6

2.0 Visibility Assessment Methodology

The visibility assessment consists of an analysis o f baseline emission impacts to determine whether the FMC Granger coal-fired boiler s are ‘subject to BART’. Source parameters, baseline emission rates and baseline mo deling 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 meteoro logical 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 (versio n 5.51, level 030709) were used. The modeling domain specified in the AQD pro tocol as applicable for southwest Wyoming was used since the PSD Class I ar eas of interest include the Bridger and Fitzpatrick PSD Class I Wilderness Area s.

2.2 Meteorological Model

The CALMET meteorological model was used to determi ne hourly windfields for years 2001 through 2003. Mesoscale model meteorolo gical data (MM5) at 12- km grid resolution, surface and upper air meteorolo gical observation data, precipitation data, and geophysical data as provide d by the AQD were used. The modeling domain was processed to a uniform hori zontal 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 he ights 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 speci fied in the AQD protocol (3500 meters) since the upper air data provided by the AQ D was not sufficient to support a 3500 meter layer height. All other CALME T input switch settings and input control values were determined following the AQD protocol.7 For the initial screening analysis AQD used the yea rs 1995, 1996, and 2001 and created windfields with CALMET using MM5 data, surf ace and upper air meteorological data, and precipitation data. Howeve r, 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 emis sions of nitrogen oxides (NO x ), sulfur dioxide (SO 2 ), primary sulfate (SO 4 ) and particulate matter (PM).

SO 4 emissions were determined from sulfuric acid mist (H 2 SO 4 ) emissions using the ratio of the molecular weights of the two compo unds. All particulate matter

was assumed to be PM 2.5 .

CALPUFF was run using the control file switch setti ngs and values as provided in the AQD protocol. Model receptors determined for t he Bridger and Fitzpatrick Wilderness Areas were provided by the AQD. Chemica l transformations were modeled based on the MESOPUFF II chemistry mechanis m for conversion of

SO 2 to SO 4 and NO x to nitric acid (HNO 3 ) and nitrate (NO 3 ). A background ammonia (NH 3 ) concentration value of 2.0 parts per billion (ppb ) was specified. Background hourly ozone concentration data from mon itoring sites throughout the modeling domain were provided by the AQD for ea ch modeled year.

2.4 Model Post-Processing

The CALPUFF model results were post-processed using the CALPOST processor to estimate visibility impacts at the Bri dger and Fitzpatrick Wilderness Areas. Visibility impacts were determined using CA LPOST method 6. Method 6 calculates visibility impacts from CALPUFF predicte d concentration data, relative humidity factors, and aerosol components. Relative humidity factors and background concentrations of aerosol components for the 20 percent best days8 applicable to the Bridger and Fitzpatrick Wildernes s Areas and all CALPOST switch settings were provided from the AQD protocol

.

CALPOST was used to determine both the 98 th percentile,or 8 th highest, 24-hour visibility impacts for each year and the maximum nu mber of days per year above 0.5 delta deciviews ( **∆** dv).

2.5 Modeled Sources

The source parameters for the FMC Granger coal-fire d 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 inco rrect UTM N coordinate for UIN-15 (4614077 meters) and none of the UTM coordin ates 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 coa l-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 SO 2 ,

lbs/hr NOx,

lbs/hr H 2 SO 4 ,

lbs/hr

UIN-14 #1 coal-fired

boiler 175 134,600 35.85 71.7 250.95 3.279 UIN-15 #2 coal-fired

boiler 175 134,600 35.85 71.7 250.95 3.27

The baseline emission rates represent the maximum p otential to emit values for each pollutant for each of the boilers. Particula te matter is assumed to be PM 2.5 .

For the initial screening analysis AQD used stack t emps of 146 deg F and exhaust flow rates of 138,400 ACFM. The data in Tab le 2-2 reflects the maximum design for the boilers and is consistent wi th the information in Operating Permit 30-083.

Also, based on the information contained in the Div ision’s BART Air Modeling Draft Final Report, it appears that sulfuric acid m ist (H 2 SO 4 ) was not included in the initial AQD CALPUFF screening analysis.

2.6.2 Baseline Modeling Results

The baseline modeling results for the FMC Granger c oal-fired boilers for each modeled year are shown below in Table 2-3. Th e number of days

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

Table 2-3 Baseline Visibility Modeling Results

2001 2002 2003

Class I Area 98 th

Percentile Value ( ∆ dv) No. of days

exceeding 0.5 ( ∆ dv) 98 th

Percentile Value ( ∆ dv) No. of days

exceeding 0.5 ( ∆ dv) 98 th

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 t he FMC Granger boilers do not exceed the visibility impact thresho ld of a 98 th percentile change in visibility of 0.5 deciviews (dv) above ba ckground conditions. Therefore, FMC Granger is not subject to BART.