relative to other source contributions in the Class I area. Thus, we disagree that the degree of 

improvement should be contingent upon perceptibility. Failing to consider less-than-perceptible 

contributions to visibility impairment would ignore the CAA’s intent to have BART requirements 

apply to sources that contribute to, as well as cause, such impairment.” (70 FR 39129) 

That is, impacts smaller than this to contribute to impairment. Conversely, an 

improvement of 0.5 dv or less contributes to improvement in visibility impairment. As stated in 

the proposal, the modeled improvements in visibility are large enough to warrant requiring the 

proposed BART controls. While the actual improvements may be larger, from 0.6 to 2.8 

deciviews, even an improvement of 0.5 dv is a contribution toward improving visibility, 

especially when the benefits at multiple Class I areas are considered. In conjunction with 

improvements from other sources, this will help toward the Clean Air Act goal of remedying 

manmade visibility impairment. 

Comment: 

One environmental advocacy group commenter (0182) stated that EPA underestimated 

visibility improvement from installing NOx controls because it overestimates the production of 

sulfuric acid by the SCR and underestimated the amount of sulfuric acid removed downstream of 

the SCR. The commenter cited reports attached to the comments (0182) to argue that sulfuric 

acid does not limit SCR NOx control efficiency. The commenter also states that modeling shows 

that greater NOx removal rates are not offset by sulfuric acid emissions but instead yield greater 

visibility improvements than those proposed by EPA. The commenter argues that there will be a 

significant visibility benefit from increasing the SCR NOx efficiency from 80 percent to 90 

percent and therefore concludes that a higher level of NOx control than 80 percent should be 

determined BART. 

Response: 

EPA disagrees with the comment that we overstated the production of sulfuric acid from 

the SCR catalyst and underestimated the amount of sulfuric acid removed downstream of the 

SCR. In the TSD for our proposed BART determination, we estimated sulfuric acid emissions 

using the EPRI methodology and provided detailed explanations for all of the assumptions we 

applied as well as a discussion to compare measured and calculated values of sulfuric acid 

emissions from the Navajo Generating Station (see TSD p.55-59, 64-65, and 68). While we fully 

acknowledge and understand that the generalized EPRI methodology does not precisely 

represent true sulfuric acid emissions for a given facility, this method is a commonly used 

calculation methodology for estimating sulfuric acid emissions under a future operating scenario 

involving SCR. The assumptions we applied in estimating sulfuric acid emissions after the 

installation and operation of SCR using the EPRI methodology were justified and appropriate. 

EPA assumed in our BART proposal a 3+1 system (4 layers of catalyst) would achieve 

80% NOx removal. Greater reduction efficiencies would likely require an additional layer of 

catalyst, which would likely increase sulfuric acid emissions. Based on the SO2 to SO3 

conversion rate guarantee we received from Hitachi for its CX series catalyst (ultra-low 

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conversion) of 0.167% per layer, the use of an additional catalyst layer would equal 5 layers of 

catalyst and a 0.835% conversion rate. EPA is not aware of SCR systems that use 5 layers of 

catalyst, and the addition of a 5th layer would certainly affect the cost and operation of the unit. 

Although EPA agrees that the modeling referenced by the commenters appears to indicate 

greater visibility improvement from an SCR system achieving 90% removal compared to 80% 

removal despite higher sulfuric acid emissions57F

58, EPA does not agree that this requires EPA to 

determine that 90% control be determined BART. In Section 8.1 of this document, we respond to 

the comment that the emission limitation for BART should be based on 90% control with SCR 

rather than 80% control with SCR. Based on the rationale outlined in that response, EPA 

determined that 80% control, achievable with SCR, is appropriate for BART for FCPP. 

Comment: 

One of the owners of FCPP (0176/0177) submitted the results of additional modeling and 

concluded that the proposed alternative strategy would result in greater visibility improvements 

in the Class I areas surrounding FCPP than would the proposed BART FIP; however, the 

commenter continued to disagree that the improvement would be humanly perceptible. 

According to the commenter, the commenter’s additional visibility modeling using “valid 

background ammonia values and the revised CALPUFF model” show that none of the modeled 

NOx emission reduction strategies at FCPP will result in humanly perceptible improvement in 

visibility in the 16 surrounding Class I areas. The commenter indicated that highest predicted 

visibility improvement (change in deciview) by applying SCR, compared to baseline emissions, 

is 0.28 dv, while the incremental improvement in dv resulting from application of advanced 

combustion control technology and SCR is predicted to be 0.21 dv. Because both these numbers 

are well below the EPA-threshold for human perception of 1.0 dv, SCR does not constitute 

BART for the Plant. 

Response:

 EPA agrees with the commenter that the alternative emission control strategy (of 

shutting down units 1, 2, and 3, and placing SCR on units 4 and 5) may “result in greater 

visibility improvement at the surrounding Class I areas” (EPA's supplemental proposal, 76 FR 

10532, February 25, 2011). However, as discussed in the response to another comment, EPA 

does not agree that the visibility improvement from the original or alternative NOx reduction 

strategies would be imperceptible; EPA anticipates the improvements from either strategy would 

be perceptible. 

Comment: 

One of the owners of FCPP (0176/0177) stated that EPA’s TSD Table 21 (TSD p.44) 

mischaracterized the measured visibility impairment at nearby Class I areas as 10th and 90th 

58 EPA notes that the baghouses on Units 4 and 5 are assumed to provide a significant amount of control of sulfuric 

acid emissions, therefore, such slight increases in sulfuric acid emissions would not be expected on units that are not 

equipped with baghouses. 

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