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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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.