## Nebraska Public Power District

Sulfur Dioxide 1

- Hour NAAQS Designations Modeling

for

Gerald Gentleman Station (Facility ID # 34385 )

Sutherland , Nebraska

For submittal to:

Lisa Alam c/o Records Management
Nebraska Department of Environmental Quality
Suite 400, The Atrium

1200 N Street

P.O. box 98922
Lincoln, Nebraska 69509

- 8922

Prepared by:

HDR Engineering, Inc.

September 4

, 20 15GGS 1

- Hour SO 2 Modeling 1 September 2015 INTRODUCTION

This modeling report summarizes dispersion modeling of Gerald Gentleman Station to demonstrate
attainment with the sulfur dioxide (SO 2 ) National Ambient Air Quality Standard ( NAAQS ) for the 1

- hour

averag ing period , which is equal to 75 parts per billion (p pb) or approximately 196.5 micrograms per
cubic meter (μg/m 3 ) . This modeling has been completed for submi ssion to the Nebraska Department of
Environmental Quality (NDEQ) for review and approval. The results of the dispersion modeling analysis
will be used by the NDEQ to formulate recommendations to EPA on the NAAQS designations

.

MODELING PROTOCOL

Th e dispersion modeling results summarized in this report were obtained using the EPA’s AERMOD
dispersion modeling system, using the modeling procedures and data as described in a modeling protocol
developed in collaboration with NDEQ, as provided in Attachment 1 to this report. In addition to this
report, this submittal to NDEQ includes a compact disk/DVD containing all the data files listed at the end

of the modeling protocol in Attachment 1.

MODEL INPUTS

Th is modeling report contains figures detailing the model ed sources, receptors, background SO 2 concentration and meteorological data locations, and the resulting 1

- hour SO 2 modeled impact s . The SO 2 modeling was conducted in accordance to the attached detailed dispersion modeling protocol.GGS 1

- Hour SO 2 Modeling 2 September 2015 Figure 1 displays the source locations of the two point sources included in the model, which are the coal

fired boiler stacks for Gerald Gentleman Station (GGS) Unit 1 ( GGS 1 ) and GGS Unit 2 ( GGS 2 )

.

Figure 1: Source LocationsGGS 1

- Hour SO 2 Modeling 3 September 2015 Figure 2 displays the receptor locations as specified in the attached dispersion modeling protocol in

accordance with NDEQ guidance. Note that receptors are not included over water bodies in accordance
with EPA guidance, and therefore, receptors have bee n excluded over the waters of Sutherland Reservoir.

Figure 2: Receptor LocationsGGS 1

- Hour SO 2 Modeling 4 September 2015 Figure 3 displays the N PPD facility location in relation to the background SO 2 monitor location (Trego
County, Kansas) and meteorological data stations for surface data ( Imperial , Nebraska) and upper

- air data

( North Platte, Nebraska )

.

Figure 3: Site, Monitor, and Meteorological Station Locations.GGS 1

- Hour SO 2 Modeling 5 September 2015 The stack parameter inputs for GGS1 and GGS 2 are provided in Table 1. The stack temperature and
velocity are placeholder design values. Actual temperature and velocity data w ere input to the AERMOD
model on an hourly basis with an hourly emission file that contains emission rates, stack temperatures and

exit vel ocities for both stacks for each hour of the model simulation, which spanned the years 2012

- 2014.

Table 1: Source Input Data for Modeling Analysis

UTM Coordinates Base

Elevation Stack Height Stack Temperature Stack

Exit Velocity Stack

Diameter Source ID X Y

(m) (m) (m) (m) (Deg. K) (m/s) (m)

Unit 1 320171.7 4550011 954.1 168.56 432 23.14 8.54

Unit 2 320192.2 4549894 957.5 167.64 417 25.62 8.54

The hourly emissions data files input to AERMOD were based on continuous emissions and stack
parameter monitoring data, which have been collected and reported consistent with Acid Rain program
monitoring procedures as provided in 40 CFR 75. The hourly emissions data file (including sta c k exhaust
temperature and velocity data) is provided electron ically on the CD/DVD accompanying this report.

MODEL RESULTS

Modeled design impact for the 3

- year average 1

- hour SO 2 concentration is summarized in Table 2 and
presented visually in Figure 4. The 4 th high 3

- year average concentrations are compared with the 1

- hour

SO 2 NAAQS of 75 parts per billion (ppb) or approximately 196.5 micrograms per cubic meter (μg/m 3 ).
The modeled concentration is the combined impact of Units 1 and 2. The background 1

- hour SO 2 concentration of 9 μg/m 3 was added to the modeled concentration. The m odeled p lus b ackground

concentration is well below the NA AQS as shown in Table 2

.

Table 2: Modeled Impacts ( with background of 9 μg/m 3 )

Pollutant Averaging

Period Year s UTM Coordinates Modeled

Concentration Modeled Plus Background NAAQS Easting Northing

(m) (m) (μg/m 3 ) (μg/m 3 ) (μg/m 3 )

SO 2 1

- hour 2012

- 14 321900 4548800 135.8 144.8 196.5GGS 1

- Hour SO 2 Modeling 6 September 2015 Figure 4: Modeled Result vs. SO 2 S tandard

Figure 5 is a contour plot of the modeled 1

- hour design concentration s (not including background)

, representing the average of the daily maximum 4 th high values over the 3 years of meteorological data

. The black box surrounding the contour lines indicates the extent of the receptor grid. The contour levels

ar e: Orange : 100 μ g/m 3 , Yellow : 110 μ g/m 3

, and Pink : 120 μ g/m 3 . The location of the maximum 4 th high

3

- year average modeled (without background) concentration ( 135.8 μ g/m 3 ) is displayed for reference. 0 50 100 150 200 250

SO2 Modeling Result SO2 Standard SO 2 μg/m 3 Standard

Modeled Result

BackgroundGGS 1

- Hour SO 2 Modeling 7 September 2015 Figure 5: Contour Plot of SO 2 1

- Hour Design ConcentrationsGGS 1

- Hour SO 2 Modeling 8 September 2015 CONCLUSION

This modeling analysis shows that the design maximum SO 2 impact from GGS , when added to the
estimated background concentration of 9 μg/m 3 , would cause a total maximum impact of 144.8 μg/m 3

, which is well below (under 75 %) of the SO 2 1

- hour NAAQS of 196.5 μ g/m3 (75 ppb).

# Attachment 1

## ATTACHMENT 1 NPPD, Gerald Gentleman Station

NDEQ F acility ID #34385, 07/15/2015

1 | Page HDR Engineering, Inc. Sulfur Dioxide 1

- Hour NAAQS Designations Modeling Protocol

Nebraska Public Power District, Gerald Gentleman Station ( NDEQ Facility ID # 34385 )

Sutherland , NE

Ju ly 1 5 , 201 5

INTRODUCTION

This modeling protocol addresses proposed dispersion modeling for the Nebraska Public Power

District (NPPD) Gerald Gentleman Station (GGS) , in L incoln County, near Sutherland , Nebraska

. The protocol summarizes the information that will be used to conduct dispersion modeling with respect to the National Ambient Air Quality Standard (NAAQS) for 1

- hour average sulfur dioxide (SO 2 ) concentration, which is equal to 75 parts per billion (ppb) o r approximately 196.5 micrograms per cubic meter (μg/m 3 )

Th is protocol has been prepared for 1

- hour SO 2 State Implementation Plan (S IP ) dispersion modeling and is being submitted to the Nebraska Department of Environmental Quality ( NDEQ ) for review and approval. The results of th e dispersion modeling analysis will be used by the NDEQ to formulate

recommendations to EPA on the NAAQS attainment/nonattainment area designations for SIP purposes. EPA will review and use this information, alo ng with any available monitoring data, to propose and finalize attainment/nonattainment designations for affected areas with respect to the 1

- hour SO 2 NAAQS.

MODELING SOFTWARE

The following EPA modeling software will be used for this analysis.

• AERMOD ( Version 14134 )

• BPIP

- Prime ( V ersion 04274 )

• AERMAP ( V ersion 11103 )

The AERMOD model will be executed using the rural dispersion mode, given the predominantly rural character of the land surrounding the subject facility.

METEOROLOGICAL DATA

Meteorological data for this analysis w ill be provided by NDEQ in preprocessed format, based on the

most recent versions of AERMET (Version 14134) , AERMINUTE (Version 14337) , and

AERSURFACE (Version 13016)

.

The surface meteorological will be from Imperial , Nebraska and the upper air data will be from North

Platte , Nebraska , which are considered representative of the Sutherland area. This analysis will use three years of meteorological data for the years 2012 through 2014.

POLLUTANT AND AVERAG ING PERIOD

The AERMOD model will be executed for SO 2 for 1

- hour averages. By selecting SO 2 as the pollutant and 1

- hour as the averaging period, AERMOD will automatically average the results over the three years of meteorology. The model result for comparison with t he 1

- hour SO 2 NAAQS of 196.5 μg/m 3 (75 parts per billion) will be the maximum of the 3

- year average of the 4 th highest ( 99th

## ATTACHMENT 1 NPPD, Gerald Gentleman Station

NDEQ F acility ID #34385, 07/15/2015

2 | Page HDR Engineering, Inc. percentile ) daily 1

- hour maximum concentration, as automatically output by AERMOD from the multiyear (3

- year) model run.

POINT SOUR CES

Emission points to be modeled for GGS w ill include only the Unit 1 and Unit 2 coal boiler stack s

. The NDEQ and EPA have reviewed the other emissions sources in the region and determined that there are no nearby sources with large enough emissions to be included in a modeling analysis together with GGS . Thus, total impact for comparison with the NAAQS will consist of the Unit 1

and Unit 2 combined impact plus the background concentration (see below). The AERMOD output will be set up to produce source contributions for each unit, plus the total (“ALL” source group)

concentrations.

The actual, hourly SO 2 emissions measured by the continuous emissions monitoring system (CEMS)

on the Unit 1 and 2 stack s will be used for this analysis, by using the option al hourly emissions input file for input to AERMOD. The single hourly emissions file will correspond with the same period of record represented by the three year period of meteorological data (2012

- 2014) input to AERMOD.

In addition to hourly SO 2 emissions in grams/second, the hourly emissions file will include hourly average stack gas exhaust temperature and exhaust gas exit velocity. These additional hourly parameters will be based on measurements re corded by the same CEMS systems being used to track hourly SO 2 emissions for each stack , in accordance with the routine monitoring requirements under 40 CFR 60 (New Source Performance Standards) and 40 CFR 75 (Continuous Emission Monitoring

under the Acid Rain program).

In addition to the stack heights and the hourly emissions and stack parameters described above, the only other stack parameters needed by A E RMOD are the stack exit diameters. The stack diameter s to be input for Unit s 1 and 2 will be 28 feet ( 8.535 m), based on the current design of the se stack s

.

BUILDING DOWNWASH IN PUTS

The AERMOD input will include building downwash parameters calculated using the EPA’s Building Profile Input Pro gram “PRIME” (BPIPPRIME) software (Version 04274). The BPIPPRIME input and output (I/O) files will be provided along with all the other modeling I/O files on CD with the final modeling report.

TERRAIN ELEVATIONS

Terrain data will be processed to dete rmine receptor elevations and “hill heights” for input to

AERMOD using AERMAP , Version 11103 . The AERMAP input will include terrain elevation data from the National Elevation Dataset (NED). The NED data available on

- line in 1 arc

- second spacing

from the US Geological Survey will be used for this analysis. The receptor grid (extent defined below) will include receptors only in UTM Zone 14. The NED data for this analysis will be based on North American Datum (NAD) 83 for horizontal locations and NAD88 for vertical locations (elevations).

The NED terrain file downloaded from the USGS will be provided on CD along with all other model

I/O files.

## ATTACHMENT 1 NPPD, Gerald Gentleman Station

NDEQ F acility ID #34385, 07/15/2015

3 | Page HDR Engineering, Inc. RE CEPTOR GRID

Given there will be no nearby facilities included in this analysis, the receptor grid can be focused on just Gerald Gentleman Station

. The receptor grid will include the following spacing on the fence

lines and at downwind distances from the nearest fence lines.

• 50 meter spacing on the fence line

• 50 meter spacing from the fence to 1 kilometer from the fence

• 100 meter spacing from 1 kilometer to 2 kilometers from the fence

• 250 meter spacing from 2 kilometer to 5 kilometers from the fence

• 500 meter spacing from 5 kilometer to 7 kilometers from the fence

• 1000 meter spacing from 7 to 10 kilometers from the fence

The extent of this receptor grid is shown in the figure at the end of this protocol , and based on prior modeling experience , is expected to encompas s areas of maximum 1

- hour SO 2 concentration. Note that in accordance with EPA guidance 1 , receptors that would be over water (Sutherland Reservoir, just north of the facility) are excluded from analysis, given that it would not be practical to monitor at

these locations.

Any hot spots in the 250 meter and coarser receptor spacing will be refined by performing a separate model run centered on the hot spot, with a 1000

- meter by 1000 meter grid of 50

- meter spacing centered on the highest impact receptor from the initial model run.

BACKGROUND CONCENTRA TION

The b ackground 1

- hour SO 2 concentration will be based on data from the rural monitor located at Cedar Bluff Reservoir (EPA Site ID number 201950001) in Trego County in western Kansas. This

monitor is far fr om any nearby large SO 2 sources, so is representative of background concentrations in rural areas of Nebraska exclusive of nearby major source impacts.

Given that there are apparent monitor operational problems evidenced in the data for the most recent

ye ar of complete data, 2014, the background concentration will be based on the next most recent three years of available data, those being 2011

- 2013. The table below shows the calculated average of the

99 th percentile daily maximum 1

- hour value as 8.7 μg/m 3 across the three years. Therefore, a background 1

- hour SO 2 concentration of 9 μg/m 3 will be used for this analysis.

Year Daily Maximum 1

- hour,

99th Percentile

(ppm) ( μ g/m 3 )

2011 3 7.9

2012 4 10.5

2013 3 7.9

Average 3.3 8.7

1 SO 2 NAAQS Designations Modeling Technical Assistance Document, U.S. EPA Office of Air and Radiation Office of Air Quality Planning and Standards, Air Quality Assessment Division, December 2013, DRAFT (“EPA TAD”)

## ATTACHMENT 1 NPPD, Gerald Gentleman Station

NDEQ F acility ID #34385, 07/15/2015

4 | Page HDR Engineering, Inc. MODEL ING REPORT

A final m odeling report will be submitted to NDEQ for review, describing modeling procedures (attaching this protocol), mitigation features (design changes), if any, proposed by the utility, and including all model and preprocessor input and outpu t files on CD/DVD , with the exception of the meteorological data preprocessing files given that the NDEQ performed the meteorological data preprocessing. The data on CD/DVD will include all the hourly emissions (CEM) data files used to input actual emissi ons to AERMOD.

The modeling report will contain graphics displaying, at a minimum,

• source locations

,

• receptor locations

,

• meteorological data locations

,

• background monitor location,

• contour plots displaying modeled design values (for general receptor grid and any refined grid model runs), and

• a bar chart showing background plus source impact for comparison with the NAAQS.

A copy of t he final modeling files used to support the analysis will be provided on CD/DVD to :

Lisa Alam c/o Records Management Nebraska Department of Environmental Quality 1200 "N" Street, Suite 400

P.O. Box 98922 Lincoln, Nebraska 68509

- 8922

The data and graphics files included on the CD/DVD will include as a minimum:

• AERMOD input and output files (source and receptor input data file, hourly emissions file, output listing file , and output graphics/plot file )

• Contour plot and bar chart graphics file(s)

• Source location graphic file (*.kml) from Google Earth

• Source, met data and background monitor location map/graphic

• AERMAP terrain data processor input file

• Preprocessed meteorological data (*.sfc and *.pfl) files provided by NDEQ

• BPIP

- PRIME preprocessor input and output files

## ATTACHMENT 1 NPPD, Gerald Gentleman Station

NDEQ F acility ID #34385, 07/15/2015

5 | Page HDR Engineering, Inc. Receptor Grid for Gerald Gentleman Station SO 2 Dispersion Modeling



### Town of Sutherland NE