Technical Support Document 

For the Final Rule 

on 

Source-Specific Federal Implementation Plan for Navajo Generating Station; 

Navajo Nation 

December 7, 2009 

Contacts: 

Steve Frey 

Sarvy Mahdavi 

Colleen McKaughan 

Air Division 

U.S. Environmental Protection Agency – Region 9 

Ann Lyons 

Julie Walters 

Office of Regional Counsel 

U.S. Environmental Protection Agency – Region 9 
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ABBREVIATIONS AND ACRONYMS 

Pollutants 

CO2 Carbon dioxide 

NOx Nitrogen oxides 

O3 Ozone 

PM Particulate matter 

SO2 Sulfur dioxide 

Units 

MW megawatt 

TPY tons per year 

Acronyms 

APS Arizona Public Service 

ASTM American Society for Testing and Materials 

BART Best Available Retrofit Technology 

CAA Clean Air Act 

CAM Compliance Assurance Monitoring 

CAMR Clean Air Mercury Rule 

CEM Continuous Emissions Monitor 

EDF Environmental Defense Fund 

EGU Electric Generating Unit 

EIA Environmental Impact Assessment 

EPA United States Environmental Protection Agency 

FCPP Four Corners Power Plant 

FIP Federal Implementation Plan 

GHG Greenhouse Gas 

NAAQS National Ambient Air Quality Standards 

NGS Navajo Generating Station 

NPS National Park Service 

PSD Prevention of Significant Deterioration 

SIP State Implementation Plan 

TAR Tribal Authority Rule 

TIP Tribal Implementation Plan 

WELC Western Environmental Law Center 
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Table of Contents: 

I. Background 

II. Responses to Significant Comments 

A. Concerns About Air Quality, Public Health, and Other Environmental Media 

B. Concerns About Visibility and Mercury Emissions 

C. Concerns About Greenhouse Gas Emissions 

D. Comments on Desert Rock Power Plant 

E. Concerns About Public Health 

F. Comments on the Scope of the FIP 

G. Comments on the Preamble to the Proposed Rule 

H. Comments on Emissions Limits 

I. Comments on Control Requirements 
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I. Background: 

In an accompanying Federal Register notice, the Environmental Protection Agency 

(EPA) is finalizing a source-specific Federal Implementation Plan (FIP) to regulate emissions of 

several air pollutants from the Navajo Generating Station (NGS), a privately owned and operated 

coal-fired power plant located on the Navajo Indian Reservation near Page, Arizona. Based on 

lease agreements signed several decades ago, NGS was constructed and has been operating on 

real property held in trust by the federal government for the Navajo Nation. The facility consists 

of three coal-fired electric utility steam generating units with a total capacity in excess of 2000 

megawatts (MW). NGS burns coal mined from the Navajo Coal Mine. 

In 1999, EPA initially proposed to promulgate a FIP to regulate emissions from NGS. At 

that time, NGS had historically followed certain emissions limits in the Arizona SIP. However, 

because the Arizona SIP is not approved to apply on the Navajo Indian Reservation, and because 

the Navajo Nation did not have a federally applicable tribal implementation plan (TIP), EPA 

proposed to promulgate a FIP to remedy the existing regulatory gap. 64 FR 48725 (September 8, 

1999). The proposed FIP would have, in essence, federalized the requirements applicable to NGS 

contained in the Arizona SIP. In explaining the basis for its proposed action, EPA stated that 

given the magnitude of emissions from the plant, the Agency believed the proposed FIP 

provisions were necessary and appropriate to ensure the protection of air quality on the 

Reservation. 64 FR at 48726. 

EPA did not finalize the proposed 1999 FIP. Instead, EPA proposed a new FIP in 

September 2006. In the 2006 proposed FIP, EPA again explained that the Agency was proposing 

to remedy an existing regulatory gap with a source-specific FIP for NGS. EPA proposed to 

establish federally enforceable emission limits for NOx and PM based on the Arizona SIP limits, 
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to lower the opacity limit from 40% to 20% and to add control measures for dust. 

II. Responses to Significant Comments:

A. Concerns About Air Quality, Public Health and Other Environmental Media

Comment: The majority of commenters stated that EPA should require NGS to install 

the most stringent or best available air pollution controls because the air quality in the Four 

Corners region is generally poor, visibility is deteriorating, and there are high rates of cancer, 

asthma and other respiratory and public health problems. 

Response: EPA disagrees that we are required to impose more stringent emission limits 

for NGS than those contained in the FIP. EPA is exercising its discretion to close the regulatory 

gap that exists with respect to NGS. There is no approved implementation plan covering NGS or 

the Navajo Reservation, and in this action, EPA is not promulgating a reservation-wide FIP. 

EPA’s exercise of authority in issuing this FIP is based on the Agency’s conclusion that it is 

appropriate to protect air quality on the Reservation by remedying the lack of federally 

enforceable limits applicable to this facility. As such, our action is making enforceable those 

emissions limits NGS has historically followed. 

In any event, NGS is located near Page, Arizona and not in the Four Corners area. Both 

areas are designated attainment for each of the NAAQS, including the secondary standard for 

PM2.5 which was determined by the Administrator to be “requisite to protect the public welfare” 

from adverse visibility effects under section 109 of the CAA. In addition, visibility in the 

mandatory Class I areas near Page, Arizona and in the Fours Corners area is being separately 

addressed through EPA’s nationally applicable Regional Haze Rule at 40 CFR 51.308-309. 

These two issues are discussed separately below. 
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B. Concerns About Visibility and Mercury Emissions

Comment: Several commenters urged EPA to take regulatory action in addition to the 

proposed FIP to require reductions of NOx,PM and mercury emissions from NGS. In particular, 

several commenters urged EPA to undertake a determination of best available retrofit technology 

(BART) for NGS’s NOx emissions. See 40 U.S.C. 7491(b)(2)(A). One commenter noted that 

NGS is the 8th largest NOx emitter in the U.S. and that the FIP was not addressing NOx or the 

environmental impact from the NOx emissions. The commenter also requested an explanation of 

when and at what levels BART limits would be applied to PM, mercury, VOC and other 

pollutants. 

Response: EPA agrees that it may be necessary or appropriate in a future rulemaking to 

require NGS to reduce its NOx or PM emissions below those levels which were historically 

contained in the Arizona SIP (and are now contained in this FIP) or which are necessary to 

comply with the Acid Rain program. In the 1991 revision of the visibility FIP that created SO2

emission limits for NGS, EPA concluded that those limits achieved greater reasonable progress 

than would BART, but did not address emissions of NOx or PM from NGS. Today’s rule does 

not address the requirements of EPA’s nationally applicable Regional Haze rule, codified at 40 

CFR 51.308, which contains specific implementation plan requirements regarding BART 

determinations.1

 EPA recognizes, however, the importance of addressing emissions of NOx and 

PM from NGS for purposes of addressing NGS’s contribution to visibility impairment. EPA has 

 

1 Such implementation plans were not required from the States until December 17, 2007. Tribes are not subject to 

any mandatory deadlines to submit regional haze implementation plans. See 40 CFR 49.4; 64 FR at 35758 (“For 

example, unlike States, tribes are not required by the TAR to adopt and implement CAA plans or programs, thus 

tribes are not subject to mandatory deadlines for submittal of implementation plans.”); see also Arizona Public 

Service Company v. USEPA, 562 F.3d at 1119).
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requested and SRP has submitted an analysis of the NOx and PM control options to address 

BART. This document and supplemental submittals are available on the docket EPA has 

prepared for the BART rulemaking available at: 

http://www.regulations.gov/fdmspublic/component/main?main=DocketDetail&d=EPA-R09-

OAR-2008-0454 

EPA is reviewing the information provided, and consulting with the Federal Land 

Manager(s) and States with Class I areas impacted by NGS, to determine the appropriate BART 

limits for NGS. On August 28, 2009, EPA issued an Advance Notice of Proposed Rulemaking 

(“ANPR”) concerning the anticipated visibility improvements and the cost effectiveness for 

different levels of air pollution controls as BART for NGS and for another coal-fired power plant 

located on the Navajo Nation, Four Corners Power Plant (“FCPP”). EPA issued the ANPR for 

the specific purpose of collecting additional information that EPA may consider in modeling the 

degree of anticipated visibility improvements in the Class I areas surrounding the two power 

plants and for determining whether BART controls are cost effective at this time. EPA also 

requested any additional information that commenters believe the agency should consider in 

promulgating a FIP establishing BART for the two power plants. 

After considering the information received in response to the ANPR and other relevant 

information, EPA intends to publish separate FIPs proposing EPA’s BART determinations for 

FCPP and NGS under the Regional Haze rules. After evaluating all comments on the proposed 

BART determination for NGS, EPA will take final action regarding the BART requirements at 

NGS. 

Although it is unlikely that VOC emitted from NGS will be regulated for visibility 

protection under the Regional Haze rules, comments concerning the contribution of VOCs to 
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visibility impairment are more appropriately considered during the regional haze rulemaking 

discussed above. Historically VOC emissions from coal fired electric generating units (EGUs) 

have not been considered a significant contributor to visibility impairment, and EPA knows of no 

states in the West that are considering setting limits on coal-fired EGU VOC emissions for 

regional haze. In the West, the quantity of emissions of VOC from EGUs is relatively 

insignificant compared to the quantity of VOC emissions from biogenic sources, fires, or mobile 

sources. 

EPA is not considering setting a BART limit for mercury as there is no evidence that 

mercury contributes to visibility impairment. On October 28, 2009, pursuant to CAA section 

113(g), EPA published in the Federal Register for comment a proposed Consent Decree that 

would require the Agency to propose CAA section 112(d) standards to control hazardous air 

pollutants, including mercury, from coal- and oil-fired electric utility steam generating units by 

March 16, 2011, and issue final section 112(d) standards by November 16, 2011. EPA will 

request public comment on that rulemaking and will consider any significant comments on this 

issue that are raised during our section 112(d) rulemaking. 

NGS is located near Page, Arizona, which is relatively distant from the Four Corners 

region. Nevertheless, EPA appreciates that the Four Corners region has been a center of energy 

development during the past several years. However, we also recognize that the area near Page, 

Arizona and the Four Corners region are currently designated as being attainment areas for all 

criteria air pollutants which EPA currently regulates under the CAA. Please see 

http://www.epa.gov/region09/air/maps/maps_top.html for Region 9 air quality designations. 

 Section 108 of the CAA directs the Administrator to identify and list “air pollutants” that 

“in his judgment, may reasonably be anticipated to endanger public health and welfare” and 
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whose “presence . . . in the ambient air results from numerous or diverse mobile or stationary 

sources” and, if listed, to issue air quality criteria for them. These air quality criteria are intended 

to “accurately reflect the latest scientific knowledge useful in indicating the kind and extent of 

identifiable effects on public health or welfare which may be expected from the presence of [a] 

pollutant in ambient air. . .” Section 109, in turn, directs the Administrator to issue “primary” and 

“secondary” NAAQS for pollutants identified under section 109. The CAA defines a primary 

standard as one “the attainment and maintenance of which is the judgment of the Administrator, 

based on such criteria and allowing an adequate margin of safety, are requisite to protect the 

public health.” A secondary standard must “specify a level of air quality the attainment and 

maintenance of which, in the judgment of the Administrator, based on such criteria, is requisite 

to protect the public welfare from any known or anticipated adverse effects associated with the 

presence of [the] pollutant in the ambient air.” 

EPA’s NAAQS regulations further provide: “The promulgation of national primary and 

secondary ambient air quality standards shall not be considered in any manner to allow 

significant deterioration of existing air quality in any portion of any State or Indian country.” 

(See 40 CFR 50.2(c)).

In setting a primary NAAQS, EPA takes into account the effects of an air pollutant on 

individuals who are particularly sensitive to the effects of pollution, such as children or those 

with respiratory problems. See Lead Industries v. EPA, 647 F.2d 1130, 1153 (D.C. Cir. 1980). 

EPA’s evaluation for setting the secondary standards, used to protect public welfare, includes 

the pollutant’s “effects on soils, water, vegetation, man-made materials, animals, wildlife, 

weather, visibility, climate, damage and deterioration to property, hazards to transportation, as 

well as effects on economic values and personal comfort and well-being.” 42 U.S.C. 7602(h). 
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C. Concerns About Greenhouse Gas Emissions

Comment: Numerous commenters requested EPA to take action reducing greenhouse 

gases emissions from power plants. 

Response: As many commenters may be aware, on April 2, 2007, the U.S. Supreme 

Court held that greenhouse gases fit within the definition of “air pollutant” under the Clean Air 

Act. Massachusetts v. EPA, 127 S.Ct. 1438 (2007). In response to the Supreme Court’s 

decision, on December 7, 2009, the Administrator of the U.S. EPA signed the final 

endangerment and cause or contribute findings for greenhouse gases under Section 202(a) of the 

Clean Air Act. Specifically, she found that the current and projected concentrations of the six 

key well-mixed greenhouse gases—carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), 

hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6)—in the 

atmosphere threaten the public health and welfare of current and future generations. She also 

found that the combined emissions of these well-mixed greenhouse gases from new motor 

vehicles and new motor vehicle engines contribute to the greenhouse gas pollution which 

threatens public health and welfare. Note that these findings do not themselves impose any 

requirements on industry or other entities. However, this action is a prerequisite to finalizing the 

EPA’s proposed greenhouse gas emission standards for light-duty vehicles, which were proposed 

by EPA as part of a joint rulemaking with the Department of Transportation’s National Highway 

Safety Administration in September 2009. 74 FR 49454 (September 28, 2009). 

In light of the light-duty vehicles rule, greenhouse gases will become “subject to 

regulation” for purposes of the Prevention of Significant Deterioration (PSD) and title V permit 

programs. Thus, in October 2009 EPA proposed a rule that would “tailor” the permit programs 

to limit which facilities would be required to obtain permits by initially focusing on large 
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facilities emitting over 25,000 tons of greenhouse gases a year. 74 FR 55292 (October 27, 

2009). 

Also, EPA recently finalized a rule that requires reporting of greenhouse gas emissions 

from large sources and suppliers in the United States. 74 FR 56260 (October 30, 2009). This 

rule will allow EPA to collect accurate and timely greenhouse gas emissions data to inform 

future policy decisions. 

These actions show that EPA is moving forward in a careful and deliberate manner to fashion a 

reasonable and common-sense approach to address greenhouse gas emissions and climate 

change. EPA has and will continue to take a measured approach to address greenhouse gas 

emissions. Because EPA is continuing to develop its approach to addressing greenhouse gas 

emissions more broadly, and his FIP is narrow in scope, primarily developed to make 

enforceable those emissions levels that NGS has historically followed for traditional pollutants, 

this FIP is not the appropriate vehicle to address greenhouse gases. See, e.g., Ariz. Public 

Service Co. v. EPA, 562 F.3d 1116 (10th Cir. 2009) (EPA is not required to address all potential 

air quality issues in a FIP issued under 40 C.F.R. 49.11(a)). As discussed further in the preamble 

to the final rule, EPA is issuing this FIP under CAA sections 301(a) and 301(d)(4), and 40 CFR 

49.11(a), and has determined this FIP to be an appropriate and important step in protecting air 

quality on the Reservation. Moreover, even assuming that EPA were required to address all 

potential requirements of the CAA in this FIP, there is no NAAQS for CO2, and under section 

110 of the CAA, our FIP obligations are generally limited to pollutants for which the agency has 

established a NAAQS. While section 110 does address non-NAAQS pollutants through 

110(a)(2)(C) and (J)'s requirement for plans to meet the requirements of the PSD and visibility 

programs, NGS is covered by the federal PSD program (see 40 CFR 52.144) that will cover 
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greenhouse gases when such pollutants become "regulated air pollutants." We are not aware of 

any evidence that ambient concentrations of greenhouse gases are themselves direct contributors 

to significant visibility in any Class I areas. Thus, we do not believe it would be appropriate to 

address GHGs in this FIP. 

D. Comments About Desert Rock Power Plant

Comment: Some commenters objected to construction of the Desert Rock power plant. 

Response: EPA notes that the scope of this rulemaking is promulgation of the sourcespecific FIP for NGS. 

Although NGS is not located near the site of the proposed Desert Rock Energy Facility, 

EPA is aware of the impacts that are projected to occur if Desert Rock Energy Facility is 

constructed and begins operation. That facility is subject to PSD permitting requirements, which 

ensure that permitted facilities install and operate BACT and that their emissions will not cause a 

violation of any NAAQS or applicable increment. EPA received approximately 1000 comments 

on the proposed PSD permit for the Desert Rock Energy Facility, most of which raised concerns 

with air quality in the Four Corners area. EPA responded to those comments as part of its PSD 

permitting process, and those responses are available for public review. EPA will be 

reconsidering elements of the permit after a remand of the permit by the EPA Environmental 

Appeals Board. For further information on the PSD permitting process for the Desert Rock 

Energy Facility, see http://www.epa.gov/region09/air/permit/desertrock/. 

E. Concerns About Public Health

Comment: Some comments requested EPA to conduct a health study and stated that 

rates for cancer, asthma and other public health problems were worse in the Four Corners region 

than elsewhere. 
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Response: NGS is not located in the Four Corners region. In any event and as discussed 

more fully above, EPA regularly evaluates public health in the context of promulgating 

nationally applicable standards such as the NAAQS. 

F. Comments on the Scope of the FIP

Comment: The majority of commenters objecting to both the FCPP and NGS FIPs 

indicated that EPA should go beyond merely federalizing the emission limits which NGS has 

historically followed. Other commenters urged EPA to take regulatory action to regulate or to 

further reduce emissions of SO2, NOx, PM, mercury, and “toxic emissions.” Commenters raised 

a variety of general concerns regarding impacts associated with coal fired power plants such as 

NGS, including public health and/or environmental impacts of fugitive dust from coal mining, 

mercury (Hg) and carbon dioxide (CO2, greenhouse gases). Another commenter argued that in 

issuing a FIP for NGS, EPA must comply not only with all of the requirements of section 301 of 

the CAA but also ensure through the FIP process that NGS is in compliance with all applicable 

federal and state ambient standards by complying with the requirements of section 110 of the 

CAA addressing State implementation plans. 

Response: As stated above, EPA’s authority to promulgate this source-specific FIP is 

based on CAA sections 301(a) and (d)(4) and the regulations implementing these provisions at 

40 CFR Part 49. Today’s action is not based on the requirements of CAA section 110. CAA 

section 301(d)(4) provides EPA with broad discretion to promulgate regulations directly for 

sources located in Indian country. The Tribal Authority Rule provides EPA with “discretion to 

determine what rulemaking is necessary or appropriate to protect air quality in Indian country. 

Arizona Public Service Company v. USEPA, 562 F.3d 1116, 1125 (10th Cir. 2009)(upholding 

EPA’s interpretation of its discretionary authority in the context of a source-specific FIP for the 
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Four Corners Power Plant). 

EPA is exercising its discretion to promulgate emission limitations for NGS to close the 

regulatory gap that exists with respect to NGS. As explained above, at present there is no 

approved implementation plan covering NGS because the Arizona SIP does not apply to sources 

located on the Navajo Indian Reservation and the Navajo Nation has not promulgated an 

applicable Tribal Implementation Plan. EPA’s exercise of authority in issuing this FIP is based 

on the Agency’s conclusion that it is appropriate to protect air quality on the Reservation by 

remedying the lack of federally enforceable limits applicable to NGS. As such, our action is 

largely limited to making enforceable those emissions limits which NGS has historically 

followed and re-codifying the limitations applicable to NGS in the visibility FIP for Arizona. 

We have also finalized our proposal to lower the opacity limit and to add certain material 

handling measures to provide additional benefits to air quality and visibility, and to conform to 

revisions that have been approved into the Arizona SIP. Today’s action is an important step in 

protecting air quality on the Reservation. As noted in the proposal, this action will contribute 

towards ensuring continued maintenance of the NAAQS and towards protecting visibility. EPA 

acknowledges that additional regulatory actions by EPA may be necessary or appropriate in the 

future to further protect air quality on the Navajo Reservation, depending on, among other 

things, conditions on the Reservation and the decisions of the Navajo Nation to implement air 

quality programs. Our detailed response to comments on mercury, CO2 and other emissions is 

discussed further below and in our Response to Comments document: 

G. Comments on the Preamble to the Proposed Rule 

Comment: SRP submitted several clarification comments on the language in the 

preamble to the proposed rule. Clarifications included corrections to our description of NGS’ 
15

ownership, pollutant emissions, the particulate matter limit, SRP’s previous method of 

compliance determination for SO2 emissions, and discussion of coal sulfur content. 

Response: Although EPA agrees with SRP’s clarification comments on the preamble to 

the proposed rule, these comments do not affect the language in the final rule. Although we are 

not making any changes to the FIP’s regulatory language as a result of these comments, we have 

made changes, where appropriate, in the preamble to this final action to reflect these clarifying 

comments. 

H. Comments on Emissions Limits

Comment: Several commenters urged EPA to take regulatory action in addition to the 

proposed FIP to require reductions of NOx and PM emissions from NGS. In particular, several 

commenters urged EPA to undertake a determination of best available retrofit technology 

(BART) for NGS’s NOx emissions. See 40 U.S.C. 7491(b)(2)(A). One commenter noted that 

NGS is the 8th largest NOx emitter in the U.S. and that the FIP was not addressing NOx or the 

environmental impact from the NOx emissions. The commenter also requested an explanation of 

when and at what levels BART limits would be applied to PM, mercury, VOC and other 

pollutants. 

Response: As noted in detail above, EPA agrees that it may be necessary or appropriate 

in a future rulemaking to require NGS to reduce its NOx or PM emissions below those levels 

which were historically contained in the Arizona SIP (and are now contained in this FIP) or 

which are necessary to comply with the Acid Rain program. EPA recognizes, however, the 

importance of addressing emissions of NOx and PM from NGS for purposes of addressing 

NGS’s contribution to visibility impairment. For further information, see the previous response 
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to comments on visibility impairment. 

Although it is unlikely that VOC emitted from NGS will be regulated for visibility 

protection under the Regional Haze rules, comments concerning the contribution of VOCs to 

visibility impairment are more appropriately considered during the regional haze rulemaking 

discussed above. Historically VOC emissions from coal fired electric generating units (EGUs) 

have not been considered a significant contributor to visibility impairment, and EPA knows of no 

states in the West that are considering setting limits on coal-fired EGU VOC emissions for 

regional haze. In the West, the quantity of emissions of VOC from EGUs is relatively 

insignificant compared to the quantity of VOC emissions from biogenic sources, fires, or mobile 

sources. 

EPA is not considering setting a BART limit for mercury as there is no evidence that 

mercury contributes to visibility impairment. On October 28, 2009, pursuant to CAA section 

113(g), EPA published in the Federal Register for comment a proposed Consent Decree that 

would require the Agency to propose CAA section 112(d) standards to control hazardous air 

pollutants, including mercury, from coal- and oil-fired electric utility steam generating units by 

March 16, 2011, and issue final section 112(d) standards by November 16, 2011. EPA will 

request public comment on that rulemaking and will consider any significant comments on this 

issue that are raised during our section 112(d) rulemaking. 

Comment: SRP requested that the particulate matter limit in the proposed rule be revised 

for better clarity. The requested changes included that compliance would be determined from at 

least three test runs over a 60 minute duration at each stack. 

Response: EPA agrees with SRP’s proposed changes to the particulate matter limit and 

has made the appropriate revisions in the final rule which include specifying at least three 60 
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minute sampling runs for each stack. This also changes the averaging time for the particulate 

matter limit from the proposed 6 hour average to a three hour average based on three runs lasting 

approximately one hour each. This is not a significant change from the proposal and results in 

increasing the enforceability of the limit. 

Comment: One commenter noted inconsistencies with NGS’s excess opacity reporting to 

the Arizona Division of Environmental Quality (ADEQ) and to the Navajo EPA. The commenter 

noted that the excess opacity reporting to the Navajo EPA was based on 40% and not the 

proposed 20%, while reporting to ADEQ was based on 20% opacity. 

Response: The commenter’s observation is correct that past reports to the Navajo EPA 

were based on 40% opacity exceedances. This FIP proposed and is finalizing a 20% opacity 

limit. By making NGS subject to a 20% opacity limit, the units will be operated in a manner 

consistent with the change that was recently made to the Arizona SIP to require 20 % opacity. 

The 20% opacity standard is more stringent (i.e., environmentally beneficial) than 40%. 

Comment: One commenter noted that the language in the opacity provision should be 

clarified to "condensed uncombined water." The commenter also suggested that the word 

“normal” be dropped from the phrase “normal six (6) minute period.” In addition, the 

commenter thought that the exemption allowing 40% opacity for “scrubber transition periods” 

was subjective and allows NGS to define these periods. 

Response: EPA agrees with the first part of the comment. EPA has changed the 

language from the proposed, “excluding condensed water vapor”, to the suggested “condensed 

uncombined water”, and has also added the word “droplets” after "condensed uncombined 

water,” as EPA believes the term “condensed uncombined water droplets” is more accurate. 

EPA notes that adding the word “droplets” to this language is consistent with the use of the word 
18

in the analogous opacity provision in the FCPP FIP. EPA also agrees with the second part of the 

comment; EPA believes that that the word “normal” is ambiguous and unnecessary and has 

deleted it from the provision.

However, EPA does not agree with the third part of the comment referring to the scrubber 

transition periods. One purpose for monitoring opacity is to ensure the particulate matter 

standard is being met on a continuous basis without requiring installation of a PM CEMS. 

However, for the particular PM emissions control system at NGS (Electrostatic Precipitators 

followed by SO2 scrubbers), there are periods of opacity that cannot be necessarily correlated to 

improper operation of the control system. EPA has added to the definition of "scrubber 

transition period" language that further defines it as a period when the scrubber module is not in 

use. This is a very well defined period and is easily identified by the high readings of the SO2

CEMs. NGS has found that during the very infrequent occasions when a scrubber module is out 

of service and the unit is operating (i.e., the scrubber transition period), the hot flue gas 

(normally cooled by the scrubber) collects salts that have accumulated in the stack over extended 

periods of wet stack conditions. This has led to increased opacity even when the Electrostatic 

Precipitators (ESPs) upstream of the scrubbers are operating normally and properly controlling 

PM emissions. This potential excess opacity is not the result of poor performance of the 

particulate matter control device, but is due to the inevitable minor carryover of droplets from the 

scrubber which deposit on the stacks. 

I. Comments on Control Requirements

Comment: SRP requested the startup termination limit for NGS be increased from 300 to 

400 MW to maintain consistency with the startup termination limit for FCPP. 

Response: Other than noting that EPA allowed a startup termination limit of 400 MW for 
19

FCPP, SRP has not provided an explanation as to why a startup termination limit of 400 MW is 

more appropriate for NGS than 300 MW. The critical factor in the startup is that the hot side ESP 

reaches 400o so that it may be expected to operate properly. This temperature can be reached 

when the NGS units reach 300 MW. To allow the startup to extend beyond this operating level 

simply because EPA agreed to it for FCPP, which has completely different control technology 

with different operational limitations, is not reasonable. Given that the control technology at 

NGS is different from the control technology at FCPP, and that NGS provided no technical 

justification for making the change from 300 MW to 400 MW, EPA maintains the 300 MW 

startup termination limit for NGS along with the proposed 400o precipitator temperature. 

Comment: SRP requested a change to the shutdown definition, because they claimed 

that the first sentence, which referred to cessation of coal burning, was incorrect. 

Response: EPA agrees with this comment and dropped the first sentence of the definition 

referring to cessation of coal burning, since coal may still be combusted when a unit load reaches 

300 MW or less and the intention is to remove the unit from service. This is not a significant 

change from the proposal. 

Comment: SRP requested that NGS be exempt from opacity monitoring requirements, 

consistent with 40 CFR 75.14(b) which exempts units equipped with a wet flue pollution control 

system for SO2 or particulates from the monitoring requirements of Part 75, if the source “can 

demonstrate that condensed water is present in the exhaust flue gas stream and would impede the 

accuracy of opacity measurements.” 

Response: EPA agrees with SRP’s comments that when the stack is saturated and has 

uncombined water droplets, the Continuous Opacity Monitoring Systems (COMs) cannot 

correctly read the opacity due to particulate matter and has updated the final rule to reflect this 
20

change; however, NGS will continue to have a requirement to operate COMs on each stack. 

NGS is required to report those time periods when saturated stack conditions impact the opacity 

monitoring. EPA believes that it is reasonable to continue to require the COMs since there are 

times that NGS is allowed to bypass the scrubbers under the visibility FIP, and continued 

operation of the COMs will allow assessment of opacity and proper operation of the particulate 

matter controls during these periods. 

Comment: SRP requested that a new section be added to the NGS FIP to address the 

method to be used to demonstrate compliance with the 20 percent opacity limit for dust related 

activities. 

Response: EPA agrees with SRP’s comments that the rule should cite the appropriate 

EPA reference method and with SRP’s proposed addition to the NGS FIP for this purpose. EPA 

has added this new requirement to the final rule at 40 CFR 49.24(e)(8), and has also added a 

reference to EPA Reference Method 9 at 40 CFR 49.24(d)(3). This is not a significant change 

from the proposal and increases enforceability. 

Comment: SRP commented that because SRP is already operating CEMS for nitrogen 

oxides (NOx) and SO2 and COMs on Units 1, 2, and 3, the requirement to maintain and operate 

these systems sixty days after promulgation of the rule is unnecessary. SRP also commented that 

because this equipment has already been certified, it does not need to be re-certified. 

Response: EPA agrees with these comments and notes that the requirements in 40 CFR 

Part 75 govern operation and maintenance of the CEMS and COMs equipment at NGS. EPA has 

removed the requirements for the equipment to be maintained and operated sixty days after the 

promulgation of the rule and the certification requirement. This is not a significant change from 

the proposal because NGS must continue to operate its CEM and COMs pursuant to the 
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requirements of 40 CFR Part 75. 

Commment: SRP requested that the final FIP for NGS only require the use of the 

Quality Assurance procedures in 40 CFR Part 75. 

Response: In the proposal, EPA added quality assurance provisions in addition to the 

quality assurance procedures in Part 75. EPA believes that these procedures and performance 

specifications are needed for very low emitting SO2 sources such as NGS where the monitor is 

used for determining compliance. However, EPA has determined that the 1991 visibility FIP 

already requires the use of CEMs for SO2 compliance determinations and requires 40 CFR Part 

60 Appendix F quality assurance (See 40 CFR 52.145(d)(4)(1991)) for these CEMs. Further, the 

Quality Assurance procedures in 40 CFR Part 75 are also already applicable to the facility. 

Therefore, this final rule has been modified from the proposal to delete these quality assurance 

provisions. 

Comment: SRP requested several revisions to the language in the rule regarding 

emissions tests for auxiliary boilers to better accommodate the limited use of these boilers. SRP 

requested that the testing be required if the unit operated more than 720 hours in any calendar 

year as opposed to the proposed requirement to test after a cumulative 720 hours of operation. 

Response: Due to the limited use of these boilers at NGS, EPA agrees with SRP’s 

comments and has updated the final rule to incorporate SRP’s requested changes. In addition, the 

final rule states that EPA has the authority to require testing at any time EPA believes it is 

necessary. This is not a significant change from the proposal and increases enforceability of the 

rule.