## Big Brown Steam Electric Station

Fairfield, Texas

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



## December 15, 2015

Conducted by:

Steven Klafka, P.E., BCEE

Wingra Engineering, S.C.

Madison, WisconsinEvaluation of Compliance with the 1-hour NAAQS for SO 2 December 15, 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 Big Brown Steam Electric Station located in F airfield, Texas.

To ensure the modeling analysis reflected the cumulative concen tration of SO 2 emissions, it included
emissions from the Limestone Ele ctric Generating Station in Jew ett, Texas, which is an additional
source of SO 2 emissions located within 50 kilome ters of the Big Brown Steam Electric Station.

This report updates th e modeling results presented in an earlie r report dated September 11, 2015. The analysis was updated to include three years of meteorologic al data.

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

It was determined that based on m easured actual emissions or cu rrent allowable limits, the Big
Brown Steam Electric Stati on is estimated to create SO 2 concentrations which exceed the 1-hour NAAQS.

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

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

2 http://www.epa.gov/oaqps001/sulfurdioxide/pdfs/SO2ModelingTAD. pdfEvaluation of Compliance with the 1-hour NAAQS for SO 2 December 15, 2015 Page 3

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

2.2 Modeling Results

Model results for both sou rces included in the SO 2 analysis are summarized in Table 1. Results are provided for each source alone , and for all sources combined.

Modeling results for B ig Brown Steam Elect ric Station and Limes tone Electric Generating Station
are summarized in Table 1. It wa s determined that based on eith er current allowable emissions or measured actual emissions, the B ig Brown Steam Electric Station 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

In addition, the emissions from L imestone Electric Generating S tation significantly contribute to the
ambient SO 2 concentration in the area impact ed by Big Brown Steam Electric Station.

Air quality impacts in Texas are based on a background concentr ation of 7.8 μg/m 3 . This is the 2011-13 design value for El Paso, Tex as—the lowest measured bac kground concentration 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.

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. 15181, sub routine Modules. The conversion calculation is 75/0.3823 = 196.2 μg/m 3

. 5 http://ampd.epa.gov/ampd/Evaluation of Compliance with the 1-hour NAAQS for SO 2 December 15, 2015 Page 4

Table 1 - SO 2 Modeling Results for Big Brown Steam Electric 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 Big Brown 817.9 7.8 825.7 817.9 No

Actual 380.1 7.8 387.9 380.1 No

Allowable Limestone 1,396.6 7.8 1,404.4 1,396.6 No

Actual 200.5 7.8 208.3 200.5 No

Allowable Both 1,398.9 7.8 1,406.7 1,398.9 No

Actual 380.3 7.8 388.1 380.3 No

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

Table 2 - Modeled SO 2 Emissions from Big Brow n Steam Electric Station 6

Stack

ID Unit
ID Allowable Emissions 3-hour Average (lbs/hr)
S01 Unit 1 17,790.0 S02 Unit 2 17,790.0 Big Brown Subtotal 35,580.0 S01 Unit 1 26,790.0 S02 Unit 3 24,390.0 Limestone Subtotal 51,180.0 Both Stations Total 86,760.0

Based on the modeling results, Table 3 provides the emission re ductions from current allowable rates necessary to achieve com pliance with the 1-hour NAAQS. T his assumes a one-hour averaging period for the emission rate and that the emission rate is bind ing at all times. However, given the conservative aspects of this modeling protocol, it is extremely likely that this limit is too high to protect the NAAQS. For example , startup or shutdown periods wer e not evaluated. During these periods, decreased gas velocities and temperatures may lead to greater ambient impacts at ground level. Further, the hypothetic al emission limitation in Table 3 would allow Big Brown Steam
Electric Station to consume the entire NAAQS, leaving little to no room for any other source of SO 2 in the area. No margin of saf ety has been included in the hypot hetical emission limitation.

6 Big Brown allowable emissions are taken from TXCEQ, Federal Op erating Permit, No. O65. January 15, 2014. Limestone allowable emissions are taken from TXCEQ, Federal Ope rating Permit, No. O75, January 24, 2012.Evaluation of Compliance with the 1-hour NAAQS for SO 2 December 15, 2015 Page 5

Table 3 - Required Emission Reductions from Current Allowable Limits for Big Brown Steam Electric Station to Comply 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)

188.4 88% 4,791.8 0.40

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 Limestone Electric Generating Station, no considerat ion of off-site sources. These other sources of SO 2 will increase the predicted impacts.

 Air quality impacts are based on a background SO 2 concentration of 7.8 μg/m 3 , which is the lowest measured background concentr ation in the state. Given t he proximity of the Big Brown facility to other major sources of SO 2 , the actual background con centration is likely much higher.

 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.Evaluation of Compliance with the 1-hour NAAQS for SO 2 December 15, 2015 Page 6

Figure 1 - Regional View of Impacts Base d on Allowable Emissions from Both SourcesEvaluation of Compliance with the 1-hour NAAQS for SO 2 December 15, 2015 Page 7

Figure 2 - Regional View of Impacts Based on Actual Emissions for 2012-14 from Both SourcesEvaluation of Compliance with the 1-hour NAAQS for SO 2 December 15, 2015 Page 8

3. Modeling Methodology

3.1 Air Dispersion Model

The modeling analysis used U SEPA’s AERMOD program, v. 15181. 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 December 15, 2015 Page 9

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. 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 0.8% 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 December 15, 2015 Page 10

4.2 Emission Rates and Source Parameters

The modeling analysis considered SO 2 emissions from the Big Brown S team Electric Station and Limestone Electric Generating Sta tion. Other off-site sources w ere not considered. Concentrations were predicted for the scena rios 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

. 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

Facility Big Brown Limestone

Stack B01 B02 L01 L02 Description Unit 1 Unit 2 Unit 1 Unit 2 X Coord. [m] 778731.22 778774.22 761099 761130 Y Coord. [m] 3524371.07 3524295.07 3479813 3479694 Base Elevation [m] 98.61 98 141.35 139.17 Release Height [m] 122 122 171.6 171.6 Gas Exit Temperature [°K] 459 459 333.15 333.15

Gas Exit Velocity [m/s] 23.7 23.7 21.294 21.294

Inside Diameter [m] 6.77 6.77 8.23 8.23 Allowable Emission Rate [g/s] 2,242 2,242 3,375 3,073

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 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 December 15, 2015 Page 11

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 Big Brown Steam Electric St ation, three rece ptor grids were employed:

1. A 100-meter Cartesian receptor grid centered on Big Brown Steam Electric Station and extending out 5 kilometers.

2. A 500-meter Cartesian receptor grid centered on Big Brown Steam Electric Station and extending out 10 kilometers.
3. A 1,000-meter Cartesian receptor grid centered on Big Brown Ste am Electric Station and extending out 50 kilometers. 50 kilome ters is the maximum dista nce 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.

The receptor grids were not changed to remove receptors located on source property or above water bodies. The highest predicted c oncentrations occurred off facil ity property so eliminating on-site receptors would not change the modeling results. While USEPA te chnical guidance suggests receptors should not be placed where ambient monitors cannot be located, such as above water bodies, the concentrations predic ted above water bodies still s how potential exposure to the general public.

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

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.Evaluation of Compliance with the 1-hour NAAQS for SO 2 December 15, 2015 Page 12

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. 15272 is used for these tasks. AE RMET was used to randomize
wind directions for NWS wind data if one-minute ASOS wind speed s were not available for any periods.

This section discusses how the meteorological data was prepared for use in the 1-hour SO 2 NAAQS modeling analyses. The USEPA software program AERMET v. 15181 is used for these tasks.

4.5.1 Surface Meteorology

Surface meteorology was obtained for Corsicana Campbell Field l ocated near the Big Brown Steam Electric Station. Integrated Surf ace Hourly (ISH) data for the 2012-2014 period were obtained from the National Climatic Data Center (NCDC). The ISH surface dat a 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 Big Brown Steam Electric Station, the conc urrent 2012-2014 upper air data from twice-daily
radiosonde measurements obtained at the most representative loc ation were used. This location was the Fort Worth, Texas measurement station. These data are in Fo recast Systems Laboratory (FSL)
format and were downloaded in ASC II text format from NOAA’s FSL website. 14 All reporting
levels were downloaded a nd 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

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

14 Available at: http://esrl.noaa.gov/raobs/Evaluation of Compliance with the 1-hour NAAQS for SO 2 December 15, 2015 Page 13

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 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 1.63% 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 Corsicana Campbell Field is located close t o Big Brown Steam Electric Sta tion, this meteorological data set was considered appropriate for this modeling analysis. 16 This weather station provided high quality surface measurements for the most recent 3-year time, and had s imilar land use, surface characteristics, terra in features and climate.

Finally, TCEQ provides pre-proces sed meteorological data suitab le for modeling for each county. 17 For Freestone County, TCEQ recommends using data from the same surface and upper air stations used for this modeling analysis . The TCEQ data were not used fo r this project because they were not from the most recent years need ed for this analysis and had not been processed using the latest
versions of USEPA m odeling software.

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

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 Texas Commission on Environmental Quality, Meteorological Data for Refined Screening with AERMOD, http://www.tceq.texas.gov/permit ting/air/modeling/aermod-datase ts.html , Last updated November 22, 2013.

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.Evaluation of Compliance with the 1-hour NAAQS for SO 2 December 15, 2015 Page 14

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

Texas. 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.

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