POLK 2-5 COMBINED CYCLE 

CONVERSION PROJECT 

SUPPLEMENTAL NITROGEN DIOXIDE 

AIR QUALITY IMPACT ANALYSIS 

SIMPLE CYCLE UNITS 2 THROUGH 5 

Prepared for: 

Tampa, Florida 

Prepared by: 

3701 Northwest 98th Street 

Gainesville, Florida 32606

ECT No. 110888-0200

March 2013 
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TABLE OF CONTENTS

Section Page

1.0 INTRODUCTION 1

2.0 MODELING PROCEDURES 3

2.1 MODELING APPROACH 3

2.2 MODELS 3

2.3 MODEL OPTIONS: 1-HOUR AND ANNUAL NO2 3

2.4 METEOROLOGICAL DATA 4

2.5 BUILDING WAKE EFFECTS 4

2.6 RECEPTOR GRIDS 4

2.7 MODELED EMISSIONS SOURCES—SCCT UNITS 2 

THROUGH 5 4

2.8 MODELED EMISSIONS SOURCES—OFFSITE 

SOURCES 5

2.9 BACKGROUND 1-HOUR NO2 CONCENTRATION 5

3.0 MODELING RESULTS 6

3.1 1-HOUR NO2 SIL ANALYSIS 6

3.2 ANNUAL NO2 SIL ANALYSIS 6

3.3 1-HOUR NO2 NAAQS ANALYSIS 6
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1.0 INTRODUCTION

An application for an air construction/prevention of significant deterioration (PSD) permit for the Tampa Electric Company (Tampa Electric) Polk 2-5 Conversion Project (Project) was submitted to the Florida Department of Environmental Protection (FDEP) in 

October 2012. This application included an air quality impact assessment for the four existing Polk Power Station (PPS) simple cycle combustion turbine (SCCT) Units 2 through 

5 following their conversion to combined cycle mode operation. The dispersion modeling 

analysis demonstrated that the Project will comply with all applicable ambient air quality 

standards.

The Project will be designed to allow the existing combustion turbine generators (CTGs)

to operate in simple cycle mode when the combined cycle mode steam turbine generator 

is out of service or to meet peak power demands. For each CTG, a diverter damper in 

combination with the existing SCCT stack will allow the CTG exhaust gas to bypass the 

heat recovery steam generator and flow directly to the atmosphere. The Project also includes the addition of fuel oil-firing for existing SCCT Units 4 and 5.

As part of its review of the air construction/PSD permit application, FDEP requested a 

dispersion modeling analysis to assess nitrogen dioxide (NO2) ambient air quality impacts for existing SCCT Units 2 through 5 when firing ultra-low sulfur diesel (ULSD)

fuel oil. This supplemental air quality analyses to the October 2012 air construction/PSD 

permit application provides the additional NO2 modeling requested by FDEP.

The results of the SCCT Units 2 through 5 supplemental NO2 modeling analysis are 

summarized as follows:

 The highest 1-hour average NO2 impact is 27.9 micrograms per cubic meter 

(μg/m3), which exceeds the 7.5-μg/m

3

PSD Class II 1-hour average NO2 significant impact level (SIL).

 The maximum annual average NO2 impact is 0.046 μg/m

3

, which is below 

the 1.0-μg/m

3

PSD Class II annual average NO2 SIL.
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 The cumulative NO2 model results indicate potential exceedances of the

1-hour NO2 national ambient air quality standards (NAAQS). However, 

SCCT Units 2 through 5 did not have a significant contribution to any of the 

modeled 1-hour average NO2 NAAQS exceedances. The maximum SCCT 

Units 2 through 5 contribution to any potential 1-hour average NO2 NAAQS 

exceedance was 0.31 μg/m

3

, well below the 7.5-μg/m

3 1-hour average NO2

SIL.

Section 2.0 of this supplemental report provides the modeling procedures used to assess 

SCCT Units 2 through 5 1-hour and annual average NO2 impacts. Section 3.0 presents the 

results of the modeling analyses.
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2.0 MODELING PROCEDURES

2.1 MODELING APPROACH

The supplemental NO2 dispersion modeling assessment was conducted in accordance 

with the methodology described in Section 6.0 of the October 2012 air construction/PSD 

permit application.

2.2 MODELS

The supplemental NO2 modeling analyses were conducted using the current version of 

the U.S. Environmental Protection Agency (EPA)-approved American Meteorological 

Society (AMS)/EPA Regulatory Model (AERMOD) modeling system. Specifically, 

AERMOD Version 12345 (December 11, 2012) was used.

2.3 MODEL OPTIONS: 1-HOUR AND ANNUAL NO2

Emissions of nitrogen oxides (NOx) from combustion sources consist of nitric oxide (NO) 

and NO2. At stack exit conditions, the primary species is NO, which typically comprises 

90 percent or more of total NOx.

AERMOD includes three options for estimating NO2 impacts:

 Tier 1—Assumes complete (i.e., 100-percent) conversion of NO to NO2.

 Tier 2—Ambient ratio method, which represents the average ambient 

NO2/NOx ratio. Current EPA guidance recommends using a ratio of 0.80.

 Tier 3—Uses the ozone limiting method and plume molar volume ratio 

method.

The Tier 1 option is an AERMOD regulatory default option that may be used without additional regulatory agency approval. The Tier 2 option has been historically accepted for 

regulatory modeling applications using an average ambient NO2/NOx ratio of 0.75. In accordance with EPA’s March 1, 2011, guidance, Tier 2 will be accepted for regulatory 

modeling applications if the EPA recommended average ambient NO2/NOx ratio of 0.80 

is used for 1-hour average NO2 impacts.
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For the SCCT Units 2 through 5 1-hour NO2 modeling, the Tier 2 approach with an average ambient NO2/NOx ratio of 0.80 was used. For annual NO2 impacts, the Tier 2 approach with an average ambient NO2/NOx ratio of 0.75 was used. As noted previously, 

the approach is acceptable without additional regulatory agency approval.

2.4 METEOROLOGICAL DATA

The same meteorological data as employed in the prior Polk 2-5 Conversion Project 

modeling (see Section 6.9 of the October 2012 air construction/PSD permit application)

was used for the supplemental NO2 modeling. This dataset consists of 5 years (i.e., 2006 

through 2010) of National Weather Service data from the Orlando International Airport 

surface and Tampa International Airport upper air weather stations.

2.5 BUILDING WAKE EFFECTS

The same assessment of building effects as described in Section 6.7 of the October 2012 

air construction/PSD permit application was used in the supplemental NO2 modeling 

analysis.

2.6 RECEPTOR GRIDS

The same receptor grids as described in Section 6.8 of the October 2012 air construction/PSD permit application was used in the supplemental NO2 modeling analysis. The 

total receptor grid included a total of 10,830 individual discrete receptors.

2.7 MODELED EMISSIONS SOURCES—SCCT UNITS 2 THROUGH 5

The modeled emissions sources consisted of the PPS four existing SCCT Units 2 through 

5 firing ULSD fuel oil. SCCT Units 2 through 5 may fire ULSD fuel oil for up to 

750 hours per year (hr/yr) under a variety of combustion turbine loads and ambient temperatures. The SCCTs may also use inlet air evaporative cooling. The October 2012 air 

construction/PSD permit application identified 11 operating cases for the SCCTs when 

firing ULSD fuel oil (reference Table B-1 of Appendix B). SCCT NOx emissions rates 

and stack parameters (i.e., stack exit velocity and temperature) will vary depending on 

these operating cases.
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To determine the SCCT operating case that will result in the highest 1-hour average NO2

impacts, one input file was created that included each SCCT ULSD fuel oil-firing case 

together with the stack parameters and hourly NOx emissions rate for each case. Emissions source groups were defined for each of the 11 operating cases. The AERMOD 

model was then run using this input file together with 5 years of meteorological data and 

the full receptor grid. Model output consisted of the maximum 1

st highest maximum daily 

1-hour results averaged over 5 years.

To determine the SCCT operating case that will result in the highest annual average NO2

impacts, five input files were created (one for each year of meteorological data) that included each SCCT ULSD fuel oil-firing case together with the stack parameters and annual NOx emissions rate for each case. Model output consisted of the maximum annual 

impact for each of the 5 years of meteorological data.

2.8 MODELED EMISSIONS SOURCES—OFFSITE SOURCES

The same offsite NO2 emissions inventory as described in Section 6.10.2 of the October 

2012 air construction/PSD permit application was used in the supplemental NO2 modeling analysis.

2.9 BACKGROUND 1-HOUR NO2 CONCENTRATION

For the cumulative modeling analysis, total ambient NO2 impacts consist of the modeled 

impacts together with a representative background concentration. The same 1-hour NO2

background concentration as described in Section 7.3 of the October 2012 air construction/PSD permit application was used in the supplemental NO2 modeling analysis; i.e., 

the 3-year average of the 8th highest daily maximum ambient air values (69.0 μg/m3) 

from the Hillsborough County Environmental Protection Commission (HCEPC) monitoring station located at 5121 Grandy Boulevard in Tampa (ID 12-057-1065) was used as 

background.
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3.0 MODELING RESULTS

Model results for the 1-hour and annual NO2 SIL and cumulative NAAQS analyses are 

discussed in the following sections.

3.1 1-HOUR NO2 SIL ANALYSIS

Table 3-1 summarizes the maximum 1

st highest maximum daily 1-hour average NO2 impacts averaged over 5 years for each of the 11 SCCT operating cases. The highest Tier 2 

1-hour average NO2 impact of 27.9 μg/m3occurred for SCCT ULSD fuel oil-firing 

Case SC-12; i.e., 100-percent load and 20 degrees Fahrenheit [°F] ambient temperature. 

Since this impact exceeds the PSD Class II 1-hour average NO2 SIL of 7.5 μg/m3, cumulative modeling is required to assess total 1-hour average NO2 impacts for comparison to 

the 1-hour average NO2 NAAQS.

3.2 ANNUAL NO2 SIL ANALYSIS

Table 3-2 summarizes the maximum annual average NO2 impacts for each SCCT operating case and for each of the 5 years of meteorology. The highest annual average NO2 impact of 0.046 μg/m3

occurred for SCCT ULSD fuel oil-firing Case SC-12 during 2007 

year meteorological conditions. Since this impact is below the PSD Class II annual average NO2 SIL of 1.0 μg/m

3

, no further modeling is required.

3.3 1-HOUR NO2 NAAQS ANALYSIS

Cumulative modeling that included SCCT Units 2 through 5, other PPS NOx emissions

sources, and the offsite NOx emissions inventory was conducted to obtain 1-hour NO2 air 

quality impacts for these emissions sources. The model results were then added to the 

representative background concentrations to obtain a total impact for comparison to the 

1-hour NO2 NAAQS. The cumulative modeling was conducted using the full 

10,830 individual receptor grids. Table 3-3 summarizes the results of the cumulative 

1-hour average NO2 modeling.

For modeling purposes, the 1-hour NO2 NAAQS is evaluated based on the 5-year average 

of the 98th percentile (i.e., highest, 8th highest) of the 1-hour daily maximum NO2
Table 3-1. 1st Highest Maximum Daily 1-Hour Average NO2

 SIL Impacts—Simple Cycle ULSD Fuel Oil

Number of SCCTs: 4

Tier 2 NO2/NOx ratio (1-hour): 0.80

Evaporative Ambient

Load Cooling Temperature

(%) (Yes/No') (°F) Tier 1 Tier 2

100 No 20 34.9 27.9 

75 No 20 33.3 26.7 

50 No 20 31.8 25.4 

100 No 59 34.3 27.5 

100 Yes 59 34.4 27.5 

75 No 59 32.1 25.7 

50 No 59 31.7 25.3 

100 No 90 33.4 26.7 

100 Yes 90 33.1 26.5 

75 No 90 31.2 25.0 

50 No 90 30.8 24.7 

Maximums 34.9 27.9 

Units Value

— SC-12

2006-2010

μg/m3 27.9

Annual Impact Location

Receptor UTM easting 

coordinate (X)

meters 402,470 

Receptor UTM northing 

coordinate (Y)

meters 3,065,883 

Receptor elevation meters 41.6 

Distance from stack* meters 1,371 

Direction vector from stack† degrees 184 

PSD SIL μg/m3 7.5 

Exceed SIL Yes/No Yes

Percent of SIL % 372.4

*Distance from SCCT Unit 3 stack to the location of highest impact.

†Direction from stack toward impact location. For example, 90° means the highest impact is located due east

 of the SCCT Unit 3 stack.

Source: ECT, 2013.

Case

Year

Tier 2 1-hour NO2 impact

5-Year Average

2006 through 2010 (μg/m3)

Case

Parameter

SC-12

SC-13

SC-14

SC-15

SC-16

SC-17

SC-18

SC-19

SC-20

SC-21

SC-22

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Table 3-2. Annual Average NO2 SIL Impacts—Simple Cycle ULSD Fuel Oil

Number of SCCTs: 4

Maximum annual hours: 750 hr/yr

Tier 2 NO2/NOx ratio (annual): 0.75

Evaporative Ambient

Load Cooling Temperature

(%) (Yes/No) (°F) Tier 1 Tier 2 Tier 1 Tier 2 Tier 1 Tier 2 Tier 1 Tier 2 Tier 1 Ti100 N 20 0.043 0.032 0.062 0.046 0.033 0.025 0.043 0.032 0.042 0

75 N 20 0.042 0.031 0.059 0.045 0.033 0.025 0.043 0.032 0.042 050 N 20 0.038 0.029 0.053 0.040 0.031 0.023 0.039 0.029 0.038 0

100 N 59 0.042 0.031 0.060 0.045 0.032 0.024 0.042 0.032 0.041 0100 Y 59 0.042 0.031 0.060 0.045 0.032 0.024 0.042 0.032 0.041 0

75 N 59 0.040 0.030 0.057 0.043 0.032 0.024 0.041 0.031 0.040 050 N 59 0.037 0.028 0.052 0.039 0.030 0.023 0.038 0.029 0.037 0

100 N 90 0.040 0.030 0.057 0.043 0.031 0.023 0.041 0.030 0.039 0100 Y 90 0.040 0.030 0.058 0.043 0.031 0.023 0.041 0.031 0.040 0

75 N 90 0.038 0.029 0.054 0.041 0.031 0.023 0.039 0.029 0.038 050 N 90 0.035 0.026 0.049 0.037 0.029 0.022 0.036 0.027 0.035 0

0.043 0.032 0.062 0.046 0.033 0.025 0.043 0.032 0.042 0

Units Value Value

Case — SC-12 PSD SIL 1 Year 2007 Exceed SIL NoTier 2 annual NO2 impact μg/m3 0.046 Percent of SIL 4.6

Annual Impact Location PSD de minimis ambient impact 14 

Receptor UTM easting coordinate (X) meters 400,845 Exceed PSD de minimis ambient impact NoReceptor UTM northing coordinate (Y) meters 3,067,301 Percent of PSD de minimis ambient impact 0.33Receptor elevation meters 43.2 Distance from stack* meters 1,724 Direction vector from stack† degrees 272 

*Distance from SCCT Unit 3 stack to the location of highest impact.†Direction from stack toward impact location. For example, 90° means the highest impact is located due east of the SCCT Unit 3 stack.

Source: ECT, 2013.

%

Parameter Units

μg/m3

Yes/No

%

μg/m3

Yes/No

SIL Impacts (μg/m3)

2006 2007 2008 2009 2010 

SC-21SC-22

Maximums

Parameter

SC-16SC-17SC-18SC-19SC-20SC-12SC-13SC-14SC-15

Case

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Table 3-3. 1-Hour NO2

 Cumulative Air Quality Impacts - Simple Cycle ULSD Fuel Oil

Units 2 

through 5

Parameter All Sources Contribution

Tier 1 highest 98th percentile of 1-hour daily maximum NO2 impacts (μg/m3)* 368.9 0.00413

Tier 2 highest 98th percentile of 1-hour daily maximum NO2 impacts (μg/m3)† 295.1 0.00330

Percent of impact (%) 100.0 0.0011

Receptor UTM easting (meter) 408,028 

Receptor UTM northing (meter) 3,071,964 

Receptor elevation (meter, amsl) 86.6

Distance from SCCT Unit 3 stack (meter)‡ 7,212 

Direction from SCCT Unit 3 stack (degrees)‡ 49 

Background 1-hour NO2 concentration (μg/m3) 69.0

Total 1-hour NO2 concentration (μg/m3) 364.1

1-hour NO2 NAAQS (μg/m3) 188 

Exceed 1-hour NO2 NAAQS? (Yes/No) Yes

Percent of 1-hour NO2 NAAQS (%) 193.7

Number of modeled exceedances of the 1-hour NO2 NAAQS (μg/m3)§ 6,963

Number of SCCT Units 2-5 significant impacts to modeled exceedances 0

Highest PPS Units 2-5 Contribution to an Exceedance

Total 1-hour NO2 concentration (μg/m3) 199.0

Receptor UTM easting (meter) 406,028 

Receptor UTM northing (meter) 3,057,214 

Receptor elevation (meter, amsl) 38.1

Distance from SCCT Unit 3 stack (meter)‡ 10,617 

Direction from SCCT Unit 3 stack (degrees)‡ 161 

SCCT Units 2 through 5 contribution (μg/m3) 0.307

SCCT Units 2 through 5 contribution (%) 0.13

1-Hour NO2 significant impact level (μg/m3) 7.5

*Assumes total conversion of NOx to NO2.

†Tier 1 impact multiplied by EPA-recommended NO2/NOx ratio of 0.80.

‡Distance and direction measured from SC Unit 3 stack located at UTM 402,568 meters easting and

 3,067,251 meters northing.

§For 5 years including all ranks and background concentrations.

Source: ECT, 2013.

2006 through 2010 5-Year

Average Impacts

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impacts. The AERMOD assessment of SCCT Units 2 through 5 contributions to the 

modeled exceedances was conducted by: (a) specifying NO2 on the AERMOD 

COPOLLUTID keyword, (b) selecting 1-hour averaging using the AVERTIME keyword, 

and (c) using the MAXDCONT keyword with defined source groups of ALL and SC.

The ALL source group includes all (offsite and onsite) modeled emissions sources. SC is 

the NOx emissions source code used in AERMOD for SCCT Units 2 through 5 with 

Case SC-12 emissions rates and stack parameters.

As shown in Table 3-3, the model results indicate potential exceedances of the 1-hour 

NO2 NAAQS. However, SCCT Units 2 through 5 did not have a significant contribution 

to any of the modeled NO2 NAAQS exceedances. The maximum SCCT Units 2 through 

5 contribution to any potential 1-hour average NO2 NAAQS exceedance was 0.31 μg/m

3

, 

well below the 7.5-μg/m

3

1-hour average NO2 SIL.