December13,2016 Ms.NancyVehrAirQualityAdministratorWyomingDepartmentofEnvironmentalQuality200W.17thStreet,3rdFloorCheyenne,WY82002RE: GilletteWyomingArea1‐HourSO2ModelingAnalysisReportDearMs.Vehr,OnbehalfofBasinElectricPowerCooperative,BlackHillsCorporationandPacifiCorp,TrinityConsultantsissubmittingareporton1‐hourSO2ModelingconductedforthemajorSO2sourcesinthevicinityofGilletteinCampbellCounty,Wyoming.IndiscussionwithWDEQ,thethreeutilitiescontractedwithTrinityConsultantstoperformthemodelingusingthelatestversionofAERMOD(pertheprotocolsubmittedandapprovedinNovember2016)forthesixcoal‐firedelectricgenerationunitsintheGilletteareafortheperiodof2012through2014usingactualhourlyCEMSdataforeachoftheunitsandlocalrepresentativemeteorologicaldata.Themodelingresultedinthehighestreceptorconcentrationbeinglessthan50%ofthe1‐hourSO2NAAQS.Thus,themodelinghasdemonstratedcompliancewiththe1‐hourSO2NAAQS.EnclosedwiththissubmittalaretwocopiesofthefinalreportandonesetofelectronicmodelingfilesonCD.IfyouhaveanyquestionsorneedfurtherinformationpleasecontactAnnaUnruhat(720)638‐7647,ext.114oremailataunruh@trinityconsultants.com.Sincerely,TRINITYCONSULTANTS

AnnaUnruhSeniorConsultantEnclosures:cc: JoshNall,WDEQ MikePaulandCrisMiller,BasinElectric FredCarl,TimMordhorst,andTimRogers,BlackHills BillLawsonandCraigLucke,PacifiCorp

Environmental solutions delivered uncommonly well

REVISED 1-HOUR SO2 AIR DISPERSION MODELING PROTOCOL FOR

GILLETTE WYOMING POWER PLANT ANALYSIS

Preparedfor

Basin Electric Power Cooperative Black Hills Corporation

PacifiCorp

Submittedto

Wyoming Department of Environmental Quality Air Quality Division

Preparedby

TRINITYCONSULTANTS1391NorthSpeerBoulevard,Suite350

Denver,CO80204

RevisedDecember13,2016

Project150601.0033

Gillette Wyoming Power Plant 1-Hour SO2 Modeling Analysis Trinity Consultants 1

TABLE OF CONTENTS

1. EXECUTIVE SUMMARY 4 

2. INTRODUCTION 5 

3. DESCRIPTION OF UTILITY SOURCES 6 

4. AIR QUALITY MODELING METHODOLOGY 8 

4.1.ModelSelection 8 

4.2.MeteorologicalData 8 

4.2.1.SurfaceData 8 4.2.2.UpperAirData 9 4.2.3.LandUseAnalysis 9 4.2.4.AERMETDataProcessing 11 

4.3.CoordinateSystem 11 

4.4.ReceptorLocations 11 

4.5.TerrainElevations 15 

4.6.EmissionSources 15 

4.7.OtherSources 15 

4.8.BuildingInfluences 17 

4.9.BackgroundConcentration 22 

4.10.CharacterizationofModeledArea 22 

5. PRESENTATION OF MODELING RESULTS 26 

6. ELECTRONIC FILES 28 

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LIST OF FIGURES

Figure3‐1.Locationof1‐hourSO2ModeledSources...........................................................................................................................7 Figure4‐1.ReceptorLocations–FullExtent........................................................................................................................................12 Figure4‐2.ReceptorLocations–Wygen,Wyodak,andNeilSimpsonArea............................................................................13 Figure4‐3.ReceptorLocations–DryForkArea.................................................................................................................................14 Figure4‐4.OtherSO2EmissionSources>100tonswithin150km...........................................................................................16 Figure4‐5.DryForkStationGeneralArrangement...........................................................................................................................18 Figure4‐6.WyodakGeneralArrangement............................................................................................................................................19 Figure4‐7.WyGenIandNeilSimpson2GeneralArrangement..................................................................................................20 Figure4‐8.WyGenIIandWyGenIIIGeneralArrangement...........................................................................................................21 Figure4‐9.SO2AmbientMonitoringStationsinWyoming............................................................................................................23 Figure4‐10.AerialImage‐Wygen,Wyodak,andNeilSimpsonArea........................................................................................24 Figure4‐11.AerialImage–DryForkArea............................................................................................................................................25 Figure5‐1.SO21‐HourConcentrationPlot............................................................................................................................................27 

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LIST OF TABLES

Table3‐1.UtilityUnitsincludedinModelingStudy.............................................................................................................................6 Table3‐2.UtilityUnitStackParameters....................................................................................................................................................6 Table4‐1.BlackHillsPowerMeteorologicalTowerMeasuredParameters..............................................................................9 Table4‐2.AERSURFACEInputParameters...........................................................................................................................................10 Table4‐3.HourlyCEMSDataCollectedforEachUnit......................................................................................................................15 Table4‐4.HeightofModeledDownwashStructures........................................................................................................................17 Table6‐1.SummaryofElectronicFiles...................................................................................................................................................28 Table6‐2.AERMODFileDescriptions......................................................................................................................................................28 Table6‐3.MetDataFileDescriptions......................................................................................................................................................29 

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1. EXECUTIVE SUMMARY

TheWyomingDepartmentofEnvironmentalQuality,AirQualityDivision(WDEQ)requestedtheassistanceofBasinElectricPowerCooperative(BasinElectric),BlackHillsCorporation(BlackHills)andPacifiCorpindeterminingcompliancewiththeone‐hoursulfurdioxide(SO2)NationalAmbientAirQualityStandard(NAAQS)intheGillette,Wyomingarea.ThemodelinganalysisincludedthemajorSO2sourcesinthevicinityofGillette,Wyomingspecificallythesixcoal‐firedelectricutilityunitslocatedattheDryForkStation,WyodakandtheNeilSimpson/WyGengenerationcomplex.Allsixcoal‐firedelectricalgeneratingunitshaveSO2airpollutioncontrolsystems.TheairqualityanalysiswasconductedwiththelatestAERMODVersion15181modelingsystem.Theanalysisincludedthecalendaryears2012,2013and2014.Meteorologicaldatafortheperiodwassuppliedbythe30‐metertoweroperatedbyBlackHillsPowerneartheWyodak/NeilSimpson/WyGencomplex.HourlySO2emissionsandstackflowparameterswereutilizedforthethreeyearperiodfromeachoftheutilityunit40CFRPart75ContinuousEmissionMonitoringSystems(CEMS).FormodelingpurposestheSO2backgroundconcentrationwasprovidedbyWDEQ.Theairqualitymodelinganalysisshowednoexceedancesofthe1‐hourSO2NAAQSfortheperiod2012through2014.Thehighestreceptorconcentrationincludingbackgroundwas93.72µg/m³comparedtothe196µg/m³NAAQS.Thus,themodelinghasdemonstratedcompliancewiththe1‐hourSO2NAAQS.

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2. INTRODUCTION

InJune2010,theU.S.EnvironmentalProtectionAgency(EPA)promulgatedthenew1‐hourSO2NAAQSatalevelof75partsperbillion(196µg/m3)basedonthe3‐yearaverageoftheannual99thpercentileof1‐hourdailymaximumconcentrations.WDEQrequestedtheassistanceofBlackHills,BasinElectricandPacifiCorpincharacterizingtheairqualityoftheGillette,Wyomingareathroughambientmonitoringand/orairqualitymodelingtechniques,asrequiredbytheDataRequirementsRuleforthe20101‐HourSulfurDioxidePrimaryNationalAmbientAirQualityStandard(SO2DRR)byevaluatingSO2emissionsfromtheGilletteareapowerplantslocatedeastandnorthofGillette,Wyoming.Modeling,andnotambientmonitoring,isthechosenmethodofsatisfyingtheSO2DRRfortheGillette,Wyomingarea.BlackHills,BasinElectricandPacifiCorpcontractedwithTrinityConsultantstoconductthemodelingstudy.ThemodelingfollowsapplicableEPAandWDEQguidanceandutilizestheEPAAERMODmodel.ThemodelingwasconductedpertherecommendedguidanceintheEPAdraftAugust2016document,SO2NAAQSDesignationsModelingTechnicalAssistanceDocument(TAD)(referredtohereinasthe2016SO2NAAQSModelingTAD).ThemodelingfollowsthemodelingprotocoldatedNovember3,2016,whichwasapprovedbytheEPAonNovember28,2016.1TheEPAiscurrentlygoingthroughamulti‐phasedesignationprocesswithrespecttothe1‐hourSO2NAAQS.Aninitialphaseofdesignationshasbeencompletedandresultedinsomeareasofthecountrybeingdesignatedasnonattainment.Therearethreemorephasesstilltocome.TwoofthenextthreephaseswerethesubjectoftheEPA’sproposedDataRequirementsRule,publishedinMay2014andfinalizedinAugust2015.InconsultationwithWDEQ,therearenosourcesinWyomingthatwouldtriggertheearlydesignationrequirement.Ifmodelingwasnotabletodemonstratecompliancewiththeone‐hourSO2NAAQS,thenambientSO2monitoringwouldneedtostartdatacollectionbyJanuary1,2017tocollectthenecessarythreeyearsofdata.Therefore,inlinewiththeEPA’sFinalSO2DRR,anSO2designationfortheGilletteareawillbebasedonthepredictionsofanairdispersionmodel.TheTADindicatesthatactualhourlyemissionratesfromSO2ContinuousEmissionMonitoringSystems(CEMS)shouldbeincludedinthemodeltocharacterizeemissions.Thefollowingsectionsofthisreportwilldiscusstheutilitysources,airmodelingmethodology,modelingresults,andelectronicfilesincludedonCD.

1 The November 3, 2016 modeling protocol was approved via email from Ms. Rebecca Matichuk, EPA Region 8 to Mr. Josh Nall, WDEQ on November 28, 2016.

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3. DESCRIPTION OF UTILITY SOURCES

ThemodelinganalysisincludedthesixGilletteWyomingareacoal‐firedelectricgeneratingutilityunitsshowninTable3‐1.AllsixunitshaveSO2airpollutioncontrolsystems.

Table3‐1.UtilityUnitsincludedinModelingStudy

Utility Plant/Unit Size(MW)

AirPollutionControlEquipment

BasinElectric DryForkStationUnit1 385 CirculatingDryScrubber&FabricFilterBlackHills WyGenI 80 DryScrubber&FabricFilterBlackHills WyGenII 95 DryScrubber&FabricFilterBlackHills WyGenIII 110 DryScrubber&FabricFilterBlackHills NeilSimpson2 90 CirculatingDryScrubber&ESPPacifiCorp WyodakUnit1 335 DryScrubber&FabricFilter

UnitstackparametersandlocationcoordinatesareshowninTable3‐2.

Table3‐2.UtilityUnitStackParameters

Utility Plant/Unit StackHeight(feet)

StackExitID(feet)

Elevation(feet)

Latitude Longitude

BasinElectric DryForkStationUnit1 500 19.50 4,250 44.3882 ‐105.4596BlackHills WyGenI 295 9.25 4,420 44.2861 ‐105.3843BlackHills WyGenII 397 10.25 4,420 44.2911 ‐105.3815BlackHills WyGenIII 397 10.25 4,420 44.2911 ‐105.3800BlackHills NeilSimpson2 295 9.25 4,420 44.2853 ‐105.3842PacifiCorp WyodakUnit1 400 20.00 4,430 44.2879 ‐105.3840

Figure3‐1showsthelocationofeachoftheunitsrelativetotheCityofGillette. 

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Figure3‐1.Locationof1‐hourSO2ModeledSources

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4. AIR QUALITY MODELING METHODOLOGY

4.1. MODEL SELECTION Trinityperformed1‐hourSO2modelingusingthelatestAERMODversionalongwithTrinity’sBREEZE™AERMODsoftware.TheEPAAERMODmodelisrecommendedforpredictingimpactsfromindustrialpointsourcesaswellasareaandvolumesources.TheBREEZE™AERMODgraphicaluserinterface(GUI)wasusedtosetuptheAERMODinputfile.Thefinalmodelrunswerebeperformedusingthecurrentversion(Version15181)oftheEPAAERMODexecutable.TheAERMODmodelcombinessimpleandcomplexterrainalgorithms,andincludesthePlumeRiseModelEnhancement(PRIME)algorithmstoaccountforbuildingdownwashandcavityzoneimpacts.Allregulatorydefaultoptionswereusedinthemodeling.ThepollutantIDwassettoSO2andtheoutputoptionswereconfiguredsuchthatthemodelwouldpredictanSO2designvaluebasedonthe3‐yearaverageofthe99thpercentileoftheannualdistributionofthedailymaximum1‐hourconcentrationsforcomparisonwiththe1‐hourSO2NAAQSof196μg/m³.ThecompleteAERMODmodelingsystemiscomprisedofthreeparts:theAERMETpreprocessor,theAERMAPpre‐processor,andtheAERMODmodel.TheAERMETpreprocessorcompilesthesurfaceandupper‐airmeteorologicaldataandformatsthedataforAERMODinput.TheAERMAPpreprocessorisusedtoobtainelevationandcontrollinghillheightsforAERMODinput.

4.2. METEOROLOGICAL DATA

4.2.1. Surface Data TrinityutilizedsurfacemeteorologicaldatacollectedattheBlackHillsPower30‐metermeteorologicaltowerasinputtotheAERMODmodel.The30‐metertoweroperatesneartheWyodak/WyGen/NeilSimpsoncomplexsouthofI‐90andHighway51.ThetowerlocationisshownonFigure5‐1.Thetowerisequippedwithsensorsatthe2‐meter,10‐meterand30‐meterlevelsasshowninTable4‐1.Trinityprocessedhourlydatafromthetowerfortheyears2012,2013and2014(consistentwiththethreeyearsofactualemissionsdatathatwereberelieduponforthesixutilityunits).

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Table4‐1.BlackHillsPowerMeteorologicalTowerMeasuredParameters

Parameter Units LevelWindSpeed m/s 10mWindSpeed m/s 30m

WindDirection degrees 10mWindDirection degrees 30mSigmaTheta calculated 10m,30mTemperature °C 2mTemperature °C 10mTemperature °C 30m

DeltaTemperature °C 10m–2mRelativeHumidity % 2mPrecipitation inches 2m

BarometricPressure inchesHg 2mSolarRadiation W/m2 2m

Asnecessary,processeddatafor2012,2013,and2014collectedattheNationalWeatherService(NWS)ASOSmeteorologicalstationlocatedattheGillette‐CampbellCountyAirportinGillette,Wyoming(KGCC)wasused.AdeterminationofwhetherthemeteorologicaldatafromtheGillette‐CampbellCountyAirportisappropriateforuseinthismodelinganalysisisconsideredbydeterminingwhetherthedatawererepresentativeofthelocationofthemodeledsources.Thecloseproximityoftheairportwithrespecttothesources(approximately5to10milesdistance),inadditiontothesimilarityintheclimatologyandtopography(theairportelevationisapproximately4,354feetandsourceelevationsrangefromapproximately4,250feetto4,430feet)supportthatthemeteorologicalconditionsattheairportarerepresentativeofthemeteorologicalconditionsatthesources.AERMOD‐readymeteorologicaldatawaspreparedusingthelatestversionoftheEPA’sAERMETmeteorologicalprocessingutility(Version15181).StandardEPAmeteorologicaldataprocessingguidancewasusedfortheanalysis.Asurfacedatawindrosefortheperiod2012through2014isshowninFigure4‐1.Seasonally,thepredominantwinddirectionisnorthwesterly,withtheexceptionofsummerwhenitisbimodalinthenorthwesterlyandsoutheasterlydirections.

4.2.2. Upper Air Data Inadditiontosurfacemeteorologicaldata,AERMETrequirestheuseofdatafromasunrise‐timeupperairsoundingtoestimatedaytimemixingheights.UpperairdatafromthenearestNWSupper‐airballoonstation,locatedinRapidCity,SouthDakota(KUNR),wasobtainedfromtheNationalOceanicandAtmosphericAdministration(NOAA)inFSLformat.

4.2.3. Land Use Analysis Parametersderivedfromanalysisoflandusedata(surfaceroughness,Bowenratio,andalbedo)arealsorequiredbyAERMET.InaccordancewithEPAguidance,thesevaluesweredeterminedusingthelatestversion

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oftheEPAAERSURFACEtool(version13016)2.TheAERSURFACEsettingsusedforprocessingaresummarizedinTable4‐2below.ThemetstationcoordinatesarefortheBlackHillsPower30‐metertower.NationalLandCoverDataset(NLCD)1992(CONUS)LandCoverdatathatwereusedinAERSURFACEprocessingwasobtainedfromtheMulti‐ResolutionLandUseConsortium(MRLC).EPAguidancedictatesthatonatleastanannualbasis,precipitationatasurfacesiteshouldbeclassifiedaswet,dry,oraverageincomparisontothe30‐yearclimatologicalrecordatthesite.ThisdeterminationisusedtosettheBowenratioestimatedbyAERSURFACE.Tomakethedetermination,seasonalprecipitationineachmodeledyear(2012‐2014),asmeasuredattheBlackHills30‐metertower,wascomparedtothehistoricalclimatologicalrecordfortheareasurroundingtheBlackHills30‐metertower.A30yearrecord(1981‐2010)isavailablefromtheNWSSheridanWyomingsite;however,datahasbeencollectedattheGilletteCampbellCountyAirportsitesinceJuly1998.BasedonthecloseproximitytotheBlackHills30‐metertowerlocation,Trinityrecommendedtheuseofthisdataset(July1998–May2015)forthecomparison.The30thand70thpercentilevaluesoftheseasonalprecipitationdistributionfromthedatasetwerecalculated.PerEPAguidance,eachmodeledyearwasclassifiedforAERSURFACEprocessingas“wet”ifitsseasonalprecipitationwashigherthanthe70thpercentilevalue,“dry”ifitsseasonalprecipitationwaslowerthanthe30thpercentilevalue,and“average”ifitwasbetweenthe30thand70thpercentilevalues.Snowrecordsfor2012‐2014werereviewedtodeterminewhethertheareahadcontinuouswintersnowcover.Ifallthreemonthsinagivenyearindicatedthatatleast50%ofdayshadasnowdepthofatleast1inch,thencontinuouswintersnowcoverwasassumed.

Table4‐2.AERSURFACEInputParameters

AERSURFACEParameter ValueMetStationLatitude 44.2778MetStationLongitude ‐105.3765

Datum NAD1983Radiusforsurfaceroughness(km) 1.0

VarybySector? YesNumberofSectors 12TemporalResolution Monthly

ContinuousWinterSnowCover? DeterminedbasedonobservedsnowrecordsStationLocatedatAirport? No

AridRegion? NoSurfaceMoistureClassification Determinedbasedon30thand70thpercentileof

climatenormals

2 U.S. Environmental Protection Agency. 2013. “AERSURFACE User’s Guide.” EPA-454/B-08-001, Revised 01/16/2013. Available Online: http://www.epa.gov/scram001/7thconf/aermod/aersurface_userguide.pdf

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4.2.4. AERMET Data Processing ThesurfaceandupperairdatawasprocessedwithAERMETalongwiththeoutputfromtheAERSURFACEprocessing.StandardAERMETprocessingoptionswereused.3,4AERMETprocessingincludedAERMINUTEfiles.AERMINUTEprocessingisdesignedtoreducethenumberofmissing/calmhours.ThepreparationofthemeteorologicaldatafilesusingAERMETisathreestageprocess.Thefirststageincludestheextractionofrawhourlysurfaceobservationsandupperairsoundings.TheextractedfileswerecheckedbyAERMETmoduleforconsistencyandanymissingorcalmhourswereidentified.Thesecondstagemergesthesurfaceandupperairdata.ThethirdstageestimatestheboundarylayerparametersrequiredbyAERMODusingtheAERSURFACEoutput.

4.3. COORDINATE SYSTEM Inallmodelinginputandoutputfiles,thelocationsofemissionsources,structures,andreceptorsarerepresentedinZone13oftheUniversalTransverseMercator(UTM)coordinatesystemusingdatumWorldGeodeticSystem(WGS)1984,whichiscomparabletotheNorthAmericanDatum1983(NAD83).ThelocationsforthesixcoalunitsincludedinthemodelingareshowninTable3‐2andinFigure3‐1.Thebaseelevationofthefacilitiesrangefrom4,250to4,430feetabovemeansealevel.

4.4. RECEPTOR LOCATIONS ThedispersionmodelingusedacombinationofaCartesiangridsystemcenteredonthesixfacilitiesanddiscretereceptorpointsalongthefacilityfencelines.Receptorswereplacedat25meterintervalsalongthefencelineforeachfacility,100meterintervalsouttoadistanceofatleast2.5kilometers(km)fromeachfacility,andat500meterintervalsouttoatleast10kmorfurtherfromeachfacilityifneeded.Perthe2016SO2NAAQSModelingTADandthe2015SO2AreaDesignationGuidance,thereceptorgridcoverstheentiremodelingdomain.Since,asindicatedinSection5,therewerenoelevatedlevelsofSO2(atleast90%ofthestandard)encounteredneartheedgeofthereceptorgrid,therewasnoneedtoexpandoradjustthereceptorgridtoconformtothe2016SO2NAAQSModelingTADandthe2015SO2AreaDesignationGuidance.ThemodeledreceptorgridsaredepictedinFigures4‐1,4‐2,and4‐3.PropertyfencelinesutilizedinthereceptorgridforthesixfacilitiesareshowninFigure4‐5,Figure4‐6,Figure4‐7andFigure4‐8.Asshowninthefigures,allfacilitiesexceptfortheDryForkfacilityareclusteredtogether(lessthan1kmapart),andtheDryForkfacilityisapproximately12kmfromtheotherfacilities.Assuch,thereceptorgridfortheareacontainingtheWyodak,Wygen,andNeilSimpsonsourceswascreatedusingthecombinedfencelineforthosereceptors,andthegridfortheDryForkfacilitywasgeneratedbasedonthefencelinefortheDryForkFacilityonly.SincethegridfortheDryForkfacilityoverlapsthegridfortheotherfacilities,redundantreceptorswereremoved.Cautionwastakenwhenremovingreceptorstomaintaintheminimumspacingdescribedabove.InaccordancewithSection4.2ofthe2016SO2NAAQSModelingTAD,on‐sitereceptorsforneighboringfacilitiesareincludedinthisanalysis.5Sincetheareawithineachfacility’sfencelineisconsideredambientairrelativetoemissionsgeneratedattheothermodeledfacility,noon‐sitereceptorswereremovedinthisanalysis.

3 Fox, Tyler, U.S. Environmental Protection Agency. 2013. “Use of ASOS Meteorological Data in AERMOD Dispersion Modeling.” Available Online: http://www.epa.gov/ttn/scram/guidance/clarification/20130308_Met_Data_Clarification.pdf

4 “User’s Guide for the AERMOD Meteorological Preprocessor (AERMET)”. EPA-454/B-03-002, November 2004). 5 SO2 NAAQS Designations Modeling Technical Assistance Document, EPA, August 2016.

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Figure4‐1.ReceptorLocations–FullExtent

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Figure4‐2.ReceptorLocations–Wygen,Wyodak,andNeilSimpsonArea

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Figure4‐3.ReceptorLocations–DryForkArea

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4.5. TERRAIN ELEVATIONS Theterrainelevationforeachreceptor,building,andemissionsourcewasdeterminedusingUSGS1/3arc‐secondNationalElevationData(NED).TheNED,obtainedfromtheUSGS,hasterrainelevationsat10‐meterintervals.UsingtheAERMODterrainprocessor,AERMAP(version11103),theterrainheightforeachreceptor,building,andemissionsourceincludedinthemodelwasdeterminedbyassigningtheinterpolatedheightfromthedigitalterrainelevationssurroundingeachsource.Inaddition,AERMAPwasusedtocomputethehillheightscalesforeachreceptor.AERMAPsearchesallNEDpointsfortheterrainheightandlocationthathasthegreatestinfluenceoneachreceptortodeterminethehillheightscaleforthatreceptor.AERMODthenusesthehillheightscaleinordertoselectthecorrectcriticaldividingstreamlineandconcentrationalgorithmforeachreceptor.PertheAERMAPUser’sGuide,carewastakentoensurethatthedomainoftheNEDfilewassufficientlylargeenoughtocoverallsignificantterrainnodes(i.e.,allterrainthatisatorabovea10%slopefromeachandeveryreceptor)toallowAERMAPtocorrectlycalculatethehillheightscaleforeachreceptor.6

4.6. EMISSION SOURCES Thesixcoal‐firedboilersshowninTable3‐1wereincludedintheanalysis.Hourly40CFRPart75CEMSdatafromtheEPAAirMarketProgramwerepreparedfortheyears2012,2013and2014foreachoftheunits.Thisthreeyearperiodisrepresentativeofnormaloperationsforthesixunits.TherawCEMSparameterscollectedforeachunitareshowninTable4‐3.Theflowparameterswerethencorrectedtoactualconditions(basedonstacktemperatureandpressure).ThemodelingwasconductedbasedontheactualhourlySO2emissions,stacktemperaturesandstackflowrates.

Table4‐3.HourlyCEMSDataCollectedforEachUnit

Parameter UnitsDate Hour

UnitOperation On/OffSO2MassFlow lb/hrStackFlow SCFH

StackTemperature °FBarometricPressure inchesHg

4.7. OTHER SOURCES WDEQ’semissioninventoryindicatesnootherlarge(>100ton)SO2emissionsourceswithin10kmoftheGillettearea,sonoadditionalemissionsourceswereincludedinthisanalysis.ThisisconsistentwiththeEPAMarch1,2011Memorandum7andthe2016SO2NAAQSModelingTADthattheselectionofregionalbackgroundsourcesshouldbelimitedtowithin10kilometersofthesourcelocation.ThecharacterizationmethodologyofevaluatingonlytheSO2emissionsfromthesixutilitycoalunitsisconsistentwithEPAguidanceandwas 6 U.S. EPA User’s Guide for the AERMOD Terrain Preprocessor (AERMAP), October 2004. 7 U.S. EPA, 2011, Additional Clarification Regarding the Application of Appendix W Modeling Guidance for the 1-hr NO2 National Ambient Air Quality Standard. Tyler Fox Memorandum dated March 1, 2011, RTP, NC 27711. http://www3.epa.gov/ttn/scram/guidance/clarification/Additional_Clarifications_AppendixW_Hourly-NO2-NAAQS_FINAL_03-01-2011.pdf

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confirmedwithWDEQ.Figure4‐4displaysothersourceswithemissionsofSO2greaterthan100tpywithin150kmofthemodeledsources.Allaregreaterthan50kmfromthemodeledsources.

Figure4‐4.OtherSO2EmissionSources>100tonswithin150km

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4.8. BUILDING INFLUENCES TheEPA’sBuildingProfileInputProgram(BPIP)withPlumeRiseModelEnhancement(PRIME)(version04274)wasusedtoaccountforbuilding/structuredownwashinfluencesateachofthefacilitiesinthemodel.Thepurposeofabuildingdownwashanalysisistodetermineiftheplumedischargedfromastackwillbecomecaughtintheturbulentwakeofabuildingorotherstructure,resultingindownwashoftheplume.Thedownwashofaplumecanresultinelevatedground‐levelconcentrations.AtTrinity’srequest,WDEQprovidedhistoricalBPIPfilesfromtheBasinElectricDryForkStationandtheBlackHillsWyGen3permitapplications.TheWyGen3BPIPfilesincludedthebuildings/structuresforbothWyGen2and3andsomeofthestructuresrelatedtotheWyodak,WyGen1andNeilSimpsoncomplexes.TrinityworkedwiththeutilitiestorevisetheBPIPfilesasnecessarytoincludeothermajorbuildings/structuresattheWyodak,WyGen1andNeilSimpsoncomplexes.Themodeledfacilities,includingthemajorbuildingsandstructuresforeach,areshowninFigures4‐5,4‐5,4‐6,and4‐8.ThemodeledheightsofthesestructuresareprovidedinTable4‐4.Nonon‐defaultconfigurationoptionswereusedintheBPIPanalysis.

Table4‐4.HeightofModeledDownwashStructures

IDHeight(ft) ID

Height(ft)

ID

Height(ft)

WYO001 70 WYO003 143 WYG001 104.00WYO002 101 WYO004 143 WYG002 179.76WYO006 90 WYO005 143 WYG003 24.92WYO007 90 WYG1_02 116 WYG004 108.76WYO008 136 DF001 16.73 WYG005 80.75WYO009 228 DF002 38.71 WYG006 84WYO010 189 DF003 114.50 WYG007 108.67WYO011 121 DF004 114.50 WYG008 84WYO012 86 DF005 270.34 WYG009 80.74WYO013 33 DF006 182.41 WYG010 108.75WYG1_01 70 DF007 166.67 WYG011 179.75NS2_01 70 DF008 85.30 WYG012 108.67NS2_02 81 DF009 29.86 WYG013 24.93WYG1_03 81 DF010 29.86 WYG014 104.00NS2_03 109 DF011 13.12 WYG022 116.70WYG1_04 109 DF012 114.83 WYG023 94.90NS2_04 180 DF013 18.04 WYG024 123.50WYG1_05 180 DF014 127.95 WYG025 94.90NS2_05 129 DF015 147.64 WYG026 116.70WYG1_06 129 DF016 42.65 WYG027 123.50NS2_06 150 DF017 206.69 WYG028 212.00WYG1_07 78 DF018 206.69 NS2_07 129 DF019 114.00

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Figure4‐5.DryForkStationGeneralArrangement

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Figure4‐6.WyodakGeneralArrangement

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Figure4‐7.WyGenIandNeilSimpson2GeneralArrangement

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Figure4‐8.WyGenIIandWyGenIIIGeneralArrangement

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4.9. BACKGROUND CONCENTRATION WDEQprovideda6ppmdesignvalue1‐hourSO2backgroundconcentrationasrepresentativeofthebackgroundconcentrationinthevicinityofnortheastWyomingfacilities.ThedesignvalueisfromtheNCOREsitenearCheyenneandisthe99thpercentilethreeyearaverage(2012‐2014).Thebackgroundconcentrationwasaddedtothemodelingresults,andtheresultingconcentrationwascomparedtothe1‐hourSO2NAAQS.AnareamapoftheWyomingSO2AmbientMonitoringSitesisshowninFigure4‐9.

4.10. CHARACTERIZATION OF MODELED AREA Thesourcesarelocatedapproximately5‐10kmfromthecityofGillette,Wyoming.Theareaisasemi‐aridclimatewithhotanddrysummersandcoldwinters.Theareareceivessomesnowduringthewinter,butdoesnotconsistentlyexperiencecontinuoussnowcover.ThesourcesarelocatedonrelativelyflatterraininthePowderRiverBasinbetweentheBigHornMountainsandtheBlackHills.Theareaisclassifiedasattainmentorunclassifiedforallcriteriapollutants.Inordertocategorizetheareaasruralorurbanformodelingpurposes,NationalLandCoverDataset(NLCD)1992(CONUS)LandCoverdatawasobtainedfromtheMulti‐ResolutionLandUseConsortium(MRLC).Datawithina3kmradiusofeachsourcewasanalyzedusingtheEPAAERSURFACEtool(version13016).8PerSection6.3ofthe2016SO2NAAQSModelingTAD,asourceisconsideredurbanifthelandusetypesI1(heavyindustrial),I2(light‐moderateindustrial),C1(commercial),R2(commonresidential),andR3(compactresidential)are50percentormoreoftheareawithinthe3kmradiuscircle.Otherwise,thesourceisconsideredaruralsource.9BasedontheanalysisusingNLCD1992LandCoverdata,lessthan5%ofthelandwithin3kmofeachsourcefallsintothelandusetypecategorieslistedabove.Althoughsomelanddevelopmenthasoccurredintheareasincethe1992datawaspublished,itisclearfromtheaerialimagesprovidedinFigures4‐10and4‐11thatsignificantlylessthan50%ofthelandwithin3kmofthesourcescanbeconsideredurban.Assuch,thesourceswereconsideredruralforthemodelinganalysis.

8 U.S. Environmental Protection Agency. 2013. “AERSURFACE User’s Guide.” EPA-454/B-08-001, Revised 01/16/2013. Available Online: http://www.epa.gov/scram001/7thconf/aermod/aersurface_userguide.pdf

9 SO2 NAAQS Designations Modeling Technical Assistance Document, EPA, August 2016.

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Figure4‐9.SO2AmbientMonitoringStationsinWyoming

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Figure4‐10.AerialImage‐Wygen,Wyodak,andNeilSimpsonArea

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Figure4‐11.AerialImage–DryForkArea

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5. PRESENTATION OF MODELING RESULTS

Themaximummodeledground‐levelconcentrationsobtainedusingtheapproachdescribedinSection3andcomparisontothe1‐hourSO2standardarepresentedinthissection.Therearenoexceedancesofthe1‐hourSO2NAAQSfortheperiod2012through2014.Pertheformofthe1‐hourSO2NAAQS,resultsarereportedasthe3‐yearaverageofthe99thpercentileoftheannualdistributionofdailymaximum1‐hourconcentrations(i.e.4thhighvalue).Thehighest4thhighestdailymaximum1‐hourconcentrationwas93.72µg/m³comparedtothe1‐hourSO2NAAQSvalueof196µg/m³.Thisvalueincludesthe6ppb(15.66μg/m³)backgroundconcentrationprovidedbytheWDEQ.Figure5‐1providesagraphicalrepresentationoftheSO2concentrationlevelsinthemodelingdomain.ThesixcoalunitsandtheBlackHillsPowermeteorologicaltower/SO2monitoringsitearealsodisplayedonthefigureasareference.Basedonthe2012–2014modelingresults(lessthan50%ofthestandard),furtheranalysisrelatedtopotentialambientmonitorlocationswasnotconductedsincecompliancewiththe1‐hourSO2NAAQSisdemonstratedviamodeling.

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Figure5‐1.SO21‐HourConcentrationPlot

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6. ELECTRONIC FILES

AlloftheairqualitydispersionmodelinganalysiselectronicdatafilesusedtogeneratetheresultspresentedinthisreportareprovidedintheattachedCDs.Theseelectronicdatafilesincludethefollowing:

> AllAERMODinput,output,andplotdatafiles> Alldownwashinputandoutputfiles> AERMETinputandoutputfiles> AERSURFACEfiles> Theboundaryfilespecifyingcoordinatesofthemodeledfencelines> CEMShourlydatafilesforallsixutilityunits> ElectroniccopyoftheAirQualityAnalysis

ThefollowingtablessummarizetheelectronicfilesincludedintheattachedCDs.

Table6‐1.SummaryofElectronicFiles

Folder/FileName Folder/FileDescriptionAERMOD AERMODInputandOutputFiles(seeTable6‐2)MetData Filesassociatedwithmetdataprocessing(seeTable6‐3)Fenceline FencelineboundaryfilesBPIP BPIP(Downwash)input,output,andsummaryfilesTerrainData NEDDatausedforAERMAPelevationsSO2ModelingReport12‐2016.pdf ElectroniccopyoftheairqualityanalysisWYStackandCEMSData‐CleanAirMarketDataEdits(2016‐1206).xlsx Emissionsandstackparameterdata

Table6‐2.AERMODFileDescriptions

FileName AssociatedFiles

WYElectricUtilitySO2Mod_1214H04.ami InputFile

HourlySrcData.src HourlyEmissionRateFile

WYElectricUtilitySO2Mod_1214H04.aml OutputFile*.plt Plotfiles

WYSO2_1214H04_ALL.mdc DailyMax.ContributionFile

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Table6‐3.MetDataFileDescriptions

FolderName Description

AERMINUTE> AERMINUTEInputandOutputFiles> 1‐MinuteDatausedinAERMINUTEdownloadedfrom

ftp://ftp.ncdc.noaa.gov/pub/data/asos-onemin/

AERSURFACE

> RawPrecipitationDataforGilletteAirport> RawSnowCoverDataforGILLETTE4SEStation> SummaryofAERSURFACEdatausedinAERMET

(appropriatemoisturedetermination/snowcoverforeachseason)

> Landusedatadownloadedfromhttp://www.mrlc.gov/viewerjs/

> AERSURFACEinputandoutputfilesforBlackHillsMetStationandNWSStation A/D/W=Average/Dry/WetSurfaceMoisture SC/NSC=SnowCover/NoSnowCoverduringwinter

months

FSLFormattedUAData

> Twicedailyupperairdatadownloadedfromhttp://www.esrl.noaa.gov/raobs/

ISHDData > IntegratedSurfaceHourlyData(ISHD)datadownloadedfromftp://ftp.ncdc.noaa.gov/pub/data/noaa/

AERMET > AERMETinputandoutputfilesfor2012‐2014> DataforBlackHillsMetTower