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Selective Catalytic Reduction FAQ
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Contents1. Why select SCR for low-speed engines? ............................................ 3
1.1. Why WinGD is promoting SCR for low-speed engines? ............................... 31.2.WhatarethebenefitsofSCRcomparedtoEGR? ......................................... 4
2. SCR Concept & design ....................................................................... 62.1.Whatarethedifferencesbetweenhigh-pressure(HP)and
low-pressure(LP)SCR? ............................................................................. 62.2.WhichpartiesareinvolvedinthesupplyofSCRsystemsandwhat
are their responsibilities? ........................................................................... 82.3.IsSCRapplicableforengineswithmorethanoneturbocharger? ............... 9
3. SCR and engine operation ................................................................. 93.1.WhatenginetuningsarepossiblewithSCR? .............................................. 93.2.DoeslowambienttemperatureaffecttheSCRsystem? ............................ 103.3.WhatistheacceptablebackpressureforHPSCRandLPSCR? ................. 103.4.Whatisthepurposeofthesootblowingsystem?Howdoesthesoot
blowing system operate? .......................................................................... 103.5.Isreactortraceheatingneeded? ............................................................... 103.6.Isthermalshocktothecatalystelementsaconcernwhenstartingup
undercoldambientconditions? ................................................................ 11
4. Catalyst element ............................................................................. 124.1.WhatisthelifetimeofcatalystelementsinanSCRsystem? ..................... 124.2.Howmanyrunninghours(RH)dothecatalystelementsinanSCR
system last? ............................................................................................. 124.3.Whatarethetypicalweightanddimensionsofacatalystblockinside
theSCRreactor? ....................................................................................... 124.4.Howcantheusedelementsofacatalystblockbedisposedof? ................ 12
5. UREA handling ................................................................................ 135.1.WhyisureausedforSCRandwhatistherequiredgradeofurea? ............. 135.2.Whatarethechemicalandphysicalcharacteristicsofurea? .................... 135.3.Howshouldureabestoredonboardaship? ............................................. 13
6.Rulesandcertification .................................................................... 146.1.WhoisresponsibleforarrangingtheTierIIIenginecertificationand
whoistheSCRsystemsuppliedto? .......................................................... 146.2. Is on board NOXmonitoringmandatoryinECAareas? ............................... 146.3.HowlongdoesittaketocarryouttheSchemeBtestonboard? ................ 14
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1. Why select SCR for low-speed engines?1.1. Why WinGD is promoting SCR for low-speed engines?
SCR is a well-proven and consolidated technologySelectivecatalyticreduction(SCR)isaproventechnologylaunchedmorethan40yearsago,forcleaningNOXfromexhaustgasemittedbyinternalcombustionengines.
Duringthepastdecade,theautomotiveindustryhaslargelyreplacedexhaustgasrecirculation(EGR)technologywithSCRtoachievethelowestNOX limits requiredbyregulations.
SCR is the marine industry standard for medium and high speed enginesSCRistheindustrystandardandthemostusedtechnologyforremovingNOX fromexhaustgasoffour-strokemainandauxiliaryenginesinmarineandstationaryapplications.
WinGDhasextensiveandbroadexperiencewithSCRsystems,withover600unitsordered/installedinalargevarietyofenginesandapplications:
• Dieselandhigh-pressure(HP)gasengines
• Marineandstationaryapplications
• fourandtwo-strokeengineapplications
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1.2. WhatarethebenefitsofSCRcomparedtoEGR?
SCR has a lower initial investment cost than EGRTheinvestmentcostofaTierIIIsolutionforalow-speedenginedependsonthetypeofengineandship,thepoweroftheengine,theamountofexhaustgastoclean,etc.
Formostshipsegments,theinitialinvestmentcostoftheSCRislowerthanthatofEGR.
Figure1:IndicativeinitialinvestmentcostcomparisonforHPEGR,HPSCRandLPSCR.
SCR and EGR have similar total consumable costsBothSCRandEGRcreateoperationalcostswhencleaningNOXfromexhaustgas.Thesourceoftheoperationalcostsaredifferentasshowninthetablebelow.ThetotaloperationalcostsforSCRandEGRaresimilar.
HP SCR HP EGR
Ureaconsumption (averageabout18g/kWh)
NaOHconsumption
Fuelpenaltyisupto2g/kWhatlow engine load only
FuelpenaltyforEGRisbetween3-8g/kWh(acrosstheentireenginepowerrange)
Negligibleenergyconsumptionfortheancillarysystems,suchastheureasystemand soot blowing
Highenergyconsumptionforthewatertreatment system, blower, additional coolingwaterandsludge
Nosecondpollutantiscreated EGRproducesasecondpollutantdischargedagainstafee
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Figure2:IndicativeconsumablecostcomparisonforHPEGRandHPSCR.
SCR is a less complex system to install and maintain than EGRThefollowingtableshowsthemaincomponentsofHPSCRandHPEGR
HP SCR HP EGR
SCRon/offvalvesandcontrol Exhaust Gas Recirculation system:cylinderbypass,EGRon/offvalves,EGRblower,EGRcoolerandcontrol
SCR reactor and catalyst elements Scrubbing system:Scrubber,freshwaterproduction,tank,pump,filterandinjectionsystem
Urea system:tank,pump,filterandinjection
Caustic soda:tank,pump,filterandmixingunit
Soot blowing system: bottles,filter,injectors
Black water treatment:Pumps,filter,sludgefilter,aircompressor,drain&sludgetank
SCR has no impact on steam production
WithSCRinoperation,itisstillpossibletogeneratesteamusingtheexhaustgaseconomizerwhichproducesthesamesteamproductionpower.WithEGR,steamproductionisreducedbyabout40%duetoexhaustgasrecirculation.
SCR does not impact piston running and combustion behaviourSCRisapost-combustionsystemandcausesnochangetothein-cylinderairandpistonrunning.EGRmayhaveanimpactonpistonrunningbehaviourbecauseofthechangedcombustionandSulphurcontentintherecirculatedgas.
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2. SCR Concept & design2.1. Whatarethedifferencesbetweenhigh-pressure(HP)and
low-pressure(LP)SCR?ThereareseveraldifferencesbetweenHPandLPSCR.Belowarethemostimportant:
Reactor position
HP SCR LPSCR
Thereactorisbeforetheturbocharger.Theexhaustgasinthereactorhasahigherpressure(max4bar(g))andtemperature.
Thereactorisaftertheturbocharger.Theexhaustgasinthereactorhasarelativelylowerpressure(about0.6bar(g))andtemperature.
AnHPSCRsystemhas3maindesignfeatures.TheSCRreactor(1),apipesection,knownastheEvaporationorMixingPipeforureainjectionandevaporation(2),andtheinterfacevalvestotheengine(3a/3b)
AnLPSCRsystemhasthefollowingdesignfeatures:theSCRreactor,thedecompositionunitforammoniagenerationandthecontrolvalves(V1,V2,V3)
TheHPSCRreactoristypicallyinstalledintheengineroom.Theshipdesignerhasthechoicetopositionthereactorinahorizontalorverticalpositionaccordingtotheengineroomlayoutandavailablespace.
TheLPSCRreactoristypicallyinstalledoutsidetheengineroominthestack.Thisprovidesmoreflexibilityfortheshipdesigner.Howeveradditionalcomponentssuchasthefuelburnerandblowermustalsobeintegratedinthe ship design.
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Ammonia generation
HP SCR LPSCR
Ureaisinjectedintothemixedpipingwithhightemperatureexhaustgas,noenergyconsumption
Throughammoniagenerationusingafueloilburner
Minimum working temperature
HP SCR LPSCR
Exhaustgastemperaturebeforeturbocharger:• 310°Cforfuel<0.1%S
Exhaustgastemperatureafterturbocharger:• 230°Cforfuel<0.1%S
Fuel penalties
HP SCR LPSCR
≈0-3g/kWh between25-40%ofenginepower
≈0-3g/kWh between25-50%ofenginepower≈0-2g/kWh between50-80%ofenginepower
Engine interfaces
HP SCR LPSCR
Enginemechanicalmodifications:• Exhaustgasmanifold• Enginegallery• Turbochargerconnections
Noenginemodification
SCRbypassvalvescontrolledbytheenginecontrolsystem
SCRbypassvalvescontrolledbytheSCRsystemcontrol
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SCR components
HP SCR LPSCR
UreaTank UreaTank
Compressedairbottletank Compressedairbottletank
Compressed air system Compressed air system
Ureasupply Ureasupply
Soot blowing system Soot blowing system
SCRvalves SCRvalves
- Burner
- Blower
2.2. Which parties are involved in the supply of SCR systems and what are their responsibilities?Theenginemanufacturer,theenginedesigner(WinGD),theSCRsupplierandtheshipyardareallinvolvedinSCRprojectdevelopments.
WinGD’s responsibility and tasksWinGDdefinesthemandatorytechnicalrequirementsandthesystemdesignguidelinesfortheSCRsystemandtheenginetuning.
Inaddition,WinGDdesignstheengines’mechanicalandelectricalinterfacestotheHPSCRsystem.Asalicensor,WinGDisresponsibleforengineperformanceanddeliverstheenginedesignandtuningoptimisedforTierIIIcompliance.
InthecaseofHPSCR,WinGDalsodeliverstheSCRvalvecontrolsystem.
Engine manufacturerTheenginemanufacturerproducestheengineinaccordancetoWinGD’sdesignandincludesthemechanicalandautomationinterfaces(inthecaseofHPSCR).
TheenginemanufacturerisresponsiblefortheIMOTierIIINOXcertification.
SCR supplierTheSCRsupplierdesignsandmanufacturesthekeySCRelements.Thereactor,mixingpipe,ammoniagenerator,thecatalystelementwithappropriatesizeandvolume,ureainjectionsystem,sootblowingsystem,ureadosingsystem,etc.
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TheSCRsupplierprovidesdetailedinterfaceandmountingspecificationstotheenginemanufacturerandshipyards.
ShipyardTheshipyarddesignsthestructuralsupportforthereactorhousingandducting,andarrangestheSCRsystemontheship.
Theshipyarddesigns,procuresandmanufacturestheconnectingpiping,compensator,flangesetc.andcarriesoutthefinalassemblyoftheSCRsystem on the ship.
2.3. Is SCR applicable for engines with more than one turbocharger?BothLPandHPSCRarepossiblesolutionsforengineswithmorethanone(1)turbochargerapplication.WinGDhasdevelopedseveralconfigurationsolutionsthatareavailableondemand.
3. SCR and engine operation3.1. What engine tunings are possible with SCR?
AlltheavailabletuningsforWinGD’slow-speedenginesareapplicableforboth,HPandLPSCR.
Available engine tuningsTuning Options
HP SCR LPSCR
Standard:highloadtuning,TierIIcompliant,optimisedforengineloadsabove90%.(notrecommendedforcurrentoperatingscenarios)
P P
Delta:partloadtuning,TierIIcompliant,optimisedforengineloadsbetween75%–90%.–electronictuningoftheengine P P
Delta Bypass:partloadtuningwithalowerBSFCbelow50%engineload,TierIIcompliant,optimisedforincreasingsteamproductionabove50%engineloadandreducedfuelconsumptionbelow50%engineload.
P P
Lowload:TierIIcompliant,optimisedforengineloadsbelow75%. P P
Thetuningoptions“HPSCR”and“LPSCR”willensurethattheexhaustgastemperatureisalwaysabovetheminimumrequiredtemperatureforreliableoperationoftheSCRsystem.Incasetheexhaustgastemperaturegoesbelowthisminimumrequiredtemperature,theenginecontrolsystemreactsandadequatelyincreasesthetemperature.
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3.2. DoeslowambienttemperatureaffecttheSCRsystem?ColdexternalconditionshavenoimpactontheSCRsystembecausetheengine’sTierIIItuningwillcontroltheexhaustgastemperaturesothatitstaysabovetheminimumrequiredlowtemperature.
3.3. WhatistheacceptablebackpressureforHPSCRandLPSCR?
HP SCR back pressureThemaximumacceptablepressuredropfortheentireHPSCRsystemoveritslifetimemustbekeptbelowamaximumacceptablelimit.
Aproperpre-calculationandpipingsystemdesignmustensurethatthepressuredropoftheSCRsystemincludingthepiping(withoutthebypassvalves)doesnotexceed70mbar.
An“SCRValveControlSystem”monitorsthepressuredropvaluesacrossthemaincomponents.Itwilltriggeranalarmandsetupcounter-measuresshouldthepressuredropexceedthesetvalues.
LPSCRbackpressureThemaximumallowedbackpressurecreatedbytheLPSCRsystemis300 mm WC.Incombinationwiththeeconomizer,silencerandotherpiping,themaximumbackpressureis600 mm WConanewinstallation.
3.4. What is the purpose of the soot blowing system? How does the soot blowing system operate?Theaccumulationofdeposits,likesootandash,isverylikelytooccuronthesurfaceofthecatalyticelements.Thesootblowingsystemregularlyremovessuchdepositswithadedicatedsootblowinglineprovidedforeachcatalystlayerinthereactor.
ThesootblowingiscontrolledandenforcedbytheSCRcontrolsystem.Typically,thesootblowingisoperatedautomaticallyandsequentiallyatapressureof8bar(g).
3.5. Is reactor trace heating needed?TheSCRsupplierwilldecidewhetherreactortraceheatingisrequiredandwillsupplytherelevantspecifications.
Currently,therearenoregulationsthatrequiretheinstallationoftraceheating.Thismaydependonthecustomer’soperationalprofileandindividualrequirements.
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How much power is needed for trace heating?Theenergyrequireddependsonhowfastthecustomerwouldliketoheatthecatalystelementandonthevolumeofthecatalystelements.Accordingtoourexperienceandcurrentprojects,thepre-heaterpoweriswellbelow50 kW.
How is trace heating for the HP SCR’s reactor used in an ECA port?Iftheship’sportstayisshort,thecatalystelementswillstillbehotaftertheenginehasbeenswitchedoff.Thereforethepre-heater(ifswitchedonatthismoment)willprovideasmallamountofenergytocompensateforthethermaldissipationsofthereactor,andwillkeepthecatalystelementattherighttemperature.
How long does it take to pre-heat the HP SCR’s reactor when the ship enters an ECA area?InthecaseoftheshipenteringanECAarea,thecatalystelementsareheatedbypartiallyopeningtheSCRvalvesaccordingtoaspecificheatingprocedure.
BeforetheSCRsystemisswitchedon,thecatalystelementsneedtobeheatedtotheminimumSCRoperationaltemperaturebeforeanyreducingagentisinjected.
3.6. Is thermal shock to the catalyst elements a concern when starting up under cold ambient conditions?Thermalshocktothecatalystelementsisavoidedbytakingintoconsiderationthethermalgradientrecommendationfromthecatalystelementsupplier.
Thecatalystelementsareheatedaccordingtoagiventime-temperaturelaw;boththetracedheaterandtheSCRvalvecontrolsystemwilltakeintoaccounttheheatinggradientparametersduringtheheatingprocedure.
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4. Catalyst element4.1. What is the lifetime of catalyst elements in an SCR system?
Itiscommonpracticetoassumethatthelifetimeofthecatalystelementsis5years,i.e.thetimecorrespondingtotheship’sdry-dockingschedule.
Thepropermaintenanceofthecatalystelementsisthekeytoextendedlifeandawell-functioningSCRinstallation.Aswellassootblowingandtheammoniabi-sulphate(ABS)regenerationmeasures,manualmaintenanceisalso important.
4.2. Howmanyrunninghours(RH)dothecatalystelementsinanSCRsystem last?Themaximumnumberofrunninghours(RH)forthecatalystelementsdependsonthedesignoftheSCRsystem(e.g.thevolumeofthecatalystelement,flowvelocity,layers,fueltype,engineoperatingprofile,etc.).
Therefore,themaximumRHofcatalystelementsistheresultoftheSCRsupplier’sdesign,takingintoaccountthecustomer’soperatingprofileforalifetimeof5years,andtheneedtokeepthereactorsizewithinreasonabledimensions.
4.3. What are the typical weight and dimensions of a catalyst block inside the SCR reactor?Thecatalystelementblocksaredesignedsothattheycanbedismountedandremovedwithoutanyspecialtools.Thetypicalcatalystelementblockmeasures300x300x500mmandweighs24kg.
4.4. How can the used elements of a catalyst block be disposed of?Metalfromthecanisters/frames/cassettesisrecycledasnormalmetal.Localdisposalofthecatalystblocksismanagedthroughrecyclingwastedisposalcompanies.Thesupplierofthecatalystcanprovideadviceregarding disposal globally.
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5. UREA handling5.1. Why is urea used for SCR and what is the required grade of urea?
ThechemicalelementrequiredforthereactionofNOX to water and Nitrogen isAmmonia(NH4).However,ammoniaisdifficulttostoreandhandleonashipandisthereforegeneratedfromurea.Ureaisstoredandtransformedintoammoniaafterinjectionintotheexhaustbeforeenteringthereactor.Ureaisinjectedintotheexhaustgasstreammixedwithcompressedairorfresh water.
ThestandardAqueousUreaSolution(AUS)formarineapplicationsis40%ureainwater(Marineurea/AUS40).TheappropriatequalityrequirementsfortheureasolutionarepresentedinISO22241.Onboardthevessel,areadyureawatermixturecanbebunkeredorstored.Ureacanalsobestoredonboardinpowderorpelletformandcanbemixedwithpurifiedwaterwhenneeded.
5.2. What are the chemical and physical characteristics of urea?AUS40ureahasthefollowingcharacteristics:
• Colourlessliquid
• Notflammable
• Notclassifiedasharmfulbytheclasssocietiesrulesorotherapplicablerules.
• Notrecordedashazardouswaste.Recoverypossiblethroughuseasfertilizer
• Lightammoniacodour,theammoniaccontentislessthan0.1%
• Density:1.1-1.15ton/m3
• Decomposition/boilingtemperatureat105°Cat1bara
5.3. How should urea be stored on board a ship?ThegeneralguidelinesforureahandlingandstorageareprescribedinISO18622-3:2014.
ThestorageproceduresofureaAUS40areprovidedbytheoriginalSCRorureasupplier.
Urea(AUS40)mustbestoredatatemperaturebetween5°Cand35°Ctoavoiddecompositionorsaltdeposits.
Thestoragetimeforureasolutionistemperaturedependent.Astoragetemperatureoflessthan35°Cwillkeeptheureasolutionusableformorethanayear.Forexample,atatemperatureofabout20°C,ureacanbestoredforupto2years.
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Ureatanksaretobekeptclearfromfireorhightemperaturesourcestoavoidevaporation.Ureaiscorrosiveoncarbonsteels,nickelandnon-ferrousmetals(copper,zinc,etc.).Therefore,theureatanksmustbesuitablycoatedormadeofstainlesssteelorselectedplastics.Furthermore,therecommendationsfromtheclasssocietiesmustbeconsideredwhendesigningtheureatank.
6. Rulesandcertification6.1. WhoisresponsibleforarrangingtheTierIIIenginecertificationand
who is the SCR system supplied to?TheapprovedSCRsupplierdeliverstheSCRsystemtotheenginemanufacturer(onlyfortheparentengine).TheenginemanufacturerneedstoensurethattheparentenginewithSCRmeetsTierIIINOX emissions levelsduringshoptest.TheTierIIIcertificationisissuedbytheClassificationSociety,accordingtoIMOscheme“A”testprocedure.
Formemberengines,theSCRsystemisdelivereddirectlytotheshipyard
6.2. Is on board NOX monitoring mandatory in ECA areas?NOXmonitoringsystemisnotmandatoryandisnotexplicitlyrequestedbytheapplicablerulesinforce.
6.3. How long does it take to carry out the Scheme B test on board?Discussionsregardingthevarioustestproceduresareongoingandarenotyetfinalised.WinGDisactivelyparticipatinginthesediscussions,whichalsoincludethetestproceduresaccordingtoschemeB,withIACS(andIMO).
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WinGD® is a registered trademark.
© Copyright, 2018 Winterthur Gas & Diesel Ltd.
www.wingd.com
Winterthur Gas & Diesel Ltd. (WinGD) is a leading developer of two-stroke low-speed gas and diesel engines used for propulsion power in merchant shipping.
WinGD sets the industry standard for reliability, efficiency and environmental sustainability. WinGD provides designs, training and technical support to engine manufacturers, shipbuilders and ship operators worldwide.
WinGD is headquartered in Winterthur, Switzerland, where, as one of the earliest developers of diesel technology, it started the design of large internal combustion engines in 1893 under the “Sulzer” name.
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