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2.0 Visibility Assessment Methodology 

The visibility assessment consists of an analysis of baseline emission impacts to 

determine whether the FMC Granger coal-fired boilers are ‘subject to BART’. Source 

parameters, baseline emission rates and baseline modeling results are presented in this 

section. 

2.1 Modeling Methodology 

The protocol indicates that visibility assessments are to be conducted using the 

CALPUFF modeling system, and three years of meteorological data, years 2001 

through 2003. The modeling system is comprised of the CALMET 

meteorological model, the CALPUFF dispersion model, and the CALPOST post 

processor. For this analysis CALMET (version 5.53a, level 040716), CALPUFF 

(version 5.711a, level 040716), and CALPOST (version 5.51, level 030709) were 

used. The modeling domain specified in the AQD protocol as applicable for 

southwest Wyoming was used since the PSD Class I areas of interest include the 

Bridger and Fitzpatrick PSD Class I Wilderness Areas. 

2.2 Meteorological Model 

The CALMET meteorological model was used to determine hourly windfields for 

years 2001 through 2003. Mesoscale model meteorological data (MM5) at 12-

km grid resolution, surface and upper air meteorological observation data, 

precipitation data, and geophysical data as provided by the AQD were used. 

The modeling domain was processed to a uniform horizontal grid using 4-lm 

resolution, based on a Lambert Conformal Projection defined with a central 

longitude/latitude at (-108.55°, 42.55°) and first and second latitude parallels at 

30° and 60°. Ten vertical layers were used with heights of 20, 40, 100, 140, 320, 

580, 1020, 1480, 2220, and 3400 meters. The height used for the top layer 

(3400 meters) differed slightly from what was specified in the AQD protocol (3500 

meters) since the upper air data provided by the AQD was not sufficient to 

support a 3500 meter layer height. All other CALMET input switch settings and 

input control values were determined following the AQD protocol. 
 

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For the initial screening analysis AQD used the years 1995, 1996, and 2001 and 

created windfields with CALMET using MM5 data, surface and upper air 

meteorological data, and precipitation data. However, the MM5 data was at a 

much coarser grid resolution (36-km vs. 12-km). By using finer scale MM5 data, 

there is more variability in met conditions between each grid cell in the CALMET 

model domain. 

2.3 Dispersion Model 

The CALPUFF dispersion model was used to model emissions of nitrogen oxides 

(NOx), sulfur dioxide (SO2), primary sulfate (SO4) and particulate matter (PM). 

SO4 emissions were determined from sulfuric acid mist (H2SO4) emissions using 

the ratio of the molecular weights of the two compounds. All particulate matter 

was assumed to be PM2.5. 

CALPUFF was run using the control file switch settings and values as provided in 

the AQD protocol. Model receptors determined for the Bridger and Fitzpatrick 

Wilderness Areas were provided by the AQD. Chemical transformations were 

modeled based on the MESOPUFF II chemistry mechanism for conversion of 

SO2 to SO4 and NOx to nitric acid (HNO3) and nitrate (NO3). A background 

ammonia (NH3) concentration value of 2.0 parts per billion (ppb) was specified. 

Background hourly ozone concentration data from monitoring sites throughout 

the modeling domain were provided by the AQD for each modeled year. 

 

2.4 Model Post-Processing 

The CALPUFF model results were post-processed using the CALPOST 

processor to estimate visibility impacts at the Bridger and Fitzpatrick Wilderness 

Areas. Visibility impacts were determined using CALPOST method 6. Method 6 

calculates visibility impacts from CALPUFF predicted concentration data, relative 

humidity factors, and aerosol components. Relative humidity factors and 

background concentrations of aerosol components for the 20 percent best days 
 

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applicable to the Bridger and Fitzpatrick Wilderness Areas and all CALPOST 

switch settings were provided from the AQD protocol. 

CALPOST was used to determine both the 98th percentile,or 8th highest, 24-hour 

visibility impacts for each year and the maximum number of days per year above 

0.5 delta deciviews (∆dv). 

2.5 Modeled Sources 

The source parameters for the FMC Granger coal-fired boilers units are 

summarized below: 

 Table 2-1 

Source Parameters 

Source 

ID 

Source 

Name 

UTM E, 

meters 

UTM N, 

meters 

Base 

Elevation, 

ft 

Stack 

Height 

ft. 

Stack 

Dia., 

ft. 

UIN-14 

#1 coal-fired 

boiler 591757 4613883 6360.99 150 6.5 

UIN-15 

#2 coal-fired 

boiler 591770 4613884 6360.99 150 6.5 

For the initial screening analysis AQD used an incorrect UTM N coordinate for 

UIN-15 (4614077 meters) and none of the UTM coordinates were properly 

converted to Lambert Conformal coordinates. 

2.6 Baseline Analyses 

2.6.1 Baseline Emission Rates 

The baseline emission rates for the FMC Granger coal-fired boilers are 

summarized below: 

Table 2-2 

Baseline Emission Rates 

Source 

ID 

Source 

Name 

Stack 

Temp, 

deg F 

Exhaust 

Rate, 

ACFM 

PM 

lbs/hr 

SO2, 

lbs/hr 

NOx, 

lbs/hr 

H2SO4, 

lbs/hr 

UIN-14 

#1 coal-fired 

boiler 175 134,600 35.85 71.7 250.95 3.27 
 

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UIN-15 

#2 coal-fired 

boiler 175 134,600 35.85 71.7 250.95 3.27 

The baseline emission rates represent the maximum potential to emit values for 

each pollutant for each of the boilers. Particulate matter is assumed to be PM2.5. 

For the initial screening analysis AQD used stack temps of 146 deg F and 

exhaust flow rates of 138,400 ACFM. The data in Table 2-2 reflects the 

maximum design for the boilers and is consistent with the information in 

Operating Permit 30-083. 

Also, based on the information contained in the Division’s BART Air Modeling 

Draft Final Report, it appears that sulfuric acid mist (H2SO4) was not included in 

the initial AQD CALPUFF screening analysis. 

2.6.2 Baseline Modeling Results 

The baseline modeling results for the FMC Granger coal-fired boilers for 

each modeled year are shown below in Table 2-3. The number of days 

above 0.5 ∆dv and the 98th percentile values are provided. 

Table 2-3 

Baseline Visibility Modeling Results 

2001 2002 2003 

Class I 

Area 

98th

Percentile 

Value 

(∆dv) 

No. of days 

exceeding 

0.5 (∆dv) 

98th

Percentile 

Value 

(∆dv) 

No. of days 

exceeding 

0.5 (∆dv) 

98th

Percentile 

Value 

(∆dv) 

No. of days 

exceeding 

0.5 (∆dv) 

Bridger 0.33 1 0.26 2 0.39 5 

Fitzpatrick 0.14 0 0.20 1 0.17 0 

The results in Table 2-3 clearly demonstrate that the FMC Granger 

boilers do not exceed the visibility impact threshold of a 98th percentile 

change in visibility of 0.5 deciviews (dv) above background conditions. 

Therefore, FMC Granger is not subject to BART.