Nebraska Public Power District

Sulfur Dioxide 1-Hour NAAQS Designations Modeling

for

Sheldon Station (Facility ID #33563)

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, 2015
Sheldon 1-Hour SO2 Modeling 1 Sept. 2015

INTRODUCTION

This modeling report summarizes dispersion modeling of Sheldon Station to demonstrate attainment with 

the sulfur dioxide (SO2) National Ambient Air Quality Standard (NAAQS) for the 1-hour averaging 

period, which is equal to 75 parts per billion (ppb) or approximately 196.5 micrograms per cubic meter 

(μg/m3

). This modeling has been completed for submission 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. 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 for SO2 NAAQS designations.

The modeling in this report demonstrates compliance with the SO2 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 scenario 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 design 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 whether one or both units continue with operation on coal.

MODELING PROTOCOL

The 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

This modeling report contains figures detailing the modeled sources, receptors, background SO2

concentration and meteorological data locations, and the resulting 1-hour SO2 modeled impacts. The SO2

modeling was conducted in accordance to the attached detailed dispersion modeling protocol.
Sheldon 1-Hour SO2 Modeling 2 Sept. 2015

Figure 1 displays the source locations of the two point sources included in the model, which are the coalfired boiler stacks for Sheldon Station Unit 1 and Unit 2.

Figure 1: Source Locations
Sheldon 1-Hour SO2 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, NE
Sheldon 1-Hour SO2 Modeling 4 Sept. 2015

Figure 3 displays the Sheldon Station location in relation to the background SO2 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 SO2 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 were 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 represent 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 

Source ID X Y Diameter

(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 stack 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 SO2 concentration is summarized in 

Table 2 and presented visually in Figure 4, for both operating scenarios. The 4th high 3-year average 

concentrations are compared with the 1-hour SO2 NAAQS of 75 parts per billion (ppb) or approximately 

196.5 micrograms per cubic meter (μg/m3

). The background 1-hour SO2 concentration of 9 μg/m3 was 

added to the modeled concentration for each scenario. The modeled plus background concentrations are

below the NAAQS for each scenario, as shown in Table 2. 

Table 2: Modeled Impacts (with background of 9 μg/m3

)

Pollutant

Avg.

Period Scenario

UTM Coordinates Modeled 

Concentration

Modeled Plus 

Easting Northing Background NAAQS

(m) (m) (μg/m3) (μg/m3) (μg/m3)

SO2 1-hour Unit 1 Only 687950 4491850 176.6 185.6 196.5

SO2 1-hour Unit 1 & 2 688050 4491750 171.0 180.0 196.5
Sheldon 1-Hour SO2 Modeling 6 Sept. 2015

Figure 4: Modeled Result vs. SO2 Standard

Figures 5 and 6 are contour plots of the modeled 1-hour design concentrations (not including 

background), for operating scenarios 1 and 2, respectively. These plotted data represent the average of 

the daily maximum 4th 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/m3

,

Yellow: 130 μg/m3

, and Pink: 160 μg/m3.

The location of the maximum 4th 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 Standard

SO2 g/s

Standard

Modeled Results

Background
Sheldon 1-Hour SO2 Modeling 7 Sept. 2015

Figure 5: Contour Plot of SO2 1-Hour Design Concentrations (μg/m3) – Unit 1 Only
Sheldon 1-Hour SO2 Modeling 8 Sept. 2015

Figure 6: Contour Plot of SO2 1-Hour Design Concentrations (μg/m3) – Units 1 & 2
Sheldon 1-Hour SO2 Modeling 9 Sept. 2015

CONCLUSION

This modeling analysis shows that the design maximum SO2 impact from Sheldon Station, when added to 

the estimated background concentration of 9 μg/m3

, would comply with the SO2 1-hour NAAQS of 196.5 

μg/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 stack height will demonstrate compliance with the 1-hour SO2 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, Facility ID #33563

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HDR Engineering, Inc.

Sulfur Dioxide 1-Hour NAAQS Designations Modeling Protocol

Nebraska Public Power District, Sheldon Station (Facility ID #33563)

Hallam, NE

July 15, 2015

INTRODUCTION

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

District (NPPD) Sheldon Station, in Lancaster County, near Hallam, 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 (SO2) 

concentration, which is equal to 75 parts per billion (ppb) or approximately 196.5 micrograms per 

cubic meter (μg/m3

)

This protocol has been prepared for 1-hour SO2 State Implementation Plan (SIP) dispersion modeling 

and is being submitted 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 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 designations for affected areas with respect to the 1-

hour SO2 NAAQS.

MODELING SOFTWARE

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

• AERMOD (Version 14134) 

• BPIP-Prime (Version 04274) 

• AERMAP (Version 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 will 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 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 AVERAGING PERIOD

The AERMOD model will be executed for SO2 for 1-hour averages. By selecting SO2 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 the 1-hour SO2 NAAQS of 

196.5 μg/m3 (75 parts per billion) will be the maximum of the 3-year average of the 4th highest (99th 
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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 will include only the Unit 1 and 2 coal boiler 

stacks. 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 with 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 group) concentrations.

The actual, hourly SO2 emissions measured by the continuous emissions monitoring system (CEMS) 

on the Unit 1and 2 stacks will be used for this analysis, by using the optional 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 SO2 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 SO2 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 AERMOD are the stack exit diameters. The stack diameters to 

be input for Units 1 and 2 will be identical at 11.6 feet (3.536 m), based on the current design of these

stacks. 

BUILDING DOWNWASH INPUTS

The AERMOD input will include building downwash parameters calculated using the EPA’s 

Building Profile Input Program “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 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 files.
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RECEPTOR 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 expected to encompass areas of maximum 1-hour SO2 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 spacing 

centered on the highest impact receptor from the initial model run.

BACKGROUND CONCENTRATION

The background 1-hour SO2 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 SO2 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 

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 the 

99th percentile daily maximum 1-hour value as 8.7 μg/m3 across the three years. Therefore, a 

background 1-hour SO2 concentration of 9 μg/m3 will be used for this analysis. 

Year

Daily Maximum 1-hour, 

99th Percentile

(ppm) (μg/m3)

2011 3 7.9

2012 4 10.5

2013 3 7.9

Average 3.3 8.7
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MODELING REPORT 

A final modeling 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 emissions (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 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 the 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, Sheldon Station, Facility ID #33563

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Receptor Grid for Sheldon Station SO2 Dispersion Modeling

Town of Hallam, NE