## Sheldon Station Hallam, Nebraska Evaluation of Compliance with the 1-hour NAAQS for S0 2 

## September 17, 2015

Co11ducted by: 

### Steve11 Klajka, P.E., BCEE

Wi11gra E11gilleeri11g, S.C. Madiso11 , Wisco11si11Evaluation of Compliance with the 1-hour NAAQS for S0 1 Sep temb e r 1 7 , 2015 Pagel 1. 

### Introduction Wingra Eng ineering , S.C. was hired by Sierra Club to conduct an air modeling impact analysis to help the U.S. Environmental Protectio n Agency (USEPA) , state and loca l air agencies identify facilities that are likely causing exceedence s of the 1-hour sulfur dioxide (S02) National Ambient A ir Quality Standard (NAAQS). This document describes the re sul ts and procedures for an evaluation co ndu cted for the She ldon Station located in H allam, Nebraska. The dispersion modeling ana lys i s predicted ambient air concentrations for comparison with the 1- hour S02 NAAQS. The modeling was performed u sin g the most recent version of AERM OD , AERMET, and AERMINUTE, w ith data provided to S ierra Club by regulatory air agenc ie s or obtained through other publicly-available sources as documented below. The analysis was co nducted in adherence to a ll availab le USEPA gu idance for evaluating source impacts on attainment of the 1-hour S02 NAAQS via aerial di spersion modeling, including the AERMO D Implementation Guide ; USEPA's Applicab ili ty of Appendix W Mo deling Guidance for the 1-hour S0 2 National Ambient A ir Quality Standard, August 23, 2010; modeling guidance promulgated by USEPA in Ap pendix W to 40 CFR Part 51; USEPA's March 2 011 Modeling Guid ance for S02 NAAQS Designations; 1 and USEPA's D ecember 20 1 3 S 0 2 NAAQS D esignations Technical Assistance Document. 2 

### 2. Compliance with the 1-hour 802 

### NAAQS 2.1 1-hour SOz NAAQS The 1-hour S 0 2 NAAQS takes the form o f a three-year average of the 99 1h-percentile of the annual distribution of daily maximum 1-hour concentratio ns, which cannot exceed 75 parts per billion (ppb). 3 Comp liance with this standard was verified using USEPA's AERMOD air dispersion model, which produces air concentrations in units

**of~g/m**

**.** The 1-hour S0 2 NAAQS of75 ppb equals 196.2

**g/**

**,** and this i s the value used for determining whether modeled impacts exceed the NAA QS. 4 The 99 th -percentile of the annual distribution of daily maximum 1-hour concentrations corresponds to the fourth-highest value at each receptor for a given year.

1 http:/ / www.epa.gov/scramOO 1 /so2
_ modcling_guidance
. htm 2 http: //www.epa.gov/o aqpsOO 1 / s ulfurdioxide!pdfs/S02ModelingT AD.pdf 3 USEPA, Applicability of Appendi
. x W Modeling Guidance for the ! -hour S0 2 National Ambient Air Quality Standard , August 23, 2010. 4 The ppb to J.1g/m3 conversion is found in the source code to AERMOD v . 14134, s ubroutine Modules
. The conversion calculation i s 75 / 0.3823 = 196 .2 J.lg/m 3

.Evaluation of Compliance with the 1-hour NAAQS for S01 September 17 , 2015 Page3 

### 2.2 Modeling Results Modeling results for Sheldon Station are summarized in Table 1. It was determined that based on either current allowab le emissions or measured actual emissions, the Sheldon Station is estimated to create downwind S02 concentrations which exceed the 1-hour NAAQS. More specifically, the modeling results presented in Table 1, show the plant's a llowable and actual emissions cause exceedances ofthe NAAQS. "Allowable " is the peak emission rate from each unit as approved by the current air quality operation permit for the facility. "Actual" are the measured emissions for each hour between January 1, 2012 and December 31, 2014 as taken from US EPA Air Markets Program Data. 5 Air quality impacts in Nebraska are based on a background concentrati on of 88.9 Jlg/m3

. This i s the 2011-13 design value for Douglas County, Nebraska

- the lowest measured background concentration in the state. This is the most recently avai l able design value. See Section 5 for further discussion of the background concentrations used for this analysis
. Table 1
- S02 Modeling Results for Sheldon Station Modeling Analysis Averaging 99th Percentile !-hour Daily Maximum (f.1g/m 3 ) Complies with Emission Rates Period Impact Background Total NAAQS NAAQS?

Allowable 1-hour 841.2 88.9 930.1 196.2 No

Actual !-hour 119.7 88.9 208.6 196.2 No The emissions used for the modeling analysis are summarized in Table 2. Table 2 -Modeled S02Emissions from Sheldon Station 6 Stack Unit Allowab le Emissions

ID ID 2-hour Average (lbs/hr) SOl Unit 1 3,435.0 S02 Unit2 3,435.0 Stack Total All Units 6 , 870.0

5 http: //ampd.epa.gov/ ampd/ 6 Allowable emissions are based on the 2-hour average 2.5 lbs/ mmbtu limitation in Title V Air Contaminant Source Operating Permit issued by the City of Lincoln, Nebraska, December 6, 2010 .Evaluation of Compliance w ith the 1
- hour NAAQS for S0 1 Sep tember 17, 2 01 5 Page4 Based on the modeling results, Table 3 provides the necessary emission reductions from current a llow able rates necessary to achieve compliance with the 1
- hour NAAQS. This assumes a one-hour averaging period for the emission rate , and that the emission rate is binding at all times. However, it i s extremely likely that even this limit is too high to protect the NAAQS, given the conservative aspects ofthis modeling protocol. For example, startup or shutdown periods were not evaluated. During these periods, decreased gas ve locitie s and temperatures may lead to greater ambient impacts at ground level. Further, the hypothetical emission limitation in Table 3 w ould allow Sheldon Station to consume the entire NAAQS, leaving little to no room for any other so urc e of S02 in the area. No consideration has been given to other sources and no margin of safety has been included in the hypothetical emission limitation. 

### Table 3
- Required Emission Reductions from Sheldon Stationfor Compliance with the 1-hour NAA QS for S02 Acceptab le Impact Required R equired Required (NAAQS
- Background) Total Facility Total Facility 99th Percentile Reduction Based on Total Facility 1-hour Average Emission Rate 1-hour Daill Max Allowable Emissions (lbslhr) Emission Rate (J.tg/m) (%) (lbs/mmbtu)

107 . 3 87% 876.3 0.32 Predicted exceedences of the 1-hour NAAQS for S0 2 based on allowable emissions extend throughout the region to a maximum distance of 42 kilometers. Figure 1 s hows the extent ofNAAQS violations based on a llowabl e emissions. Figure 2 s hows the extent ofNAAQS vio lations based on actual hourly emissions. 

### 2.3 Conservative Modeling Assumptions A dispersion modeling analysis requires the selection of numerous parameters which affect the predicted concentration s . For the enclosed analysis , several parameters were selected which underpredict facility impacts. Ass umptions used in this modeling analysis which like ly under-estimate concentrations include the following:
• A llowable emissions are based on a limitation with an averaging period whi ch is greater than the 1-hour average used for the 

### SO z air quality standard. Emi ss ion s and impacts during any 1-hour p eriod may be higher than assumed for the modeling analysis.
• No consideration of facility operation at less than 100 % load. Stack parameters such as exit flow rate and temperature are typically lower at les s than full load , reducing pollutant di s persion and increasing predicted air quality impacts.Evaluation of Compliance with the 1-hour NAAQSfor S0 1 September 17, 20 1 5 Page5

• No consideration of building or structure downwash. These downwash effects typically increase predicted concentrations near the facility.
• No consideration of off-site sources. These other sources of S02 will increa se the predicted impacts.Evaluation of Compliance with the !-hour NAAQSfor S02 September 17, 2015 Page6

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0 8**~** ::! 8 s .. ¢ ;J 

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.... J'**~>0000** 655000 oe.oooo €>€St»l e:oooo ol$000 ESOOOO oe;ooo sg.:;ooo 695000 1oooco 1osooo 1 1 0000 1 1 :.000 noooo 12;c.oo r;ocoo**TI.IE (ml**  1-hour average S02 concentrations (ug per cubic meter ) -All colored areas exceed the NAAQS. 196 400 600 800 Figure 1 -Regional View of Impacts Based on Allowable EmissionsEvaluation of Compliance with the 1-hour NAAQS for S01 September 17 , 2015 Page 7

.. Sheldon Station t

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f81 > o() "];IJ('C)

**~0**

**vn.•E~o;~(m(** 1-hour average S02 concentrations (ug per cubic meter) - All colored areas exceed the NAAQS
. 196.2 205.0 

### Fi g ure 2- Regional View of Impacts Based on Actual Emissions for the 2012-14 PeriodE v aluation of Compliance with th e 1
-hour NAAQS[or S0 1 Se

pt

ember 17, 2015 Pa g e 8 

### 3. Modeling Methodo lo gy 3.1 Air Dispersion Model The mode ling analysis u sed USEPA's AERMOD program , v. 14134. AERMOD, as available from the Support Center for Regulatory Atmospheric Modeling (SCRAM) website, was used in conjunction with a third-party modeling software program , AERMOD View, so ld by Lakes Environmental Software. 3.2 

### Control Options The AERMOD mode l was run with the following control options: • 1-hour average air concentrations • Regulatory defaults • Flagpole receptors To reflect a representative inh a lation level , a flagpol e height of 1.5 meters was used for a ll mode led receptors . This parameter was adde d to the receptor file when running AERMAP, as described in Section 4.4. An evaluation was conducted to determine 

### ifthe modeled facility was located in a rural or urban setti ng using USEPA's methodology outlined in Section 7.2.3 ofthe Guideline on Air Quality Models. 7 For urban sources, the URBANOPT opt ion is u sed in conjunction with the urban p o pulation from an appropriate nearby city and a default surface roughness of 1 .0 meter. Metho ds describ ed in Sect io n 4.1 were u sed t o determine whether rural or urb an dispersion coefficients were appropriate for the modeling analysis. 3 . 3 

### Output Options The AERMOD analys is was based on three years of recent meteorolo gic al data. The modeling analyses used one run with three years of sequential meteoro lo gical data from 20 1 2-2014. Consistent with USEPA's Mode ling Guidance for S0 2 NAAQS Designations, AERMOD provided a table of fourt h-hi gh 1-hour S02 impacts concentrations co ns istent with the form of the 1
-hour S02 NAAQS. 8 Please refer to Table 1 fo r the modeling results.

7 US EPA, Revision to the Guideline on Air Quality Models: Adoption of a Preferred General Purpose (Flat and Complex Terrain) D ispers ion Model and Other Revisions, Appendix. W to 40 CFR Part 51, November 9, 2005
. 8 USE PA , Area Designations for the 2010 Revi se d Primary Su lfur Dioxide National Ambient Air Quality Standards, Attachment 3, March 24, 201 1 , pp. 24
- 26.E valuation of Compliance w ith th e 1
-hour NAAQS for S02 Sept emb e r 17, 2015 Page 9 

### 4. Model Inputs 4.1 Geog raphical Inputs The "ground floor" of all a ir dispersion modeling analyses is establishing a coordin ate system for identifying the geographica l location of emission sources and receptors. These geographical locations are used to determine local characteristics (such as land u se and elevation), and also to ascertain source to re ceptor distances and relation s hip s. The Universal Tran sverse Mercator (UTM) NAD83 coordinate system was used for identifying the easting (x) and northing (y) coordinates ofthe modeled sources and receptors. Stack locations were obtained from facility pennits and prior modeling files provided by the state regulatory agency. The s tack locations were then verified using aerial photographs. The facility was evaluated to determine if it should be modeled using the rural or urban dispersion coefficient option in AERMOD. A Geographic Information System (GIS) was used to determine whet her rural or urban dispersion coefficients app ly to a s ite. Land use within a three-kilometer radius circle surrounding the facility was considered. USEPA guidance states that urban dispersion coefficients are us ed if more than 50% of the area within 3 kilometers has urban land uses. Otherwise, rural dispersion coefficients are appropriate. 9 USEPA's AERSURFACE v. 13016 was used to develop the meteoro logical data for the modeling analysis . This model was a lso used to evaluate surroundin g land use within 3 kilometers. Based on the output from th e AERSURFACE, approximately 1.2% of surrounding land use around the modeled faci lity was of urban land use types including Type 21 -Low Inten s ity Residential , Type 22
- High Intensity Residential and Type 23
- Commerc ial 

### I Industrial 

### I Transportation. This is less than the 50 % value considered appropriate for the use of urban dispersion coefficients. Based on the AERSURFACE analysis, it was concluded that the rural option would be used for the mode ling summarized in this report. Please refer to Section 4.5.3 for a discussion of the AERSURFACE ana lysis.

9 USEPA, Revision to the Guideline on Air Quality Models: Adoption of a Preferred General Purpose (Flat and Comp lex T errain) Dispersion Mode l and Other Revisions, Appendix W to 40 CFR Part 51, November 9, 2005, Sectio n 7.2.3.Eva luation of Complian ce with th e 1
- hour NAAQSfor S02 Sep temb e r 17, 2015 Page 10 4.2 Emission Rates and Source Parameters The modeling analyses only considered S0 2 emissions from the facility. Off-site sources were not considered. Concentrations were predicted for the scenarios shown in Tables 1 and 2: 1) a llowable emissions based on the current permit issued by the regulatory agency, and 2) actual hourly emissions measured each hour between January 1, 2012 and December 31, 2014 as taken from 

### USEPAAir Markets Program Data. 10 Stack parameters and emissions used for the modeling analysis are summarized in Table 4. 

### Table 4- Facility Stack Parameters and Emissions 11 Stack SOl S02 De scription Boiler 1 Boiler 2 X Coord. [m] 687602 687589 Y Coord. [m] 4492175 4492215 Base Elevation [m] 440.5 440.46 Release Height [m] 53.04 53.04 Gas Exit Temperature [° K] 424.261 424.261

Gas Exit Velocity [m/s] 22.25 22.25

Inside Diameter [ m] 3.536 3.536 A ll owable E mi ssion Rate [g/s] 432.8 476.9

Actual Emission Rate [g/s] 

## - - The above stack parameters an d emissions were obtained from regulatory agency documents and databases identified in Section 2.2. The analysis was conducted based on 100% operating load using maximum exhaust flow rates and temperatures. Operation at less than full capacity loads was not c on s idered. This assumption tends to under-predict impacts since stack parameters such as exit flow rate and temperature are typically lower at less than fu ll load , reducing pollutant dispersion and increasing predicted air quality impacts. Stack location , height and diameter were verified using aerial photographs, and flue gas flow rate and temperature were verified u sing co mbu st ion calcu lations
.

10 http: //ampd.epa.gov/ampd/ 1 1 Email, G. Bergstrom
- Lincoln-Lancaster Co. Health Dept., Request for Title V Permit and Stack Paramaters for She ldon Station

- Hallam, Nebraska, January 18 ,2 01 2 .Evaluation of Compliance with the 1-hour NAAQSfor S0 2 Se

ptember 17 , 2015 Page 11 

### 4.3 Building Dimensions No building dimensions or prior downwash evaluations were availab le
. Therefore this modeling analysis did not address the effects of downwash and this may under-predict impacts. 

### 4.4 Receptors For She ldon Station, three receptor grids were employed: 1. A 1 00-meter Cartesian receptor grid centered on She ldon Station and extending out 5 kilometers. 2. A 500
- meter Cartesian receptor grid centered on Sheldon Station and extending out 10 kilometers. 3. A 1,000-meter Cartesian receptor grid centered on Sheldon Stat ion and extending out 50 kilometers. 50 kilometers is the maximum distance accepted by USEPA for the use of the AERMOD dispersion model. 12 A flagpole height of 1 .5 meters was used for all these receptors. Elevations from stacks and receptors were obtained from National E levation Dataset (NED) GeoTiff data. GeoTiff is a binary file that includes data descriptors and gee-referencing information necessary for extracting terrain elevations. These elevations were extracted from 1 arc-second (30 meter) resolution NED files. The USEPA software program AERMAP v. 11103 i s u sed for these tasks. 

### 4.5 Meteorological Data To improve the accuracy of the modeling analysis, re cent meteorological data for the 2012
- 2014 period were prepared using the USEPA's program AERMET which creates the model-ready surface and profile data fi les required by AERMOD. Requ ire d data inputs to AERMET included surface meteorological measurements, tw ice-daily soundings ofupper air measurements, and the micrometeorological parameters surface roughness , albedo, and Bowen ratio. One -minqte ASOS data were available so USEPA methods were used to reduce calm and missing hours. 13 The USEPA software program AERMINUTE v. 14237 is used for these tasks. This section discusses how the meteorological data was prepared for use in the 1-hour 

### SOz NAAQS

12 USEPA , Revision to the Guideline on Air Quality Models: Adoption of a Preferred General Purpose (Flat and C omplex Terrain) Di spersion Model and Other Revisions , Appendix W to 40 CFR Part 51, Section A. I.( I) , November 9 , 2005. 1 3 USEPA , Area Designations for the 2010 Revised Primary Su lfur Dioxide National Ambient Air Quality Standards , Attachme nt 3 , March 24 , 2011 , p . 19.Evaluation of Compliance with the I -hour NAAQS for S0 1 S e

ptemb e r I 7 , 2015 Pa g e 12 modeling analyses
. The USEPA software program AERMET v . 14134 is used for these tasks
. 

### 4.5.1 Surface Meteorology Surface meteorology was obtained for Lincoln Municipal Airport located near the Sheldon Stat ion. Integrated Surface Hourly (ISH) data for the 2012-2014 period were obtained from the National Climatic Data Center (NCDC). The ISH surface data was processed through AERMET Stage 1 , which performs data extraction and quality control checks. 

### 4.5 .2 Upper Air Data Upper-air data are collected by a "weather balloon" that is released twice per day at selected locations. As the balloon is released, it rises through the atmosphere, and radios the data back to the surface. The measuring and transmitting device i s known as either a radiosonde, or rawindsonde
. Data co ll ected and radioed back include: air pressure, height, temperature, dew point, wind speed , and wind direction. The upp er air data were processed through AERMET Stage 1 , which performs data extraction and quality control checks. For Sheldon Station, the concurrent 2012 -2014 upper air data from twice-daily radio sonde measurements obtained at the most representative location were used. This location was the Omaha, Nebraska measurement station. These data are in Forecast Systems Laboratory (FSL) format and were downloaded in ASCII text format from NOAA's FSL website. 14 All reporting level s were downloaded and processed w ith AERMET. 

### 4.5.3 AERSURF ACE AERSURFACE is a program that extracts surface roughness, a lb edo, and daytime Bowen ratio for an area surrounding a given location. AERSURFACE uses land use and land cover (LULC) data in the U.S. Geological Survey's 1 992 National Land Cover Dataset to extract the necessary micrometeorological data. LULC data was used for processing meteorological data sets u sed as input to AERMOD . AERSURFACE v. 13016 was used to develop surface roughness, albedo, and daytime Bowen ratio values in a region surrounding the meteorological data co llection site. AERSURF ACE was used to develop surface roughness in a one kilometer radius surrounding the data co ll ection sit e. Bowen ratio and albedo was developed for a 10 kilometer by 10 kilometer area centered on the meteorological data collection site. These micrometeorological data were processed for seasonal periods using 30-degree sectors . Seasonal moisture conditions were considered average w ith winter

14 Av ailab le at : http: //esrl.noaa
.gov/raob slEvaluation ofCompliance with th e 1
- hour N AAQSforS0 1 Se

pt

embe r 1 7, 2015 Pa ge 13 months having no continuo us snow cover. 

### 4.5.4 Data Revi ew Missing meteorological data were not filled as the data file met USEPA's 90% data completeness requirement. 1 5 The AERMOD output file show s there were 0.33% missing data. To confirm the representativeness of the airport meteorological data, the surface characteristics of the airport data collection s ite and the modeled source location were compared. Since the Lincoln Municipal Airport is located close to S he ldon Station, this meteorological data set was consi d ered appropriate for this modeling analys i s. 16 Additionally, this weather station provided high quality s urface measurements for the most recent 3-year time , and had simi lar land use, surface characteristics , terrain features and climate. 

### 5. Background S02 

### Concentrations Background concentrations were determined consistent with USEPA's Model ing Guidance for S0 2 NAAQS Designations. 17 To preserve the form of the ! -hour S0 2 standard, based on the 99th p ercentile of the annual distribution of daily maximum 1-hour concentrations averaged across the numb er o f years modeled, the background fourth-highe st dai ly maximum 1-hour 

## so 2 concentration wa s ad d ed t o the modeled fourth-highest daily maximum 1 -hour S02 concentration. 1 8 Background concentrations were based on the 201 1 -13 design value measured by the ambient monitors located in Nebra s ka. 19 

### 6. Rep orti ng A ll files from the programs used for this modeling analysis are availab le to regulatory agencies. Thes e include analyses prepared with AERSURFACE , AERMET, AERMAP , and AERMOD.

15 USEPA , Meteorological Monitorin g Guidance for Regulatory Modeling Applications , EPA-454/R-99-05 , February 200 0 , S ection 5.3.2 , pp. 5-4 to 5
- 5 . 16 USE PA, AER.i\ilOD Impl e mentati o n Guide , March 19 ,2 00 9, pp . 3 -4 . 17 USE PA , S 02 NAAQS De s ign ati o ns Modelin g Techn i cal A ss is tan ce Document, Dec. 2013 , s ection 8.1, pp 2 7-28; USE PA , Area Designation s for the 2 010 Re vised Primary S ulfur Dioxide National Ambient Air Quality Standard s, Attac hment 3 , March 24 ,2 011 , pp. 2 0-23. 1 8 US EPA , Applicability of App endix W Modeling Guidance for the !-hour S0 2 National Ambient Air Quality Standard , Au gu s t 23 , 2010, p. 3. 1 9 http: //www.epa.gov/airtrends/values.btml