## Gibson Generating Station

Owensville, Indiana

Evaluation of Compliance wi th the 1-hour NAAQS for SO 2


## September 3, 2015

Conducted by:

Steven Klafka, P.E., BCEE

Wingra Engineering, S.C.

Madison, WisconsinEvaluation of Compliance with the 1-hour NAAQS for SO 2 September 3, 2015 Page 2

1. Introduction

Wingra Engineering, S.C. was hi red by Sierra Club to conduct an air modeling impact analysis to help the U.S. Environmental Prot ection Agency (USEPA), state an d local air agencies identify
facilities that are likely causing exceedances of the 1-hour su lfur dioxide (SO 2 ) national ambient air quality standard (NAAQS). This doc ument describes the results and procedures for an evaluation
conducted for the Gibson Generat ing Station located in Owensvil le, Indiana.

To ensure the modeling analysis reflected the cumulative concen tration of SO 2 emissions, it included emissions from the Petersburg Gen erating Station in Petersburg, Indiana, which is an additional
source of SO 2 emissions located within 50 kilometers of the Gibson Generatin g Station.

The dispersion modeling analysis predicted ambient air concentr ations for comparison with the 1-
hour SO 2 NAAQS. The modeling was performe d using the most recent versi on of AERMOD, AERMET, and AERMINUTE, with data provided to Sierra Club by reg ulatory air agencies or obtained through other publicly-a vailable sources as documented below. The analysis was
conducted in adherence to all ava ilable USEPA guidance for eval uating source impacts on
attainment of the 1-hour SO 2 NAAQS via aerial dispersion m odeling, including the AERMOD Implementation Guide; USEPA's A pplicability of Appendix W Model ing Guidance for the 1-hour
SO 2 National Ambient Air Qual ity Standard, August 23, 2010; modeli ng guidance promulgated by USEPA in Appendix W to 40 CFR Part 51; USEPA’s March 2011 Model ing Guidance for SO 2 NAAQS Designations; 1 and, USEPA’s December 2013 SO 2 NAAQS Designations Technical
Assistance Document. 2

2. Compliance with the 1-hour SO 2 NAAQS

2.1 1-hour SO 2 NAAQS

The 1-hour SO 2 NAAQS takes the form of a t hree-year average of the 99 th -percentile of the annual distribution of daily maximum 1- hour concentrations, which cann ot exceed 75 parts per billion (ppb). 3 Compliance with this standar d was verified using USEPA’s AERM OD air dispersion model, which produces air concentr ations in units of μg/m 3 . The 1-hour SO 2 NAAQS of 75 ppb equals 196.2 μg/m 3 , and this is the value used for determining whether modeled im pacts exceed the NAAQS. 4 The 99 th -percentile of the annual dist ribution of daily maximum 1-hour concentrations

1 http://www.epa.gov/scram001/ so2_modeling_guidance.htm

2 http://www.epa.gov/oaqps001/sulfurdioxide/pdfs/SO2ModelingTAD. pdf

3 USEPA, Applicability of Appendix W Modeling Guidance for the 1- hour SO 2 National Ambient Air Quality Standard, August 23, 2010.

4 The ppb to μg/m 3 conversion is found in the source code to AERMOD v. 14134, sub routine Modules. The conversion calculation is 75/0.3823 = 196.2 μg/m 3

.Evaluation of Compliance with the 1-hour NAAQS for SO 2 September 3, 2015 Page 3

corresponds to the fourth-highest value at each receptor for a given year.

2.2 Modeling Results

Model results for both sources i ncluded in the analysis of SO 2 are summarized in Table 1. Results are provided for each source alone, and for both sources combin ed.

Modeling results for Gibson and P etersburg Generating Stations are summarized in Table 1. It was
determined that based on either c urrent allowable emissions or measured actual emissions, the Gibson Generating Station is es timated to create downwind SO 2 concentrations which exceed the 1- hour NAAQS.

More specifically, the modeling results presented in Table 1, s how exceedances of the NAAQS by the plant’s allowable and actua l emissions. “Allowable” is the peak emission rate from each unit as approved by the current air quali ty operation permit for the fa cility. “Actual” are the measured emissions for each hour between January 1, 2012 and December 31 , 2014 as taken from USEPA Air Markets Program Data

. 5

In addition, the emissions from t he Petersburg Generating Stati on significantly contribute to the
ambient SO 2 concentration in the area imp acted by Gibson Generating Statio n.

Air quality impacts in India na are based on a background concen tration of 47.1 μg/m 3 . This is the 2011-13 design value for Vanderburgh County, Indiana - the lowe st measured background
concentration in the state. This is the most recently availabl e design value. See S ection 5 for further discussion of the background concen trations used for this analy sis.

Table 1 - SO 2 Modeling Results for Gibson Generating Station Modeling Analysis

Emission Rates Averaging Period 99 th Percentile 1-hour Daily Maximum (μg/m 3 ) Complies with NAAQS? Impact Background Total NAAQS

Allowable Gibson 1,046.6 47.1 1,093.7 196.2 No

Actual 229.7 47.1 276.8 196.2 No

Allowable Petersburg 1,978.9 47.1 2,026.0 196.2 No

Actual 726.5 47.1 773.6 196.2 No

Allowable Both 1,980.7 47.1 2,027.8 196.2 No

Actual 726.9 47.1 774.0 196.2 No

5 http://ampd.epa.gov/ampd/Evaluation of Compliance with the 1-hour NAAQS for SO 2 September 3, 2015 Page 4

The emissions used for the mode ling analysis are summarized in Table 2.

Table 2 - Modeled SO 2 Emissions from Gibson Generating Station 6

Stack

ID Unit ID Allowable Emissions 30-day Average (lbs/hr) G01 Unit 1 5,875.0 G02 Unit 2 5,875.0 G03 Unit 3 8,845.5 G04 Unit 4 3,538.2 G05 Unit 5 6,490.0 Gibson Subtotal 30,623.7

P01 Unit 1 13,200.0 P02 Unit 2 24,864.0 P03 Unit 3 6,648.0 P04 Unit 4 6,660.0 Petersburg Subtotal 51,372.0 Both Stations Total 81,995.7

Based on the modeling results, Table 3 provides the necessary e mission reductions from current allowable rates necessary to ach ieve compliance with the 1-hour NAAQS.

This assumes a one-hour averagi ng period for the emission rate, and that the emission rate is binding at all times. However, it is e xtremely likely that this limit is too high to protect the NAAQS, given the conservative aspects of thi s modeling protocol. For example , startup or shutdown periods were not evaluated. During these peri ods, decreased gas velocities a nd temperatures may lead to greater ambient impacts at ground level . Further, the hypothetical emi ssion limitation i n Table 3 would
allow Gibson to consume the en tire NAAQS, leaving little to no room for any other source of SO 2 in
the area. No consideration has b een given to other sources asid e from Petersburg Generating Station, and no margin of safety has been i ncluded in the hypothetical e mission limitation.

6 For Gibson, allowable emissions are based on Indiana Departmen t of Environmental Management, Part 70 Operating Permit Renewal No. T 051-27086-00013, June 8, 2009. The emissio n limitations for Units 1 to 5 are 1.0, 1.0, 1.5, 0.6 and 1.1 lbs/mmbtu, respectively . For Petersburg, allowable emis isons are based on Indiana Department of Environmental Management, Part 70 Operating Permit Renewal No. T125-30045-000 02, First Significant Modification No. 125-33773- 00002, January 10, 2014. The emission limitation for Units 1 to 2 is 6.0 lbs/mmbtu based on a 30-day rolling average. The emission limitation for Units 3 and 4 is 1.2 lbs/mmbtu base d on a 3-hour average.Evaluation of Compliance with the 1-hour NAAQS for SO 2 September 3, 2015 Page 5

Table 3 - Required Emission Reductions from Gibson Generating Station for Compliance with
the 1-hour NAAQS for SO 2 Acceptable Impact (NAAQS - Background) 99th Percentile 1-hour Daily Max (μg/m 3 ) Required Total Facility Reduction Based on Allowable Emissions (%) Required Total Facility Emission Rate (lbs/hr) Required Total Facility 1-hour Average Emission Rate

(lbs/mmbtu)
149.1 92% 2,305.2 0.08

Predicted exceedances of the 1-hour NAAQS for SO 2 based on allowable emissions extend throughout the region to a maxim um distance of 50 kilometers.

Figure 1 shows the extent of NAA QS violations based on allowabl e emissions from both sources.

Figure 2 shows the extent of NAA QS violations based on actual h ourly emissions from both sources.

2.3 Conservative Modeling Assumptions

A dispersion modeling analysis requires the selection of numero us parameters which affect the predicted concentrations. For the enclosed analysis, several pa rameters were selected which underpredict facility impacts.

Assumptions used in this modeli ng analysis which likely under-e stimate concentrations include the following:

 Allowable emissions are base d on a limitation with an averaging period which is greater than the 1-hour average used for the SO 2 air quality standard. Emissions and impacts during any 1-hour period may be higher than assumed for the modeling analy sis.

 No consideration of facility operation at less than 100% load. Stack parameters such as exit flow rate and temperature are typically lower at less than full load, reducing pollutant dispersion and increasing predic ted air quality impacts.

 No consideration of building or s tructure downwash. These downw ash effects typically increase predicted concentr ations near the facility.

 Except for the Petersburg Gen erating Station, no consideration of off-site sources. These other sources of SO 2 will increase the predicted impacts.

2.4 Comparison with Existing Ambient Monitoring

An ambient monitor for SO 2 is operated approximately 2.7 km or 1.7 miles northeast of the Gibson Generating Station. The USEPA da tabase for ambient monitoring m easurements identifies MonitorEvaluation of Compliance with the 1-hour NAAQS for SO 2 September 3, 2015 Page 6

#5180510002 as the Gibson Coal Road - PSI/ Duke Energy- Gibson Network/ E. Mt. Carmel monitor. 7 The 2012-14 design value measur ed at this monitor was 72 ppb o r 188 μg/m 3

.
Based on actual hourly emissions from the Gibson plant and othe r regional sources, the modeling analysis predicted a concentra tion at the monitor location of 2 29 μg/m 3 . Minus the assumed background of 47.1 μg/m 3 the predicted impact due to regional SO 2 sources is 181.9 μg/m 3 . Since the predicted and actual measur ed design values are in close ag reement, this comparison supports the accuracy of the modeling results presented in this report.

7 http://www.epa.gov/airtrends/pdfs /SO2_DesignValues_20122014_FIN AL_8_3_15.xlsxEvaluation of Compliance with the 1-hour NAAQS for SO 2 September 3, 2015 Page 7

Figure 1 - Regional View of Impacts Base d on Allowable Emissions from Both SourcesEvaluation of Compliance with the 1-hour NAAQS for SO 2 September 3, 2015 Page 8

Figure 2 - Regional View of Impacts Based on Actual Emissions for the 2012-14 Period from Both SourcesEvaluation of Compliance with the 1-hour NAAQS for SO 2 September 3, 2015 Page 9

3. Modeling Methodology

3.1 Air Dispersion Model

The modeling analysis used U SEPA’s AERMOD program, v. 14134. A ERMOD, as available from the Support Center for Regulator y Atmospheric Modeling (SCRAM) website, was used in conjunction with a third-par ty modeling software program, AERMOD View , sold by Lakes
Environmental Software.

3.2 Control Options

The AERMOD model was run with t he following control options:

 1-hour average air concentrations

 Regulatory defaults

 Flagpole receptors
To reflect a representative i nhalation level, a flagpole height of 1.5 meters was used for all modeled receptors. This parameter was added to the receptor file when running AERMAP, as described in
Section 4.4.

An evaluation was conducted to det ermine if the modeled facilit y was located in a rural or urban setting using USEPA’s methodology outlined in Section 7.2.3 of the Guideline on Air Quality
Models. 8 For urban sources, the URBANOPT option is used in conjunction with the urban population from an appropriate ne arby city and a default surfac e roughness of 1.0 meter. Methods
described in Section 4.1 were used t o determine whether rural o r urban dispersion coefficients were appropriate for the modeling analysis.

3.3 Output Options

The AERMOD analysis was based on three years of recent meteorol ogical data. The modeling analyses used one run with three ye ars of sequential meteorolog ical data from 2012-2014. Consistent with USEPA’s Modeling Guidance for SO 2 NAAQS Designations, AER MOD provided a table of fourth-high 1-hour SO 2 impacts concentrations consiste nt with the form of the 1-hour SO 2 NAAQS. 9

Please refer to Table 1 for the modeling results.

8 USEPA, Revision to the Guidelin e on Air Quality Models: Adopti on of a Preferred General Purpose (Flat and Complex Terrain) Dispersion Model and Other Revisions, Appendix W to 40 CFR Part 51, November 9, 2005.

9 USEPA, Area Designations for the 2010 Revised Primary Sulfur Di oxide National Ambient Air Quality Standards, Attachment 3, March 24, 2011, pp. 24-26.Evaluation of Compliance with the 1-hour NAAQS for SO 2 September 3, 2015 Page 10

4. Model Inputs

4.1 Geographical Inputs

The “ground floor” of all air dis persion modeling analyses is e stablishing a coordinate system for identifying the geographical loc ation of emission sources and r eceptors. These geographical
locations are used to determine local characteristics (such as land use and elevation), and also to ascertain source to receptor di stances and relationships.

The Universal Transverse Merca tor (UTM) NAD83 coordinate system was used for identifying the easting (x) and northing (y) coordi nates of the modeled sources and receptors. Stac k locations were obtained from facility permits and prior modeling files provide d by the state regulatory agency. The
stack locations were then veri fied using aerial photographs.

The facility was evaluated to de termine if it should be modeled using the rural o r urban dispersion coefficient option in AERMOD. A Geographic Information System (GIS) was used to determine whether rural or urban dispersi on coefficients apply to a site. Land use within a three-kilometer radius circle surrounding the f acility was considered. USEPA gu idance states that urban dispersion
coefficients are used if more t han 50% of the area within 3 kil ometers has urban land uses. Otherwise, rural dispersion c oefficients are appropriate. 10

USEPA’s AERSURFACE v. 13016 was use d to develop the meteorologi cal data for the modeling analysis. This model was also us ed to evaluate surrounding land use within 3 kilometers. Based on the output from the AERSURFAC E, approximately 0.6% of surroundi ng land use around the modeled facility was of urban l and use types including Type 21 – Low Intensity Residential, Type 22 – High Intensity Residential and Type 23 – Commercial / Indu strial / Transportation.

This is less than the 50% value c onsidered appropriate for the use of urban dispersion coefficients. Based on the AERSURFACE analysis, i t was concluded that the rur al option would be used for the modeling summarized in this re port. Please refer to Section 4. 5.3 for a discussion of the AERSURFACE analysis.

10 USEPA, Revision to the Guidelin e on Air Quality Models: Adopti on of a Preferred General Purpose (Flat and Complex Terrain) Dispersion Model and Other Revisions, Appendix W to 40 CFR Part 51, No vember 9, 2005, Section 7.2.3.Evaluation of Compliance with the 1-hour NAAQS for SO 2 September 3, 2015 Page 11

4.2 Emission Rates and Source Parameters

The modeling analyses only considered SO 2 emissions from the Gibs on and Petersburg facilities.
Off-site sources were not consid ered. Concentrations were predi cted for the scenarios shown in
Tables 1 and 2:

1) allowable emissions based on t he current permit issued by th e regulatory agency, and

2) actual hourly emissions meas ured each hour between January 1 , 2012 and December 31,
2014 as taken from USEPA Air Markets Program Data

. 11

Stack parameters and emissions use d for the modeling analysis a re summarized in Table 4.

Table 4 – Facility Stack Parameters and Emissions 12

Facility Gibson Petersburg

Stack S01 S02 S03 S04 S05 P0 1 P02 P03 P04

Description Unit 1 Unit 2 Unit 3 Unit 4 Unit 5 Unit 1 Unit 2 Un it 3 Unit 4

X Coord. [m] 433023.73 433018.24 432936.33 432914.36 432867.49 477854.96 477855.47 477960 477982.44

Y Coord. [m] 4247196.11 4247199. 49 4247264.51 4247334.35 424743 6.95 4264361.51 4264374.25 4264468.08 4264532.33

Base Elevation [m] 125.1 125.41 124.34 124.47 124.54 127.58 128 .26 130.86 130.81

Release Height [m] 188.98 188.98 188.98 152.4 152.4 186.54 181. 66 187.45 187.45

Gas Exit Temperature [°K] 327.594 327.594 327.594 324.817 335.9 28 324.817 323.706 347.594 338.706

Gas Exit Velocity [m/s] 21.464 21.464 21.464 21.398 21.398 22.4 69 21.72 26.727 25.391

Inside Diameter [m] 7.62 7.62 7.62 7.163 7.163 4.654 6.279 6.70 6 6.706

Allowable Emission Rate [g/s] 740.2 740.2 1,115 445.8 817.7 1,6 63 3,133 837.6 839.1

Actual Emission Rate [g /s] - - - - - - - - -

The above stack parameters and emissions were obtained from reg ulatory agency documents and
databases identified in Secti on 2.2. The analysis was conducted based on 100% operating load using
maximum exhaust flow rates and t emperatures. Operation at less than full capacity loads was not considered. This assumption te nds to under-predict impacts sinc e stack parameters such as exit flow rate and temperature are typically lower at less than full load , reducing pollutant dispersion and increasing predicted air qualit y impacts. Stack location, heigh t and diameter were verified using aerial photographs, and flue gas fl ow rate and temperature were verified using combustion
calculations.

11 http://ampd.epa.gov/ampd/

12 Stack parameters were obtained from the annual survey compiled by the U.S. Energy Information Administration. http://www.eia.gov/electricity/data/eia860/Evaluation of Compliance with the 1-hour NAAQS for SO 2 September 3, 2015 Page 12

4.3 Building Dimensions

No building dimensions or prior dow nwash evaluations were avail able. Therefore this modeling analysis did not address the e ffects of downwash and this may u nder-predict impacts.

4.4 Receptors

For Gibson Generating Station, three receptor grids were employ ed:

1. A 100-meter Cartesian receptor grid centered on Gibson Generati ng Station and extending
out 5 kilometers.
2. A 500-meter Cartesian receptor grid centered on Gibson Generati ng Station and extending
out 10 kilometers. 3. A 1,000-meter Cartesian receptor grid centered on Gibson Genera ting Station and extending out 50 kilometers. 50 kilometers is t he maximum distance accept ed by USEPA for the use of the AERMOD dispersion model. 13

A flagpole height of 1.5 meters w as used for all these receptor s. Elevations from stacks and recep tors were obtained from Nationa l Elevation Dataset (NED) GeoTiff data. GeoTiff is a binary file tha t includes data descriptors a nd geo-referencing information necessary for extract ing terrain elevations. These elevations w ere extracted from 1 arc-second (30 meter) resolution NED files. The U SEPA software program AERMAP v. 11103 is used for these
tasks.

4.5 Meteorological Data

To improve the accuracy of the modeling analysis, recent meteor ological data for the 2012-2014 period were prepared using the U SEPA’s program AERMET which cre ates the model-ready surface and profile data files requir ed by AERMOD. Required data inpu ts to AERMET included surface meteorological measurements, t wice-daily soundings of upper air measurements, and the micrometeorological parameters surface roughness, albedo, and B owen ratio. One-minute ASOS
data were available so USEPA met hods were used to reduce calm a nd missing hours. 14 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 SO 2 NAAQS

13 USEPA, Revision to the Guideline on Air Quality Models: Adopti on of a Preferred General Purpose (Flat and Complex Terrain) Dispersion Model and Other Revisions, Appendix W to 40 CFR Part 51, Section A.1.(1), November 9, 2005.

14 USEPA, Area Designations for the 2010 Revised Primary Sulfur D ioxide National Ambient Air Quality Standards, Attachment 3, March 24, 2011, p. 19.Evaluation of Compliance with the 1-hour NAAQS for SO 2 September 3, 2015 Page 13

modeling analyses. The USEPA software program AERMET v. 14134 is used for these tasks.

4.5.1 Surface Meteorology

Surface meteorology was obtained for Evansville Regional Airpor t located near the Gibson Generating Station. Integrated S urface Hourly (ISH) data for th e 2012-2014 period were obtained
from the National Climatic Data Center (NCDC). The ISH surfac e data was processed through AERMET Stage 1, which performs data extraction and quality cont rol checks.

4.5.2 Upper Air Data

Upper-air data are collected by a “ weather balloon” that is rel eased twice per day at selected
locations. As the balloon is rel eased, it rises through the at mosphere, and radios the data back to the surface. The measuring and transmitting device is known as eit her a radiosonde, or rawindsonde.
Data collected and radioed back i nclude: air pressure, height, temperature, dew point, wind speed, and wind direction. The upper air data were processed through AERMET Stage 1, which performs data extraction and quality control checks.

For Gibson Generating Stati on, the concurrent 2012-2014 upper a ir data from twice-daily radiosonde
measurements obtained at the most representative location were used. This location was the Lincoln, Illinois measurement st ation. These data are in Foreca st Systems Laboratory (FSL) format
and were downloaded in ASCII text format from NOAA’s FSL websit e. 15 All reporting levels were downloaded and processed with AERMET.

4.5.3 AERSURFACE

AERSURFACE is a program that ex tracts surface roughness, albedo , and daytime Bowen ratio for an area surrounding a given locatio n. AERSURFACE uses land use and land cover (LULC) data in the U.S. Geological Survey’s 1992 National Land Cover Dataset t o extract the necessary micrometeorological data. LULC data was used for processing me teorological data sets used as input to AERMOD.

AERSURFACE v. 13016 was used to deve lop surface roughness, albe do, and daytime Bowen ratio values in a region surrounding the meteorological data collecti on site. AERSURFACE was used to develop surface roughness in a one kilometer radius surrounding the data collection site. Bowen ratio and albedo was developed for a 10 kilometer by 10 kilomet er area centered on the meteorological data collection si te. These micrometeorological data were processed for seasonal periods using 30-degree sectors . Seasonal moisture conditions w ere considered ave rage with winter

15 Available at: http://esrl.noaa.gov/raobs/Evaluation of Compliance with the 1-hour NAAQS for SO 2 September 3, 2015 Page 14

months having continuous snow cover.

4.5.4 Data Review

Missing meteorological data were not filled as the data file me t USEPA’s 90% data completeness requirement. 16 The AERMOD output file shows th ere were 0.75% missing data.

To confirm the representativene ss of the airport meteorological data, the surface characteristics of the airport data collection s ite and the modeled source locatio n were compared. Since the Evansville Regional Airport is located clo se to Gibson Generating Station, this meteorological data set was considered appropriate for this modeling analysis. 17 This weather stati on provided high quality surface measurements for the most recent 3-year time, and had s imilar land use, surface characteristics, terrain featur es and climate. Finally, the met eorological data from the selected surface and upper air stations were recommended by the Indiana Department of Environmental Management for modeling facili ties located in Gibson County. 18

5. Background SO 2 Concentrations

Background concentrations were determined consistent with USEPA ’s Modeling Guidance for SO 2 NAAQS Designations. 19, 20 To preserve the form of the 1-hour SO 2 standard, based on the 99 th percentile of the annual distri bution of daily maximum 1-hour c oncentrations averaged across the number of years modeled, the b ackground fourth-highest daily ma ximum 1-hour SO 2 concentration
was added to the modeled fourth-h ighest daily maximum 1-hour SO 2 concentration. 21 Background
concentrations were based on the 2011-13 design value measured by the ambient monitors located in
Indiana. 22

6. Reporting

All files from the programs used for this modeling analysis are available to regulatory agencies. These include analyses prepared w ith AERSURFACE, AERMET, AERMAP , and AERMOD.

16 USEPA, Meteorological Monitoring Guidance for Regulatory Modeli ng Applications, EPA-454/R-99-05, February 2000, Section 5.3.2, pp. 5-4 to 5-5.

17 USEPA, AERMOD Implementation Guide, March 19, 2009, pp. 3-4.

18 Indiana DEM, Air Dispersion Me teorological Data, http://www.in .gov/idem/airquality/2376.htm

19 USEPA, Area Designations for the 2010 Revised Primary Sulfur D ioxide National Ambient Air Quality Standards, Attachment 3, March 24, 2011, pp. 20-23.

20 USEPA, SO 2 NAAQS Designations Modeling Te chnical Assistance Document, Dec . 2013, section 8.1, pp 27-28.

21 USEPA, Applicability of Appendix W Modeling Guidance for the 1- hour SO 2 National Ambient Air Quality Standard, August 23, 2010, p. 3.

22 http://www.epa.gov/airtrends/values.html