Appendix 11.2 

Holcomb Class I Visibility Modeling Report 
Visibility Impact Modeling Results 

 

Holcomb Station Expansion

prepared for: 

Sunflower Electric Power Corporation 

Hays, Kansas 

for submission to: 

Kansas Department of Health & Environment 

prepared by: 

HDR Engineering, Inc. 

701 Xenia Ave. South, Suite 600 

Minneapolis, MN 55416 

(763)-591-5452 

Attn: Ed Liebsch 

Ed.Liebsch@hdrinc.com

June 8, 2007
Holcomb Station Expn., Visibility Results 1 June 8, 2007 

Visibility Impact Modeling Results 

 Holcomb Station Expansion

1.0 INTRODUCTION 

This report briefly summarizes the results of Class I area dispersion modeling for the proposed 

expansion of Holcomb Station, located in western Kansas, west of Garden City. The proposed 

expansion would consist of two nominal 700 MW electric generating units and support 

equipment. Holcomb Station currently has one nominal 360 MW electric generating unit, with 

air emission controls consisting of lo-NOX burners for nitrogen oxide (NOX) control, a lime spray 

dryer (dry scrubber) for sulfur dioxide (SO2) control and a fabric filter (baghouse) for particulate 

matter control. The proposed new units at the site would use state-of-the-art emission controls to 

minimize NOX, SO2 and other acid gases, particulate matter, and mercury. 

The analysis presented in this report was requested by the Kansas Department of Health & 

Environment (KDHE), which is the agency responsible for issuance of a construction (Prevention 

of Significant Deterioration) air permit, in consultation with US Environmental Protection 

Agency (EPA) Region 7 staff, and US Fish & Wildlife Service (FWS) staff. 

EPA’s New Source Review (NSR) guidance (DRAFT, October 1990 New Source Review 

Workshop Manual) states that Class I impact analysis are necessary for major sources locating 

within 100 km of a Class I area. While the proposed project, which will be a major modification 

of the facility, is located much farther away from any Class I area, the EPA NSR guidance states 

that sources more distant than 100 km should be considered for analysis if they are large and 

considered to have a potential for adverse impact at Class I areas. At the request of FSW and 

KDHE, Sunflower Electric has completed a Class I visibility impact analysis with the assistance 

of HDR Engineering, Inc. 

The proposed project is quite distant from the nearest Class I areas, at nearly 400 km from Great 

Sand Dunes National Monument in Colorado, and slightly over 400 km from Wichita Mountains 

National Wildlife Refuge in south-central Oklahoma. While the Wichita Mountains area is 

slightly farther away from the plant site than the Great Sand Dunes, Wichita Mountains is in a 

more predominant wind direction. This analysis focuses primary on Wichita Mountains because 

of higher expected impacts than at Great Sand Dunes, but the impacts at both areas are 

summarized in this report. 

2.0 METHODOLOGY 

The atmospheric dispersion and atmospheric chemistry modeling for estimating visibility impacts 

was accomplished with the current regulatory versions of the CALPUFF modeling system, which 

is an EPA-guideline approach for estimating visibility impacts at distances beyond 50 km and up 

to a few hundred km from a Class I area. CALPUFF has generally been recognized as being 

applicable up to 300 km downwind, and is considered to become over-conservative (high) in its 

impact predictions beyond 300 km. Given that the Class I areas analyzed here are nearly 400 km 

away, the CALPUFF visibility impacts presented here are considered to be overestimates of the 

impacts that should be expected from the proposed facility. 
Holcomb Station Expn., Visibility Results 2 June 8, 2007 

The methodology for estimating impacts with the CALPUFF system (CALMET meteorological 

preprocessor, CALPUFF model, POSTUTIL postprocessor and CALPOST postprocessor) is 

detailed in the attached, final modeling protocol. This protocol was the subject of two review 

cycles over the past months with staff from KDHE, EPA Region 7, and FWS providing comment 

on the protocol. Changes recommended by these agencies have been incorporated into the final 

protocol, which included as Attachment 1 of this report. 

3.0 RESULTS 

3.1 Model Input and Results Summary

The results of the CALPUFF visibility analysis are presented in this section. This analysis is 

based on the emissions factors and rates shown in Table 1 as input to CALPUFF. These emission 

rates are based on a maximum heat input rate, for both units combined, of 13,002 mmBtu/hour. 

Table 1. Emission Rates Input to CALPUFF Visibility Analysis 

Pollutant 

Emission Factor (lb/mmBtu) Emission Rate 

(lb/hr, both units) 

Sulfur dioxide 0.101 1313.2 

Nitrogen oxides (as NO2) 0.05 650.1 

Coarse Particulate Matter 0.00384 49.9 

Fine Particulate Matter 0.0103 129.2 

Elemental Carbon PM 0.000382 5.0 

 

Tables 2 and 3 present the CALPUFF (actually CALPOST postprocessor) results for visibility 

impact at the Great Sand Dunes and Wichita Mountains Class I areas, respectively. 

At Great Sand Dunes, there are three episodes (days) when modeled light extinction exceeded the 

Federal Land Managers’ (FLMs) de-minimis threshold of 5%. This would normally be grounds 

for triggering a more refined analysis. Meteorological data for these days at the nearest weather 

site, Alamosa, Colorado, were reviewed to determine conditions in the area on these days. The 

numbers in parentheses next to the listed values over 5% represent the Julian Day number of the 

subject episode. For all three of the days over 5% visibility modeled impact, the weather 

conditions were generally clear at the Alamosa observing station. 

While the FLMs’ guidance for visibility modeling has for the past few years specified light 

extinction as the metric for assessing visibility impact, they are drafting guidance to soon change 

this to a deciview metric, based on a 98th percentile impact. This would be consistent with EPA’s 

recommended visibility metric for Best Available Retrofit Technology (BART) analyses. In 

general, states and EPA have been using a 0.5 deciview impact level (98th percentile, for a refined 

analysis) as a basis for assessing whether a source is exempt from BART retrofits. At the request 

of the FWS, the “Method 6” results in the above table are presented for comparison with the 

anticipated new FLM metric. The results in Table 2 show that for Great Sand Dunes, the 98th

percentile impacts would be well below the 0.5 deciview threshold. 

Thus, there appears little potential for adverse impact at Great Sand Dunes, especially when 

considering the more statistically robust 98th percentile deciview metric, and there does not 
Holcomb Station Expn., Visibility Results 3 June 8, 2007 

appear to be a need for refined analysis. Also, considering that CALPUFF likely significantly 

overestimates the impacts, there does not appear to be cause for concern at Great Sand Dunes. 

Table 2. Summary of Great Sand Dunes Visibility Impact Results, in Percent Light 

Extinction Increase, and in Deciview Increase 

Background Great Sand Dunes 

Visibility Method Rank 2001 2002 2003 

1 2.60 11.17 (307) 8.27 (311)

2 1.60 8.12 (296) 4.08 

3 1.40 4.00 3.59 

4 1.11 3.18 2.90 

5 0.78 2.92 1.59 

6 0.67 2.76 1.40 

7 0.56 2.66 1.20 

Method 2 (all 

values in % light 

extinction)

8 0.49 2.62 1.10 

1 0.171 0.549 0.572 

2 0.153 0.404 0.361 

3 0.146 0.234 0.261 

4 0.096 0.216 0.200 

5 0.093 0.212 0.118 

6 0.062 0.203 0.093 

7 0.057 0.190 0.092 

Method 6 (all 

values in deciviews)

8 0.048 0.138 0.091 

At Wichita Mountains, the model predicted higher exceedances of the FLMs’ 5% light extinction 

de-minimis threshold, especially for 2001 meteorology. A review of meteorology at Lawton, 

Oklahoma, for the episodes (days) of these predicted impacts (Julian Day in parentheses), shows 

that all three days were affected by very high humidity, fog and precipitation. These conditions 

would not only make it difficult to see great distances, but there is concern in the scientific 

community that the calculations of visibility impacts on such days are not accurate. 

For the 2002 and 2003 episodes that are over 5% light extinction (more marginally than for 

2001), there was less precipitation than for the 2001 episodes, and two of the episodes in 2002 

and 2003 had no reported precipitation at Lawton on the subject days. 

Using the newer metric of deciviews, assessed on a 98th percentile basis (see “Method 6” results 

above), the impacts at Wichita Mountains are all below the 0.5 deciview threshold commonly 

being used to determine a potentially adverse visibility impact in BART analyses nationwide. 

The 98th percentile impacts at Wichita Mountains are higher than at Great Sand Dunes, which is 

expected, given the greater predominance of winds from Holcomb Station toward Wichita 

Mountains. However, even at Wichita Mountains, the 98th percentile deciview values for each 

year are significantly lower than the 0.5 deciview BART threshold. As explained above, the 

tendency of CALPUFF to overestimate impacts beyond 300 km means that there is little potential 

for adverse visibility impact from the proposed facility at the Wichita Mountains. 
Holcomb Station Expn., Visibility Results 4 June 8, 2007 

Table 3. Summary of Wichita Mountains Visibility Impact Results, in Percent Light 

Extinction Increase, and in Deciview Increase 

Background Wichita Mountains 

Visibility Method Rank 2001 2002 2003 

1 30.58 (47) 7.20 (19) 6.36 (62) 

2 18.36 (46) 5.99 (38) 5.43 (61) 

3 13.48 (347) 5.71 (270) 4.78 

4 4.76 5.01 (342) 4.43 

5 4.48 3.83 3.63 

6 3.77 3.43 3.54 

7 2.45 3.07 3.39 

Method 2 (all 

values in % light 

extinction)

8 2.26 2.93 3.32 

1 1.107 0.487 0.614 

2 0.921 0.486 0.524 

3 0.517 0.468 0.312 

4 0.369 0.427 0.299 

5 0.229 0.418 0.268 

6 0.216 0.366 0.229 

7 0.212 0.363 0.222 

Method 6 (all 

values in deciviews)

8 0.212 0.331 0.213 

3.2 Analysis of Worst-Case Modeled Episodes

At the request of the FWS, this section of the report presents the results of an analysis to estimate 

impacts at receptors intermediate between Holcomb Station and Wichita Mountains, on the two 

days (Julian Days 46 & 47, or February 15 & 16, 2001) that generated the maximum impacts. 

For this analysis, receptors were placed ever 25 km from 250 km downwind to the nearest point 

of the Wichita Mountains Class I area, just over 400 km downwind. Table 4 presents the results 

of this analysis for both days. 

The two sets of results in Table 4 are somewhat different for these episodes, although they are 

both reasonable based on the CALPUFF model design, depending on atmospheric chemistry and 

dispersion. For Day 46, the impacts decrease uniformly with downwind distance. This is not 

unusual with dispersion models not including any chemistry interactions and this pattern can 

typically be attributed simply to increasing dispersion with distance. For Day 47, modeled 

impacts increase up to 325 km downwind, and then begin decreasing uniformly. Such a pattern 

could be caused by multiple effects simulated by the model, including: 

1) meandering plume with transport downwind, 

2) simple dispersion of an elevated, stable, plume down to the ground, thus peaking the 

impact at 325 km, with increasing dispersion thereafter, 

3) chemistry & dispersion interactions, wherein chemical conversion using background 

constituents is “exhausted” at 325 km, and then dispersion takes over, or 

4) elevation changes with distance. 
Holcomb Station Expn., Visibility Results 5 June 8, 2007 

Any of these factors could affect predicted concentration patterns with downwind distance. 

However, it is clear that the impact data in Table 4 are consistent with the reported worst-case 

impacts at Wichita Mountains. 

Table 4. Visibility Impacts in 2001 at Intermediate Receptors Located Between Holcomb 

Station and Wichita Mountains 

Day 46 Day 47 

Bkgd. Light Ext. (1/106 m) => 18.495 19.185 

Receptor Location 

Distance Elevation Total Ext. Total Ext. 

Receptor ID (km) (m) 1/mega-m Ext. (%) 1/mega-m Ext. (%) 

1235 250 643 26.900 45.44 24.832 29.43

1236 275 564 26.409 42.79 25.465 32.73

1237 300 522 25.558 38.19 25.828 34.63

1238 325 546 24.648 33.27 25.965 35.34

1239 350 500 23.779 28.57 25.913 35.07

1240 375 433 22.891 23.77 25.658 33.74

Wichita Max. 400+ 606 21.891 18.36 25.052 30.58

4.0 CONCLUSIONS 

Modeled visibility impacts at both the Great Sand Dunes National Monument in Colorado, and 

the Wichita Mountains National Wildlife Refuge in Oklahoma areas, assessed according to the 

98th percentile deciview metric, are below the threshold of 0.5 deciviews. This threshold is now 

being used in regulatory reviews of existing sources to determine if they are potentially subject to 

retrofit emissions controls. The FWS and other FLMs are expected to soon begin using a similar 

approach to Class I analysis for purposes of PSD/New Source Review permits. 

This analysis of Class I area visibility impacts has demonstrated that the proposed Holcomb 

Station expansion is not expected to adversely affect visibility at the Great Sand Dunes National 

Monument in Colorado, or the Wichita Mountains National Wildlife Refuge in Oklahoma.