## Nebraska Public Power District

Sulfur Dioxide 1

- Hour NAAQS Designations Modeling

for

Sheldon Station (Facility ID # 3 3563 )

Hallam , 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 11, 20 15Sheldon 1

- Hour SO 2 Modeling 1 Sept. 2015 INTRODUCTION

This modeling report summarizes dispersion modeling of Sheldon 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 (ppb) 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 th e NDEQ to formulate recommendations to EPA on the NAAQS designations

. Although

the Lincoln

- Lancaster Health Department has air emissions permitting authority for Sheldon Station, the
NDEQ has responsibility for review of all Nebraska dispersion modeling f or SO 2 NAAQS designations.

The modeling in this report demonstrates compliance with the SO 2 1

- hour NAAQS by inclusion of stack
height increases for the Unit 1 and 2 boiler stacks. Given that NPPD has not yet decided if or when Unit
2 might be converted to gaseous fuel containing negligible sulfur, this modeling report provides results
for two operating scenarios. One scenario includes only Unit 1 operating on coal, and assumes an
increase in Unit 1 stack height from 174 feet to 199 feet. The second scena rio assumes both Unit 1 and
Unit 2 are operating on coal, and assumes an increase in the height of both stacks to 215 feet. Note that
while the de minimis good engineering practice (GEP) stack height is 65 meters (213 feet), the
prospective height of the stacks for the second operating scenario is well below the calculated GEP height

of 102.3 meters ( 335.6 feet ) for each stack.

While the modeling for each of the two scenarios described above demonstrates compliance with the
NAAQS, NPPD has not made final d esign decisions on actual stack heights or other modifications to be
installed, but plans to make sufficient facility modifications to demonstrate compliance with the NAAQS,
regardless of wh ether one or both units continue with operation on coal

.

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 f igures 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.Sheldon 1

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

fired boiler stacks for Sheldon Station Unit 1 and Unit 2

.

Figure 1: Source LocationsSheldon 1

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

accordance with NDEQ guidance.

Figure 2: Receptor Locations

Town of Hallam, NESheldon 1

- Hour SO 2 Modeling 4 Sept. 2015 Figure 3 displays the Sheldon Station location in relation to the background SO 2 monitor location (Trego
County, Kansas) and meteorological data stations for surface data ( Lincoln , Nebraska) and upper

- air data

( Omaha , Nebraska )

.

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

- Hour SO 2 Modeling 5 Sept. 2015 The stack parameter inputs for Sheldon Station Units 1 & 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 velocities for both stacks for each hour of the model simulation, which spanned the
years 2012

- 2014. Note that the stack heights listed repres ent increases from the current 174 feet (53
meters) stacks on both units. Under Scenario 1, only the Unit 1 stack would be raised to 199 feet (60.66
meters) and under Scenario 2, Unit 1 and Unit 2 stacks would be raised to 215 feet (65.53 meters).

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 687597.9 4492175.0 438.9 60.66 /65.53 452 26.88 3.54

Unit 2 687585.2 4492213.3 438.9 65.53 450 26.81 3.54

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 electronically on the CD/DVD accompanying this report.

MODEL RESULTS

Modeled design impact for the 3

- year average (2012

- 2014) 1

- hour SO 2 concentration is summarized in

Table 2 and presented visually in Figure 4 , for both operating scenarios . 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 background 1

- hour SO 2 concentration of 9 μg/m 3 was

added to the modeled concentration for each scenario . The m odeled p lus b ackground concentration s are

below the NA AQS for each scenario, as shown in Table 2

.

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

Pollutant Av g. Period Scenario 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 Unit 1 Only 687950 4491850 176.6 185.6 196.5

SO 2 1

- hour Unit 1 & 2 688050 4491750 171.0 180.0 196.5Sheldon 1

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

Figure s 5 and 6 are contour plot s of the modeled 1

- hour design concentration s (not including
background)

, for operating scenarios 1 and 2, respectively. These plotted data represent 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 are: Orange : 100 μg/m 3

, Yellow : 1 3 0 μg/m 3

, and Pink: 1 6 0 μg/m 3

.

The location of the maximum 4 th high 3

- year average modeled ( without background) is displayed for

reference in each plot

. For each operating scenario, this maximum was only a few hundred meters

southeast of the boiler stacks. 0 50 100 150 200 250

199 - Unit1 215 - Unit 1&2 StandardSO2g/s Standard

Modeled Results

BackgroundSheldon 1

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

- Hour Design Concentrations ( μg/m 3 )

– Unit 1 OnlySheldon 1

- Hour SO 2 Modeling 8 Sept. 2015 Figure 6 : Contour Plot of SO 2 1

- Hour Design Concentrations ( μg/m 3 )

– Units 1 & 2Sheldon 1

- Hour SO 2 Modeling 9 Sept. 2015 CONCLUSION

This modeling analysis shows that the design maximum SO 2 impact from Sheldon Station , when added to
the estimated background concentration of 9 μg/m 3

, would comply with th e SO 2 1

- hour NAAQS of 196.5

μ g/m3 (75 ppb) for each operating scenario, with the associated adjustments in stack heights

. NPPD may
elect to modify stack heights differently than those analyzed here, but these results show that relatively

modest increases in st ack height will demonstrate compliance with the 1

- hour SO 2 NAAQS for only the

Unit 1 boiler operating on coal, or both the Unit 1 and 2 boilers operating on coal.

## ATTACHMENT 1 NPPD, Sheldon Station , F acility ID #3 3563

07/15/2015

1 | Page HDR Engineering, Inc. Sulfur Dioxide 1

- Hour NAAQS Designations Modeling Protocol

Nebraska Public Power District, Sheldon Station ( Fa cility ID # 3 3563 )

Hallam , NE

Ju ly 1 5 , 201 5

INTRODUCTION

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

District (NPPD) Sheldon S tation, in Lancaster County, near Hallam , Nebraska

. The protocol

summarizes the information that will be used to conduct dispersion modeli ng 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) or 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, along with any available monitoring data, to propose and finalize attainment/nonattainment designation s 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 rec ent versions of AERMET (Version 14134) , AERMINUTE (Version 14337) , and

AERSURFACE (Version 13016)

.

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

Omaha , Nebraska , which are considered representative of the Hallam 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 aver aging period, AERMOD will automatically average the results over the three years of meteorology. The model result for comparison with the 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, Sheldon Station , F acility ID #3 3563

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 SOURCES

Emission points to be modeled for Sheldon Station w ill include only the Unit 1 and 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 Sheldon Station . Thus, total impact for comparison wit h the NAAQS will consist of Unit 1 for Scenario 1, and Unit 1 and Unit 2 combined for Scenario 2, 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 g roup) 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 sing le 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 recorded 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 abov e, 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 identical at 11.6 feet ( 3.536 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 modelin g I/O files on CD with the final modeling report.

TERRAIN ELEVATIONS

Terrain data will be processed to determine 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 fil es.

## ATTACHMENT 1 NPPD, Sheldon Station , F acility ID #3 3563

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 Sheldon 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 exp ected to encompass areas of maximum 1

- hour SO 2 concentration.

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 spaci ng 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 from 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 operati onal problems evidenced in the data for the most recent year 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 t he

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

## ATTACHMENT 1 NPPD, Sheldon Station , F acility ID #3 3563

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 output 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 e missions (CEM) data files used to input actual emissions to AERMOD.

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

• source locations

,

• receptor locations

,

• meteorological data locations

,

• background monitor location,

• contour plots di splaying 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 provid ed 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 in put 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, Sheldon Station , F acility ID #3 3563

07/15/2015

5 | Page HDR Engineering, Inc. Receptor Grid for Sheldon Station SO 2 Dispers ion Modeling



### Town of Hallam, NE