (cid:2)Fexitgastemperaturefromasingle
pressureunfiredgasturbineHRSGgeneratingsaturatedsteamat600psig
usingfeedwateratabout230 (cid:2)F.Asimpleanalysisrevealsthatonlyabout
340–350(cid:2)Fisthermodynamicallyfeasible.
(cid:1)SupplementaryfiringingasturbineHRSGsisanefficientwaytogenerate
steamcomparedwithsteamgenerationinapackagedboiler.Thebookexplains
whythisisso,withexamplesinChapters1and8.Cogenerationengineerscan
make use of this information to minimize fuel costs in their plants.
(cid:1) Afewwasteheatboilerspecificationsprovidethefluegasflowinvolumetric
units instead of mass units, leading to confusion. Lack of information on
molecular weight or gas pressure can lead to incorrect evaluation of density
andhencethemassflow.Also,volumeoffluegasisoftengivenincfm(cubic
feet per minute) and one is not sure whether it is acfm (actual cubic feet per
minute)orscfm(standardcubicfeetperminute).Thedifferenceinmassflow
can be significant depending on the basis.
(cid:1) Althoughfluegasanalysisaffectsgasspecificheat,heattransfer,boilerduty,
and temperature profiles, these data are often not given in specifications for
waste heat boilers. Forexample,theratioofspecificheats offluegases from
combustion of natural gas and fuel oil is about 3.5%, which is not insignif-
icant.Thisisduetothe18%volumeofwatervaporinnaturalgasproductsof
combustion versus 12% in fuel oil combustion products.
(cid:1)AfewconsultantsselectboilersandHRSGsbasedonsurfacearea,although
itcanvarysignificantlybasedontubegeometryorfinconfiguration.With
finnedtubes,ascanbeseenfromseveralexamplesinthisbook,thevariation
insurfaceareascouldbeintherangeof200–300%forthesameduty.
(cid:1)Operatingcostduetofuelconsumptionorgaspressuredropacrossheating
surfacesisoftenignoredbymanyconsultantsintheirevaluationandonly
initialcostsarecomparedwhilepurchasingsteamgeneratorsorHRSGs,
resultinginapoorselectionfortheenduser.Afewplantsarenowrealizing
thattheitemsofsteamplantequipmenttheypurchasedyearsagobasedonlow
initialcostsaredrainingtheircashreservesthroughcostlyfuelandelectricity
billsandhencearescramblingtoimprovetheirdesignandperformance.
(cid:1)Manyengineersarenotawareofrecentdevelopmentsinoil-andgas-fired
packagedboilersandarestillspecifyingboilersusingrefractorylinedfurnace
wallsandfloors!
(cid:1)Plantengineersoftenassumethataboilerdesignedfor600psig,forexample,
canbeoperatedat200psigandatthesamecapacity.Thepotentialproblems
associatedwithsignificantchangesinsteampressureandspecificvolumein
boileroperationarediscussedinChapters1and3.
(cid:1) Condensing exchangersarebeing considered in boilers andHRSGsnotonly
forimprovementinefficiencybutalsotorecoverandrecyclethewaterinthe
flue gases, which is a precious commodity in some places.
(cid:1) Emission control methods such as flue gas recirculation increase the mass
flowof flue gases through the boiler; yet standard boilers are being selected
thatcanbeexpensivetooperateintermsoffanpowerconsumption.Manyare
not aware of the advantages of custom-designed boilers, which can cost less
to own and operate.
(cid:1) Afewsteamplantprofessionalsdonotappreciatetherelationbetweenboiler
efficienciesandhigherandlowerheatingvalues,andthusspecifyvaluesthat
are either impossible to accomplish or too inefficient.
As a result of this ‘‘knowledge=information gap’’ in process engineering
aspects of boilers or HRSG, the end user may need to settle for a product with
substandardperformanceandhighcosts.Thisbookelaboratesonvariousdesign
and performance aspects of steam generators and heat recovery boilers so that
anyone involved with them will become more informed and ask the right
questions during the early stages of development of any steam plant project.
This will give the best chance of selecting the steam generator with the right
design and parameters. Even a tiny improvement in design, efficiency, operating
costs, or performance goes a long way in easing the ‘‘energy crunch.’’
The first four chapters describe some of the recent trends in power
generation systems, a few aspects of steam generator and HRSG design and
performance, and the impact of emissions on boilers in general. The remaining
chaptersdealwithcalculationsthatshouldbeofinteresttosteamplantengineers.
IauthoredtheSteamPlantCalculationsManual(MarcelDekker,Inc.)several
yearsagoandhadbeenthinkingofaddingmoreexamplestothisworkforquite
sometime.Thisbookbuildsonthatfoundation.
Chapter1isanintroductorydiscussionofpowerplantsanddescribessome
oftherecentdevelopmentsinpowersystemssuchasthesupercriticalRankine
cycle,theKalinacycle,theChengcycle,andtheintegratedcoalgasificationand
combinedcycle(IGCC)plantthatisfastbecomingareality.
Thesecondchapterdescribesheatrecoverysystemsinvariousindustries.
TheroleoftheHRSGinsulfurrecoveryplants,sulfuricacidplants,gasturbine
plants,hydrogenplants,andincinerationsystemsiselaborated.
Chapter3,onsteamgenerators,describesthelatesttrendsincustom-
designedpackageboilersandthelimitationsofstandardboilersdeveloped
decadesago.Emissionregulationshaveresultedinchangesinboileroperating
parameterssuchashigherexcessairandFGRratesthatimpactboilerperfor-
mancesignificantly.Itshouldbenotedthattherecanbeseveraldesignsfora
boilersimplybecausetheemissionlevelsaredifferent,althoughthesteam
parametersmaybeidentical.IfanSCRsystemisrequired,itnecessitatesthe
additionofagasbypasssystem,addingtothecostandcomplexityofboiler
design.Theseareexplainedthroughquantitativeandpracticalexamples.
Chapter4,onemissions,describesthevariousmethodsusedinboilersand
HRSGstolimitNOxandCOandhowtheirdesignsareimpacted.Forexample,
theHRSGevaporatormayhavetobesplituptoaccommodatetheselective
catalyticreduction(SCR)system;gasbypassdampersmayhavetobeusedin
packagedsteamgeneratorstoachievetheoptimalgastemperatureatthecatalyst
forNOxconversionatvariousloads.Fluegasrecirculation(FGR)addstothefan
powerconsumptionifthestandardboilerisnotredesigned.Itmayalsoaffectthe
boilerefficiencythroughhigherexitgastemperatureduetothelargermassflow
offluegases.Othermethodsforemissioncontrol,suchassteaminjectionand
burnermodifications,arealsoaddressed.
Chapters4–8,whichpresentcalculationspertainingtovariousaspectsof
boilersandHRSGsandtheirauxiliaries,elaborateonthesecondeditionofthe
SteamPlantCalculationsbook.Severalexampleshavealsobeenadded.Chapter
5dealswithcalculationssuchasconversionofmasstovolumetricflowrates,
energyutilizationfromboilerblowdown,generalASMEcodecalculations,and
lifecyclecostingmethods.(ASMEhasbeenupdatingtheallowablestressvalues
forseveralboilermaterialsandoneshouldusethelatestdata.)Alsoprovidedare
ABMAandASMEguidelinesonboilerwater,forevaluatingtheblowdownor
estimatingthesteamfordeaeration.Lifecyclecostingisexplainedthroughafew
examples.
Chapter6dealswithcombustioncalculations,boilerefficiency,and
emission conversion calculations. Simplified combustion calculation procedures
suchastheMMBtumethodareexplained.Oftenboilerefficiencyiscitedona
HigherHeatingValuebasis,whileafewengineersusetheLowerHeatingValue
basis.Therelationbetweenthetwoisillustrated.TheASMEPTC4.1methodof
calculatingheatlossesforestimatingboilerefficiencyiselaborated,andsimpli-
fiedequationsforboilerefficiencyarepresented.Examplesillustratetherelation
betweenoxygeninturbineexhaustgasesandfuelinput.Correlationsfordew
pointofvariousacidvaporsaregivenwithexamples.
Chapter7explainsboilercirculationcalculationsinbothfiretubeandwater
tubeboilers.Fluidflowinblowoffandblowdownlines,whichinvolvetwo-phase
flowcalculations,canbeestimatedbyusingtheproceduresshown.Theproblem
offlowinstabilityinboilingcircuitsisexplained,alongwithmeasuresto
minimizethisconcern,suchasuseoforificesattheinlettothetubes.
Calculationsinvolvingorificesandsafetyvalvesshouldalsobeofinterestto
plantengineers.
Chapter8onheattransferhasover65examplesofsizing,off-design
performance calculations pertaining to boilers, superheaters, economizers,
HRSGs, and air heaters. Tube wall temperature calculations and calculations
with finned tubes for insulation performance will help engineers understand the
design concepts better and even question the boiler supplier. HRSG temperature
profiles are also explained, with methods described for evaluating off-design
HRSG performance.
The last chapter deals with pumps, fans, and turbines and examples show
the effect of a few important variables on their performance. The impact of air
density on boiler fan operation is illustrated, and the effect of elevation and
temperature on flow and head are explained. With flue gas recirculation being
used in almost all boilers, the effect of density on the volume is important to
understand.Theeffectof inletairtemperatureonBraytoncycleefficiencyisalso
explained and plant engineers will appreciate the need for inlet air-cooling in
summer months in large gas turbine plants. The efficiency of cogeneration is
explained, as are also power output calculations using steam turbines.
Asimplequizisgivenattheendofthebook.Itspurposeistorecapitulate
important aspects of boiler and HRSG performance discussed in the book.
In sum, the book will be a valuable addition to anyone involved in steam
plants,cogenerationsystems,orcombinedcycleplants.Manyexamplesarebased
on my personal experience and hence, the conclusions drawn do not reflect the
viewsofanyorganization.Itispossible,duetolackof informationonmypartor
totherapiddevelopmentsinsteamplantengineeringandtechnology,thatIhave
expressedsomeviewsthatmaynotbecurrentormaybeagainstthegrain;ifso,I
expressmyregrets.Iwouldappreciatereadersbringingthesetomyattention.The
calculations have been checked to the best of my ability; however if there are
errors,Iapologizeandwouldappreciateyourfeedback.Itismyferventhopethat
this book will be the constant companion of professionals involved in the steam
generation industry.
I would like to thank ABCO Industries for allowing me to reproduce
severalofthedrawingsandphotographsofboilersandHRSGs.Ialsothankother
sources that have provided me with information on recent developments on
various technologies.
V. Ganapathy