relative to other source contributions in the Class I area. Thus, we disagree that the degree of improvement should be contingent upon perceptib ility. Failing to consid er less-than-perceptible contributions to visibility im pairment would ignore the CAA’s in tent 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 c ontribute to impairment. Conversely, an improvement of 0.5 dv or less contributes to impro vement in visibility impai rment. As stated in the proposal, the modeled improvements in visi bility are large enough to warrant requiring the proposed BART controls. While the actual im provements 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 commente r (0182) stated that EPA underestimated visibility improvement from installing NO

x 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 NO

x control efficiency. The commenter also states that modeling shows that greater NO

x removal rates are not offset by sulfuric acid emissions but instead yield greater visibility improvements than those proposed by EP A. The commenter argues that there will be a significant visibility benef it from increasing the SCR NO

x efficiency from 80 percent to 90 percent and therefore conclude s that a higher level of NO

x control than 80 percent should be

determined BART.

Response :

EPA disagrees with the comment that we overst ated 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 determi nation, 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 measur ed and calculated values of sulfuric acid emissions from the Navajo Generating Stati on (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 gi ven facility, this met hod is a commonly used calculation methodology fo r 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% NO

x removal. Greater reduction efficiencies would likely require an additional layer of catalyst, which would likely increase sulfuri c acid emissions . Based on the SO 2 to SO 3 conversion rate guarantee we received from Hita chi for its CX series catalyst (ultra-low

Page 95 of 171

August 2012 – Four Corners Power Plant Final Rulemaking – Response to Commentsconversion) of 0.167% per layer, the use of an add itional 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 5 th layer would certainly affect the cost and operation of the unit. Although EPA agrees that the modeling referen ced by the commenters appears to indicate greater visibility improvement from an SCR sy stem achieving 90% removal compared to 80% removal despite higher sulfuric acid emissions

5 7 F 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 bas ed 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) subm itted the results of a dditional 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 CALP UFF model” show that none of the modeled

NO

x emission reduction strategies at FCPP will re sult in humanly perceptible improvement in visibility in the 16 surrounding Class I areas. The commenter i ndicated 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 app lication 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 a lternative 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 Cla ss 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 improvem ent from the original or alternative NO

x reduction strategies would be imperceptibl e; EPA anticipates the improvements from either strategy would be perceptible.

Comment:

One of the owners of FCPP (0176/0177) st ated that EPA’s TSD Table 21 (TSD p.44) mischaracterized the measured visibility impairment at nearby Class I areas as 10 th and 90 th

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 incr eases in sulfuric acid emissions would not be expected on units that are not equipped with baghouses.

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