Tail-End SCR Technology ConsiderationsEPRI SCR Workshop 2009Tom WeirMarilynn MartinHans Hartenstein5 bituminous coal-fired units:Duke Energy’s Allen Steam Station, North Carolina•Units 1 and 2 (165 MW each),Units 3 (265 MW), 4 (280 MW)and 5(270 MW).)•Tangentially-fired CE boilerswith low-NOx burners and OFA.•All 5 units combine into 2 wetLSFO Alstom FGDs and a wetstack, start-up in 2009.•SNCRs on all 5 units resultingin additional air heater washes.Mi tNO•ayrequreagreaterxremoval efficiency technologysuch as SCR.Duke Energy’s Allen Steam Station, North CarolinaSite considerations for a highdust SCR retrofit include:•No room between the boilersand the ESPs or the ESPsand the stacks.•Extended downtime of eachunit if high-dust SCR reactorsto be placed on the top ofthe boiler house. Load issuesmay make this infeasible.•Constructability problemsdue to no accessibility of thethree center units.•High specific retrofit costs.Duke Energy’s Allen Steam Station, North CarolinaOther SCR retrofit options for Allen Steam Station include:•2 Cold-side low-dust SCRs for all 5 units.BoilerStackESPNH3SCRRegenerativeGas-Gas HeatSteamReheaterBoilerESPFGD•2 tail-end SCRs for all 5 units.StackAirAir HeaterSteamExchangerFGDESPSCRSteamReheaterBoilerAirAir HeaterSteamNH3RegenerativeGas-Gas HeatExchangerDuke Energy’s Allen Steam Station, North CarolinaImmediately perceiveddisadvantages of 2 cold-sidelow-dust or tail-end SCRretrofits compared to 5 highd t SCR t fit i l ddust SCRretrofitsinclude:•Additional energy/steamconsumption for flue gasreheating.•Additional pressure dropfrom a regenerative gas/gasheat exchanger and steamcoil for flue gas reheating.•Higher capital investmentcosts due to the need of thisadditional equipment.Duke Energy’s Allen Steam Station, North CarolinaActually expected advantages of 2 cold-side low-dust or tailend SCR retrofit compared to high-dust SCR retrofits for all 5units would include:•Lower expected capital equipment costs for only 2 tail-end SCRs for all 5 units compared to 5 high-dust SCRs.•Much lower expected construction costs due to no realspace constraints for easy constructability and tie-ins.•No interference with current unit operation duringconstruction and tieins-.•Short downtime of the units for tie-ins only. Each of the 2SCRscanbetiedinseparately.s ca be t ed sepa ate y•In case of tail-end SCR, dry stack and elimination of avisible wet plume.Evonik’s Tail-End SCR Experience – EquipmentEvonik has operated 1 cold-side low-dust and 9 tail-end SCRsfor more than 20 years:Cold-side low dust and tail-end SCRs typically include:•one 2-layer SCR reactor filled with 3 half-layers•catalyst with a pitch of 4.2 mm (35 x 35 cells) or less•a regenerative gas/gas heat exchanger (GGHE) with agradient of less than 25 – 45 F (15 – 25 K)•a steam coil for overcoming the GGHE’s gradienta steam coil for overcoming the GGHE s gradientEvonik’s Tail-End SCR Experience – FootprintTail-end SCRs typically have a small footprint:•A tail-end SCRfor a 350 MWunit has anactual footprint ofonly 80 ft x 80 ftincluding a GGHEand booster fanEvonik’s Tail-End SCR Experience – CatalystTail-end SCRs typically have a small reactor:•Small catalyst volume of only about0.75 m3/MW due to:•Use of small catalyst pitch(4.3 mm, 35 x 35 cells).•higher tolerable NH3slip (5 ppm)due to low SOxconcentrations

l PHerne 3and no NH3in fly ash concerns.•Catalyst deactivationis extremely slow.

Ox potentia NH 3 -slipCatalyst life of150,000 hours ofSCR operation and020000400006000080000100000120000140000

DeNpmore is normal.020000400006000080000100000120000140000Tail-end SCRs typically have a higher pressure drop:Tail-End SCR Experience – Pressure Drop•The higher pressure drop of the smaller pitch catalyst isoffset by fewer catalyst layers and shorter log length.•Additional pressure drop of 7–8 in. w.c. is caused bythe GGHE and steam coil for flue gas reheating.Tail-end SCRs typically have a lower operating temperature:Tail-End SCR Experience – Temperature•Due to very low SO2, lower SCR operating temperaturesof around 550 F (290°C) are suitable.L SCR ti t t lt i•LowerSCRoperatingtemperaturesresult in:•Lower actual flue gas volumeflow rateSmaller reactor cross1.0%•sectional area•Smaller GGHE diameters•Very low SO/SOconversion23conversionrate•Low SO3concentrationsdownstream of the SCR and0.5%in the stack•No SO3plume issuesTail-End SCR Experience – Steam UsageTail-end SCRs typically have a higher steam consumption:•Steam consumption for flue gas reheating is determinedby the gradientof the GGHE,typically 1 – 3%of unit steamflow dependenton the gradientof the GGHE.Id l f t bi•Ideal forturbinelimited units asno reduction ingeneration isincurred.•No sootblowers.Tail-End SCR Experience – Real Life ExampleHigh-Dust Versus Tail-End – Qualitative ComparisonAllen Steam Plant high‐dust cold‐side low‐dust tail‐endSCR reactor height 3+1 layers 2+1 layers 2+0 layersSCR reactor cross sectional area 100% 100% 85%Economizer bypassfor low loadneededEconomizerbypassforlowload‐ ‐Sootblowers/sonic horns needed‐ ‐LPA screens needed‐ ‐Catalyst module transport system needed‐ ‐NOx analyzer maintenance high low very lowRegenerativegasas heat exchanger‐needed needed/g gSteam coil for flue gas reheating‐needed needed•Lower SCR reactor height for less required catalyst volumed t ll t l t it hduetosmallercatalystpitch.•Smaller SCR reactor cross sectional area due to lower fluegas temperature and actual flue gas volume flow rate.Allen Steam Plant high‐dust cold‐side low‐dust tail‐endHigh-Dust Versus Tail-End – Qualitative ComparisongCapital equipment cost $$$ $ $Constructability / installation cost $$$ $ $Costoflost generation / tieintime$$$$$lostgeneration/tie‐Energy consumption (dp, steam) $ $$ $$$Catalyst cost / life $$$ $$ $Cost for catalyst exchanges $$$ $$ $SO3plume issues $$$ $ 0Visibleplume$$0LPA issues $$$ 0 0NOx analyzer maintenance $$$ $ $•In case of Allen Steam Plant, the only tangible disadvantageremains the higher energy cost due to steam consumptionfor flue gas reheat in the range of about 2% of steam flow.Cold-side low-dust or tail-end SCR retrofits are ideallysuitedHigh-Dust Versus Tail-End SCR – Conclusionsfor combining several smaller units into one SCR reactor.This can very effectively:•Reduce unit downtime and thus lost generation duringconstruction and tie-in.R d it l i t t t lt f f ll•Reducecapital investmentcostsasaresultof fewsmallerrather than multiple larger SCR reactors.•Eliminate space constraints and constructability issues.•Reduce retrofit construction time and minimize field laboras a result of ease of construction.•Eliminate impact of SCR on unit operation.High-Dust Versus Tail-End SCR – ConclusionsAdvantages include:•Minimal catalyst deactivation results in a very long catalystlife and very little cost for catalyst exchanges.•Elimination of operational issues such as LPA pluggage,catalyst erosion, SO3plume, NOx analyzer maintenanceissues catalyst exchange logistics issues etcissues,catalyst exchange logistics issues,etc..•Virtually fly ash free flue gas results in very littlemaintenance requirements and costs.•High adaptability to site specifics and selection of location.Only really tangible disadvantage in a case like Allen remainsthe higher energy cost due to steam consumption for flue gasreheat in the range of about 2% of unit steam flow.Questions?