Four Corners FIP Revisions | Response to Comments for Final Rule | January 2017 Page 1 of 18

Revisions to the Source-Specific Federal 

Implementation Plan for Four Corners Power 

Plant, Navajo Nation

Final Rule

Responses to Comments

January 2017
Four Corners FIP Revisions | Response to Comments for Final Rule | January 2017 Page 2 of 18

The EPA’s proposed rule, titled “Revisions to the Source-Specific Federal Implementation Plan 

for Four Corners Power Plant, Navajo Nation,” published in the Federal Register on December 2, 2016. 

The 30-day comment period for our proposed rule closed on January 3, 2017. We received 3 comment 

letters prior to the close of the comment period.1 All comment letters we received were generally 

supportive of our proposed action. Two of the letters requested additional clarification or 

recommended additional revisions to the FCPP FIP. 

Comment 1: A consortium of non-governmental organizations commented in support of the 

proposed revisions to the FCPP FIP and agreed with our assessment that the proposed revisions would 

strengthen the FIP. The consortium encouraged the EPA to finalize the proposed revisions to the FCPP 

FIP.

Response 1: The EPA is taking final action on our proposed rule to revise the FCPP FIP.

Comment 2: One commenter recommended several revisions to 40 CFR 49.5512(f) pertaining to 

the requirements for notifications. The commenter first stated that the result of the EPA’s proposed 

revision to 40 CFR 49.5512(f), i.e., the requirement to submit notifications to both the EPA and the 

NNEPA (“dual notification requirement”), would be to limit the dual notification requirement only to 

those submittals required under paragraph (f), such that submittals required under other paragraphs in 

40 CFR 49.5512, including paragraphs (e) and (j), would not be subject to the dual notification 

requirement. The commenter recommended that the EPA revise paragraph (f) to apply the dual 

notification requirement to all notifications required under “this section” rather than under “this 

paragraph (f).” Next, the commenter noted that some provisions in the FIP require APS to notify the 

 1 See letter dated December 22, 2016, from Andrea Issod, Sierra Club Environmental Law Program, Dan Olson, San 

Juan Citizens Alliance, Erik Schlenker-Goodrich, Western Environmental Law Center, and Carol Davis, Dine Citizens 

Against Ruining our Environment, to Anita Lee, EPA; see letter dated December 29, 2016, from Donald Benn, 

Navajo Nation Environmental Protection Agency, to Anita Lee, EPA; see letter dated January 3, 2017 from Chas 

Spell, Arizona Public Service, to Gina McCarthy, EPA. 
Four Corners FIP Revisions | Response to Comments for Final Rule | January 2017 Page 3 of 18

EPA of specific actions, but that the dual notification requirement only applies to required notifications 

to the Regional Administrator or Administrator. The commenter recommended that the EPA revise 

paragraph (f) to add that required notifications to the EPA are also subject to the dual notification 

requirement. Finally, the commenter noted that if the EPA revises paragraph (f) to apply to all provisions 

in 40 CFR 49.5512, we should add the term “petitions” to the types of documents that would be 

submitted under paragraph (f), because the term “petitions” appears in a similar notification 

requirement in paragraph (k)(7).

2

Response 2: The EPA agrees with the commenter that the proposed revision to 40 CFR 

49.5512(f) would have limited the dual notification requirement. The EPA did not intend to limit the dual 

notification requirement to only those things described in paragraph (f). We also agree with the 

commenter that there are provisions in the FCPP FIP that require APS to notify the EPA of specific 

actions, and that the term “the EPA” is not included in the proposed revisions to paragraph (f). Finally, 

we agree with the commenter that the term “petitions” should be included in the types of documents 

that would be subject to the dual notification requirement because these petitions refer to provisions of 

the Consent Decree, which we proposed to include in the FCPP FIP in 40 CFR 49.5512(k)(2)(iii), that allow 

APS to petition the EPA for a revised NOX emission limitation. Therefore, in this final action, we are 

amending the beginning portion of paragraph (f), as recommended by the commenter, to read “All 

requests, reports, submittals, notifications, petitions, and other communications to the Regional 

Administrator, Administrator, or the EPA, required by this section and references therein, shall be 

submitted . . .”3 As a result, all required notices must be sent to both the EPA and NNEPA.

 2 These “petitions” refer to the process established in the Consent Decree that allows APS to petition the EPA for a 

revised NOX emission limitation. 3 For clarity, we have included a document titled “FCPP FIP reg text RLSO for NFR.docx” in the docket for this 

rulemaking to indicate in red-line/strike-out text the changes to the original FCPP FIP that we are promulgating in 

this final rulemaking. 
Four Corners FIP Revisions | Response to Comments for Final Rule | January 2017 Page 4 of 18

Comment 3: One commenter noted that the proposed revision to 40 CFR 49.5512(e)(3) was 

ambiguous because it did not specify to whom the notifications required in the paragraph should be 

sent. The commenter recommended that the EPA revise paragraph (e)(3) to specify that the required 

written notifications be sent by APS to the Regional Administrator, and therefore would be subject to 

the dual notification requirement in paragraph (f). 

Response 3: The EPA agrees with the commenter that proposed revisions to 40 CFR 

49.5512(e)(3) did not specify to whom the required notification should be sent. Therefore, in this final 

action, the EPA is amending the relevant sentences in paragraph (e)(3), as recommended by the 

commenter, to specify that written notification must be submitted to the Regional Administrator.

Comment 4: One commenter stated that the proposed dual notification requirement in 40 CFR 

49.5512(k)(7) is redundant to the notification requirements in paragraph (f), particularly if the EPA 

revises paragraph (f) as recommended by the commenter to apply to all provisions in 40 CFR 49.5512 (as 

discussed in Comment 2). 

Response 4: The EPA agrees with the comment that the reporting requirement proposed in 40 

CFR 49.5512(k)(7) is redundant with the requirements in 40 CFR 49.5512(f). Therefore, in this final 

action, the EPA is removing the proposed reporting requirement in paragraph (k)(7).

Comment 5: One commenter recommended that the EPA revise the definition of PM in 40 CFR 

49.5512(k)(1)(xvii) to include both filterable and condensable PM because paragraph (k)(5) requires 

annual source testing for filterable PM and condensable PM. The commenter suggests that it is 

inaccurate to define PM as only filterable PM when the Consent Decree also addresses condensable PM.

Response 5: The EPA disagrees with the comment that the definition of PM in 40 CFR 

49.5512(k)(1)(xvii) is inaccurate. We note that the definition of PM in the Consent Decree in Section 

III.40 is identical to the definition we proposed in 40 CFR 49.5512(k)(1)(xvii). The emission limitation for 
Four Corners FIP Revisions | Response to Comments for Final Rule | January 2017 Page 5 of 18

PM in the Consent Decree is based only on filterable PM. The inclusion of condensable PM in the 

definition of “PM” as recommended by the commenter would thus be inaccurate, and would change the 

stringency of the numerical emission limitation for PM established in the Consent Decree. Although 

paragraph (k)(5)(v) requires the owner or operator to conduct an annual “PM stack test for condensable 

PM at FCPP Units 4 and 5, using the reference methods and procedures set forth at 40 CFR part 51, 

Appendix M, Method 202”, this requirement is separate from the requirement at paragraph (k)(5)(iv) to 

conduct annual stack tests for PM using the reference methods and procedures (filterable portion only) 

specified in 40 CFR part 60, App. A-3, Method 5.” In addition, paragraph (k)(5)(v) applicable to testing for 

condensable PM further states that “the results of the PM stack test conducted pursuant to this 

paragraph shall not be used for the purpose of determining compliance with the PM Emission Rates 

required by paragraph (k).” Thus, the specific language in paragraph (k)(5)(v) stating that the results of 

source tests for condensable PM shall not be used for compliance with the PM emission limitations in 

paragraph (k) make clear that the Consent Decree defines PM as filterable PM. We recognize that the 

use of the term “PM” within the requirements to conduct stack tests for condensable PM in paragraph 

(k)(5)(v) creates confusion because the term “PM” is specifically defined in the proposed FIP as filterable 

PM. Therefore, in this final action, the EPA is amending paragraph (k)(5)(v) to read:

Once each calendar year, the owner or operator shall conduct a stack test

for condensable particulate matter at FCPP Units 4 and 5, using the

reference methods and procedures set forth at 40 C.F.R. Part 51, Appendix M,

Method 202 and as set forth in paragraph (iv). This test shall be conducted

under as similar operating conditions and as close in time as reasonably

possible as the test for PM in paragraph (k)(5)(iv). Each test shall consist of 

three separate runs performed under representative operating conditions not

including periods ofstartup,shutdown, or Malfunction. The sampling time for

each run shall be at least 120 minutes and the volume of each run shall be at 

least 1.70 dry standard cubic meters (60 dry standard cubic feet). The owner

or operator shall calculate the number of pounds of condensable particulate 

matter emitted in lb/MMBtu of heat input from the stack test resultsin

accordance with 40 CFR 60.8(f). The results of the condensable particulate 

matter stack test conducted pursuant to this paragraph shall not be used for the 

purpose of determining compliance with thePM Emission Rates required by
Four Corners FIP Revisions | Response to Comments for Final Rule | January 2017 Page 6 of 18

paragraph (k). The results of each condensable particulate matter stack test

shall be submitted to EPA within sixty (60) Days of completion of each test. If

EPA approves a request to demonstrate continuous compliance with an 

applicable PM Emission Rate at a Unit using PM CEMS under paragraph 

(k)(5)(iii), annual stack testing for condensable particulate matter using the

reference methods and procedures set forth at 40 CFR part 51, Appendix

M, Method 202 is not required for that Unit.

Comment 6: One commenter noted two typographical errors in 40 CFR 49.5512(k), at 

paragraphs (k)(2)(i) and (k)(4)(i). The commenter recommended that the EPA replace an “or” in the first 

sentence of paragraph (k)(2)(i) with a “the.” The commenter further recommended that the EPA add the 

word “by” to the first sentence of paragraph (k)(4)(i), to refer to “any emission limitation required by

paragraph (k).”

Response 6: We agree with the commenter that there are typographical errors in 40 CFR 

49.5512(k)(2)(i) and (k)(4)(i). Therefore, in this final action, we are amending the first sentence in 

paragraph (k)(2)(i) to remove the word “or” and we are amending the first sentence in paragraph 

(k)(4)(i) to add the word “by.”

Comment 7: One commenter recommended that the EPA incorporate Section VII of the Consent 

Decree, entitled “Prohibition on Netting Credits or Offsets” into the FCPP FIP. The commenter noted 

that paragraph 152 of the Consent Decree specifically stated that “limits on use of emission credits” are 

to be included in the FCPP FIP. 

Response 7: We agree with the comment that Section VII of the Consent Decree should be 

included in the FCPP FIP. In this final action, we are adding the provisions pertaining to the prohibition 

on netting credits and offsets to paragraph (k)(7). Adding these provisions from the Consent Decree to 

the revised FIP will ensure continued enforceability after the Consent Decree is terminated.

Comment 8: One commenter disagreed with the EPA’s characterization in the proposed rule 

that the existing FIP provisions provide an exemption or exclusion from the opacity standard during 
Four Corners FIP Revisions | Response to Comments for Final Rule | January 2017 Page 7 of 18

periods of saturated stack. The commenter argued that the opacity limit does apply continuously, but 

the prior FIP provisions merely provided FCPP with “flexibility” to demonstrate compliance with the 

opacity limit by different means. The commenter stated that in the event a high opacity reading occurs, 

the existing FIP provides a “presumption” of compliance with the opacity limit during periods of 

saturated stack conditions if the baghouse is operating within its normal operating parameters and if the 

baghouse is not fully closed. The commenter opined that the language in the existing FIP is thus not 

inconsistent with the 2015 SSM Action because at no time are the units at FCPP actually exempt from 

compliance with the opacity emission limitation. Aside from disagreeing with the EPA’s analysis of the of 

the prior FIP provisions, the commenter primarily focused on what it considers more appropriate 

requirements for monitoring compliance with the opacity limit. The commenter thus opined that a 

continuous VE performance test, during the duration of saturated stack conditions, is not necessary and 

recommended that the EPA assure compliance with the opacity limitation through the combined use of 

COMS, demonstrations that the baghouse is operating within its normal operating parameters, 

demonstrations that the baghouse is not fully closed, and periodic (i.e. monthly) VE performance tests in 

accordance with EPA Reference Method 9 whenever saturated stack conditions occur. 

Response 8: The EPA and commenter appear to agree that the opacity emission limitation 

should be read to apply continuously and that any form of credible evidence should be useable to 

determine compliance with the opacity emission limitation. The EPA and commenter also agree that the 

revisions will improve the FIP. The commenter appears to agree with the EPA’s proposed replacement of 

the provisions related to the COMS in 40 CFR 49.5512(e) with revised opacity provisions in 40 CFR 

49.5512(e)(6), provided the EPA accepts the commenter’s recommendation to require monthly VE 

performance tests if the baghouse is operating normally and is not fully closed in the event of a high 

opacity reading in paragraph (e)(6)(i). The commenter disagrees with the EPA’s characterization in the 

preamble of our proposal that the existing FIP provision related to water vapor results in an 
Four Corners FIP Revisions | Response to Comments for Final Rule | January 2017 Page 8 of 18

“exemption” from continuous compliance and also disagrees with the EPA’s proposal to require 

continuous VE performance tests during saturated stack conditions. 

We proposed revisions in 40 CFR 49.5512(e)(6) so that if the opacity standard applies (i.e. APS 

has not chosen to demonstrate compliance with the PM emission limitation using PM CEMS), APS would 

have three options for demonstrating compliance with the opacity standard. The first option, in 

paragraph (e)(6)(i), which is the subject of this comment, would have required the use of the existing 

COMS during dry (unsaturated) stack conditions, and required VE performance testing for the duration 

of saturated stack conditions.4 As noted in our proposed rule, because condensed water vapor in the 

stack impedes the accuracy of the COMS, we consider VE performance testing for the duration of the 

saturated stack condition to provide a reasonable demonstration of compliance with the opacity 

standard, assuming these saturated stack conditions occur infrequently.5 As noted by the commenter, 

the stacks at FCPP Units 4 and 5 are currently dry and only in rare instances have saturated stack 

conditions occurred in the past.6

However, APS is in the process of converting the ductwork and stacks for Units 4 and 5 to 

withstand wet stack conditions, and therefore, saturated stack conditions are expected to be more 

frequent during future operations.7 The Consent Decree requires this conversion to be completed on 

one unit by March 31, 2018, and on the second unit by July 31, 2018. After that conversion, the stacks 

are likely to be saturated more frequently. Therefore, after the conversion to wet stacks, when the 

stacks at FCPP experience saturated stack conditions with greater frequency and/or consistency, 

continuous VE performance testing (for the duration of the saturated stack condition) would be 

 4 See proposed rule at 81 FR 86988 at 86994 (December 2, 2016). 5 Id.

6 See APS comment letter dated January 3, 2017 at page 3. 7 Id.
Four Corners FIP Revisions | Response to Comments for Final Rule | January 2017 Page 9 of 18

impractical.8 Therefore, in this final action, we are amending 40 CFR 49.5512(e)(6) to make the option in 

paragraph (e)(6)(i) (the use of COMS during dry stack conditions and VE performance testing during the 

duration of the saturated stack conditions) available only until March 31, 2018 on Unit 5 and July 31, 

2018 on Unit 4, the dates by which APS must convert those units to wet stacks. Because the option in 

paragraph (e)(6)(i) will only be available when the stacks are consistently dry, the requirement to 

conduct VE performance testing during the duration of saturated stack conditions (which are infrequent 

and typically short in duration), provides a reasonable and feasible method for demonstrating

compliance with the opacity standard. 

After the stacks are converted to wet stacks in 2018, the option in paragraph (e)(6)(i) to use 

COMS with VE performance testing for the duration of saturated stack conditions will no longer be 

available. This means that in the rule we are finalizing today, APS will have only three options for 

addressing the opacity standard after March and July 2018. The three options are: (1) use PM CEMS to 

demonstrate compliance with the 0.015 lb/MMBtu PM emission limitation, in which case the opacity 

standard does not apply; (2) use of CPMS with periodic VE performance testing to demonstrate 

compliance with the opacity standard; or (3) use of a bag leak detection system with periodic VE 

performance testing to demonstrate compliance with the opacity standard. As discussed in Response 9

below, there will be a brief period after APS completes its modifications to the ductwork and stacks 

when APS will be testing and tuning the new equipment. During that period, we are finalizing a provision

in 40 CFR 49.5512(e)(6) to allow APS either: (i) to use the COMS during dry stack conditions and conduct 

weekly VE performance testing during saturated stack conditions to demonstrate compliance with the 

opacity standard, or (ii) to use the PM CEMS to demonstrate compliance with a PM emission limitation 

 8 We also noted in our proposed rule that condensed water vapor in the stack impedes the accuracy of the COMS, 

but continuous VE performance testing if the stacks are consistently saturated may be impractical. See proposed 

rule at 86994.
Four Corners FIP Revisions | Response to Comments for Final Rule | January 2017 Page 10 of 18

of 0.030 lb/MMBtu, in which case the opacity standard does not apply. This provision is limited to the 

period between the conversion to the wet stacks and the compliance dates in the Consent Decree of 

March 31, 2018 and July 31, 2018. 

These revisions to the FCPP FIP improve the clarity and enforceability of the compliance 

determination for the opacity standard during saturated stack conditions, and are consistent with the 

NSPS for EGUs. The NSPS for EGUs includes methods that the EPA considers adequate for purposes of 

determining compliance and supporting enforcement for opacity limits at this type of source. We are 

finalizing some changes to our proposal as discussed above that are consistent with the comments we 

received, specifically comments recommending a different frequency for VE performance testing.

We are also adding a provision that will require APS to notify the EPA and NNEPA of the method 

that APS will use to demonstrate compliance with the opacity standard (i.e. CPMS or bag leak detection 

in conjunction with periodic VE performance testing) or the use of the PM CEMS for demonstrating 

compliance with the PM emission limitation of 0.015 lb/MMBtu.

We are also correcting a provision in the regulatory text that will be inconsistent with the 

revisions we are finalizing in this action. 40 CFR 49.5512(d)(4) states that the opacity standard in 

paragraph (d)(4) and associated requirements in paragraphs (e) and (f) (i.e., Testing and Monitoring, and 

Reporting and Recordkeeping Requirements), would not apply if APS operates its PM CEMS under 

paragraph (e)(3). Paragraph (e)(3) provides three options for demonstrating compliance with the 

emission limitation in paragraph (d)(2). Paragraph (d)(2), however, is the PM emission limitation from 

the 2007 FIP of 0.050 lb/MMBtu. APS cannot be relieved of complying with the opacity standard if its 

PM CEMS are only used for determining compliance with the emission limitation of 0.050 lb/MMBtu in 
Four Corners FIP Revisions | Response to Comments for Final Rule | January 2017 Page 11 of 18

paragraph (d)(2). 9

 In this final rule, the EPA is amending paragraph (e)(3) to state that the provisions of 

paragraph (e)(3) are used to assure continuous compliance with the PM limits in paragraphs (d)(2) and 

(i)(1), which is the 2012 emission limitation of 0.015 lb/MMBtu. This revision is necessary to ensure that 

the provisions in the opacity standard in paragraph (d)(4) are consistent with the 2015 SSM Action. For 

additional clarity, the EPA is also amending paragraph (d)(4) to add the underlined provisions: “The 

opacity standard in this paragraph (d)(4) and associated requirements in paragraphs (e) and (f) to 

demonstrate compliance with the opacity standard shall not apply to any unit for which the owner or 

operator installs, calibrates, maintains, and operates particulate matter CEMS under paragraph (e)(3) to 

demonstrate compliance with its PM emission limitation in paragraph (i)(1).”

Comment 9: One commenter requested that the EPA provide time to the operator to recertify 

the PM CEMS after completion of the conversion of Units 4 and 5 to wet stacks. The proposed FIP 

provisions incorporating the requirements of the Consent Decree require the operator to install 

selective catalytic reduction (SCR) and convert the existing duct work and stacks to wet stacks on one 

unit by no later than March 31, 2018, and on the other unit by no later than July 31, 2018. The 

commenter stated that the operator plans to complete the installation of SCR and conversion to wet 

stacks earlier than required in the Consent Decree to allow sufficient time for tuning and testing of the 

new equipment. The commenter stated that although the proposed FIP provisions provide several 

options for demonstrating compliance with the opacity standard, including use of PM CEMS as a 

continuous parametric monitoring system (CPMS), the operator has no experience operating a PM CEMS 

under wet stack conditions and it would be unlikely that the operator could use CPMS until the operator 

 9 See 2015 SSM Action at 80 FR 33840 at 33891 and 33892 (June 12, 2015), stating that “States evaluating how best 

to replace impermissible SSM exemptions from opacity standards may wish to consider a similar approach 

conditioned upon the use of PM CEMS and a sufficiently stringent PM emission limitation,” and footnote 148, 

which indicates that 0.030 lb/MMBtu is deemed sufficiently stringent because the contribution of filterable PM to 

opacity at PM levels of 0.030 lb/MMBtu or less is generally negligible and that those units will therefore operate 

with little or no visible emissions (i.e., less than 5 percent opacity).
Four Corners FIP Revisions | Response to Comments for Final Rule | January 2017 Page 12 of 18

identifies the appropriate range for the parameter being monitored by the CPMS. Therefore, the 

commenter requested that the EPA establish a compliance date for the optional methods for 

determining compliance with the opacity standard in 40 CFR 49.5512(e)(6) of March 31, 2018 on one 

unit, and after July 31, 2018 for the second unit.

Response 9: The EPA proposed that the revisions to the FCPP FIP would apply upon the effective 

date of the final rule. Therefore, as proposed, the operator would be required to comply with the 

opacity standard consistent with the options provided in 40 CFR 49.5512(d)(4) and (e)(6), prior to and 

following the installation of SCR and conversion to wet stacks. The commenter requested that the EPA 

amend 40 CFR 49.5512(e)(6) so that that the alternative opacity compliance demonstrations in 

paragraph (e)(6) commence after March 31, 2018 for one unit, and July 31, 2018 for the other unit. 

Although the commenter cites to the need for additional time to recertify its PM CEMS following the 

conversion to wet stack conditions, 40 CFR 49.5512(e)(6) does not address PM CEMS. The optional use 

of PM CEMS to comply with the PM emission limitation (0.015 lb/MMBtu) is found in 40 CFR 

49.5512(e)(3) and 40 CFR 49.5512(d)(4). The latter section provides that the opacity standard shall not 

apply if the operator chooses the use PM CEMS to comply with the PM emission limitation. Thus, the 

revisions recommended by the commenter, to establish compliance dates for the alternative opacity 

compliance demonstrations in paragraph (e)(6), do not appear to remedy the concern addressed related 

to PM CEMS, because the commenter’s recommended revision does not address any provisions related 

to PM CEMS.10

We recognize that the operating conditions at FCPP will change following the installation of SCR 

 10 The commenter’s recommended revision would provide only one way to comply with the opacity standard (i.e.,

the use of PM CEMS such that the opacity standard does not apply consistent with 40 CFR 49.5512(d)(4)) until 

March 31, 2018 for one unit or July 31, 2018 for the other unit, and would only make the alternative opacity 

compliance demonstrations in paragraph (e)(6) available after the installation of SCR and conversion to wet stack 

conditions.
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and the conversion to wet stacks on each unit, and that these changes may require the operator to 

recalibrate the PM CEMS, or to calibrate the PM CEMS for use as a CPMS, as well as require additional 

time to tune and test the new equipment. Therefore, we agree with the commenter that additional time 

is necessary. However, we find that the recommended revision from the commenter does not 

completely achieve this intended purpose. The EPA understands that the modifications to convert Units 

4 and 5 to wet stacks have not yet been completed. Unless APS elects to use PM CEMS to comply with 

its PM emission limitation in paragraph (i)(1), the opacity standard for each unit would apply. Therefore, 

the options for demonstrating compliance with the opacity limit (i.e., COMS and VE performance testing, 

or CPMS or a bag leak detection system and periodic VE performance testing), should be available 

options upon the effective date of the final rule if APS does not elect to use PM CEMS for demonstrating 

compliance with the PM emission limit. However, in the interest of providing APS additional time to 

recertify the PM CEMS or CPMS, as applicable, following the conversion to wet stacks (but prior to the 

compliance dates for the stack modifications in the Consent Decree), in this final action, we are adding

provisions to paragraphs (e)(3) and (e)(6) of 40 CFR 49.5512 to provide that the requirements of 

paragraph (e)(3) and (e)(6) apply upon the effective date of this final rule, but that following conversion 

of the ductwork and stack for Units 4 and 5 to wet stacks, the compliance dates for paragraph (e)(3) and 

(e)(6) shall be March 31, 2018 for Unit 5 and after July 31, 2018 for Unit 4.11 This revision would allow 

the operator to use the options for demonstrating compliance with the opacity standard (if applicable) 

immediately upon the effective date of the final rule, but would also establish a new compliance date 

after the conversion to wet stacks to provide time to recertify the PM CEMS (under paragraph (e)(3)) or

develop parameters to use the PM CEMS as a CPMS (under paragraph (e)(6)). This revision is consistent 

with the Consent Decree because it does not change any compliance dates in the Consent Decree for 

 11 The provisions in paragraph (k)(2) and (3) require the installation of SCR and conversion to wet stacks on Unit 5 

by no later than March 31, 2018 and on Unit 4 by no later than July 31, 2018. 
Four Corners FIP Revisions | Response to Comments for Final Rule | January 2017 Page 14 of 18

the conversion to wet stacks required in 40 CFR 49.5512(k)(ii) and (iii). 

As discussed in Response 8, we are modifying the provision at 40 CFR 49.5512(e)(6)(i) to make 

this option for determining compliance with the opacity standard (i.e., use of COMS during dry stack 

conditions and use of VE performance testing during the duration of the wet stack condition) available 

only until March 31, 2018 for Unit 5 and July 31, 2018 for Unit 4, because this option would no longer be 

reasonable for demonstrating compliance with the opacity standard in stacks after those dates because 

we anticipate the stacks will experience saturated conditions more frequently and/or consistently. 

However, during the period following the completion of the modifications for the wet stacks and the 

compliance dates in the Consent Decree, APS will be testing and tuning the new equipment, as well as 

recertifying its PM CEMS or CPMS. Therefore, we anticipate that during this period, the use of the 

provisions in paragraph (e)(6)(i) may be necessary because the PM CEMS and/or CPMS may not yet be 

available for this purpose. As noted in Response 8, continuous VE performance testing (during the 

duration of saturated stack conditions) when the stacks are more frequently and/or consistently 

saturated as a result of the conversion to wet stacks would be impractical. Therefore, for the short 

period between the completion of the modifications to convert the stacks to withstand wet conditions, 

and the compliance date in the Consent Decree, we are adding a provision in paragraph (e)(6) to require 

APS to either demonstrate compliance with the opacity standard using COMS if the stacks are dry, and 

weekly VE performance testing if the stacks are saturated, or use the PM CEMS to demonstrate 

compliance with a PM emission limitation of 0.030 lb/MMBtu, in which case the opacity standard does 

not apply during this period.

12

 12 Although we anticipate that the modifications to the ductwork to allow the stacks to withstand wet conditions in 

order to increase the SO2 removal efficiency of the scrubbers will result in stacks that experience wet stack 

conditions more consistently, we recognize there may be some uncertainty that the stacks will be saturated at all 

times following the modifications. On page 3 of its comment letter, APS stated: “As a result of these changes APS 

anticipates that saturated stack conditions will be more prevalent during future operations.” Therefore, during this 
Four Corners FIP Revisions | Response to Comments for Final Rule | January 2017 Page 15 of 18

Comment 10: One commenter requested that the EPA provide additional discussion of a CPMS 

and how a non-certified PM CEMS could be approved as a CPMS.

Response 10: We proposed several options for demonstrating compliance with the PM emission 

limitation and the opacity standard, including the use of a CPMS in accordance with 40 CFR part 63 

subpart UUUUU. Provisions for a CPMS are included in 40 CFR 63.10010(h) and 40 CFR 63.10023, and 

they state that the operating principle of a PM CPMS must be based on, e.g., in-situ or extractive light 

scatter or beta attenuation detection of the exhaust gas and be expressed as milliamps, PM 

concentration, or other raw data signal value. A certified PM CEMS must meet “Performance 

Specification 11 – Specifications and Test Procedures for Particulate Matter Continuous Emission 

Monitoring Systems at Stationary Sources” (“PS-11”).13 The provisions of PS-11 state that several 

different types of PM CEMS technologies (e.g. light scattering, beta attenuation, etc.) can be designed 

with in-situ or extractive sample gas handling systems. Thus, a PM CEMS and a CPMS are based on the 

same technologies, but, as stated in the preamble to the NSPS for EGUs, a “PM CPMS does not need to 

meet the requirements for a PM CEMS under PS 11,” and the “PM CPMS will not be correlated as a PM 

CEMS under PS 11 and will produce data in terms of a signal you define.”14 Therefore, the EPA generally 

considers a PM CEMS to be approvable as a CPMS if it meets the requirements in 40 CFR part 63 subpart 

UUUUU, even if it does not meet PS-11.

Comment 11: One commenter requested that the EPA revise the proposed provisions at 40 CFR 

49.5512(d)(6)(ii) and (iii) to remove the requirement to conduct periodic VE performance tests from two 

options for demonstrating compliance with the opacity standard, the CPMS option and the option 

requiring a bag leak detection system. The commenter argued that the use of either a CPMS or a bag 

 

brief period after wet stack conversion but before the compliance dates in the Consent Decree, we retain the use 

of COMS in the event the stacks are dry.

13 See 40 CFR part 60 appendix B, Performance Specification 11. 14 See 77 FR 9303 at 9372 and 9384 (February 16, 2012).
Four Corners FIP Revisions | Response to Comments for Final Rule | January 2017 Page 16 of 18

leak detection system alone are sufficient for demonstrating compliance with the opacity standard and 

that neither of those options need to be supplemented with periodic VE performance tests.

Response 11: The EPA disagrees with this comment. We note that the NSPS for EGUs at 40 CFR 

60.49Da(a)(3) provides that operators of facilities that meet the conditions of 40 CFR 60.49Da(a)(2), e.g.,

facilities that use a baghouse and install and operate a bag leak detection system under 40 CFR 

60.49Da(a)(2)(i), may elect to conduct periodic VE performance testing as an alternative to using a 

COMS. The NSPS for EGUs, at 40 CFR 60.49Da(a)(3), allows periodic VE performance testing in lieu of 

COMS if the facility meets certain conditions (e.g. use of a baghouse with a bag leak detection system 

that is installed and operated according to the requirements in paragraph 40 CFR 60.48Da(o)(4)(i) 

through (v)).15 Thus, the use of a bag leak detection system, or the other alternatives under 40 CFR 

60.49Da(a)(2), does not completely replace the need for opacity monitoring because the opacity 

standard would still apply. The revisions we proposed at 40 CFR 49.5512(d)(6)(ii) and (iii) require 

periodic VE performance testing using the procedures specified in 40 CFR 60.49Da(a)(3). This is 

consistent with the NSPS for EGUs. Although FCPP is not subject to the NSPS for EGUs, the NSPS for 

EGUs represents the Agency’s current view on what is adequate for compliance assurance and 

enforcement of opacity limits at such sources. Therefore, in this final action, the EPA is not removing the 

periodic VE performance testing requirements under 40 CFR 49.5512(d)(6)(ii) and (iii). 

Comment 12: One commenter requested that the EPA amend 40 CFR 49.5512(e)(2) to provide 

for the operator to use its SO2 CEMS at the FGD inlet for determining the SO2 concentration at the inlet 

to the FGD, in addition to the current method that relies on calculations of the daily average percent 

sulfur and the heat content (in British Thermal Units or BTUs) of the coal combusted in the boilers. The 

 15 The frequency of required VE performance testing in the NSPS for EGUs ranges from every 45 days if the 

maximum 6-minute average opacity is greater than 10 percent during the initial performance test, to every 12 

months if the maximum 6-minute opacity is less than or equal to 5 percent during the initial test.
Four Corners FIP Revisions | Response to Comments for Final Rule | January 2017 Page 17 of 18

commenter recommended the following additions (underlined) for the relevant provisions at 40 CFR 

49.5512(e)(2):

(2) Sulfur Dioxide. For the purpose of demonstrating compliance with this 

section, the sulfur dioxide inlet concentration (in lb/MMBtu) shall be calculated 

using the daily average percent sulfur and BTU content of the coal combusted, 

or after the installation of the SO2 and any diluent CEMS required under

paragraph (k)(3)(v), compliance with the provisions of paragraph (k)(3)(vi). If the 

sulfur dioxide inlet concentration is calculated [t]he inlet sulfur concentration 

and BTU testing shall be. . .

Response 12: The existing FIP requirements in 40 CFR 49.5512(d)(1) provide that the owner or 

operator shall not discharge SO2 in excess of 12.0 percent of the potential combustion concentration 

assuming all the sulfur in the coal is converted to SO2. The provisions at 40 CFR 49.5512(e)(2) require the 

owner or operator to calculate the SO2 inlet concentration to use the daily average percent sulfur and 

BTU content of the coal combusted. The proposed FIP revisions to incorporate the requirements of the 

Consent Decree into paragraph (k)(3) require the owner or operator to maintain a 95.0 percent SO2

removal efficiency and to determine the SO2 removal efficiency using an SO2 and diluent CEMS in 

addition to the SO2 outlet CEMS.16 The commenter is requesting the ability to comply with the 

requirement in 40 CFR 49.5512(e)(2) using the inlet SO2 and diluent CEMS required in the proposed 

provisions at paragraph (k)(3)(v). Because the requirements in paragraphs (d)(1), (e)(2), and (k)(3) 

pertain to SO2 removal efficiencies or methods for determining compliance with the SO2 removal 

efficiency requirements, the EPA considers it appropriate to streamline the methods for determining 

compliance with the removal efficiencies such that the method required in paragraph (k)(3), i.e., SO2 and 

diluent CEMS at the FGD inlet. In the final rule, APS can use the SO2 and diluent CEMS at the FGD inlet in 

lieu of the requirements in paragraph (e)(2), i.e., coal sampling and characterization, to determine 

compliance with the requirement in paragraph (d)(1). Therefore, in this final action, the EPA is amending 

 16 See proposed FIP provisions for 40 CFR 49.5512(k)(3)(iii) and (v).
Four Corners FIP Revisions | Response to Comments for Final Rule | January 2017 Page 18 of 18

40 CFR 49.5512(e)(2) as recommended by the commenter.

Comment 13: One commenter expressed concerns with the provision in the proposed FIP 

requiring APS to provide all submittals to both the Director of the Enforcement Division and the Director 

of the Air Division at Region IX of the EPA. The commenter opined that this expands the compliance 

burden of the operator without achieving any benefits for ensuring compliance or protecting the 

environment. The commenter recommended that the EPA require submittal of notifications to the 

Regional Administrator of Region IX and the Director of the Navajo Nation Environmental Protection 

Agency.

Response 13: We agree with the commenter that our proposed revision to require the operator 

to provide all submittals to both the Director of the Enforcement Division and the Director of the Air 

Division is not necessary. Although the EPA considers any additional burden for the proposed 

notifications to two offices of the agency to be small, we are amending the provisions at 40 CFR 

49.5512(f) as recommended by the commenter.