## Scherer Steam-Electric Generating Plant

Juliette, Georgia

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



## July 30, 2015

Conducted by:

Steven Klafka, P.E., BCEE

Wingra Engineering, S.C.

Madison, WisconsinEvaluation of Compliance with the 1-hour NAAQS for SO 2 July 30, 2015 Page 2

1. Introduction

Wingra Engineering, S.C. was hi red by the Sierra Club to conduc t an air modeling impact analysis to help USEPA, state and local air a gencies identify facilities th at are likely causing exceedances of the 1-hour sulfur dioxide (SO 2 ) national ambient air qualit y standard (NAAQS). This document describes the results and pro cedures for an evaluation conducte d for the Scherer Steam-Electric Generating Plant locate d in Juliette, Georgia.

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 the Sierra Club by regulatory air agencies and through other publicly-available so urces as documented below. The analysis was conducted in
adherence to all available USEP A guidance for evaluating source impacts on attainment of the 1-
hour SO 2 NAAQS via aerial dispersion mode ling, including the AERMOD Imp lementation Guide; USEPA's Applicability of Appendi x W Modeling Guidance for the 1 -hour SO 2 National Ambient Air Quality Standard, August 23, 2010; modeling guidance promul gated by USEPA in Appendix W to 40 CFR Part 51; USEPA’s Ma rch 2011 Modeling 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 ppb. 3 Compliance
with this standard w as verified using USEPA’s AERMOD air disper sion model, which produces air concentrations 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 impacts exceed t he NAAQS. 4 The 99 th -percentile
of the annual distri bution of daily maximum 1-hour concentratio ns corresponds to the fourth-highest value at each receptor for a given year.

2.2 Modeling Results

Modeling results for Scherer St eam-Electric Generating Plant ar e summarized in Table 1. It was

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 July 30, 2015 Page 3

determined that based on either c urrent allowable emissions or measured actual emissions, the Scherer Steam-Electric Generating Plant is estimated 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

Air quality impacts in Georg ia are based on a background concen tration of 23.5 μg/m 3 . This is the 2011-13 design value for Muscogee, Georgia - the lowest measure d background concentr ation in the
state. This is the most recen tly available design value. See S ection 5 for further discussion of the background concentrations used for this analysis.

Table 1 - SO 2 Modeling Results for Scherer Steam-Electric Generating Plant Modeling Analysis

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

Allowable 1-hour 1,001.2 23.5 1,024.7 196.2 No

Actual 1-hour 210.7 23.5 234.2 196.2 No

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

Table 2 - Modeled SO 2 Emissions from Scherer Stea m-Electric Generating Plant 6

Stack

ID Unit
ID Allowable Emissions 24-hour Average (lbs/hr)
S01 Unit 1 9,834 S02 Unit 2 9,834
S03 Unit 3 9,834
S04 Unit 4 9,834
Total All Units 39,336.0

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.

5 http://ampd.epa.gov/ampd/

6 Georgia Department of Natural Resources, Pa rt 70 Operating Per mit No. 4911-207-0008-V-03-0, May 8, 2012.Evaluation of Compliance with the 1-hour NAAQS for SO 2 July 30, 2015 Page 4

Table 3 - Required Emission Reduc tions 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)

172.7 83% 6,785.2 0.21

Predicted exceedances of the 1-hour NAAQS for SO 2 based on allowable emissions extend throughout the region to a maxim um distance of 44 kilometers.

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

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

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.

 No evaluation has been conducted t o determine if the stack heig ht exceeds Good Engineering Practice or GEP height. If the st ack height exceeds GEP, the pr edicted concentrations will increase.

 No consideration of off-site sources. These other sources of SO 2 will increase the predicted impacts.Evaluation of Compliance with the 1-hour NAAQS for SO 2 July 30, 2015 Page 5

Figure 1 - Regional View of Impac ts Based on Allowable EmissionsEvaluation of Compliance with the 1-hour NAAQS for SO 2 July 30, 2015 Page 6

Figure 2 - Regional View of Impacts Base d on Actual Emission s for the 2012-14 PeriodEvaluation of Compliance with the 1-hour NAAQS for SO 2 July 30, 2015 Page 7

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. 7 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. 8

Please refer to Table 1 for the modeling results.

7 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.

8 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 July 30, 2015 Page 8

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 GIS was used to determine whet her rural or urban dispersion coefficients apply to a site. L and use within a three-kilomete r radius circle surr ounding the facility was considered. USEPA guidance st ates that urban dispersion coe fficients are used 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 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 0.2% 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.

9 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, Section 7.2.3.Evaluation of Compliance with the 1-hour NAAQS for SO 2 July 30, 2015 Page 9

4.2 Emission Rates and Source Parameters

The modeling analyses only considered SO 2 emissions from the facilit y. Off-site sources were not
considered. Concentrations were predicted for the scenarios sho wn 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 , 2011 and December 31,
2014 as taken from USEPA Air Markets Program Data

. 10

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

Table 4 – Facility Stack Parameters and Emissions 11

Stack S01 S02 S03 S04 Description Unit 1 Unit 2 Unit 3 Unit 4 X Coord. [m] 237957 237957 237971.26 237982.8 Y Coord. [m] 3661152 3661162.86 3661822.09 3661827.52 Base Elevation [m] 141.94 141.95 141.95 141.96 Release Height [m] 258.17 258.17 258.17 258.17 Gas Exit Temperature [°K] 329.261 329.261 329.261 329.261

Gas Exit Velocity [m/s] 17.345 17.345 17.345 17.345

Inside Diameter [m] 10.363 10.363 10.363 10.363 Allowable Emission Rate [g/s] 1,239 1,239 1,239 1,239

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.

10 http://ampd.epa.gov/ampd/

11 Stack parameters were obtained from the U.S. Energy Information Agency, http://www.eia.gov/elec tricity/data/eia860/, 2013 Form EIA-860 Data - Schedule 6, 'Stac k & Flue Data' , 6_2_EnviroEqui p_Y2013.xlsxEvaluation of Compliance with the 1-hour NAAQS for SO 2 July 30, 2015 Page 10

4.3 Building Dimensions and GEP

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 Scherer Steam-Electric Gene rating Plant, three receptor gri ds were employed:

1. A 100-meter Cartesian receptor grid centered on Scherer Steam-E lectric Generating Plant and extending out 5 kilometers.

2. A 500-meter Cartesian receptor grid centered on Scherer Steam-E lectric Generating Plant and extending out 10 kilometers.
3. A 1,000-meter Cartesian receptor grid centered on Scherer Steam -Electric Generating Plant and extending out 50 kilometers. 50 kilometers is the maximum d istance accepted by USEPA for the use of the AERMOD dispersion model. 12

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. 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 SO 2 NAAQS

12 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.

13 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 July 30, 2015 Page 11

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

4.5.1 Surface Meteorology

Surface meteorology was obtained for Middle Georgia Regional Ai rport located near the Scherer Steam-Electric Generating Plant. I ntegrated Surface Hourly (ISH ) data for the 2012-2014 period
were obtained from the National C limatic Data Center (NCDC). The ISH surface data was processed through AERMET Stage 1, which performs data extractio n and quality control 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 Scherer Steam-Electric Gene rating Plant, the concurrent 201 2-2014 upper air data from twicedaily radiosonde measurements obtai ned at the most representati ve location were used. This location was the Peachtree City / Falcon Field measurement stat ion. These data are in Forecast Systems Laboratory (FSL) format and were downloaded in ASCII te xt format from NOAA’s FSL
website. 14 All reporting levels were downlo aded and processed with AERME T.

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

14 Available at: http://esrl.noaa.gov/raobs/Evaluation of Compliance with the 1-hour NAAQS for SO 2 July 30, 2015 Page 12

months having no 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. 15 The AERMOD output file shows th ere were 0.46% 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 Middle Georgia Regional Airport is locat ed close to Scherer Steam-Elec tric Generating Plant, this meteorological data set was consid ered appropriate for this mod eling analysis. 16 This weather station provided high quality surf ace measurements for the most recent 3-year time, and had similar land use, surface characteristic s, terrain features and climate . Lastly, the weather stations selected for the modeling analysis were those recommended for use of the AER MOD dispersion model by the Georgia Department of Natural Resources. 17

5. Background SO 2 Concentrations

Background concentrations were determined consistent with USEPA ’s Modeling Guidance for SO 2 NAAQS Designations. 18,19 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. 20 Background
concentrations were based on the 2011-13 design value measured by the ambient monitors located in Georgia. 21

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.

15 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.

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

17 http://www.georgiaair.org/airpermit/html/sspp/modeling/aermetd ata.htm

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

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

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

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