## Omaha Public Power District

Sulfur Dioxide 1

- Hour NAAQS Designations Modeling

for

Nebraska City Station (Facility ID #58343)

Nebraska City, 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.

August 20 15NCS 1

- Hour SO 2 Modeling 1 Aug. 2015

INTRODUCTION

This modeling report summarizes dispersion modeling 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 m eter (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

reviewed and approved by 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.NCS 1

- Hour SO 2 Modeling 2 Aug. 2015

Figure 1 displays the source locations of the two point sources included in the model, which are the coal

fired boiler stacks for Nebraska City Unit 1 ( NC1 ) and Nebraska City Unit 2 ( NC2 )

.

Figure 1: Source LocationsNCS 1

- Hour SO 2 Modeling 3 Aug. 2015

Figure 2 displays the receptor locations as specified in the attached dispersion modeling protocol in

accordance with NDEQ guidance.

Figure 2: Receptor LocationsNCS 1

- Hour SO 2 Modeling 4 Aug. 2015

Figure 3 displays the O PPD facility in relation to the background SO 2 monitor location and

meteorological data stations for surface data (Falls City, Nebraska) and upper

- air data (Topeka, Kansas)

.

Figure 3: Site, Monitor, and Meteorological Station Locati ons.

The stack parameter inputs for NC1 and NC2 at OPPD are provided in Table 1. The stack temperature

and velocity are placeholder average 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.

Table 1: Source Input Data for Modeling A nalysis

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)

NC1 265387 4500392 281.9 213.36 415.37 30.02 7.19

NC2 265447 4500318 281.9 121.92 347.04 24.29 7.01NCS 1

- Hour SO 2 Modeling 5 Aug. 2015

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. Th e hourly emissions data files (including sta c k

exhaust temperature and velocity data) are provided electronically 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 264750 4505000 69.5 78.5 196.5

Figure 4: Modeled Result vs. SO 2 S tandard

0 50 100 150 200 250

SO2 Modeled Result SO2 Standard SO

2 μg/m 3 Standard

Modeled Result

BackgroundNCS 1

- Hour SO 2 Modeling 6 Aug. 2015

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

are: Orange : 50 μ g/m 3 , Yellow : 55 μ g/m 3

, and Pink : 60 μ g/m 3 . The location of the maximum 4 th high 3

-

year average modeled (without background) concentration (69.5 μ g/m 3 ) is displayed for reference.

Figure 5: Contour Plot of SO 2 1

- Hour Design Concentrations

50 μg/m 3

55 μg/m 3

60 μg/m 3NCS 1

- Hour SO 2 Modeling 7 Aug. 2015

CONCLUSION

This modeling analysis shows that the SO 2 impact from NCS, when added to the estimated background

concentration of 9 μg/m 3 , would cause a total maximum impact of 78.5 μg/m 3 , which is well below

(under 50%) of the SO 2 1

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