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Rule2026-00677

New Source Performance Standards Review for Stationary Combustion Turbines and Stationary Gas Turbines

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Published

January 15, 2026

Effective

January 15, 2026

Issuing agencies

Environmental Protection Agency

Abstract

The U.S. Environmental Protection Agency (EPA, or Agency) is finalizing amendments to the new source performance standards (NSPS) for stationary combustion turbines and stationary gas turbines pursuant to a review required by the Clean Air Act (CAA). As a result of this review, the EPA is establishing subcategories for new, modified, or reconstructed stationary combustion turbines based on size, rates of utilization, design efficiency, and fuel type. The EPA determined that combustion controls are the best system of emission reduction (BSER) for nitrogen oxide (NO<INF>X</INF>) emissions for most new, modified, or reconstructed stationary combustion turbines. For one subcategory, the BSER for NO<INF>X</INF> is combustion controls with the addition of selective catalytic reduction (SCR). The EPA further determined that the BSER for sulfur dioxide (SO<INF>2</INF>) emissions has not changed since the last NSPS review. Based on these determinations, the Agency is promulgating standards of performance in a new subpart of the Code of Federal Regulations (CFR). The Agency is also adding a subcategory for stationary combustion turbines that are used in temporary applications, exempting certain sources from title V requirements, and finalizing other provisions. The EPA is finalizing amendments to existing regulations to address or clarify specific technical and editorial issues.

Full Text

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[Federal Register Volume 91, Number 10 (Thursday, January 15, 2026)]
[Rules and Regulations]
[Pages 1910-2005]
From the Federal Register Online via the Government Publishing Office [<a href="http://www.gpo.gov">www.gpo.gov</a>]
[FR Doc No: 2026-00677]

[[Page 1909]]

Vol. 91

Thursday,

No. 10

January 15, 2026

Part III

Environmental Protection Agency

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40 CFR Part 60

New Source Performance Standards Review for Stationary Combustion
Turbines and Stationary Gas Turbines; Final Rule

Federal Register / Vol. 91, No. 10 / Thursday, January 15, 2026 /
Rules and Regulations

[[Page 1910]]

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 60

[EPA-HQ-OAR-2024-0419; FRL-11542-02-OAR]
RIN 2060-AW21

New Source Performance Standards Review for Stationary Combustion
Turbines and Stationary Gas Turbines

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

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SUMMARY: The U.S. Environmental Protection Agency (EPA, or Agency) is
finalizing amendments to the new source performance standards (NSPS)
for stationary combustion turbines and stationary gas turbines pursuant
to a review required by the Clean Air Act (CAA). As a result of this
review, the EPA is establishing subcategories for new, modified, or
reconstructed stationary combustion turbines based on size, rates of
utilization, design efficiency, and fuel type. The EPA determined that
combustion controls are the best system of emission reduction (BSER)
for nitrogen oxide (NO<INF>X</INF>) emissions for most new, modified,
or reconstructed stationary combustion turbines. For one subcategory,
the BSER for NO<INF>X</INF> is combustion controls with the addition of
selective catalytic reduction (SCR). The EPA further determined that
the BSER for sulfur dioxide (SO<INF>2</INF>) emissions has not changed
since the last NSPS review. Based on these determinations, the Agency
is promulgating standards of performance in a new subpart of the Code
of Federal Regulations (CFR). The Agency is also adding a subcategory
for stationary combustion turbines that are used in temporary
applications, exempting certain sources from title V requirements, and
finalizing other provisions. The EPA is finalizing amendments to
existing regulations to address or clarify specific technical and
editorial issues.

DATES: This final rule is effective on January 15, 2026. The
incorporation by reference of certain publications listed in the rule
is approved by the Director of the Federal Register as of January 15,
2026. The incorporation by reference of certain other material listed
in the rule was approved by the Director of the Federal Register as of
July 8, 2004, and July 6, 2006.

ADDRESSES: The EPA has established a docket for this action under
Docket ID No. EPA-HQ-OAR-2024-0419. All documents in the docket are
listed on the <a href="https://www.regulations.gov">https://www.regulations.gov</a> website. Although listed,
some information is not publicly available, e.g., Confidential Business
Information (CBI) or other information whose disclosure is restricted
by statute. Certain other material, such as copyrighted material, is
not placed on the internet and will be publicly available only as
portable document format (PDF) versions that can only be accessed on
the EPA computers in the docket office reading room. Certain databases
and physical items cannot be downloaded from the docket but may be
requested by contacting the docket office at (202) 566-1744. The docket
office has up to 10 business days to respond to these requests. Except
for such material, all documents are available electronically in
<a href="http://Regulations.gov">Regulations.gov</a> or on the EPA computers in the docket office reading
room at the EPA Docket Center, WJC West Building, Room Number 3334,
1301 Constitution Ave. NW, Washington, DC. The Public Reading Room
hours of operation are 8:30 a.m. to 4:30 p.m. Eastern Standard Time
(EST), Monday through Friday. The telephone number for the Public
Reading Room is (202) 566-1744, and the telephone number for the EPA
Docket Center is (202) 566-1742.

FOR FURTHER INFORMATION CONTACT: For information about this final rule,
contact John Ashley, Industrial Processing and Power Division (D243-
02), Office of Clean Air Programs, U.S. Environmental Protection
Agency, 109 T.W. Alexander Drive, P.O. Box 12055, RTP, North Carolina
27711; telephone number: (919) 541-1458; and email address:
<a href="/cdn-cgi/l/email-protection#4726342f2b223e692d282f290722372669202831">[email&#160;protected]</a>.

SUPPLEMENTARY INFORMATION:
Preamble acronyms and abbreviations. Throughout this document the
use of ``we,'' ``us,'' or ``our'' is intended to refer to the EPA. We
use multiple acronyms and terms in this preamble. While this list may
not be exhaustive, to ease the reading of this preamble and for
reference purposes, the EPA defines the following terms and acronyms
here:

ANSI American National Standards Institute
ASME American Society of Mechanical Engineers
ASTM American Society for Testing and Materials
BPT benefit-per-ton
BSER best system of emission reduction
Btu British thermal unit
CAA Clean Air Act
CAMPD Clean Air Markets Program Data
CBI Confidential Business Information
CDX Central Data Exchange
CEDRI Compliance and Emissions Data Reporting Interface
CEMS continuous emissions monitoring system
CFR Code of Federal Regulations
CHP combined heat and power
CMS continuous monitoring system
CO carbon monoxide
CO<INF>2</INF> carbon dioxide
DLE dry low-emission
DLN dry low-NO<INF>X</INF>
EIA Economic Impact Analysis
EPA Environmental Protection Agency
ERT Electronic Reporting Tool
FR Federal Register
GE General Electric
GHG greenhouse gas
GJ gigajoule(s)
gr grains
HAP hazardous air pollutant
HHV higher heating value
HRSG heat recovery steam generator
ICR information collection request
ISA Integrated Science Assessment
kW kilowatt
LAER lowest achievable emission rate
LCOE levelized cost of electricity
lb/MWh pounds per megawatt-hour
lb/MMBtu pounds per million British thermal units
MJ megajoules
MMBtu/h million British thermal units per hour
MW megawatt
MWh megawatt-hour
NAICS North American Industry Classification System
NEI National Emissions Inventory
NESHAP national emission standards for hazardous air pollutants
NETL National Energy Technology Laboratory
ng/J nanograms per joule
NO<INF>X</INF> nitrogen oxide
NSPS new source performance standards
NSR New Source Review
NSSN National Standards System Network
NTTAA National Technology Transfer and Advancement Act
O<INF>2</INF> oxygen gas
O&M operating and maintenance
OEM original equipment manufacturers
OMB Office of Management and Budget
PDF portable document format
PM particulate matter
PM<INF>2.5</INF> particulate matter (diameter less than or equal to
2.5 micrometers)
ppm parts per million
ppmv parts per million by volume
ppmvd parts per million by volume dry
ppmw parts per million by weight
PRA Paperwork Reduction Act
PSD Prevention of Significant Deterioration
RATA relative accuracy test audit
RFA Regulatory Flexibility Act
RICE reciprocating internal combustion engines
scf standard cubic feet
scm standard cubic meter
SCR selective catalytic reduction
SO<INF>2</INF> sulfur dioxide
SSM startup, shutdown, and malfunction
ULSD ultra-low-sulfur diesel
UMRA Unfunded Mandates Reform Act
U.S.C. United States Code
VCS voluntary consensus standard

[[Page 1911]]

VOC volatile organic compound(s)

Table of Contents

I. General Information
A. Does this action apply to me?
B. Where can I get a copy of this document and other related
information?
C. Judicial Review and Administrative Review
II. Background
A. What is the statutory authority for this final action?
B. How does the EPA perform the NSPS review?
C. What is the source category regulated in this final action?
D. The Role of the NSPS
III. What changes did we propose for the stationary combustion
turbines and stationary gas turbines NSPS?
IV. What actions are we finalizing and what is our rationale for
such decisions?
A. Applicability
B. NO<INF>X</INF> Emissions Standards
C. SO<INF>2</INF> Emissions Standards
D. Consideration of Other Criteria Pollutants
E. Additional Amendments
F. NSPS Subpart KKKKa Without Startup, Shutdown, and Malfunction
Exemptions
G. Testing and Monitoring Requirements
H. Electronic Reporting
I. Other Final Amendments
J. Effective Date and Compliance Date
K. Severability
V. Summary of Cost, Environmental, and Economic Impacts
A. What are the air quality impacts?
B. What are the secondary impacts?
C. What are the cost impacts?
D. What are the economic impacts?
E. What are the benefits?
VI. What actions are we not finalizing and what is our rationale for
such decisions?
A. Clarification to the Definition of Stationary Combustion
Turbine
B. Definition of Noncontinental Area
C. Affected Facility
VII. Statutory and Executive Order Reviews
A. Executive Order 12866: Regulatory Planning and Review and
Executive Order 13563: Improving Regulation and Regulatory Review
B. Executive Order 14192: Unleashing Prosperity Through
Deregulation
C. Paperwork Reduction Act (PRA)
D. Regulatory Flexibility Act (RFA)
E. Unfunded Mandates Reform Act (UMRA)
F. Executive Order 13132: Federalism
G. Executive Order 13175: Consultation and Coordination With
Indian Tribal Governments
H. Executive Order 13045: Protection of Children From
Environmental Health Risks and Safety Risks
I. Executive Order 13211: Actions Concerning Regulations That
Significantly Affect Energy Supply, Distribution, or Use
J. National Technology Transfer and Advancement Act (NTTAA) and
1 CFR Part 51
K. Congressional Review Act (CRA)

I. General Information

A. Does this action apply to me?

The source category that is the subject of this final action is
composed of stationary combustion turbines and stationary gas turbines
regulated under CAA section 111. Based on the number of sources of
stationary combustion turbines listed in the 2020 National Emissions
Inventory (NEI), most, but not all, are accounted for by the following
2022 North American Industry Classification System (NAICS) codes. These
include 2111 (Oil and Gas Extraction), 2211 (Electric Power Generation,
Transmission, and Distribution), 2212 (Natural Gas Distribution), 3251
(Basic Chemical Manufacturing), 4862 (Pipeline Transportation of
Natural Gas), and 518210 (Data Processing, Hosting, and Related
Services). The NAICS codes serve as a guide for readers outlining the
types of entities that this final action is likely to affect.
The NSPS codified in 40 CFR part 60, subpart KKKKa, are directly
applicable to affected facilities that began construction,
modification, or reconstruction after December 13, 2024. Federal,
State, local, and Tribal government entities that own and/or operate
stationary combustion turbines subject to 40 CFR part 60, subpart
KKKKa, are affected by these amendments and standards. If you have any
questions regarding the applicability of this action to a particular
entity, you should carefully examine the applicability criteria found
in 40 CFR part 60, subparts GG, KKKK, and KKKKa, and consult the person
listed in the FOR FURTHER INFORMATION CONTACT section of this preamble,
your State air pollution control agency with delegated authority for
NSPS, or your EPA Regional Office.

B. Where can I get a copy of this document and other related
information?

In addition to being available in the docket, an electronic copy of
this final action is available on the internet at <a href="https://www.epa.gov/stationary-sources-air-pollution/stationary-gas-and-combustion-turbines-new-source-performance">https://www.epa.gov/stationary-sources-air-pollution/stationary-gas-and-combustion-turbines-new-source-performance</a>. Following publication in the Federal
Register, the EPA will post the Federal Register version of the final
rule and key technical documents at this same website.

C. Judicial Review and Administrative Review

Under CAA section 307(b)(1), judicial review of this final action
is available only by filing a petition for review in the United States
Court of Appeals for the District of Columbia Circuit by March 16,
2026. Under CAA section 307(b)(2), the requirements established by this
final rule may not be challenged separately in any civil or criminal
proceedings brought by the EPA to enforce the requirements.
CAA section 307(d)(7)(B) further provides that ``[o]nly an
objection to a rule or procedure which was raised with reasonable
specificity during the period for public comment (including any public
hearing) may be raised during judicial review.'' This section also
provides a mechanism for the EPA to convene a proceeding for
reconsideration ``[i]f the person raising an objection can demonstrate
to the EPA that it was impracticable to raise such objection within
[the period for public comment] or if the grounds for such objection
arose after the period for public comment, (but within the time
specified for judicial review) and if such objection is of central
relevance to the outcome of the rule.'' Any person seeking to make such
a demonstration to us should submit a Petition for Reconsideration to
the Office of the Administrator, U.S. Environmental Protection Agency,
Room 3000, WJC South Building, 1200 Pennsylvania Ave. NW, Washington,
DC 20460, with a copy to both the person(s) listed in the preceding FOR
FURTHER INFORMATION CONTACT section, and the Associate General Counsel
for the Air and Radiation Law Office, Office of General Counsel (Mail
Code 2344A), U.S. Environmental Protection Agency, 1200 Pennsylvania
Ave. NW, Washington, DC 20460.

II. Background

A. What is the statutory authority for this final action?

The EPA's authority for this final rule is CAA section 111, which
governs the establishment of standards of performance for stationary
sources. CAA section 111(b)(1)(A) requires the EPA Administrator to
promulgate a list of categories of stationary sources that the
Administrator, ``in his judgment,'' finds ``causes, or contributes
significantly to, air pollution which may reasonably be anticipated to
endanger public health or welfare.'' The EPA has the authority under
this section to define the scope of the source categories; to
determine, consistent with the statutory requirements, the pollutants
for which standards should be developed; and to distinguish among
classes, types, and sizes within categories in establishing

[[Page 1912]]

the standards.\1\ Once the EPA lists a source category that contributes
significantly to dangerous air pollution, the EPA must, under CAA
section 111(b)(1)(B), establish ``standards of performance'' for ``new
sources'' in the source category. These standards are referred to as
new source performance standards, or NSPS. The NSPS are national
requirements that apply directly to the sources subject to them.
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\1\ 42 U.S.C. 7411(b)(2) provides the EPA the authority to
establish subcategories.
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Under CAA section 111(a)(1), a ``standard of performance'' is
defined as ``a standard for emissions of air pollutants'' that is
determined in a specified manner. When the EPA establishes or revises a
performance standard, CAA section 111(a)(1) provides that such standard
must ``reflect[ ] the degree of emission limitation achievable through
the application of the best system of emission reduction which (taking
into account the cost of achieving such reduction and any nonair
quality health and environmental impact and energy requirements) the
Administrator determines has been adequately demonstrated.'' Thus, the
term ``standard of performance'' as used in CAA section 111 makes clear
that the EPA must determine both the ``best system of emission
reduction . . . adequately demonstrated'' (BSER) for emissions of the
relevant air pollutants by regulated sources in the source category and
the ``degree of emission limitation achievable through the application
of the [BSER].'' \2\ As explained further below, to determine the BSER,
the EPA first identifies the ``system[s] of emission reduction'' that
are ``adequately demonstrated,'' and then determines the ``best'' of
those adequately demonstrated systems, ``taking into account'' factors
including ``cost,'' ``nonair quality health and environmental impact,''
and ``energy requirements.'' The EPA then derives from that system an
``achievable'' ``degree of emission limitation.'' The EPA must then,
under CAA section 111(b)(1)(B), promulgate ``standard[s] for
emissions''--the NSPS--that reflect that level of stringency. The EPA
may determine that different sets of sources have different
characteristics relevant for determining the BSER for emissions of the
relevant air pollutants and may subcategorize sources accordingly.\3\
CAA section 111(b)(5) generally precludes the EPA from prescribing a
particular technological system that must be used to comply with a
standard of performance. Rather, sources can select any measure or
combination of measures that will achieve the standard.
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\2\ West Virginia v. EPA, 597 U.S. 697, 709 (2022).
\3\ 42 U.S.C. 7411(b)(2).
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Pursuant to the definition of new source in CAA section 111(a)(2),
standards of performance apply to facilities that begin construction,
modification, or reconstruction after the date of publication of the
proposed standards in the Federal Register. Under CAA section
111(a)(4), ``modification'' means any physical change in, or change in
the method of operation of, a stationary source which increases the
amount of any air pollutant emitted by such source or which results in
the emission of any air pollutant not previously emitted. Changes to an
existing facility that do not result in an increase in emissions are
not considered modifications. Under the provisions in 40 CFR 60.15,
reconstruction means the replacement of components of an existing
facility such that: (1) the fixed capital cost of the new components
exceeds 50 percent of the fixed capital cost that would be required to
construct a comparable entirely new facility; and (2) it is
technologically and economically feasible to meet the applicable
standards. Pursuant to CAA section 111(b)(1)(B), the standards of
performance or revisions thereof shall become effective upon
promulgation.
1. Key Elements of Determining a Standard of Performance
Congress first defined the term ``standard of performance'' when
enacting CAA section 111 in the 1970 Clean Air Act, amended the
definition in the Clean Air Act Amendments (CAAA) of 1977, and then
amended the definition again in the 1990 CAAA to largely restore the
definition as it read in the 1970 CAA. The D.C. Circuit has reviewed
CAA section 111 rulemakings on numerous occasions since 1973 and has
developed a body of caselaw that interprets the term.\4\
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\4\ Portland Cement Ass'n v. Ruckelshaus, 486 F.2d 375 (D.C.
Cir. 1973); Essex Chemical Corp. v. Ruckelshaus, 486 F.2d 427 (D.C.
Cir. 1973); Sierra Club v. Costle, 657 F.2d 298 (D.C. Cir. 1981);
Lignite Energy Council v. EPA, 198 F.3d 930 (D.C. Cir. 1999);
Portland Cement Ass'n v. EPA, 665 F.3d 177 (D.C. Cir. 2011);
American Lung Ass'n v. EPA, 985 F.3d 914 (D.C. Cir. 2021), rev'd in
part, West Virginia v. EPA, 597 U.S. 697 (2022). See also Delaware
v. EPA, 785 F.3d 1 (D.C. Cir. 2015).
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The basis for standards of performance is the ``degree of emission
limitation'' that is ``achievable'' by sources in the source category
by application of the ``best system of emission reduction'' that the
EPA determines is ``adequately demonstrated'' (BSER). As explained
further below in this section, the D.C. Circuit has explained that
systems are not ``adequately demonstrated'' if they are ``purely
theoretical or experimental.'' \5\ The D.C. Circuit has stated that in
determining the ``best'' adequately demonstrated system for the
pollutants at issue, the EPA must also take into account ``the amount
of air pollution'' reduced.\6\ The D.C. Circuit has also stated that
the EPA may weigh the various factors identified in the statute and
caselaw to determine the ``best'' system and has emphasized that the
EPA has significant discretion in weighing the factors.\7\
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\5\ Essex Chem. Corp. v. Ruckelshaus, 486 F.2d 427, 433-34 (D.C.
Cir. 1973).
\6\ See Sierra Club v. Costle, 657 F.2d 298, 326 (D.C. Cir.
1981). The D.C. Circuit has stated that EPA must also take into
account ``technological innovation.'' See id. at 347.
\7\ See Lignite Energy Council, 198 F.3d at 933 (``Because
section 111 does not set forth the weight that should be assigned to
each of these factors, we have granted the agency a great degree of
discretion in balancing them.'').
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After determining the BSER, the EPA sets an achievable emission
limit based on application of the BSER.\8\ For a CAA section 111(b)
rule, the EPA determines the standard of performance that reflects the
achievable emission limit. For a CAA section 111(d) rule, the States
have the obligation of establishing standards of performance for the
affected sources that reflect the degree of emission limitation that
the EPA has determined and provided to States as part of an emission
guideline. In applying these standards to existing sources, States are
permitted to take a source's remaining useful life and other factors
into account.
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\8\ See, e.g., Oil and Natural Gas Sector: New Source
Performance Standards and National Emission Standards for Hazardous
Air pollutants Reviews (77 FR 49494; August 16, 2012) (describing
the three-step analysis in setting a standard of performance).
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In identifying ``system[s] of emission reduction, the EPA has
historically followed a ``technology-based approach'' that focuses on
``measures that improve the pollution performance of individual
sources,'' such as ``add-on controls.'' \9\ The EPA departed from its
historical approach in a significant way in the 2015 Clean Power Plan
(CPP) \10\ by setting a BSER in which the ``system'' of emissions
reduction involved shifting electricity generation from one type of
fuel to another. In West Virginia v. EPA, the Supreme Court applied the
major questions doctrine to hold that the term ``system'' did not
provide the requisite clear authorization to support the CPP's BSER,
which the Court described as ``carbon emissions

[[Page 1913]]

caps based on a generation shifting approach'' \11\ that capped
``emissions at a level that will force a nationwide transition away
from the use of coal to generate electricity[.]'' \12\ The Court
explained that the EPA's BSER ``forc[es] a shift throughout the power
grid from one type of energy source to another,'' which constituted ``
`unprecedented power over American industry' '' and was different in
kind from the type of ``system'' of emissions reduction envisioned by
CAA section 111(d).\13\
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\9\ See West Virginia v. EPA, 597 U.S. at 727 (internal
quotations removed).
\10\ 80 FR 64662 (Oct. 23, 2015).
\11\ West Virginia v. EPA, 597 U.S. at 732.
\12\ Id. at 734.
\13\ Id. at 728 (citation omitted).
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To qualify for selection as the BSER, the system of emission
reduction must be ``adequately demonstrated'' as ``the Administrator
determines.'' The plain text of CAA section 111(a)(1), and in
particular the terms ``adequately'' and ``the Administrator
determines,'' confer discretion to the EPA in identifying the
appropriate system, including making scientific and technological
determinations and considering a broad range of policy
considerations.\14\ However, the terms ``adequately'' and
``demonstrated,'' as well as applicable caselaw, make clear that the
EPA may not determine that a ``purely theoretical or experimental''
system is ``adequately demonstrated.'' \15\
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\14\ Nat'l Asphalt Pavement Ass'n v. Train, 539 F.2d 775, 786
(D.C. Cir. 1976); Essex Chem. Corp. v. Ruckelshaus, 486 F.2d 427,
434 (D.C. Cir. 1973).
\15\ Essex Chem. Corp., 486 F.2d at 433-34; see Portland Cement
Assn. v. Ruckelshaus, 486 F.2d 375, 391-92 (D.C. Cir. 1973) (EPA may
not base an ``adequately demonstrated'' determination on a ``
`crystal ball' inquiry'') (citation omitted).
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In addition, CAA section 111(a)(1) requires the EPA to account for
``the cost of achieving [the emission] reduction'' in determining the
adequately demonstrated BSER. Although the CAA does not describe how
the EPA is to account for costs to affected sources, the D.C. Circuit
has formulated the cost standard in various ways, including stating
that the EPA may not adopt a standard the cost of which would be
``excessive'' or ``unreasonable.'' \16\ The EPA has considerable
discretion in considering cost under section 111(a), both in
determining the appropriate level of costs and in balancing costs with
other BSER factors.\17\ The D.C. Circuit has repeatedly upheld the
EPA's consideration of cost in reviewing standards of performance.\18\
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\16\ Sierra Club v. Costle, 657 F.2d 298, 343 (D.C. Cir. 1981).
See 79 FR 1430, 1464 (January 8, 2014); Lignite Energy Council, 198
F.3d at 933 (costs may not be ``exorbitant''); Portland Cement Ass'n
v. EPA, 513 F.2d 506, 508 (D.C. Cir. 1975) (costs may not be
``greater than the industry could bear and survive'').
\17\ Sierra Club v. Costle, 657 F.2d 298, 343 (D.C. Cir. 1981).
\18\ See Essex Chemical Corp. v. Ruckelshaus, 486 F.2d 427, 440
(D.C. Cir. 1973); Portland Cement Ass'n v. Ruckelshaus, 486 F.2d
375, 387-88 (D.C. Cir. 1973); Sierra Club v. Costle, 657 F.2d 298,
313 (D.C. Cir. 1981).
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The Agency does not apply a brightline test in determining what
level of cost is reasonable. In evaluating whether the cost
reasonableness of a particular system of emission reduction, the EPA
considers various costs associated with the particular air pollution
control measure or a level of control, including capital costs and
operating costs, and the emission reductions that the control measure
or particular level of control can achieve. The Agency considers these
costs in the context of the industry's overall capital expenditures and
revenues. The Agency also considers cost effectiveness analysis as a
useful metric, and a means of evaluating whether a given control
achieves emission reduction at a reasonable cost. A cost effectiveness
analysis allows comparisons of relative costs and outcomes (effects) of
two or more options. In general, cost effectiveness is a measure of the
outcomes produced by resources spent. In the context of air pollution
control options, cost effectiveness typically refers to the annualized
cost of implementing an air pollution control option divided by the
amount of pollutant reductions realized annually. Notably, a cost
effectiveness analysis is not intended to constitute or approximate a
benefit-cost analysis in which benefits are compared to costs but
rather is intended to provide a metric to compare the relative cost of
different air pollution control options. The EPA typically has
considered cost effectiveness along with various associated cost
metrics, such as capital costs and operating costs, total costs, costs
as a percentage of capital for a new facility, and the cost per unit of
production. In many contexts, the cost per unit of production may be
passed on to consumers, including ratepayers in the utility context and
consumers of end products in other contexts.
Under CAA section 111(a)(1), the EPA is required to take into
account ``any nonair quality health and environmental impact and energy
requirements'' in determining the BSER. Nonair quality health and
environmental impacts may include the impacts of the disposal of
byproducts of the air pollution controls, or requirements of the air
pollution control equipment for water.\19\ Energy requirements may
include the impact, if any, of the air pollution controls on the
source's own energy needs.\20\ In addition, based on the D.C. Circuit's
interpretations of CAA section 111, energy requirements may also
include the impact, if any, of the air pollution controls on the energy
supply for a particular area or nationwide.\21\ In addition, the EPA
has considered under this statutory factor whether possible controls
would create risks to the reliability of the electricity system.
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\19\ Portland Cement Ass'n v. Ruckelshaus, 465 F.2d 375, 387-88
(D.C. Cir. 1973), cert. denied, 417 U.S. 921 (1974).
\20\ For details on the modeled energy requirements associated
with CCS, please see section 6.4 of the RIA for this rule.
\21\ See Sierra Club v. Costle, 657 F.2d at 327-28 (quoting 44
FR 33583-84; June 11, 1979); 79 FR 1430, 1465 (January 8, 2014)
(citing Sierra Club v. Costle, 657 F.2d at 351).
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After the EPA evaluates the statutory factors with respect to
adequately demonstrated control technologies, the EPA compares the
various systems of emission reductions and determines which system is
``best,'' and therefore represents the BSER. The D.C. Circuit has also
held that the term ``best'' authorizes the EPA to consider factors in
addition to the ones enumerated in CAA section 111(a)(1) that further
the purpose of the statute. In particular, consistent with the plain
language and the purpose of CAA section 111(a)(1), which requires the
EPA to determine the ``best system of emission reduction'' (emphasis
added), the EPA must consider the quantity of emissions at issue.\22\
In determining which adequately demonstrated system of emission
reduction is the ``best,'' the EPA has broad discretion. In Sierra Club
v. Costle, 657 F.2d 298 (D.C. Cir. 1981), the court explained that
``section 111(a) explicitly instructs the EPA to balance multiple
concerns when promulgating a NSPS'' \23\ and emphasized that ``[t]he
text gives the EPA broad discretion to weigh different factors in
setting the standard,'' including the amount of emission reductions,
the cost of the controls, and the non-air quality environmental impacts
and energy requirements.\24\
---------------------------------------------------------------------------

\22\ Sierra Club v. Costle, 657 F.2d 298, 326 (D.C. Cir. 1981).
The D.C. Circuit has also held that Congress intended for CAA
section 111 to create incentives for new technology and therefore
that the EPA is required to consider technological innovation as one
of the factors in determining the ``best system of emission
reduction.'' See id. at 346-47.
\23\ Sierra Club v. Costle, 657 F.2d at 319; see also AEP v.
Connecticut, 564 U.S. 410, 427 (2011).
\24\ Sierra Club v. Costle, 657 F.2d at 321; see also New York
v. Reilly, 969 F.2d at 1150.
---------------------------------------------------------------------------

The EPA then establishes a standard of performance that reflects
the degree of emission limitation achievable through the implementation
of the BSER. A standard of performance is

[[Page 1914]]

``achievable'' if a technology can reasonably be projected to be
available to an individual source at the time it is constructed so as
to allow it to meet the standard.\25\ For purposes of evaluating the
source category and determining BSER, the EPA can determine whether
subcategorization is appropriate based on classes, types, and sizes of
sources, and may identify a different BSER and establish different
performance standards for each subcategory. The result of the analysis
and BSER determination leads to standards of performance that apply to
facilities that begin construction, reconstruction, or modification
after the date of publication of the proposed standards in the Federal
Register. Because the NSPS reflect the BSER under conditions of proper
operation and maintenance, in doing its review, the EPA also evaluates
and determines the proper testing, monitoring, recordkeeping and
reporting requirements needed to ensure compliance with the emission
standards.
---------------------------------------------------------------------------

\25\ Sierra Club v. Costle, 657 F.2d at 364, n.276.
---------------------------------------------------------------------------

B. How does the EPA perform the NSPS review?

CAA section 111(b)(1)(B) requires the EPA to, ``at least every 8
years, review and, if appropriate, revise'' the standards of
performance applicable to new, modified, or reconstructed sources.
However, the Administrator need not review any such standard if the
``Administrator determines that such review is not appropriate in light
of readily available information on the efficacy'' of the standard. If
the EPA revises the standards of performance, they must reflect the
degree of emission limitation achievable through the application of the
BSER, which is selected from among adequately demonstrated technologies
after consideration of the cost of achieving such reduction and any
nonair quality health and environmental impact and energy
requirements.\26\ When conducting a review of an existing performance
standard, the EPA may, as appropriate and consistent with the statutory
requirements, add emission limits for pollutants or emission sources
not currently regulated for that source category.
---------------------------------------------------------------------------

\26\ See 42 U.S.C. 7411(a)(1).
---------------------------------------------------------------------------

In reviewing an NSPS for a source category to determine whether it
is ``appropriate'' to revise the standards of performance, the EPA
evaluates the statutory factors, which may include consideration of the
following information: \27\
---------------------------------------------------------------------------

\27\ See generally 42 U.S.C. 7411; 76 FR 65653, 65658 (Oct. 24,
2011).
---------------------------------------------------------------------------

<bullet> Expected growth for the source category, including how
many new facilities, modifications, or reconstructions may trigger NSPS
in the future.
<bullet> Pollution control measures, including advances in control
technologies, process operations, design or efficiency improvements, or
other systems of emission reduction, that the Administrator determines
have been ``adequately demonstrated'' in the regulated industry.
<bullet> Available information from the implementation and
enforcement of current requirements indicating that emission
limitations and percent reductions beyond those required by the current
standards are achieved in practice.
<bullet> Costs (including capital and annual costs) associated with
implementation of the available pollution control measures.
<bullet> The amount of emission reductions achievable through
application of such pollution control measures.
<bullet> Any non-air quality health and environmental impact and
energy requirements associated with those control measures.

C. What is the source category regulated in this final action?

The EPA first promulgated NSPS for stationary gas turbines on
September 10, 1979.\28\ These standards of performance are codified in
40 CFR part 60, subpart GG, and are applicable to sources that
commenced construction, modification, or reconstruction after October
3, 1977. The standards of performance in subpart GG regulate emissions
of NO<INF>X</INF> and SO<INF>2</INF> from all new, modified, or
reconstructed simple and regenerative cycle gas turbines and the gas
turbine portion of a combined cycle steam/electric generating system.
The EPA last reviewed and revised the NO<INF>X</INF> and SO<INF>2</INF>
standards of performance on July 6, 2006, and promulgated 40 CFR part
60, subpart KKKK, which is applicable to stationary combustion turbines
that commenced construction, modification, or reconstruction after
February 18, 2005.\29\ In subpart KKKK, the definition of the source
was expanded to include all equipment, including but not limited to the
combustion turbine; the fuel, air, lubrication, and exhaust gas
systems; the control systems (except emission control equipment); the
heat recovery system (including heat recovery steam generators (HRSG)
and duct burners); and any ancillary components and sub-components
comprising any simple cycle, regenerative/recuperative cycle, and
combined cycle stationary combustion turbine, and any combined heat and
power (CHP) stationary combustion turbine-based system.
---------------------------------------------------------------------------

\28\ See 44 FR 52792 (Sept. 10, 1979).
\29\ See 71 FR 38482 (July 6, 2006).
---------------------------------------------------------------------------

The stationary combustion turbine source category consists of
combustion turbines with design base load ratings (i.e., maximum heat
input at ISO conditions) equal to or greater than 10.7 gigajoules per
hour (GJ/h) (10 million British thermal units per hour (MMBtu/h)) \30\
based on the higher heating value (HHV) of the fuel and applies to
combustion turbines and their associated HRSG and duct burners, as
described above. The source is ``stationary'' because the combustion
turbine is not self-propelled or intended to be propelled while
performing its function. Combustion turbines may, however, be mounted
on a vehicle (or trailer) for portability and still be considered
stationary. As discussed in section IV.B.2.e of this preamble, the EPA
is amending the applicability of subparts KKKK and KKKKa to provide
that combustion turbines that are subject to applicable CAA title II
standards are not subject to the NSPS. To the EPA's knowledge, no such
stationary combustion turbines are currently being used in temporary
applications.
---------------------------------------------------------------------------

\30\ The base load rating is based on the heat input to the
combustion turbine engine. Any additional heat input from duct
burners used with HRSG units or fuel preheaters is not included in
the heat input value used to determine the applicability of this
subpart to a given stationary combustion turbine. However, this
subpart does apply to emissions from any HRSG and duct burners that
are associated with a combustion turbine subject to this subpart.
---------------------------------------------------------------------------

The NO<INF>X</INF> standards in subparts GG and KKKK are generally
based on the application of combustion controls (as the BSER) and allow
the turbine owner or operator the choice of meeting a concentration-
based emission standard or an output-based emission standard. The
concentration-based emission limits are in units of parts per million
by volume dry (ppmvd) at 15 percent oxygen gas (O<INF>2</INF>).\31\ The
output-based emission limits are in units of mass per unit of useful
recovered energy, nanograms per joule (ng/J) or pounds per megawatt-
hour (lb/MWh). Each NO<INF>X</INF> limit in subparts GG and KKKK is
based on the application of combustion controls as the BSER, but
individual standards may differ for individual

[[Page 1915]]

subcategories of combustion turbines based on the following factors:
the fuel input rating at base load, the fuel used, the application, the
load, and the location of the turbine.\32\ The fuel input rating of the
turbine does not include any supplemental fuel input to the heat
recovery system and refers to the rating of the combustion turbine
itself.
---------------------------------------------------------------------------

\31\ Throughout this document, all references to parts per
million (ppm) NO<INF>X</INF> are intended to be interpreted as ppmvd
at 15 percent O<INF>2</INF>, unless otherwise noted.
\32\ Throughout this document, all uses of the term ``turbine''
refer to a ``combustion turbine'' as defined in subparts KKKK and
KKKKa.
---------------------------------------------------------------------------

The standards of performance for SO<INF>2</INF> emissions in
subparts GG and KKKK reflect the BSER of using low-sulfur fuels for all
new, modified, or reconstructed combustion turbines, regardless of
class, size, or type. The input-based SO<INF>2</INF> standard applies
to the sulfur content of the fuel combusted in the turbine. The NSPS
also includes an optional output-based standard that limits the
discharge of excess SO<INF>2</INF> into the atmosphere as a fraction of
the gross energy output of the combustion turbine.\33\
---------------------------------------------------------------------------

\33\ See the 2024 Proposed Rule (89 FR 101310; Dec. 13, 2024)
for further discussion of the specific subcategories in previous
NSPS and the applicable limits for NO<INF>X</INF> and SO<INF>2</INF>
emissions in those rules.
---------------------------------------------------------------------------

Combustion turbines are a large and diverse source category.
Thousands of stationary combustion turbines are operating across
numerous industrial sectors. For instance, in the utility sector alone,
there are approximately 3,400 existing stationary combustion
turbines.\34\ Generally, existing combustion turbine sources are
subject to either subpart KKKK or subpart GG.
---------------------------------------------------------------------------

\34\ See the U.S. Environmental Protection Agency's (EPA)
National Electric Energy Data System database. NEEDS rev 06-06-2024.
Accessed at <a href="https://www.epa.gov/power-sector-modeling/national-electric-energy-data-system-needs">https://www.epa.gov/power-sector-modeling/national-electric-energy-data-system-needs</a>.
---------------------------------------------------------------------------

The EPA last revised the NSPS for stationary combustion turbines in
2006, when it promulgated subpart KKKK. In 2022, certain parties filed
a complaint in Federal district court pursuant to CAA section 304
alleging that the EPA had failed to fulfill a nondiscretionary duty
under CAA section 111(b)(1)(B) to review and, if appropriate, revise
this NSPS within 8 years of the 2006 revision. The EPA resolved this
litigation through entering a consent decree establishing judicially
enforceable deadlines for the EPA to propose and finalize this NSPS
review.\35\ The EPA is discharging its obligations under the consent
decree in this final rule.
---------------------------------------------------------------------------

\35\ See Consent Decree, Environmental Defense Fund et al. v.
EPA, No. 3:22-cv-07731-WHO (N.D. Cal. July 27, 2023).
---------------------------------------------------------------------------

The EPA proposed the current review of the stationary combustion
turbines NSPS on December 13, 2024. We received 167 unique comments
from private citizens, environmental and public health advocacy groups,
community organizations, Tribes, and States. The EPA also received
unique comments from numerous industrial sectors, including electric
utilities, public power cooperatives, original equipment manufacturers
(OEMs), trade groups and associations, and certain sectors of the oil
and gas industry. In addition, thousands of similar comments were
submitted by individuals as part of mass mailer campaigns. A summary of
significant comments we timely received regarding the 2024 Proposed
Rule and our responses are provided in this preamble. A summary of all
other public comments on the proposal and the EPA's responses to those
comments is available in the Summary of Public Comments and Responses:
Review of New Source Performance Standards for Stationary Combustion
Turbines and Stationary Gas Turbines, Docket ID No. EPA-HQ-OAR-2024-
0419. In this action, the EPA is finalizing decisions and revisions
pursuant to its CAA section 111(b)(1)(B) review of the NSPS for
stationary combustion turbines and stationary gas turbines that reflect
our consideration of all the comments received.

D. The Role of the NSPS

The role of NSPS in relation to other requirements of the Act is to
establish a minimum Federal baseline for pollution control performance
that all new, modified, or reconstructed facilities within a specific
source category must meet. While independently established by the EPA
and based strictly on the statutory criteria, in practice, NSPS often
act as a starting point for permitting requirements, such as emission
limits and standards that may be established through other programs
(e.g., the New Source Review (NSR) permitting program or State and
local requirements). NSPS are directly enforceable against sources.\36\
However, effective implementation is often achieved through
collaboration with State and local authorities, who may have delegated
authority to implement NSPS and who are typically responsible for
incorporating NSPS requirements into operating permits.
---------------------------------------------------------------------------

\36\ See 42 U.S.C. 7411(e).
---------------------------------------------------------------------------

Permitting decisions may result in more stringent emissions
standards for individual sources than the NSPS based on different legal
requirements and case-by-case assessments of the appropriate
requirements for individual facilities considering source-specific
information, such as the local air quality conditions.\37\ For example,
a permitting authority evaluating permit requirements for a new
combustion turbine in an area that has been designated as non-
attainment for ozone under the National Ambient Air Quality Standards
(NAAQS) program must set a standard based on the ``lowest achievable
emissions rate'' (LAER) (and also must offset new emissions with
reductions from other sources).\38\ Under a LAER analysis, a
NO<INF>X</INF> emissions standard lower than what is required in this
final rule may be appropriate (e.g., an emissions standard of less than
5 ppm NO<INF>X</INF> based on the application of SCR). That decision
does not necessarily mean the same level of emissions performance must
be required for all combustion turbines in the country through the
NSPS. The reverse is also true--it is not necessarily appropriate to
use the emission standards in an NSPS as the sole justification for not
requiring additional emissions reduction measures under facility-
specific permitting authorities.
---------------------------------------------------------------------------

\37\ Experience with emissions control technologies gained
through permitting for specific projects can often help inform the
EPA when conducting its periodic reviews of the NSPS.
\38\ 42 U.S.C. 7503.
---------------------------------------------------------------------------

III. What changes did we propose for the stationary combustion turbines
and stationary gas turbines NSPS?

On December 13, 2024, the EPA proposed the current review of, and
several revisions to, the stationary combustion turbines and stationary
gas turbines NSPS. In that action, we proposed to establish size-based
subcategories for new, modified, or reconstructed stationary combustion
turbines in 40 CFR part 60, subpart KKKKa that also recognized
distinctions between those sources that operate at varying loads or
capacity factors, those firing natural gas or non-natural gas fuels,
and those that operate in unique locations. Capacity factor or
``utilization'' level or rate is a ratio that measures how often a
stationary combustion turbine is operating at its maximum rated heat
input. The ratio is based on heat input, or actual heat input, compared
to the base load rating, or potential maximum heat input, under
specified conditions.
The EPA proposed post-combustion SCR in addition to combustion
controls to be the BSER for limiting NO<INF>X</INF> emissions from
certain combustion turbines in the small, medium, and large size-based
subcategories. The EPA proposed SCR to be adequately demonstrated and
generally cost-effective for combustion turbines in this

[[Page 1916]]

source category when those turbines are operated at higher utilization
rates. The EPA also proposed that a BSER that includes SCR satisfies
the other statutory criteria under CAA section 111(a)(1). We sought
comment on these proposed determinations, including on the issues set
forth below.
However, the EPA recognized that as the size of a combustion
turbine diminishes and/or as the level of operation (i.e., utilization
on an annual basis) of a combustion turbine diminishes or becomes more
variable, the incremental cost-effectiveness on a per-ton basis and
efficacy of SCR technology also diminishes. Thus, at smaller sizes and
at lower rates of utilization, we proposed to establish standards of
performance based on a BSER of combustion controls without SCR.
Specifically, for small combustion turbines (i.e., at proposal, those
that have a base load heat input rating less than or equal to 250
MMBtu/h) that operate at an annual capacity factor less than or equal
to 40 percent (i.e., at proposal, ``low'' and ``intermediate''
utilization combustion turbines), we proposed that the use of
combustion controls alone remains the BSER. For medium combustion
turbines (i.e., at proposal, those that have a base load heat input
rating greater than 250 MMBtu/h and less than or equal to 850 MMBtu/h)
that operate at capacity factors less than or equal to 20 percent
(i.e., low-utilization combustion turbines), we proposed that
combustion controls alone remain the BSER. Likewise, for large
combustion turbines (i.e., those that have a base load heat input
rating greater than 850 MMBtu/h) that operate at capacity factors less
than or equal to 20 percent (i.e., low-utilization combustion
turbines), we proposed that combustion controls alone remain the BSER.
Based on the application of these NO<INF>X</INF> control
technologies, the EPA proposed to lower the NO<INF>X</INF> standards of
performance for most of the stationary combustion turbines in this
source category relative to subpart KKKK. In addition, the EPA proposed
to maintain the current standards for SO<INF>2</INF> emissions after
finding that the use of low-sulfur fuels remains the BSER.
The Agency also proposed amendments or requested comment to address
several technical and editorial issues that had arisen under the
existing regulations in subparts GG and KKKK, which also could be
relevant to the new subpart KKKKa. These included, among other things,
whether to revise the definition of ``reconstruction'' for this source
category; how to address unique challenges faced by newer higher
efficiency combustion turbines in meeting the current subpart KKKK
standard of performance of 15 ppm NO<INF>X</INF> for large turbines;
whether to include alternative, optional mass-based NO<INF>X</INF>
standards of performance; whether to adjust the current approach to the
part-load NO<INF>X</INF> standards; whether to provide a process for
site-specific NO<INF>X</INF> standards of performance when burning
byproduct fuels; how to address co-firing of non-natural gas fuels,
including hydrogen; whether and how to handle certain kinds of
emergency operations; whether to include an exemption from title V
permitting for non-major sources under CAA section 502(a); whether to
address other criteria air pollutants; and whether to create a
subcategory or exemption for combustion turbines used in temporary
applications, such as for less than 1 year, similar to current NSPS and
national emission standards for hazardous air pollutants (NESHAP)
provisions for internal combustion engines and industrial boilers.\39\
---------------------------------------------------------------------------

\39\ See the proposed rule preamble for additional discussion
about these and other proposals and requests for comment (89 FR
101306; Dec. 13, 2024). See section IV of this preamble for
discussion of the proposals being finalized in subpart KKKKa and
section VI of this preamble for discussion of the proposals not
being finalized in this action.
---------------------------------------------------------------------------

IV. What actions are we finalizing and what is our rationale for such
decisions?

The EPA is finalizing revisions to the NSPS for stationary
combustion turbines and stationary gas turbines pursuant to its CAA
section 111(b)(1)(B) review. The EPA is promulgating the NSPS revisions
in a new subpart, 40 CFR part 60, subpart KKKKa. The revised NSPS
subpart is applicable to affected sources constructed, modified, or
reconstructed after December 13, 2024. A complete list of the final
subcategories and associated emissions standards being finalized in
this action is provided in Table 1 in section IV.B.5 of this preamble.
After considering comments critical of the proposed size-based
subcategory threshold between small and medium combustion turbines, the
EPA has decided to retain in subpart KKKKa the general size-based
subcategories from subpart KKKK. This includes subcategories for new,
modified, or reconstructed stationary combustion turbines with base
load ratings greater than 850 MMBtu/h of heat input (i.e., large), base
load ratings greater than 50 MMBtu/h and less than or equal to 850
MMBtu/h of heat input (i.e., medium), and base load ratings less than
or equal to 50 MMBtu/h of heat input (i.e., small). In addition,
certain subcategories of new stationary combustion turbines in subpart
KKKKa reflect the correlation between the level of utilization of a
combustion turbine and the cost effectiveness of available control
technologies in limiting NO<INF>X</INF> emissions. This correlation was
discussed in the proposed rule and generated significant input in
public comments.\40\ The final rule therefore subcategorizes large and
medium combustion turbines according to how they are operated--either
at high rates of utilization or low rates of utilization. A new large
or medium combustion turbine with a 12-calendar-month capacity factor
greater than 45 percent is subcategorized as a high-utilization source.
A new large or medium combustion turbine with a 12-calendar-month
capacity factor less than or equal to 45 percent is subcategorized as a
low-utilization source. Small combustion turbines are not being further
subcategorized based on utilization.
---------------------------------------------------------------------------

\40\ The proposal differentiated the cost effectiveness of
combustion controls and SCR for combustion turbines operating at
low, intermediate, and base load levels. See 89 FR 101315.
---------------------------------------------------------------------------

In addition, taking into consideration public comments in response
to the EPA's discussion in the proposal of the unique challenges faced
by new large higher efficiency combustion turbines, we are finalizing
two subcategories for large low-utilization turbines based on the
design efficiency of the turbine, which accounts for different levels
of emissions performance that can be achieved by combustion controls
alone (i.e., without SCR).\41\ Specifically, for new large turbines
with low rates of utilization (i.e., a 12-calendar-month capacity
factor less than or equal to 45 percent) and design efficiencies
greater than or equal to 38 percent on a HHV basis, the EPA is
finalizing a determination that the BSER is the use of combustion
controls alone.\42\ For new large turbines with low rates of
utilization (i.e., a 12-calendar-month capacity factor less than or
equal to 45 percent) and design efficiencies less than 38 percent, the
EPA is finalizing a

[[Page 1917]]

determination that the BSER is the use of combustion controls with
NO<INF>X</INF> emissions rate guarantees based on the use of
technologies such as lean premix combustion and dry low-NO<INF>X</INF>
(DLN) or ultra DLN burners.\43\
---------------------------------------------------------------------------

\41\ Efficiency for purposes of subcategorization in 40 CFR part
60, subpart KKKKa refers to the design efficiency of a specific
class or type of stationary combustion turbine according to
manufacturer specifications. Turbine manufacturers list this value
as a percentage based on the HHV of the individual turbine design.
\42\ The 38 percent HHV design efficiency is equal to 42 percent
on a lower heating value (LHV) basis. In relation to the design
efficiency rating of a combustion turbine, ratings based on the HHV
will appear lower, as the calculation includes a portion of heat
that may not be recoverable in many applications. Efficiency ratings
based on the LHV will appear higher because they exclude the energy
lost with the water vapor in the exhaust.
\43\ Dry combustion controls that include the use of lean
premix, DLN, ultra DLN, and other technologies are often referred to
as ``advanced'' combustion controls by turbine manufacturers and the
regulated community. These technologies are generally more effective
at NO<INF>X</INF> control than other dry combustion controls but are
not available for all types, sizes, and applications of new,
modified, or reconstructed stationary combustion turbines. The EPA
uses the same terminology in this preamble to make the same
distinction.
---------------------------------------------------------------------------

The EPA is finalizing a determination that the BSER is the use of
various types of combustion controls (i.e., without SCR) for all but
one subcategory of new, modified, or reconstructed stationary
combustion turbines. For that one subcategory--new large turbines with
high rates of utilization (i.e., 12-calendar-month capacity factors
greater than 45 percent)--the BSER is combustion controls with SCR.
The standards of performance for each subcategory of stationary
combustion turbine in subpart KKKKa reflect the degree of emission
limitation achievable based upon application of the BSER. For new large
high-utilization turbines firing natural gas with a BSER of combustion
controls with SCR, the NO<INF>X</INF> standard is 5 ppm. For new large
natural gas-fired turbines with low rates of utilization, the
NO<INF>X</INF> standard is 25 ppm for higher efficiency classes of
turbines and 9 ppm for lower efficiency classes.
For new medium high-utilization combustion turbines firing natural
gas, the NO<INF>X</INF> standard is 15 ppm based on the performance of
dry combustion controls. For new medium low-utilization turbines firing
natural gas, the NO<INF>X</INF> standard is 25 ppm based on the
performance of water- or steam-injection combustion controls. The high/
low utilization threshold--greater than or less than or equal to a 45
percent capacity factor--is the same for new medium combustion turbines
as for new large combustion turbines. And for all new small combustion
turbines firing natural gas, the NO<INF>X</INF> standard is 25 ppm
based on combustion controls regardless of the level of
utilization.\44\
---------------------------------------------------------------------------

\44\ See Table 1 of this preamble for a complete listing of
subcategories and associated NO<INF>X</INF> emissions standards.
---------------------------------------------------------------------------

This action maintains subcategories for modified and reconstructed
stationary combustion turbines that are generally consistent with the
subcategories in subpart KKKK. As discussed in section IV.B.6, these
subcategories are based on a BSER of combustion controls with
associated NO<INF>X</INF> standards of performance. As discussed in
section VI.A of this preamble, the EPA is not finalizing the proposed,
category-specific definition of ``reconstruction'' for combustion
turbines.
Some of the other final determinations reflected in subpart KKKKa
include: the creation of a new subcategory for stationary temporary
combustion turbines; lowering the threshold that defines part-load
operations to any hour when the heat input of the combustion turbine is
less than or equal to 70 percent of the base load rating; allowing
owners or operators to petition the Administrator for a site-specific
NO<INF>X</INF> standard when burning byproduct fuels; a provision that
operation during a ``system emergency'' (Energy Emergency Alert levels
1, 2, or 3) is not included in calculating a turbine's 12-calendar-
month utilization; an exemption from title V permitting for combustion
turbines that are not major sources or located at major sources under
CAA section 502(a); and retention of the SO<INF>2</INF> standards from
subpart KKKK for all new, modified, or reconstructed stationary
combustion turbines.45 46
---------------------------------------------------------------------------

\45\ Energy Emergency Alert levels 1, 2, and 3 are defined by
the North American Electric Reliability Corporation (NERC)
Reliability Standard EOP-011-2, or its successor, or equivalent.
\46\ See section IV.B.7.d of this preamble for discussion of
site-specific NO<INF>X</INF> standards for stationary combustion
turbines in subpart KKKKa. See sections IV.B.3-4 for discussion of
the BSER for the different subcategories of stationary combustion
turbines. See section IV.B.5 for discussion of the associated
NO<INF>X</INF> standards based on the application of the BSER.
---------------------------------------------------------------------------

The EPA is finalizing corresponding amendments in subparts GG and
KKKK with respect to several of these ancillary issues, which will be
applicable to combustion turbines subject to those subparts as of the
effective date of this final rule. Specifically:
<bullet> In subpart GG, the EPA is finalizing that turbines subject
to subparts Da, KKKK, or KKKKa are not subject to subpart GG.
<bullet> In subpart KKKK, the EPA is finalizing a clarification
that only the heat input to the combustion turbine engine is used for
applicability purposes and that combustion turbines regulated under
subpart KKKK are exempt from subparts KKKKa and GG. The EPA is also
finalizing that emergency, military, and firefighting combustion
turbines are exempt from the NO<INF>X</INF> emission standards in
subpart KKKK. Additionally, the EPA is finalizing flexibilities
regarding when performance tests must be conducted after long periods
of non-operation and that owners or operators can use fuel records to
comply with their SO<INF>2</INF> standard. The EPA is finalizing a low-
Btu alternative to the SO<INF>2</INF> standard in subpart KKKK, as well
as a concentration-based alternate SO<INF>2</INF> standard. Finally,
the EPA is finalizing the requirement for approval from the delegated
authority for certain monitoring and compliance tasks that are already
covered under 40 CFR part 75 and specifications about including duct
burners in performance tests.
<bullet> In both subparts GG and KKKK, the EPA is finalizing that
as an alternative to being subject to either of those subparts, owners
or operators of combustion turbines that otherwise meet those subparts'
applicability criteria may petition the Administrator to become subject
to subpart KKKKa instead. The EPA is also finalizing in both subparts
GG and KKKK that turbines subject to subparts J or Ja are not subject
to the respective SO<INF>2</INF> standard in subparts GG or KKKK and
that NO<INF>X</INF> continuous emissions monitoring systems (CEMS)
installed and certified according to 40 CFR part 75 can be used to
monitor NO<INF>X</INF> emissions, where approved. The EPA is finalizing
standard electronic reporting requirements for turbines subject to
subparts GG or KKKK and that an additional test method (EPA Method 320)
can be used to determine NO<INF>X</INF> and diluent concentration in
subparts GG and KKKK.
It is the EPA's understanding and intention that none of these
changes alter the stringency or increase any regulatory burdens with
respect to the existing combustion turbines subject to subparts GG and
KKKK, and nothing in this final rule is intended to have retroactive
effect (that is, to govern any conduct or activities occurring prior to
the effective date of this final rule).
This action finalizes standards of performance in subpart KKKKa
that apply at all times, including during periods of startup, shutdown,
and malfunction (SSM), and other changes such as electronic reporting
that also apply to previous NSPS subparts GG and KKKK. These topics are
discussed below in sections IV.F-H.

A. Applicability

The source category that is the subject of this final action is
composed of new, modified, or reconstructed stationary combustion
turbines with a base load rating of greater than 10 MMBtu/h of heat
input.\47\ The standards of

[[Page 1918]]

performance, codified in 40 CFR part 60, subpart KKKKa, are directly
applicable to affected sources that began construction, modification,
or reconstruction after December 13, 2024--the date of publication of
the proposed standards in the Federal Register. The final amendments to
subparts GG and KKKK are directly applicable to the affected facilities
already subject to those subparts. Stationary combustion turbines
subject to the standards in subpart KKKKa are not subject to the
requirements of subparts GG or KKKK. The HRSG and duct burners subject
to the standards in subpart KKKKa are exempt from the requirements of
40 CFR part 60, subpart Da (the Utility Boiler NSPS) as well as
subparts Db and Dc (the Industrial/Commercial/Institutional Boiler
NSPS), continuing the approach previously established in subpart KKKK.
---------------------------------------------------------------------------

\47\ The base load rating is the maximum heat input of the
combustion turbine engine at ISO conditions. The EPA uses the HHV
when specifying heat input ratings.
---------------------------------------------------------------------------

Subpart KKKKa maintains certain exemptions from NO<INF>X</INF>
emissions standards promulgated previously in subparts GG and KKKK. In
1977, in subpart GG, the EPA determined that it was appropriate to
exempt emergency combustion turbines from the NO<INF>X</INF> limits.
These included emergency-standby combustion turbines, military
combustion turbines, and firefighting combustion turbines. Subpart KKKK
further defined emergency combustion turbines as units that operate in
emergency situations, such as turbines that supply electric power when
the local utility service is interrupted. Additional exemptions being
maintained from subpart KKKK include: (1) stationary combustion turbine
test cells/stands, (2) integrated gasification combined cycle (IGCC)
combustion turbine facilities covered by subpart Da of 40 CFR part 60
(the Utility Boiler NSPS), and (3) stationary combustion turbines that,
as determined by the Administrator or delegated authority, are used
exclusively for the research and development of control techniques and/
or efficiency improvements relevant to stationary combustion turbine
emissions.
In general, and as discussed in the following sections, the EPA is
finalizing minor changes in wording and writing style to add clarity to
the applicability language in subparts GG and KKKK and to track with
language being finalized in subpart KKKKa. The Agency does not intend
for these editorial revisions to applicability and/or updates to the
test methods to substantively change any of the technical requirements
of existing subparts GG and KKKK.
1. Exemptions for Combustion Turbines Subject to More Stringent
Standards
The EPA is finalizing as proposed provisions to make clear that
stationary combustion turbines at petroleum refineries subject to 40
CFR part 60, subparts J or Ja are not subject to the SO<INF>2</INF>
performance standards in subparts GG, KKKK, or KKKKa. The
SO<INF>2</INF> standards in subparts J and Ja are more stringent than
the SO<INF>2</INF> limits in subparts GG, KKKK, or KKKKa. This
clarification simplifies compliance for owners or operators of
petroleum refineries without an increase in pollutant emissions by
minimizing overlap of competing NSPS for different source categories.
The EPA received supportive and no adverse comments on the subpart J
and Ja related amendments. The EPA is unaware of additional source
categories or facilities with stationary combustion turbines that are
subject to more stringent NSPS that should not be subject to the
SO<INF>2</INF> and/or NO<INF>X</INF> performance standards in subparts
GG, KKKK, or KKKKa. Further, no commenters identified any such
categories or facilities.
2. Petition To Comply With 40 CFR Part 60, Subpart KKKKa
The EPA is finalizing as proposed a provision that will allow
owners or operators of stationary combustion turbines currently covered
by subparts GG or KKKK, and any associated steam generating unit
subject to an NSPS, to petition the Administrator to comply with
subpart KKKKa in lieu of complying with subparts GG, KKKK, and any
associated steam generating unit NSPS. Since the applicability of
subpart KKKKa encompasses any associated heat recovery equipment,
owners or operators can have the flexibility to comply with one NSPS
instead of multiple NSPS. The Administrator will only grant the
petition if it is determined that compliance with subpart KKKKa would
be equivalent to, or more stringent than, compliance with subparts GG,
KKKK, or any associated steam generating unit NSPS.
Also, if any solid fuel as defined in subpart KKKKa is burned in
the HRSG, the HRSG is covered by the applicable steam generating unit
NSPS and not subpart KKKKa. The intent of the solid fuel exclusion in
subpart KKKKa is that it is only applicable to new turbines burning
liquid and gaseous fuels. The exclusion prevents a large solid fuel-
fired boiler from using the exhaust from a combustion turbine engine to
avoid the requirements of the applicable steam generating unit NSPS.

B. NOX Emissions Standards

1. Overview
This section discusses the EPA's final BSER determinations for
NO<INF>X</INF> emissions for each of the subcategories of new,
modified, or reconstructed stationary combustion turbines and the
associated standards of performance. The EPA explains in section IV.B.2
of this preamble the subcategory approach it is adopting in subpart
KKKKa. Sections IV.B.3 and IV.B.4 of this preamble present the EPA's
BSER analysis of the NO<INF>X</INF> control technologies the EPA
evaluated as part of this review of the NSPS, which include dry
combustion controls, wet combustion controls (e.g., water or steam
injection), and post-combustion SCR. Dry combustion controls include
``advanced'' systems that incorporate lean premix with dry low-
NO<INF>X</INF> (DLN) or ultra DLN burners to reduce the flame
temperature and further limit NO<INF>X</INF> formation. In section
IV.B.5 of this preamble, the EPA sets out the final NO<INF>X</INF>
performance standards, based on the application of a particular BSER
for each subcategory of stationary combustion turbine.
In determining the subcategories, BSER, and NO<INF>X</INF>
standards in this action, the EPA considered multiple characteristics
of combustion turbines within the source category. These included
whether the size of a new, modified, or reconstructed stationary
combustion turbine is small, medium, or large; whether the affected
source is of a type that typically operates at high or low annual
capacity factors (i.e., utilization); whether certain affected sources
are higher or lower efficiency designs; whether the affected source
operates at full load or part load; and whether the affected source
burns natural gas, non-natural gas (such as gaseous hydrogen or liquid
distillate), or a combination of fuels.
In section IV.B.6 of this preamble, the EPA explains the final BSER
determinations and NO<INF>X</INF> emission standards for modified and
reconstructed sources. The EPA is finalizing NO<INF>X</INF> emission
standards for modified and reconstructed stationary combustion turbines
that are different than those for new sources and reflect the EPA's
determination that combustion controls without SCR are the BSER for
these sources. This approach reflects comments that explained that many
existing turbines undergoing modification or reconstruction face
unique, site-specific challenges to retrofitting SCR, which can
dramatically increase costs.
Furthermore, in sections IV.B.2.d and IV.B.7.b of this preamble,
the EPA

[[Page 1919]]

discusses the NO<INF>X</INF> control technologies that the EPA has
determined to be the BSER for each of the non-natural gas subcategories
and also explains its approach to characterizing new, modified, or
reconstructed stationary combustion turbines that elect to co-fire with
hydrogen as either natural gas-fired or non-natural gas-fired.
Specifically, combustion turbines that elect to co-fire with natural
gas blended with hydrogen are subject to the same BSER and
NO<INF>X</INF> performance standards as those applicable to either
natural gas-fired or non-natural gas-fired combustion turbines,
depending on the size- and utilization-based subcategory. Section
IV.B.2.e of this preamble includes discussion of the new subcategory
for stationary combustion turbines used in temporary applications.
2. Subcategorization
This section describes the subcategorization approach being
finalized in subpart KKKKa. The discussion that follows begins with a
summary of the subcategories in the proposed rule and concludes with a
discussion of the final subcategory determinations and the Agency's
rationale in support of those decisions. As noted in the proposal, the
EPA bases subcategories on the characteristics of combustion turbines
that are relevant to the reasonableness of potential BSER controls
(i.e., characteristics that make potential controls reasonable or
unreasonable in accordance with one or more of the BSER factors in CAA
section 111(a)(1)). Therefore, the availability and performance of
NO<INF>X</INF> controls for different designs, sizes, etc., of
stationary combustion turbines have informed the Agency's
subcategorization decisions.
To this end, this section discusses the characteristics of various
combustion turbines--such as their size, utilization level, and
efficiency--and why these characteristics are appropriate bases for
subcategorization of sources, as well as how they impact the
determinations of the BSER and associated NO<INF>X</INF> standards of
performance.\48\ Summaries of significant comments received during the
public comment period and the EPA's responses to those comments are
included in the appropriate sections below. The EPA's further response
to comments on the proposal, including any comments not discussed in
this preamble, can be found in the EPA's response to comments document
in the docket for this rule.49 50
---------------------------------------------------------------------------

\48\ See Table 1 in section IV.B.5 of this preamble.
\49\ EPA-HQ-OAR-2024-0419. Summary of Public Comments and
Responses: Review of New Source Performance Standards for Stationary
Combustion Turbines and Stationary Gas Turbines.
\50\ See sections IV.B.3-7 of this preamble and Table 1 in
section IV.B.5 of this preamble for information about the final BSER
determinations and NO<INF>X</INF> standards of performance for all
new, modified, or reconstructed stationary combustion turbines in
subpart KKKKa.
---------------------------------------------------------------------------

a. Subcategorization Based on Size
At proposal, the EPA continued the approach from subpart KKKK of
determining subcategories based on combustion turbine size, as
reflected by the base load rated heat input of an individual combustion
turbine.\51\ As discussed in the proposal, the size of a combustion
turbine is related to its intended application, whether industrial or
utility, and the combination of those factors influences the
availability and performance of NO<INF>X</INF> combustion controls,
making it a relevant consideration for subcategorization and subsequent
BSER determinations.\52\ The EPA proposed to maintain some of the size
cutoffs for defining subcategories from subpart KKKK and proposed to
revise others.
---------------------------------------------------------------------------

\51\ The base load rating only includes the heat input to the
combustion turbine engine and does not include the rated input from
associated duct burners.
\52\ See 89 FR 101317 (Dec. 13, 2024).
---------------------------------------------------------------------------

The proposed subcategory of large combustion turbines included new,
modified, or reconstructed sources with base load ratings greater than
850 MMBtu/h of heat input. This subcategory of large turbines
maintained the size-based threshold from subpart KKKK. However, the
proposed size-based thresholds for medium and small combustion turbines
were revised relative to subpart KKKK. The EPA proposed that the size-
based subcategory for medium combustion turbines included new,
modified, or reconstructed sources with base load ratings greater than
250 MMBtu/h of heat input and less than or equal to 850 MMBtu/h. The
EPA proposed that the size-based subcategory for small combustion
turbines included new, modified, or reconstructed sources with base
load ratings less than or equal to 250 MMBtu/h of heat input. In
addition, for the subcategories of medium and small combustion
turbines, the EPA proposed to include both new and reconstructed units
in the same size subcategory; and the EPA proposed to determine the
same BSER and NO<INF>X</INF> emission standards for both new and
reconstructed units. This was also in contrast to subcategorizations in
subpart KKKK.
In particular, the proposed subcategorization approach for small
stationary combustion turbines represented a significant shift from
that in subpart KKKK. The EPA proposed that a separate subcategory of
combustion turbines smaller than 50 MMBtu/h of heat input is not
necessary because multiple turbine manufacturers have developed dry
combustion controls capable of limiting NO<INF>X</INF> emissions to the
same rates as those achieved by larger combustion turbines (e.g., those
up to 250 MMBtu/h of heat input) for both electrical and mechanical
drive applications. This same rationale also led the EPA to propose
that separate subcategories for new small combustion turbines, based on
whether they serve electrical or mechanical drive applications, are no
longer necessary.
The EPA received significant comments on the size-based
subcategorization approach for large, medium, and small stationary
combustion turbines.
Many commenters opposed the proposed elimination of the 50 MMBtu/h
threshold that distinguishes between the small and medium size
subcategories of combustion turbines in the previous NSPS (subpart
KKKK). Specifically, the commenters stated that the elimination of the
subcategory for very small combustion turbines impacted the EPA's
proposed determination of the BSER and associated standards of
performance, which they argued were not appropriate for the smallest
turbines, i.e., those less than 50 MMBtu/h of heat input. Separately,
commenters asserted that the proposed 250 MMBtu/h size threshold did
not meaningfully correspond with the emissions performance or other
characteristics of models of combustion turbines currently on the
market. For example, commenters from the natural gas pipeline industry
indicated that they use industrial turbines in sizes of up to 320
MMBtu/h at compressor stations and advocated that the small size
subcategory should be increased to that, while the BSER of combustion
controls from subpart KKKK should be maintained. There was consistent
agreement among these commenters that the subcategory of small
combustion turbines with base load ratings less than or equal to 50
MMBtu/h of heat input should be maintained in subpart KKKKa. One
commenter indicated that turbines with base load ratings less than 20
MMBtu/h should have their own subcategory.
The EPA agrees with the commenters that it is appropriate to
maintain a subcategory for new combustion turbines with base load
ratings less than or equal to 50 MMBtu/h of heat input.

[[Page 1920]]

As described in sections IV.B.3-5 of this preamble, the Agency has
further examined the available controls for the source category and
their reasonableness based on the varying characteristics of different
types of combustion turbines. At proposal, the EPA believed that 250
MMBtu/h represented an inflection point above which SCR would be cost-
reasonable at intermediate and high levels of utilization (and
therefore the BSER) and below which SCR would not be cost-reasonable
(and combustion controls would comprise the BSER) except for high-
utilization turbines. However, based on updated information, the Agency
is not determining that SCR is the BSER for any units smaller than 850
MMBtu/h. There is therefore no reason to define the boundary between
small and medium combustion turbines at 250 MMBtu/h.\53\
---------------------------------------------------------------------------

\53\ The EPA noted in the proposal that ``if the EPA were to
determine that SCR was not an appropriate BSER for all small
stationary combustion turbines, then it may be appropriate to adjust
the size-based thresholds such that turbines of greater than 50,
100, or 150 MMBtu/h of heat input should be treated as `medium'
turbines.'' 89 FR 101318.
---------------------------------------------------------------------------

Moreover, the EPA's review also indicates that the available
combustion controls for turbines with base load ratings less than or
equal to 50 MMBtu/h of heat input are more limited and can achieve
different emission reductions relative to combustion turbines with base
load ratings greater than 50 MMBtu/h of heat input.\54\ For example,
the manufacturer guaranteed NO<INF>X</INF> emission rates for these
small combustion turbines is generally 25 ppm based on the use of dry
combustion controls. However, as the size of the combustion turbine
increases above 50 MMBtu/h, manufacturers have developed more effective
dry combustion controls with manufacturer guaranteed NO<INF>X</INF>
emissions rates decreasing to 15 ppm. This includes many models of
industrial and frame type combustion turbines larger than 50 MMBtu/h
and smaller than 250 MMBtu/h that would have fallen into the proposed
small turbine subcategory. These differences between combustion
turbines smaller or larger than 50 MMBtu/h and the availability and
performance of the different combustion controls each sized group can
employ leads the Agency to conclude that subpart KKKK's size-based
cutoff of 50 MMBtu/h between small and medium combustion turbines
remains the appropriate threshold for differentiating between small-
and medium-sized combustion turbines in subpart KKKKa.
---------------------------------------------------------------------------

\54\ See the discussion of the determination of the BSER and
NO<INF>X</INF> standards for new small combustion turbines in
section IV.B.5.c of this preamble.
---------------------------------------------------------------------------

The EPA disagrees with commenters that a subcategory for new
combustion turbines with base load ratings less than or equal to 20
MMBtu/h of heat input is appropriate, as there are no significant
differences in the performance of new combustion controls for turbines
less than or equal to 20 MMBtu/h and combustion turbines greater than
20 MMBtu/h and less than or equal to 50 MMBtu/h.\55\ However,
combustion controls that achieve emission rates of 25 ppm or lower for
small combustion turbines are not available for certain existing small
combustion turbines that modify or reconstruct, and SCR is not cost
reasonable. Therefore, the EPA agrees that a subcategory for combustion
turbines with base load ratings less than or equal to 20 MMBtu/h of
heat input--with higher NO<INF>X</INF> standards based on application
of different BSER--is appropriate for modified and reconstructed
combustion turbines only.
---------------------------------------------------------------------------

\55\ See the manufacturer specification sheet in the rulemaking
docket for additional information about available models of
stationary combustion turbines.
---------------------------------------------------------------------------

The EPA is finalizing, as proposed, that subpart KKKKa will not
include additional subcategories for new, modified, or reconstructed
small combustion turbines to distinguish between those that are
electrical drive versus those that are mechanical drive. While the EPA
did receive comments requesting that it maintain the distinction
between electrical and mechanical drive turbines as in subpart KKKK,
the Agency does not believe it is necessary given that the final rule
does not treat new and reconstructed combustion turbines the same way,
and existing electrical or mechanical drive turbines that modify or
reconstruct can meet the final NO<INF>X</INF> standards of performance
for small modified or reconstructed units in subpart KKKKa using
combustion controls.\56\
---------------------------------------------------------------------------

\56\ See section IV.B.6 of this preamble for discussion of the
subcategory for small modified and reconstructed combustion
turbines.
---------------------------------------------------------------------------

In subpart KKKKa, after completion of the technology review and
consideration of comments provided by stakeholders, the EPA is
finalizing the same size-based subcategory approach as the previous
combustion turbine criteria pollutant NSPS (subpart KKKK). The final
subcategories in subpart KKKKa include combustion turbines with base
load ratings greater than 850 MMBtu/h of heat input (i.e., large),
those with base load ratings greater than 50 MMBtu/h and less than or
equal to 850 MMBtu/h of heat input (i.e., medium), and those with base
load ratings less than or equal to 50 MMBtu/h of heat input (i.e.,
small). Like subpart KKKK, these subcategories are based on the base
load rating of the turbine engine and do not include any supplemental
fuel input to the heat recovery system.
b. Subcategorization Based on Utilization
In the proposed rule, in addition to subcategorizing combustion
turbines according to size, the EPA proposed to subcategorize
stationary combustion turbines further depending on 12-calendar-month
capacity factors (i.e., utilization). Although the EPA had not
previously subcategorized on this basis in subparts GG or KKKK, it has
differentiated between combustion turbines on the basis of utilization
in other contexts since 2015.\57\ Subcategorizing on this basis is
appropriate for combustion turbines in the utility sector because a
source's pattern of operation (e.g., how often it is in operation over
different time frames) generally tracks with how turbines are
configured (e.g., as simple cycle versus combined cycle, etc.).
Patterns of utilization and configuration in turn impact the
feasibility, emission reductions that would be achieved by, and cost-
reasonableness of different types of NO<INF>X</INF> emissions controls.
For example, in the utility sector, project developers do not typically
construct combined cycle combustion turbine systems to serve peak
demand where they would be expected to start and stop often. Similarly,
project developers in the utility sector do not typically construct and
install simple cycle combustion turbines to operate at higher capacity
factors to provide base load power. Combustion turbines used in the
utility sector typically fall into both the medium and large
subcategories. Similar patterns exist for combustion turbines used in
the commercial, institutional, and industrial power generating sectors,
which are typically turbines in the small and medium subcategories. In
the non-utility sector, project developers typically construct CHP
turbines for high-utilization applications and simple cycle turbines
for low-utilization applications, such as providing backup power. Thus,
turbine utilization is a useful proxy for certain characteristics of
turbines--classes, types, sizes, and modes of operation--that are
relevant for the systems of emission reduction that the EPA may

[[Page 1921]]

evaluate to be the BSER and therefore for the resulting standards of
performance.
---------------------------------------------------------------------------

\57\ See, e.g., Standards of Performance for Greenhouse Gas
Emissions from New, Modified, and Reconstructed Stationary Sources:
Electric Utility Generating Units (88 FR 33318; Oct. 23, 2015).
---------------------------------------------------------------------------

While it is generally the case that utilization tracks turbine size
and mode of operation (e.g., simple versus combined cycle), there are
exceptions. Industrial mechanical drive applications (i.e., not
electric generating) primarily use turbines from the small and medium
subcategories but have different utilization characteristics. These
turbines tend to operate at more variable loads as compared to
combustion turbines used to generate electricity. Their frequent
operation may result in their subcategorization as high-utilization
facilities, but they are primarily in simple cycle configurations
because heat recovery is generally not a technically or economically
viable option. However, the amount of utilization and the mode of
operation remain relevant for the systems of emission reduction that
the EPA may evaluate to be the BSER and therefore for the resulting
standards of performance.
The EPA proposed that combustion turbines with 12-calendar-month
capacity factors greater than 40 percent would be subcategorized as
high capacity factor (i.e., base load or high-utilization) units, those
with capacity factors greater than 20 percent and less than or equal to
40 percent were proposed to be subcategorized as intermediate capacity
factor/utilization units, and those with capacity factors less than or
equal to 20 percent were proposed to be subcategorized as low capacity
factor/utilization units. The proposed capacity factor/utilization
thresholds were chosen to reflect what, at proposal, were believed to
be reasonable cut points above and below which different NO<INF>X</INF>
controls would be cost-effective based on sources' operational
characteristics. The proposed thresholds were also designed to align
with thresholds in the 2024 NSPS for greenhouse gas (GHG) emissions
from new combustion turbines.\58\
---------------------------------------------------------------------------

\58\ See 89 FR 39798, 39913 (May 9, 2024). The EPA proposed to
repeal the 2024 NSPS for GHG emissions for new combustion turbines,
as well as for other new and existing fossil fuel-fired power
plants, on June 17, 2025. 90 FR 25752.
---------------------------------------------------------------------------

The EPA received significant comments on the subcategorization of
stationary combustion turbines according to capacity factor (i.e.,
utilization). Several commenters recommended that the upper capacity
factor threshold for defining small low-utilization combustion turbines
be increased to at least 25 percent or as high as 40 percent in subpart
KKKKa to not restrict the operation of simple cycle peaking units that
will have to support higher demand variability in the future due to
increased deployment of intermittent generation. According to the
commenters, a lower capacity factor threshold coupled with an emission
limit based on SCR would exacerbate the risk and complexity of
operating combustion turbines essential for grid firming generation and
reliability during extreme weather events and seasonal demands, and
constraining these assets could lead to capacity shortfalls that
increase the potential of higher-emitting generation being called upon.
Another commenter stated that the EPA should set the capacity factor-
based subcategories in subpart KKKKa to better reflect the changing
operational characteristics for certain combustion turbines used in
simple cycle mode and the typical capacity factors of combined cycle
units. Specifically, the commenter stated that an annual capacity
factor of 60 percent is a more appropriate demarcation between simple
cycle and combined cycle turbines. The commenter expects that some
frame type simple cycle turbines will be required to operate at
capacity factors of more than 40 percent in the future as demand for
power climbs, largely due to the boom in artificial intelligence and
the associated data centers. In addition, the commenter stated that a
threshold of 60 percent would help differentiate between units that
operate in simple cycle mode and those that operate in combined cycle
mode.
Based on the EPA's updated analysis of the cost and feasibility of
available controls for combustion turbines, the Agency is determining
in this final rule that SCR does not qualify as the BSER for any
subcategory of stationary combustion turbines with 12-calendar-month
capacity factors less than or equal to 45 percent.\59\ Therefore, the
proposed ``intermediate load'' subcategory that would have covered
combustion turbines operating at annual capacity factors greater than
20 percent and less than or equal to 40 percent is no longer necessary.
Moreover, the EPA has not found differences in the reasonableness of
combustion controls based on a combustion turbine's utilization that
would make distinguishing between ``low'' and ``intermediate'' load
turbines appropriate. Therefore, the proposed low-utilization threshold
referenced by the commenter is not included in final subpart KKKKa.
---------------------------------------------------------------------------

\59\ See sections IV.B.3 and IV.B.5 of this preamble.
---------------------------------------------------------------------------

After deciding that three utilization-based subcategories are
unnecessary and shifting to just two in this final rule (``high
utilization'' and ``low utilization''), the EPA further considered the
cutoff between these two subcategories. To determine an appropriate
capacity factor that generally reflects the differences between
turbines that operate in simple cycle mode and those that operate in
combined cycle mode, the EPA evaluated the 12-calendar-month capacity
factors of simple cycle turbines in the electric utility power sector
that have commenced operation since January 1, 2020. To account for
variability, the EPA calculated the 99 percent confidence maximum
capacity factor for each combustion turbine. The 99 percent confidence
maximum 12-calendar-month capacity factor for recently constructed
simple cycle turbines was 43 percent. To account for potential future
uncertainty, the EPA is finalizing a 12-calendar-month utilization rate
threshold of 45 percent to delineate between low- and high-utilization
turbines.60 61
---------------------------------------------------------------------------

\60\ While the fleetwide average capacity factor of both medium
and large simple cycle turbines is increasing, the average and
maximum capacity factors of both medium and large simple cycle
turbines that have recently commenced operation has remained
relatively constant.
\61\ See section IV.B.2.g of this preamble for discussion of the
EPA's decision not to establish subcategories based on whether a
combustion turbine operates in a simple cycle or combined cycle
configuration.
---------------------------------------------------------------------------

In this final rule, the EPA is subcategorizing large and medium
combustion turbines as high- or low-utilization units depending on 12-
calendar-month capacity factors (i.e., utilization rates). The high-
utilization subcategories include large and medium turbines utilized at
12-calendar-month capacity factors greater than 45 percent. The low-
utilization subcategories include large and medium combustion turbines
utilized at 12-calendar-month capacity factors less than or equal to 45
percent. Large and medium combustion turbines that exceed the 12-
calendar-month capacity factor threshold of 45 percent will be subject
to the high-utilization NO<INF>X</INF> standards, and owners or
operators of such facilities must achieve the applicable NO<INF>X</INF>
standard, presumably through the operation of additional emission
control technology relative to that required for low-utilization
combustion turbines. The EPA is not subcategorizing small combustion
turbines by utilization and the same BSER and emissions standard is
applicable to all new small combustion turbines regardless of the
utilization level because utilization level is not determinative of the

[[Page 1922]]

reasonableness of NO<INF>X</INF> controls for these units.
Even combustion turbines that operate at consistent utilization
levels for the life of the facility, the 12-calendar-month utilization
rates vary over the life of the turbine. To estimate the variability in
12-calendar-month utilization rates, the EPA reviewed the maximum 12-
calendar-month capacity factors and the average capacity factors of
combined cycle and simple cycle turbines in the power sector that have
commenced operation since 2020. The median percentage that the maximum
capacity factor is greater than the average capacity factor is 11
percent for combined cycle turbines and 15 percent for simple cycle
turbines. Assuming this is the only factor impacting the relationship
between the maximum and average capacity factor, the maximum 12-
calendar-month capacity factors of combined cycle and simple cycle
turbines with average capacity factors of 40 percent is 44 and 46
percent, respectively. Therefore, the EPA used a 45 percent
applicability threshold as representative of combustion turbines with
an average capacity factor of 40 percent. The 40 percent value was used
when evaluating cost and other BSER factors for control technologies
for combustion turbines in the high-utilization subcategories. The EPA
acknowledges that this approach is conservative. Once that investment
is made, the control technology would likely be used for the life of
the facility even if the combustion turbine were to be subcategorized
as low utilization in the future. For example, in the utility sector,
the average 30-year capacity factor of combined cycle and simple cycle
combustion turbines is 51 percent and 9 percent, respectively. Combined
cycle turbines initially operate on average at a capacity factor of 66
percent, and by year 30, the capacity factor drops to 37 percent.\62\
Simple cycle combustion turbines initially operate at a capacity factor
of 13 percent and drop to 5 percent by year 30. For combined cycle and
simple cycle turbines, the maximum capacity factor is 28 percent higher
and 49 percent higher than the 30-year lifetime average capacity
factor, respectively. In conclusion, the EPA determined it is
appropriate to use a 40 percent utilization rate when evaluating the
BSER factors, but this translates for implementation purposes into a
utilization subcategory threshold of 45 percent based on the 12-
calendar-month capacity factor to accommodate for the variability of a
combustion turbine that operates at a consistent utilization over the
life of the unit.
---------------------------------------------------------------------------

\62\ At year 24, combined cycle turbines would become low-
utilization turbines and the NSPS BSER would no longer be based on
the use of SCR. The EPA costing analysis assumes the high-
utilization BSER (i.e., SCR) continues to operate the entire 30-year
period. Assuming the SCR ceases operation in year 24 would decrease
the cost effectiveness of SCR.
---------------------------------------------------------------------------

c. Subcategorization Based on Efficiency
The Agency noted in the proposed rule that ``[t]he EPA's review of
combustion turbine emissions data and applied control technologies . .
. demonstrates a correlation between the efficiency of new turbine
designs and NO<INF>X</INF> emissions using combustion controls.'' \63\
We went on to state that turbine manufacturers have endeavored to
increase the efficiency of new turbine designs, but that there is a
tradeoff between efficiency and NO<INF>X</INF> emissions such that some
models of large higher efficiency turbines cannot meet a 15 ppm
NO<INF>X</INF> standard.\64\ We discussed and requested comment on the
relationship between turbine efficiency and the effectiveness of
combustion controls in our analysis of combustion controls for large
combustion turbines.\65\ Based on comments received in response to its
requests, the EPA is determining that it is appropriate to further
subcategorize large low-utilization combustion turbines in subpart
KKKKa based on the manufacturer's design efficiency rating.
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\63\ 89 FR at 101325.
\64\ Id.
\65\ See, e.g., id. at 101333 (solicitation for comment on
whether combustion controls are being developed for large, high-
efficiency turbines currently guaranteed at 25 ppm that would reduce
the NO<INF>X</INF> emission rate).
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When subpart KKKK was finalized in 2006, the largest available
aeroderivative combustion turbine had a base load rating of less than
850 MMBtu/h of heat input, and less efficient frame units greater than
850 MMBtu were available with manufacturer guaranteed NO<INF>X</INF>
emission rates of 15 ppm or less. Thus, the subcategories in subpart
KKKK were designed to reflect the distinctions between the sizes and
feasibility of different types of combustion controls between more
efficient turbines that were less than 850 MMBtu/h and less efficient
turbines that were greater than 850 MMBtu/h.
Since subpart KKKK was finalized, incremental advances have been
made to the design of the aeroderivative turbine that had been used to
define the 850 MMBtu/h threshold, and the base load rating of that
specific turbine model is now approximately 1,000 MMBtu/h.\66\ Further,
new frame type turbines have become available that have higher
efficiencies. The most common way to increase the efficiency of a
combustion turbine is to increase the firing temperature. However, an
increase in firing temperature also results in increased formation of
thermal NO<INF>X</INF>. Several frame turbines have become commercially
available since 2013 that have design efficiencies of at least 38
percent on a HHV basis \67\ and guaranteed NO<INF>X</INF> emission
rates of 25 ppm. In essence, the state of the source category has
evolved since 2006 so that there are now more types of large combustion
turbines available, and those combustion turbines have a broader range
of efficiencies, which affects NO<INF>X</INF> formation and the
emission reductions that can be achieved using combustion controls.
Given the subsequent development of the industry and the EPA's further
understanding of how large, higher efficiency turbines are operated
today (i.e., of the intersection between size, utilization, and
efficiency), for the purposes of subpart KKKKa, the Agency is
determining it is appropriate to subcategorize large, low-utilization
combustion turbines depending on whether their design efficiency is
less than 38 percent or greater than or equal to 38 percent.\68\
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\66\ The larger version became available in 2013. See the Excel
file docket item titled combustion turbine manufacturer
specifications proposal docket number EPA-HQ-OAR-2024-0419-0020
attachment 3.
\67\ This value is equal to a design efficiency rating of 42
percent on a lower heating value (LHV) basis.
\68\ This characteristic was not analyzed or understood to be
relevant at the time the BSER analysis was conducted for subpart
KKKK.
---------------------------------------------------------------------------

Several commenters requested that the EPA consider subcategorizing
large combustion turbines further to reflect the performance of
available combustion controls in relation to the utilization and design
efficiencies of certain classes or types of available combustion
turbines. Other commenters stated that the EPA should revise the size-
based subcategories in subpart KKKKa to capture and accommodate
variations within certain classes of combustion turbines that could
bear significantly on the cost of NO<INF>X</INF> controls.
Specifically, commenters suggested that the EPA should create
additional subcategories for large combustion turbines to distinguish
between classes of turbines with distinct NO<INF>X</INF> profiles and
for which SCR has materially different marginal costs and benefits. The
commenters asserted that doing so would account for variation in the
BSER, NO<INF>X</INF> reductions, and cost effectiveness for three
classes of large

[[Page 1923]]

frame turbines used in the power industry. Specifically, the commenters
suggested the following:
<bullet> Simple cycle frame turbines (90 to 150 MW) with a
NO<INF>X</INF> performance standard of 5 ppm reflecting advanced DLN
combustion controls as BSER for intermediate and base load. The
performance standard should be 15 ppm based on DLN for the low-
utilization subcategory.
<bullet> Simple cycle frame turbines (200 to 320 MW) with a
performance standard of 9 ppm reflecting advanced DLN combustion
controls as BSER for intermediate and base load. The performance
standard should be 15 ppm based on DLN for the low-utilization
subcategory.
<bullet> Simple cycle frame turbines (greater than 320 MW) with a
performance standard of 25 ppm reflecting DLN combustion controls as
BSER for all load subcategories. There is no advanced DLN technology
for these very large turbines.
<bullet> All units in combined cycle mode (i.e., base load) with a
performance standard based on SCR as BSER.
The EPA agrees with the commenters that since subpart KKKK was
finalized in 2006, new higher efficiency classes of frame type
combustion turbines have become commercially available, and the sizes
of these large turbines range from approximately 290 MW to 450 MW.
There are also two aeroderivative turbine designs that are large higher
efficiency units with NO<INF>X</INF> emission rates of 25 ppm.\69\ As
pointed out by the commenters, these classes of combustion turbines are
generally larger than earlier generation designs and these frame type
turbines are differentiated from earlier models by their higher firing
temperatures that result in higher NO<INF>X</INF> emissions.\70\
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\69\ Variations of the General Electric (GE) LMS100.
\70\ Examples include GE's 7HA series (7HA.01, 7HA.02, and
7HA.03), Siemens' 9000HL, and Mitsubishi's M501J series that
includes the M501JAC.
---------------------------------------------------------------------------

As discussed above, the EPA is determining that it is appropriate
to further subcategorize large, low-utilization combustion turbines
according to efficiency. The new subcategorization approach for these
turbines reflects the distinctions between large, higher efficiency
turbines and large, lower efficiency turbines when those turbines are
operating at low levels of utilization. This distinction is not
relevant when these turbines are operating at high utilization because,
regardless of the efficiency of the turbine, combustion controls with
the addition of SCR is reasonable for large turbines operating at high
utilization.\71\ However, at low utilization, there is a clear
distinction between the technical feasibility of achieving different
emission rates using combustion controls based on the efficiency of the
turbine. Efficiency is thus an appropriate basis for subcategorization
for large combustion turbines operating at low utilization.
---------------------------------------------------------------------------

\71\ See section IV.B.3 of this preamble.
---------------------------------------------------------------------------

Further subcategorization according to design efficiency is only
reasonable for combustion turbines in the large subcategory. For
instance, the EPA is not aware of any commercially available models of
new medium combustion turbines with design efficiencies greater than 38
percent on a HHV basis. For the subcategory of new small combustion
turbines, the most efficient model of which we are aware achieves an
efficiency of 35 percent on a HHV basis. Regardless of the design
efficiencies of new small and medium combustion turbines, we did not
identify a distinct correlation between efficiency and the manufacturer
guaranteed NO<INF>X</INF> emission rates. On the other hand, for
combustion turbines in the large subcategory, we identified a clear
correlation between design efficiency and manufacturer guaranteed
NO<INF>X</INF> emissions.
For subpart KKKKa, the EPA determines this additional
subcategorization is appropriate because it reflects, in part,
improvements in the design efficiency of stationary combustion
turbines. These developments in the current combustion turbine
marketplace--as evidenced by a review of manufacturer specification
data and as stated in public comments--continued to evolve since the
promulgation of subpart KKKK in 2006. Additionally, distinguishing
between combustion turbines in subpart KKKKa based on utilization has
the effect of elucidating distinctions in the reasonableness of
controls when turbines are operating at low versus high utilization;
these distinctions were not evident based on the subcategorization
approach in subpart KKKK. As discussed in section IV.B.5 of this
preamble, this results in a higher NO<INF>X</INF> emissions standard
for the class of large low-utilization higher efficiency combustion
turbines relative to subpart KKKK. It also results in a lower
NO<INF>X</INF> emissions standard for the class of large low-
utilization lower efficiency combustion turbines than was determined
for other classes of large turbines in subpart KKKK.
The EPA notes that subcategorizing large low-utilization combustion
turbines by design efficiency can impact the availability of large
turbines for use as high-utilization units. For example, combined cycle
facilities can be built in stages--initially the simple cycle portion
is installed and the HRSG and steam turbine are installed later. This
occurs when developers elect to go ahead and install the simple cycle
portion to meet current low-utilization loads, and as demand increases
over time, they add the steam portion of the combined cycle facility to
meet high-utilization loads. Under this planned staging of construction
and generation, the combustion turbine could operate as a simple cycle
unit for years. For other installations, the simple cycle portion of
the combined cycle facility is completed prior to the remainder of the
combined cycle facility due to unforeseen events, such as delays in the
availability of materials necessary to complete the steam portion of
the facility or delays in the availability of a second (or third)
combustion turbine engine for a combined cycle facility with multiple
turbines serving a single steam turbine. The ability to begin operating
the simple cycle portion of the facility prior to the completion of the
combined cycle project could have significant financial benefit to the
developer and provide additional resources to assist in grid stability.
And because the SCR for combined cycle turbines is included in the
HRSG, the simple cycle turbine would be operating without SCR in both
scenarios.
Without a subcategory for large low-utilization combustion turbines
based on efficiency, developers would not be able to use models with
efficiencies of 38 percent or greater as simple cycle turbines--even on
a short-term basis. The lack of a subcategory would provide a perverse
regulatory incentive to install lower efficiency combustion turbines so
that they could be operated on a short-term basis in simple cycle mode.
This would result in higher overall emissions because when the HRSG
becomes operational, the resulting lower efficiency combined cycle
facility with a lower efficiency turbine engine would have higher
emissions compared to these higher efficiency turbine engines that
result in a more efficient and lower emitting combined cycle facility.
d. Subcategorization of Non-Natural Gas-Fired Combustion Turbines
Consistent with subpart KKKK, the EPA proposed that when a
combustion turbine fires a fuel that is more than 50 percent non-
natural gas (e.g., either a gaseous fuel, such as hydrogen, or a liquid
fuel, such as oil) while under full load for a portion of an hour of
operation, then that combustion turbine

[[Page 1924]]

is subject to the appropriate non-natural gas NO<INF>X</INF> emission
standard--based on the application of the BSER--for that entire hour of
full-load operation. However, we also solicited comment on eliminating
the 50 percent requirement so that the non-natural gas emissions
standard would apply when any amount of non-natural gas fuel is burned
in the combustion turbine engine at full load. In general, we proposed
that the BSER for most sources firing non-natural gas fuels is the use
of wet combustion controls (i.e., water or steam injection) and/or
diffusion flame combustion. (Diffusion flame combustion is where fuel
and air are injected at the combustor and are mixed only by diffusion
prior to ignition. Generally, it is not considered a type of combustion
control technology per se because the EPA is not aware of diffusion
flame combustors broadly available that are able to achieve significant
NO<INF>X</INF> reduction in combustion turbines, though for some
subcategories the EPA identifies this technology as the BSER in the
absence of any other method of control.) Accordingly, we proposed
NO<INF>X</INF> standards for non-natural gas-fired sources in subpart
KKKKa based on the application of the BSER for each size-based
subcategory.
Several commenters opposed the EPA's proposal to define sources in
subpart KKKKa as non-natural gas-fired when more than 50 percent of the
heat input is from a non-natural gas fuel at full load. For example,
according to one commenter, widespread industry practice when switching
from natural gas to oil is to reduce load and switch from lean premix/
DLN combustion controls (for natural gas) to diffusion flame (for oil).
This can lead to a short-term spike in emissions, which, according to
the commenter, necessitates a higher, less stringent NO<INF>X</INF>
limit. Should such a spike in NO<INF>X</INF> emissions occur when less
than 50 percent of the fuel being combusted is fuel oil, the source
would be subject to the (lower, more stringent) NO<INF>X</INF> standard
for natural gas.\72\ Commenters further explained that given the effect
on emissions of switching fuels, it could be difficult for a source to
meet a lower NO<INF>X</INF> standard for natural gas combustion when a
non-natural gas fuel is being combusted, including when the non-natural
gas fuel represents less than 50 percent of the total heat input during
the hour. The commenters asserted that a more reasonable approach would
be to apply the highest applicable NO<INF>X</INF> emissions standard
for any hour when any amount of non-natural gas fuel is combusted--as
in the Industrial Boiler NESHAP--and pointed out the EPA's
acknowledgement in the proposal that eliminating the 50 percent
threshold ``could provide a more accurate representation of the
performance of applicable control technologies.'' 73 74
---------------------------------------------------------------------------

\72\ See Table 1 in section IV.B.5 of this preamble for the
NO<INF>X</INF> standards for subcategories of natural gas-fired
stationary combustion turbines.
\73\ See 40 CFR part 63, subpart DDDDD.
\74\ See 89 FR 101318 (Dec. 13, 2024).
---------------------------------------------------------------------------

Other commenters stated the EPA's concern that eliminating the 50
percent requirement would incentivize operators to burn a small amount
of non-natural gas fuel to be subject to a higher NO<INF>X</INF>
emissions limit is unfounded. Specifically, the commenters asserted
that reducing load makes fuel switching impractical by causing
generation to be less efficient, meaning there is little to no
incentive for an operator to conduct a fuel switch to take advantage of
a less stringent standard.
Further, several commenters responded to a solicitation for comment
in the proposal regarding whether multiple fuels could be combusted
simultaneously in a combustion turbine engine and whether it is
necessary to temporarily cease operation or reduce load to switch from
natural gas to distillate oil. According to commenters, the design and
operation of combustion systems do not allow for multiple fuels to be
combusted simultaneously in turbines operating under full load--except
for specific designs of dual-fuel combustion turbines used in certain
industrial processes. The commenters explained that for combustion
turbines not designed to operate in dual-fuel mode, different gaseous
fuel streams can be premixed and fired (e.g., natural gas and refinery
fuel gas or natural gas and hydrogen). A combustion turbine operator
cannot simply switch between liquid and gaseous fuels while operating
at full load if the turbine is not designed for dual-fuel operation. In
general, most combustion turbines are not dual-fuel designs and either
start on gas or oil and continue to operate on the same fuel as the
unit loads, or, to improve reliability in cold weather, units will
start on gas and transition to oil at or before the full speed no load
(FSNL) operating condition. In all cases, turbines with dry or wet
combustion controls never operate at full load while simultaneously
firing both natural gas and fuel oil. The combustion characteristics of
the higher hydrocarbon, distillate oil differ from the combustion
characteristics of natural gas. These fuels are incompatible with
systems that were engineered for methane gas, most notably regarding
poor flashback margin, which can result in significant damage to
premixed, dry combustion controls.
In subpart KKKKa, the EPA is maintaining the provision from subpart
KKKK that non-natural gas hours are defined as any hour when more than
50 percent non-natural gas fuels are fired in the combustion turbine at
full load (i.e., when the heat input is greater than 70 percent of the
base load rating). In these situations, the non-natural gas
NO<INF>X</INF> standard applies for the entire reporting hour--even if
non-natural gas fuel was fired for only a portion of the hour.\75\
Specifically, if the total heat input is greater than 50 percent from
non-natural gas fuels (e.g., distillate oil, hydrogen, and fuels other
than natural gas), the combustion turbine is subject to the applicable
NO<INF>X</INF> standard in the non-natural gas-fired subcategory and
that NO<INF>X</INF> standard must be met for the entire hour. This is
consistent with the approach for subcategorizing hours based on load.
For example, if the turbine is operated at part load (i.e., 75 percent
and 70 percent of the base load rating in subparts KKKK and KKKKa,
respectively) at any point during the hour, the part-load standard is
applicable for the entire hour even if the average load exceeds the
full load threshold. While the EPA appreciates commenters' explanation
that fuel switching to obtain more lenient emissions standards is
unlikely to occur because it is not economical, the 50 percent non-
natural gas threshold has proven workable in subpart KKKK and retaining
this threshold in subpart KKKKa avoids any regulatory incentive to
unnecessarily combust small amounts of non-natural gas fuels.
Similarly, if multiple fuels are burned during an hour of operation and
the total heat input is less than or equal to 50 percent non-natural
gas (and more than 50 percent natural gas), then the turbine is subject
to a NO<INF>X</INF> limit that is prorated based on the heat input of
the fuels during the hour. For example, if a turbine burns 75 percent
by heat input natural gas and 25 percent non-natural gas, the
applicable hourly NO<INF>X</INF> standard is 0.75 times the applicable
natural gas standard, plus 0.25 times the applicable non-natural gas
standard.\76\
---------------------------------------------------------------------------

\75\ For example, an affected facility could burn 51 percent
non-natural gas fuel for 1 minute of an hour and 100 percent natural
gas for the remaining 59 minutes. In this extreme situation, the
entire hour would be considered a non-natural gas-fired hour.
\76\ This example assumes the natural gas and non-natural gas
fuels are using different fuel nozzles. If the fuels are mixed prior
to combustion, the natural gas/non-natural gas determination is
based on the fuel mixture. If the mixture meets the definition of
natural gas, the natural gas standard is applicable. And if the
mixture does not meet the definition of natural gas, the non-natural
gas standard is applicable.

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[[Page 1925]]

It is important to make clear that the NO<INF>X</INF> standards for
natural gas and non-natural gas hours apply only when combustion
turbines are operating at full load. As explained by commenters, most
combustion turbines decrease load during fuel switching, and regardless
of the heat input from a particular fuel being fired for a portion of
an operating hour, those turbines would be subject to the part-load
NO<INF>X</INF> standards, which are higher than the individual natural
gas- and non-natural gas-fired NO<INF>X</INF> standards. See section
IV.B.2.f of this preamble for an explanation of subcategorization for
turbines operating at part load.
In subpart KKKKa, the EPA is also finalizing as proposed, with one
exception, that the NO<INF>X</INF> standards of performance are based
on the type of fuel being burned in the combustion turbine engine
alone. Fuel choice impacts combustion turbine engine NO<INF>X</INF>
emissions to a greater degree than it impacts such emissions from a
duct burner. Therefore, the EPA concludes that this approach provides a
more accurate representation of the performance of applicable control
technologies. The natural gas standard applies at those times when the
fuel input to the combustion turbine engine meets the definition of
natural gas, regardless of the fuel, if any, that is burned in the duct
burners. The one exception is for byproduct fuels. For turbines burning
byproduct fuels, the applicable emissions standard is based on the
total heat input to the turbine, including and fuel burned in the duct
burners. See section IV.B.7.d of this preamble for further discussion
of turbines burning byproduct fuels.
e. Subcategory for Temporary Combustion Turbines
At proposal, the EPA requested comment on creating either a
subcategory or an exemption for stationary combustion turbines used in
temporary applications. Many commenters generally supported some form
of streamlined compliance for temporary applications. Some commenters
raised concerns that a full exemption could have unintended
consequences. After considering these comments, the Agency is
finalizing a new subcategory in subpart KKKKa for small and medium
stationary combustion turbines (i.e., up to 850 MMBtu/h in size) used
in temporary applications. This subcategory reflects a BSER
determination of combustion controls with an associated standard of 25
ppm NO<INF>X</INF> when combusting natural gas and 74 ppm
NO<INF>X</INF> when burning non-natural gas fuels, along with a
streamlined approach to compliance that primarily relies on maintaining
documentation of manufacturer certification. Such turbines may be used
in a single location for up to 24 months. The EPA is also amending
subpart KKKK to include an optional subcategory for stationary
temporary combustion turbines with the same BSER, NO<INF>X</INF>
standards, and recordkeeping and reporting requirements as for the
subcategory of stationary temporary combustion turbines in subpart
KKKKa.\77\
---------------------------------------------------------------------------

\77\ The emission standards for temporary turbines are
consistent with the standards in subpart KKKK.
---------------------------------------------------------------------------

As discussed in the proposal, a streamlined approach to NSPS
compliance for temporary combustion turbine applications would bring
this NSPS into alignment with similar approaches that are available in
the boilers NSPS and in the reciprocating internal combustion engines
(RICE) NSPS. The EPA has historically considered portable boilers and
RICE used for limited periods of time to be temporary equipment not
subject to regulation under their respective NSPS or NESHAP
subparts.\78\ The Agency observed at proposal that the absence of any
such provisions in the combustion turbines NSPS is anomalous insofar as
combustion turbines tend to have lower air pollutant emissions than are
emitted by an equivalent level of power generation from RICE. Further,
in the proposal, the EPA noted that the permitting, testing, and
monitoring requirements typically applicable for a combustion turbine
subject to an NSPS may not be appropriate or suitable for combustion
turbines needed quickly and only for limited periods of time. Temporary
combustion turbines are generally operated in short-term situations but
can also provide power during extended emergency or emergency-like
situations (e.g., a natural disaster damages the electric grid) while
the primary generating equipment is not available, while transmission
and/or generation capacity is being repaired and/or upgraded, or for
some other unforeseen event.\79\ Since permitting itself could take
longer than the need for temporary generation, the Agency solicited
comment on whether an applicability exemption or subcategorization
would be appropriate for temporary combustion turbines under subparts
GG, KKKK, and KKKKa.
---------------------------------------------------------------------------

\78\ See, e.g., 40 CFR 60.4200(a), 60.4230(a), 60.40b(m), and
60.40c(i). (We note that at proposal we inadvertently cited similar
but separate provisions of the RICE NSPS related to ``replacement''
engines. Cf. 40 CFR 60.4200(e), 60.4230(f).)
\79\ Note that a separate exemption is available for ``emergency
turbines'' in subpart KKKK, which is also being included in subpart
KKKKa. See 40 CFR 60.4310(a); id. 60.4420 (definition of ``emergency
combustion turbine''). However, this provision may not be clearly
applicable in all circumstances in which temporary turbines are
needed.
---------------------------------------------------------------------------

The EPA also requested comment at proposal on whether the BSER for
temporary combustion turbines is the use of combustion control
technology consistent with the otherwise applicable subcategory--25 ppm
NO<INF>X</INF> for units with base load ratings of 850 MMBtu/h or less
and 15 ppm NO<INF>X</INF> for larger units. Relatedly, we solicited
comment on the appropriate testing and recordkeeping criteria for such
regulatory provisions.
Multiple commenters supported the idea of a subcategory or
exemption. Comments, particularly from industry stakeholders, supported
a BSER of combustion controls and indicated that turbines used in
temporary applications are generally capable of meeting a
NO<INF>X</INF> standard of 25 ppm using combustion controls. The same
commenters also generally opposed requiring SCR for temporary turbines,
the complexity of which would tend to defeat the purpose of being able
to bring in such turbines quickly for immediate and short-term power
supply. The EPA agrees that combustion controls are the BSER for
temporary turbines, and the Agency applies the BSER analysis set forth
in section IV.B.3 of this preamble explaining why SCR is not the BSER
for small and medium turbines.
The Agency is limiting the scope of the temporary combustion
turbines subcategory so that large combustion turbines (i.e., those
with a base load rated heat input greater than 850 MMBtu/h) cannot
qualify for treatment as temporary combustion turbines. In general,
large combustion turbines are not used in temporary applications--these
turbines tend to be frame type units that are more challenging to
transport and operate without more extensive onsite preparation.
The EPA finds 25 ppm to be the appropriate standard of performance
for NO<INF>X</INF> emissions from temporary combustion turbines. (The
EPA is not establishing a separate SO<INF>2</INF> standard of
performance for this subcategory--in other words, the otherwise
applicable SO<INF>2</INF> standard will apply).) Most trailer-mounted
turbines, which would likely be intended to remain in the same location
for less than 2 years and so can be considered representative of
typical temporary turbines, have standard

[[Page 1926]]

emission guarantees of 25 ppm NO<INF>X</INF>. There are some trailer-
mounted turbines with lower standard emission guarantees, but these are
less efficient designs with lower rated outputs. For example, an
emissions standard of 15 ppm NO<INF>X</INF> would limit the
availability of temporary turbines to those less efficient models with
lower rated outputs--significantly increasing costs for the regulated
community and resulting in increased fuel use. Combustion systems
capable of achieving 15 ppm NO<INF>X</INF> are generally more complex
and physically larger than comparable combustion systems capable of
achieving 25 ppm NO<INF>X</INF>. For example, more complex combustion
control systems generally have more fuel nozzles and burners, premix
larger amounts of air with the fuel, and have more sophisticated
control systems. This increases the physical size and cost of a
combustion turbine for a given rated output. Furthermore,
aeroderivative turbines are generally physically smaller than frame
units for the same rated output. Most aeroderivative turbines have
guaranteed emission rates of 25 ppm NO<INF>X</INF>. The ability to
transport a temporary turbine is a critical feature and an emissions
standard of less than 25 ppm NO<INF>X</INF> would increase the physical
size per rated output of combustion turbines that could meet that
emissions standard and undermine the purpose of the subcategory. In
addition, as discussed in section IV.B.4 of this preamble, combustion
controls capable of achieving 25 ppm NO<INF>X</INF> qualify as the BSER
for small combustion turbines and low-utilization medium turbines--both
of which are potential temporary turbines. While some medium temporary
turbines may operate at high utilization levels for limited periods of
time, there will be periods when the turbine will be in storage, being
transported to a new location, or otherwise not operating. On balance,
the EPA anticipates that medium temporary turbines will have
utilization levels of less than 45 percent. Therefore, we conclude that
combustion controls capable of achieving 25 ppm NO<INF>X</INF> are the
BSER for the temporary turbines subcategory.
Commenters recommended increasing the allowable period of operation
at a single location to 18 months or 2 years to account for situations
where temporary power is needed for longer than the 12-month period
mentioned in the proposal. The Agency agrees with commenters that a 12-
calendar-month period may not be sufficient for all situations. In
addition, a 24-month period is consistent with a longstanding policy
within the Prevention of Significant Deterioration (PSD) permitting
program, which recognizes that emissions occurring for no longer than
that period of time may be considered temporary and therefore excluded
from modeling analysis.\80\ We note that 24 months is the total
residence time permitted from when a temporary turbine commences
operation. The final temporary turbine subcategory is for turbines used
at a single location for up to 24 months.
---------------------------------------------------------------------------

\80\ See 43 FR 26380, 26394 (June 19, 1978).
---------------------------------------------------------------------------

Some commenters also stated that the subcategory should be
available to combustion turbines used in temporary applications
regardless of whether they meet criteria for portability. To simplify
compliance and avoid potentially complicated regulatory determinations,
the EPA is not requiring temporary combustion turbines to be portable
in nature or meet indicia of portability to qualify for this
subcategory.\81\ Commenters noted there may be applications where a
temporary combustion turbine can be transported to a location and
installed onsite for a time-limited purpose, but may not meet a
definition of ``portable'' such as that included, for example, in the
definition of ``temporary boilers.'' \82\ Given other criteria the EPA
is finalizing that limit the scope of a new subcategory for temporary
combustion turbines, we agree a requirement to be portable serves
little benefit and is not needed.\83\
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\81\ Note that combustion turbines that are mounted on a vehicle
for portability continue to be subject to the NSPS, as they have
been under subparts GG and KKKK. See, e.g., 40 CFR 60.4420
(definition of ``stationary combustion turbine'').
\82\ See 40 CFR 60.41b.
\83\ Note that, as a separate matter, to be considered a
``nonroad engine'' for purposes of mobile source regulation under
Title II, a unit must, among other things, meet the criteria in the
definition at 40 CFR 1068.30, paragraph 1, such as being ``portable
or transportable.''
---------------------------------------------------------------------------

Monitoring, recordkeeping, and reporting requirements are
substantially reduced for the subcategory of temporary turbines. In the
final rule, the EPA is requiring only that the owner or operator of a
turbine falling within the temporary turbines subcategory maintain
documentation onsite that each temporary turbine has been certified by
its manufacturer to meet a NO<INF>X</INF> emissions rate of 25 ppm, and
that each turbine has been performance tested at least once in the
prior 5 years (for turbines older than 5 years, after the initial sale
by the manufacturer). Annual performance testing is not required for
turbines in the temporary subcategory. We anticipate that a test every
5 years will be sufficient to ensure that temporary turbines are
properly maintained so as to continue to meet the 25-ppm limit.
Consistent with the proposal, the EPA finds that several conditions
on the use or replacement of temporary turbines are necessary to ensure
the subcategory does not inadvertently create a means of avoiding
requirements that apply under the NSPS for turbines used in non-
temporary capacities. Under the final rule, should a temporary
combustion turbine remain in place for longer than 24 months, then it
would not be considered temporary for any period of its operation, and
any failure of the owner or operator to comply with the otherwise
applicable requirements of the relevant NSPS, even in the initial 24
months of operation, would be an enforceable violation of the Act. In
addition, the final rule does not allow the replacement of a temporary
combustion turbine with another temporary combustion turbine to
maintain temporary status beyond the 24-month period. However, as an
anticipated normal function for these types of turbines, temporary
turbines may be used to replace or substitute the generation provided
by non-temporary turbines (or other types of generators) when those
units are taken offline (e.g., for maintenance work). In addition, the
relocation of a temporary stationary combustion turbine within a
facility does not restart the 24-calendar month residence time.
The EPA is not finalizing a complete exemption from the NSPS for
temporary combustion turbines. In response to the alternative exemption
approach on which the Agency sought comment, multiple commenters
supported an exemption approach like the NSPS for RICE. However, for
RICE, the exemption from the NSPS for equipment operating in a single
location of up to 12 months works in conjunction with regulations
promulgated under title II of the Act to bring these RICE within the
definition of ``nonroad engines'' as set forth at 40 CFR 1068.30. Such
units are then subject to regulations that the EPA has promulgated for
nonroad engines pursuant to title II of the Act.\84\
---------------------------------------------------------------------------

\84\ See 42 U.S.C. 7547; see also, e.g., 40 CFR 60, subparts III
and JJJ; 40 CFR part 1039.
---------------------------------------------------------------------------

Under both the statute and EPA regulations, combustion turbines in
general are considered a kind of internal combustion engine that
therefore could in theory be regulated as nonroad engines.\85\
Historically, however, the EPA has not regulated combustion turbines,
even those that may be portable, as nonroad engines, but rather

[[Page 1927]]

as stationary sources.\86\ The current definition of ``nonroad engine''
at 40 CFR 1068.30 excludes engines that are subject to an NSPS. All
combustion turbines meeting the applicability criteria of the NSPS for
combustion turbines are subject to those NSPS standards (including
portable turbines) and thus have been excluded from the definition of
nonroad engines. An exemption from the NSPS for qualifying stationary
temporary applications would potentially bring portable combustion
turbines within the definition of nonroad engine at 40 CFR 1068.30.
However, the kinds of turbines that are used in stationary temporary
applications are not currently subject to any title II regulations or
standards. Finalizing an exemption for temporary or portable combustion
turbines without ensuring a workable framework for compliance under
title II could leave these engines subject to no emission standards at
all.
---------------------------------------------------------------------------

\85\ See 42 U.S.C. 7550(1) and 7602(z).
\86\ See 42 U.S.C. 7411(a)(3). See 40 CFR 60.331(a); 40 CFR
60.4420 (definition of ``stationary combustion turbine'').
---------------------------------------------------------------------------

Nonetheless, the Agency recognizes the significant interest several
stakeholders have expressed in treating combustion turbines used in
stationary temporary applications as nonroad engines subject to
regulation under title II. There could be benefits in the form of
reduced permitting burden and further streamlined compliance
obligations for the purchasers and users of such turbines. At the same
time, manufacturers of combustion turbines that are treated as nonroad
engines would be subject to compliance obligations under title II,
including, for example, obtaining certificates of conformity. Such
turbines would be treated as other nonroad engines under title II and
permitting requirements would not apply to emissions from the engine
because such turbines would no longer be considered a part of the
stationary source. Commenters on this rule identified concerns with the
air quality effects if many temporary combustion turbines were brought
together and operated in a single location and suggested imposing
operating or total-emissions constraints and air quality considerations
to prevent these consequences.\87\
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\87\ The EPA notes that under the subcategory approach to
temporary stationary combustion turbines, which was are finalizing
in subpart KKKKa, permitting authorities may take these kinds of
considerations into account in determining appropriate emissions
limitations or other requirements.
---------------------------------------------------------------------------

The EPA believes these matters deserve further investigation before
rulemaking action is taken to consider regulating portable combustion
turbines used in temporary applications under title II rather than
under the NSPS. The EPA is not promulgating any such regulations under
title II in this action. In this final rule, the EPA is including a
conditional exclusion in subpart KKKKa that will exclude combustion
turbines from the definition of ``stationary combustion turbine,'' if
the turbine meets the definition of ``nonroad engine'' under title II
of the Act and applicable regulations, and is certified to meet
emission standards promulgated pursuant to title II of the Act, along
with all related requirements. This provision will become operative if
the EPA in the future adopts nonroad emission standards and
certification requirements for portable combustion turbines.
Even in the absence of a complete exemption from the NSPS, the EPA
believes creating the subcategory for temporary combustion turbines in
this action can facilitate actions that reduce the permitting burden
faced both by sources and permitting authorities. Note that the EPA is
separately exercising authority granted to it under CAA section 502(a)
to exempt from title V permitting any combustion turbines that are not
major sources.\88\ The EPA expects that the application of combustion
turbines at sites with a potential to emit below the title V permitting
major source threshold (as referenced in the last sentence of CAA
section 502(a)) would also emit below major NSR emissions thresholds
and thus only be subject to minor NSR program requirements. CAA section
110(a)(2)(C) requires States to develop a program to regulate the
construction and modification of any stationary source, including minor
NSR requirements as necessary, to assure that NAAQS are achieved. Minor
NSR requirements are required to be approved into a State
Implementation Plan (SIP), Tribal Implementation Plan (TIP), or Federal
Implementation Plans (FIP) and are often mechanisms to assist in
achieving and maintaining the NAAQS.\89\ The CAA and the EPA's
regulations are less prescriptive regarding the minor NSR program
requirements. Therefore, reviewing authorities generally have
significant flexibility in designing their minor NSR programs,
including any air permitting programs for minor sources. Minor NSR
permits are almost exclusively issued by State, local, and other
authorized reviewing authorities, although the EPA issues minor NSR
permits for most areas of Indian country where Tribes have not
developed TIPs or requested delegation to administer minor NSR air
permitting programs for their jurisdictions. With the creation of the
temporary combustion turbines subcategory in this action, the EPA
believes authorized reviewing authorities may find it efficient to
pursue further streamlining of minor-source permitting for such
sources, including developing a general permit for such sources, or
issuing a permit by rule for these sources.
---------------------------------------------------------------------------

\88\ See section IV.E.5 of this preamble for further discussion.
\89\ See 42 U.S.C. 7410(a)(2)(C).
---------------------------------------------------------------------------

Even where temporary combustion turbines comprise or are part of a
major source for purposes of NSR permitting, the temporary turbines
subcategory will assist States in identifying emissions from such
sources that may be excluded from parts of the permit review because
they are temporary. Under the EPA's PSD regulations, temporary
emissions can be excluded from the analysis of whether the emissions
increases that would result from construction or modification of a
major stationary source cause or contribute to a violation of air
quality standards.\90\ As discussed above, the 24-month period we are
finalizing for this subcategory accords with the duration the EPA has
used for decades to classify temporary emissions in the PSD program.
Sources with characteristics that place them within this subcategory
will have a straightforward means of showing that emissions from these
sources are temporary to apply this PSD exemption for temporary
emissions in the review of a PSD permit application.
---------------------------------------------------------------------------

\90\ See 40 CFR 51.166(i)(3); 40 CFR 52.21(i)(3).
---------------------------------------------------------------------------

Further, the standards of performance in this final rule are
legally and practically enforceable and thus can serve to inform
calculations of the potential to emit of these sources for purposes of
determining whether they are major sources for NSR applicability
purposes. Sources may, of course, also voluntarily accept, in an
enforceable permit condition, more stringent emissions limits, or limit
their operating time, to reduce their potential to emit so as to become
synthetic-minor sources for NSR applicability purposes.
f. Subcategory for Combustion Turbines Operating at Part Loads, Located
North of the Arctic Circle, or Operating at Ambient Temperatures of
Less Than 0 [deg]F
When the EPA promulgated subpart GG (the original stationary gas
turbine criteria pollutant NSPS) in 1979, the NO<INF>X</INF> standards
and compliance requirements were based on performance testing. Based on
subsequent rulemakings, owners or

[[Page 1928]]

operators of a gas turbine subject to subpart GG with a NO<INF>X</INF>
CEMS began determining excess emissions on a 4-hour rolling average
basis. The EPA found that a 4-hour basis is the approximate time
required to conduct a performance test using the performance test
methods specified in subpart GG. This 4-hour rolling average became the
default for determining the emission rates of gas turbines, and, in
2006, the EPA retained it in the subsequent review of the stationary
combustion turbine criteria pollutant NSPS.
When the EPA proposed subpart KKKK in 2005, the NO<INF>X</INF>
performance emissions data were based on stack performance tests, which
are representative of emission rates at high hourly loads, rather than
CEMS data. The final NO<INF>X</INF> standards for high hourly loads
were consistent with the performance test data and manufacturer
guarantees. To avoid confusion with the annual ``utilization'' levels
discussed elsewhere in this document, we will refer to high hourly
loads as ``full loads,'' in contrast with ``part loads''; utilization
levels on an annual basis are referred to as ``low-utilization'' and
``high-utilization.'' Manufacturer guarantees are only applicable
during specific conditions, which include the load of the combustion
turbine (i.e., when the load meets certain specifications) and the
ambient temperature (i.e., generally above 0 [deg]F). When combustion
turbines are operated at part loads and/or at low ambient temperatures,
low-NO<INF>X</INF> combustion controls--the identified BSER in subpart
KKKK--were not as effective at reducing NO<INF>X</INF> from a technical
standpoint.\91\ At part-load operation and low ambient temperatures, it
is more challenging to maintain stable combustion using DLN and
adjustments to the combustion system are required--resulting in higher
NO<INF>X</INF> emission rates. Therefore, in subpart KKKK, the Agency
identified diffusion flame combustion as the BSER for hours of part-
load operation or low ambient temperatures.\92\
---------------------------------------------------------------------------

\91\ The ambient temperature of combustion turbines located
north of the Arctic Circle would often be below 0 [deg]F, and these
units are included in the low ambient temperature subcategory
regardless of the actual ambient temperature. As we found with
subpart KKKK, the costs of requiring combustion controls that would
rarely be used are not reasonable.
\92\ Combustion turbines have multiple modes of operation that
are applicable at different operating loads and when the combustion
turbine is changing loads. The modes are specific to each combustion
turbine model. The identified BSER of diffusion flame combustion
also includes periods of operation that use less effective DLN
compared to operation at full loads.
---------------------------------------------------------------------------

In subpart KKKK, a part-load hour is defined as any hour when the
heat input rate is less than 75 percent of the base load rating of the
combustion turbine. If the heat input rate drops below 75 percent at
any point during the hour, the entire hour is considered a part-load
hour, and the part-load standard is applicable during that hour.
Determination of the 4-hour emissions standard is calculated by
averaging the four previous hourly emission standards. Under this
approach, the ``full load'' standard (i.e., the standard of performance
that has been established for the relevant subcategory as discussed
elsewhere in this notice) would not be applicable until a minimum of 6
continuous operating hours. The initial and final hours would be
startup and shutdown, respectively, and the part-load standard is
applicable during those hours. If the combustion turbines were
operating at full load during the middle 4 hours, the full load
standard would be applicable to that 4-hour average. The emission
standards for the remaining hours would be a blended standard that is
between the part-load and full load standards. This approach was viewed
as appropriate to account for the different applicable BSERs. Subpart
KKKK also includes a 30-operating-day rolling average standard that is
applicable to combustion turbines with a HRSG. The 30-operating-day
rolling average was included in subpart KKKK because the HRSG was part
of the affected facility, and a longer averaging period is necessary to
account for variability when complying with the alternate output-based
emissions standard.
The EPA is finalizing the same short-term 4-hour standard for part
load in subpart KKKKa along with the blended standard approach.
Specifically, the applicable emissions standard is based on the heat
input weighted average of the four applicable hourly emissions
standards. However, as discussed at proposal, the EPA is finalizing two
changes to the part-load subcategory. First, the CEMS data analyzed by
the EPA indicates that emissions tend to slowly increase at lower
loads, but, in general, combustion turbines can maintain compliance
with the emissions standards at hourly loads of 70 percent and greater,
not just at loads of 75 percent and greater, as reflected in subpart
KKKK.\93\ Therefore, the EPA determines in subpart KKKKa that this
subcategory applies for any hour when the heat input is less than or
equal to 70 percent of the base load rating. The EPA notes that
lowering the part-load threshold brings more operating periods under
the otherwise-applicable standards of performance.
---------------------------------------------------------------------------

\93\ To maintain flame stability during part-load operation, dry
combustion controls must increase the relative amount of the fuel
going to the diffusion flame portion of combustion system. This
inherently results in an increase in the NO<INF>X</INF> emissions
rate. Similarly, to maintain stable operation during part-load
operation, the relative amount of water injected for wet combustion
controls must be reduced.
---------------------------------------------------------------------------

Second, the EPA is finalizing a different size threshold for
subcategorizing the part-load emission standards. Subpart KKKK
subcategorizes the part-load emissions standard based on the rated
output of the turbine (i.e., combustion turbines with outputs greater
than 30 MW have a more stringent part-load standard than smaller
combustion turbines). For subpart KKKKa, the EPA proposed to
subcategorize the part-load standard based on the heat input rating
(i.e., turbines with base load heat input ratings greater 250 MMBtu/h
would have a more stringent standard (96 ppm NO<INF>X</INF>) than
smaller combustion turbines at part load (150 ppm NO<INF>X</INF>)).
In this action, since the final size-based subcategorization
approach no longer includes the proposed 250 MMBtu/h of heat input size
threshold for combustion turbines operating at full load, and because
the proposal did not otherwise identify a basis for amending the part-
load size threshold, the EPA is retaining in subpart KKKKa a size
threshold that is comparable to the 30 MW output threshold in subpart
KKKK. However, instead of using an output metric, subpart KKKKa sets a
threshold to distinguish the two size-based, part-load subcategories at
less than, or equal to or greater than, 300 MMBtu/h of heat input. All
new combustion turbines with base load ratings of greater than 300
MMBtu/h have design rated outputs of greater than 30 MW, and all new
combustion turbines with base load ratings of less than 300 MMBtu/h
have design rated outputs of less than 30 MW. This maintains
consistency with the use of a heat-input metric for other size-based
subcategories in the NSPS.
In the proposed rule for subpart KKKKa, the EPA solicited comment
with respect to a concern that the standards for the part-load
subcategory are significantly less stringent than the otherwise
applicable standards of performance and could create a perverse
incentive to operate at part loads. The Agency also solicited comment
on possible solutions. Commenters largely disagreed that the part-load
standards substantially eroded the stringency of the NSPS or created a
perverse incentive for sources to operate at lower hourly

[[Page 1929]]

loads to obtain the higher emissions standards. One commenter submitted
graphical data illustrating that it typically will not be economically
advantageous to operate at part-load for extended periods of time, and
other commenters that own or operate combustion turbines stated that
extended part-load operations are not consistent with their practices.
After considering these comments, the EPA agrees that further
changes from subpart KKKK's approach to part-load operations are not
needed in subpart KKKKa. The EPA finds the commenters' explanations
credible that the part-load subcategory does not unduly weaken the
NSPS. Nonetheless, as the EPA discussed in the proposal, we believe the
use of an optional, alternative approach to compliance using mass-based
limits could be an effective way to simplify compliance for some
combustion turbines while also ensuring overall good emissions
performance consistent with the revised standards of performance in
subpart KKKKa.\94\
---------------------------------------------------------------------------

\94\ See section IV.E.4 of this preamble for discussion of the
optional, alternative mass-based NO<INF>X</INF> standards.
---------------------------------------------------------------------------

Additionally, in subpart KKKKa, the EPA is maintaining as proposed
the same ambient temperature subcategorization and BSER as in subpart
KKKK. If at any point during an operating hour the ambient temperature
is below 0 [deg]F, or if the combustion turbine is located north of the
Arctic Circle, the BSER is the use of diffusion flame combustion with
the corresponding part-load standard.
Dry combustion controls are less effective at reducing
NO<INF>X</INF> emissions at part-load operations and low ambient
temperatures. In addition, SCR is only effective at reducing
NO<INF>X</INF> under certain temperatures at part loads and is not as
effective at reducing NO<INF>X</INF> as at design conditions. The only
technology the EPA has identified for all part-load operations and/or
low ambient temperatures is the use of diffusion flame combustion.
Therefore, in subpart KKKKa, the EPA determines that diffusion flame
combustion is the BSER for these conditions as proposed.\95\
---------------------------------------------------------------------------

\95\ A BSER of diffusion flame combustion includes DLN that is
less effective at reducing NO<INF>X</INF> than DLN under design
conditions.
---------------------------------------------------------------------------

g. Subcategorization Based on Other Factors
In response to the proposed rule, several commenters recommended
that subpart KKKKa subcategorize stationary combustion turbines based
on whether they operate as simple or combined cycle units and/or
whether they are aeroderivative or frame type units. These commenters
recommended that the EPA re-evaluate its BSER determinations to better
address the physical and operational differences between simple and
combined cycle turbine configurations because of the technical and
economic effects these differences have on controlling emissions.
Specifically, the commenters cited the higher exhaust temperatures of
simple cycle frame turbines and noted the challenges this would create
for operating SCR. One commenter noted that due to the different
capabilities of the equipment, the base load subcategory should be
split so that simple cycle and combined cycle units are not in the same
group.
While the EPA appreciates the differences between these types of
units and discusses such differences as appropriate throughout this
preamble, it is not subcategorizing based on simple versus combined
cycle or aeroderivative versus frame type combustion turbines in
subpart KKKKa. For aeroderivative and frame type combustion turbines,
separate subcategories might not be technically viable. For example,
aeroderivative turbines share components and are adapted from aircraft
jet engines, and while they tend to be lighter and have higher pressure
ratios and efficiencies than similar-sized frame units, there is
overlap and no clear distinction between the technologies. In addition,
and critically, there are no inherent differences in the performance of
combustion controls or SCR between aeroderivative and frame type
combustion turbines.\96\
---------------------------------------------------------------------------

\96\ See the manufacturer specification sheet in the rulemaking
docket for additional information about available models of
stationary combustion turbines.
---------------------------------------------------------------------------

Further, the EPA believes it is more appropriate to address the
differences between combustion turbines operating in simple cycle and
combined cycle configurations through subcategorizing by
utilization.\97\ While there are clearly differences between simple and
combined cycle configurations, those differences are not necessarily
determinative of the reasonableness of different types of
NO<INF>X</INF> controls because they are superseded by another basis or
bases for subcategorization. That is, there are other characteristics
of turbines that, when accounted for under the EPA's subcategorization
approach in this final rule, obviate the need to subcategorize by
simple cycle versus combined cycle configuration because such
differences are already effectively accounted for by the utilization
subcategories.
---------------------------------------------------------------------------

\97\ See discussion in section IV.B.2.b of this preamble.
---------------------------------------------------------------------------

In the utility sector, simple cycle turbines tend to operate at
much lower capacity factors (e.g., the average lifetime capacity factor
is 9 percent) than combined cycle turbines (e.g., the average lifetime
capacity factor is 51 percent). However, there is some overlap in
capacity factors. For example, in 2024, 3 percent of simple cycle
turbines operated at capacity factors greater than 30 percent, and 19
percent of combined cycle turbines operated at capacity factors less
than 30 percent. As discussed in section IV.B.2.b of this preamble, the
capacity factor or utilization level impacts the cost effectiveness of
NO<INF>X</INF> controls. This is the case regardless of whether a
turbine is in a simple cycle versus a combined cycle configuration.
After accounting for utilization (in addition to the other types of
subcategorizations the EPA is providing in this final rule), there is
no further basis for differentiating between simple and combined cycle
turbines from the perspective of selecting the BSER and standards for
NO<INF>X</INF>. Furthermore, establishing separate subcategories could
create a regulatory incentive to install simple cycle turbines instead
of combined cycle turbines--although the same controls are reasonable
for both, and simple cycle turbines emit more NO<INF>X</INF> per unit
of useful energy output. To avoid this perverse environmental outcome,
the EPA is establishing standards of performance that are achievable by
both simple and combined cycle combustion turbines under the
subcategories in this final rule. In addition, to establish separate
subcategories for simple and combined cycle turbines, the Agency would
have to determine how to subcategorize CHP facilities that operate with
and without an associated steam turbine, turbines using steam
injection, and recuperated turbines. While these turbines recover
energy from the turbine exhaust, that energy is not necessarily used to
generate electricity with a steam turbine, so these would not be
considered a combined cycle since they are not using two separate
thermodynamic cycles. However, since these types of combustion turbines
are recovering thermal energy and the exhaust gas temperatures are
lower, the costs of SCR are lower compared to simple cycle turbines.
The EPA notes that new CHP facilities often replace existing boilers
(or boilers that would have been built if CHP were not installed) and
offer significant environmental benefit compared to generating the
electricity and thermal

[[Page 1930]]

energy separately. Increasing the costs of new small, medium or low-
utilization CHP to the point that sources are disincentivized from
using CHP could have the perverse environmental outcome of increasing
overall emissions. The Agency has considered these broader impacts in
determining not to subcategorize between simple and combined cycle
turbines.
3. Evaluation of SCR Under BSER Factors
In the proposal of subpart KKKKa in December 2024, the EPA proposed
to find SCR justified under the BSER factors for combustion turbines of
all sizes, albeit not below a 40 percent capacity factor for turbines
equal to or smaller than a base load rating of 250 MMBtu/h of heat
input, and not below a 20 percent capacity factor for turbines larger
than that size.\98\ Since the proposal, the EPA has undertaken a review
of the BSER criteria in relation to SCR considering the extensive
technical comments submitted. The EPA's closer evaluation of cost
information concerning SCR as well as information concerning the
difficulty of application of SCR for certain subcategories, and other
downsides of SCR in terms of its emissions and energy impacts have led
the EPA to conclude that SCR is not justified under the BSER factors
for all but new large high-utilization combustion turbines.
---------------------------------------------------------------------------

\98\ See 89 FR 101322-23.
---------------------------------------------------------------------------

The EPA is determining for subpart KKKKa that SCR is part of the
BSER for new large high-utilization stationary combustion turbines
(i.e., that are utilized at 12-calendar-month capacity factors greater
than 45 percent). For these types of combustion turbines, SCR has been
nearly universally adopted in recent years, and the EPA has determined
it is cost-effective, achieving substantial reductions in
NO<INF>X</INF> emissions at costs that are comparable to those that the
EPA has found reasonable in other rules over the past several decades.
The EPA received no significant, adverse comments asserting that SCR is
not appropriately part of the BSER for this subcategory of new
combustion turbines.
A review of recent rules and determinations, multiple relevant cost
metrics, and the adoption of SCR technology across certain types and
sizes of power sector stationary combustion turbines in recent years,
all support our determination that this technology is cost-reasonable
for the subcategory of large high-utilization turbines, to which we
apply it as BSER in subpart KKKKa.
However, for all other combustion turbine subcategories, the EPA is
determining that SCR is not part of the BSER under present
circumstances. For these other subcategories, SCR is not cost
reasonable in relation to the amount of NO<INF>X</INF> emission
reductions that can be achieved, presents implementation and
operational challenges, has high energy impacts, and has other non-air
quality and environmental impacts that are not justified in relation to
the relatively small reduction in NO<INF>X</INF> emissions beyond the
standards that can be achieved with combustion controls.
The SCR process is based on the chemical reduction of
NO<INF>X</INF> via a reducing agent (reagent) and a solid catalyst. To
remove NO<INF>X</INF>, the reagent, commonly ammonia (NH<INF>3</INF>,
anhydrous and aqueous) or urea-derived ammonia, is injected into the
post-combustion flue gas of the combustion turbine. The reagent reacts
selectively with the flue gas NO<INF>X</INF> within a specific
temperature range and in the presence of the catalyst and oxygen to
reduce the NO<INF>X</INF> into molecular nitrogen (N<INF>2</INF>) and
water vapor (H<INF>2</INF>O). SCR employs a ceramic honeycomb or metal-
based surface with activated catalytic sites to increase the rate of
the reduction reaction. Over time, however, the catalyst activity
decreases, requiring replacement, washing/cleaning, rejuvenation, or
regeneration to extend the life of the catalyst. Catalyst designs and
formulations are generally proprietary. The primary components of the
SCR include the ammonia storage and delivery system, ammonia injection
grid, and the catalyst reactor. The technology can be applied as a
standalone NO<INF>X</INF> control or combined with other technologies,
including wet and dry combustion controls.
The EPA's proposed BSER of combustion controls with the addition of
post-combustion SCR for most new and reconstructed combustion turbines
generated a significant adverse response from the regulated community
and certain States during the public comment period. Other commenters
supported broad application of SCR as the BSER.
Many commenters stated that the proposed BSER is problematic and
impractical because it would require SCR on industrial combustion
turbines as well as those that operate at variable loads. According to
the commenters, this would introduce significant operating complexity,
increase annual operating costs, and result in unreasonable costs and
operating burden for these installations. Instead, these commenters
argued that the need for SCR should be determined on a site-specific
basis as part of NSR air permitting.
Additionally, commenters stated that SCR systems on simple cycle
turbines are complicated, expensive, and pose design challenges when
compared to combined cycle operations. For example:
<bullet> SCR systems require specific temperature ranges to operate
effectively, typically between 315 [deg]C and 400 [deg]C (600 [deg]F
and 750 [deg]F). For simple cycle turbines with higher exhaust
temperatures, additional cooling air may be needed to cool the exhaust
flow and avoid damage to the SCR catalyst structure and operation. The
costs associated with installation, operation, and maintenance of such
cooling air systems were not adequately addressed by the EPA in the
proposal.
<bullet> The installation of SCR systems requires sufficient space
for the catalyst and ammonia injection systems. Therefore, it can be
infeasible to install SCR on an existing installation that is modifying
or reconstructing; the cost of SCR on a simple cycle frame turbine can
be 30 percent to 50 percent of the cost of the turbine alone while
doubling the space requirements.
<bullet> SCR is difficult even for combined cycle units in the case
of existing turbines going through modifications or reconstructions. An
existing turbine may have been installed without SCR in mind, so
replacement of the HRSG could be required for a combined cycle unit,
which is more expensive (estimated at $50 million) than the SCR system
itself (estimated at $14 million).
<bullet> SCR systems are generally more effective in steady-state
operations. Combustion turbines that frequently start and stop or
operate under variable loads could face challenges in optimizing SCR
performance.
<bullet> Implementing and operating an SCR system involves not only
engineering, design, and installation costs but also additional
maintenance and operational costs, including the handling and storage
of ammonia or urea, catalyst replacement, and monitoring. For this
reason, SCR is not viable for remote sites that have no full-time
operator (e.g., unattended compressor stations).
<bullet> The EPA developed the proposed limits based on utility
data, not data adequately characterizing industrial installations. The
EPA should revise its cost analysis, which will demonstrate the
requirement to achieve emissions rates associated with SCR is
inappropriate for non-utility units.
Due in part to these concerns, several commenters stated that the
EPA underestimated the cost for SCR relative

[[Page 1931]]

to recent cost estimates received from manufacturers and technology
providers and submitted information to that effect. Furthermore, the
commenters contended that considering more accurate cost estimates, SCR
costs would not be ``relatively low,'' as the EPA stated at proposal,
and the technology would not be the BSER for medium and small
combustion turbines, including industrial turbines, low-utilization
turbines, and existing sources that modify or reconstruct.
These commenters stated that the EPA should re-analyze its proposed
BSER determination based on the design and operational differences
among different types of combustion turbines. In addition, commenters
provided several cost estimates that result in the incremental cost
effectiveness of installing SCR at values generally greater than
$20,000/ton NO<INF>X</INF> abated to achieve the proposed
NO<INF>X</INF> emissions limits, which exceed the levels that the EPA
has historically considered to be cost effective.
Taking into consideration the SCR cost information submitted by
commenters, the EPA has updated the BSER cost analysis from proposal.
This cost analysis supports a conclusion that the BSER for most
subcategories of new, modified, or reconstructed combustion turbines
subject to subpart KKKKa is the use of combustion controls alone (i.e.,
without SCR). The updated cost analysis nonetheless also supports our
conclusion that SCR is the BSER for large high-utilization turbines--
turbines with base load ratings greater than 850 MMBtu/h of heat input
that are utilized at capacity factors greater than 45 percent on a 12-
calendar-month basis. The new combustion turbines subject to a standard
of performance based on the BSER of combustion controls with SCR have,
over the past 5 years, almost exclusively used combined cycle
technology and have operated as base load units (i.e., at high
utilization rates). This means that the technical issues associated
with SCR raised by commenters are not a factor for new large high-
utilization sources in this subcategory.
a. Adequately Demonstrated
SCR is a mature and well-understood post-combustion add-on
NO<INF>X</INF> control that has been installed on combustion turbines
(both simple and combined cycle), utility boilers, industrial boilers,
process heaters, and reciprocating internal combustion engines. Many
natural gas-fired combustion turbines in the power sector currently
utilize SCR. While costs and operational challenges can vary quite
dramatically among different types of combustion turbines in ways that
are relevant to other BSER factors (as discussed in the sections that
follow), the EPA is not aware that SCR is completely unavailable to any
type of natural gas-fired combustion turbine. Therefore, in general the
EPA considers SCR to be a technically feasible and available technology
for control of NO<INF>X</INF> emissions from natural gas-fired
stationary combustion turbines. In that sense, SCR can be considered to
be ``adequately demonstrated''; however, after considering all of the
BSER factors as described in the sections that follow, the EPA finds
that SCR in a number of combustion turbine applications is not the BSER
for most subcategories of combustion turbines.
For non-natural gas-fired combustion turbines, commenters noted
that SCR has not been demonstrated on liquid fuel-fired turbines
(including distillate and biofuels) operating at high-utilization rates
and that biofuels can poison SCR catalysts. The EPA does not have long-
term performance information for various types of non-natural gas-fired
combustion turbines and due to potential complications, such as
catalyst deactivation due to impurities in the fuel, the EPA is not
determining that SCR is technically feasible for all non-natural gas-
fired turbines.
b. Extent of Reductions in NO<INF>X</INF> Emissions
The percent reduction in NO<INF>X</INF> emissions from SCR depends
on the level of control achieved through combustion controls. For a
combustion turbine using standard combustion controls (i.e., a
guaranteed full load emissions rate of 25 p.m. NO<INF>X</INF>)
reductions can approach 90 percent. The percent reduction across SCR is
lower if the combustion turbine is equipped with advanced combustion
controls. In conjunction with dry combustion controls on natural gas-
fired combustion turbines, SCR has been demonstrated to reduce long-
term NO<INF>X</INF> emission rates to approximately 3 ppm for multiple
types of turbines.\99\
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\99\ See section IV.B.5.a.i of this preamble for discussion of
the determination of the NO<INF>X</INF> standards of performance for
the subcategory of combustion turbines subject to a BSER that
includes SCR in subpart KKKKa.
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c. Costs
In response to significant adverse comments concerning the EPA's
proposed cost analysis for SCR, the EPA has revised its cost analysis.
The full, final cost analysis is available in the SCR Costing technical
support document available in the docket for this action.\100\ This
section summarizes key findings from this updated analysis.
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\100\ See Docket ID No. EPA-HQ-OAR-2024-0419.
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In 2006, when subpart KKKK was promulgated, SCR was evaluated as a
potential BSER and was determined to not meet the statutory criteria.
The estimated cost of achieving incremental NO<INF>X</INF> reductions
with the use of SCR was $9,000/ton (adjusted to 2024$) compared to the
lean premix and DLN systems that were available at that time. The EPA
determined that these costs were not reasonable in promulgating subpart
KKKK.
SCR is widely adopted as a NO<INF>X</INF> emissions control
strategy for certain stationary combustion turbines, particularly for
large turbines in the utility sector. However, during the technology
review for this action, the EPA found that information contained in the
records of permitting actions requiring SCR on combustion turbines is
not consistent or well-developed for purposes of informing a detailed
cost analysis for an NSPS. Generally, if a source was required (or
chose voluntarily) to install SCR and went forward with a new
combustion turbine project or installation, the cost of SCR presumably
did not undermine the economic viability of that project. Nonetheless,
just because individual projects have been economically viable with SCR
installation does not necessarily mean SCR installation on all
combustion turbines is cost-justified on a national basis, nor does it
necessarily reflect the best or most cost-effective means of achieving
overall reductions in NO<INF>X</INF> emissions. These considerations
will be discussed further in sections IV.B.3.c.ii and iii below.
Before proceeding with our evaluation of SCR under the BSER
factors, the Agency first notes that standalone SCR (i.e., without
combustion controls) is not the BSER. The EPA estimates that SCR
without combustion controls would be able to reduce NO<INF>X</INF>
emissions by 90 percent and achieve emission rates like turbines with
25 ppm and 15 ppm NO<INF>X</INF> guarantees based on combustion
controls alone. The exact achievable level would depend on the
uncontrolled NO<INF>X</INF> emissions rate of the relevant turbine. The
estimated cost effectiveness of SCR without combustion controls is
approximately $5,000/ton for low-utilization large turbines and $2,000/
ton for high-utilization large turbines. However, the combustion
controls analyzed in this technology review can achieve the same level
of emissions reduction at significantly lower cost. As discussed in
greater detail in section IV.B.4.c of this

[[Page 1932]]

preamble, combustion control costs are approximately $2,000/ton for
low-utilization large turbines and $100/ton for high-utilization large
turbines, without any of the secondary environmental and energy impacts
associated with SCR.\101\ Therefore, SCR alone is not the BSER for any
subcategory. The remainder of this section considers whether SCR should
be a part of the BSER, as a technology applied in addition to
combustion controls.
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\101\ See section IV.B.3.d of this preamble.
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For this final rule, as in the proposal, the EPA estimated the
capital and operating costs of SCR primarily using information from the
U.S. Department of Energy's (DOE) National Energy Technology Laboratory
(NETL) flexible generation report.\102\ The NETL report includes
detailed costing information on aeroderivative simple cycle turbines
using hot SCR and frame combined cycle turbines using conventional SCR.
For information not available in the NETL report, the EPA used
information from its cost control manual and applied Agency engineering
judgment.\103\ One commenter provided detailed comments on the SCR
costing analysis that the EPA incorporated, as appropriate, into the
cost estimations. Other commenters provided cost comparisons that
suggest the costs of SCR for simple cycle turbines have been
underestimated.\104\
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\102\ Oakes, M.; Konrade, J.; Bleckinger, M.; Turner, M.;
Hughes, S.; Hoffman, H.; Shultz, T.; and Lewis, E. (May 5, 2023).
Cost and Performance Baseline for Fossil Energy Plants, Volume 5:
Natural Gas Electricity Generating Units for Flexible Operation.
U.S. Department of Energy (DOE). Office of Scientific and Technical
Information (OSTI). Available at <a href="https://www.osti.gov/biblio/1973266">https://www.osti.gov/biblio/1973266</a>.
\103\ EPA Air Pollution Control Manual, Chapter 2 Selective
Catalytic Reduction. June 2019. Available at <a href="https://www.epa.gov/economic-and-cost-analysis-air-pollution-regulations/cost-reports-and-guidance-air-pollution">https://www.epa.gov/economic-and-cost-analysis-air-pollution-regulations/cost-reports-and-guidance-air-pollution</a>.
\104\ For detailed information on the costing analysis, see the
SCR Costing technical support document included in the docket for
this action.
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The EPA determines for purposes of subpart KKKKa that the costs of
SCR are reasonable on a nationwide basis for new large high-utilization
stationary combustion turbines (i.e., with base load ratings greater
than 850 MMBtu/h of heat input and utilized at 12-calendar-month
capacity factors greater than 45 percent) and therefore that SCR is
part of the BSER for this subcategory. However, for new large low-
utilization stationary combustion turbines (i.e., utilized at 12-
calendar-month capacity factors less than or equal to 45 percent), and
for all medium and small combustion turbines, the EPA determines that
the costs of SCR are not reasonable and therefore that SCR is not part
of the BSER for these subcategories, particularly in light of the other
factors discussed in the following sections.
i. Large High-Utilization Combustion Turbines
Based on information reported to EPA's Clean Air Markets Program
Data (CAMPD), most new construction of large high-utilization
combustion turbines is projected to be combined cycle facilities. As
described in section IV.B.5 of this preamble, the maximum 12-calendar-
month capacity factor of recently constructed large simple cycle
turbines is less than 45 percent. Large turbines are almost exclusively
used to generate electrical power, and at high levels of utilization,
the levelized cost of electricity (LCOE) of combined cycle turbines is
approximately the same as or lower than the LCOE for simple cycle
turbines. Therefore, the EPA's primary costing analysis for large high-
utilization turbines is based only on the impacts and costs of using
SCR on combined cycle turbines. The costs for large high capacity
factor simple cycle turbines are provided for completeness, and while
these costs are higher than for combined cycle turbines, simple cycle
turbines are generally not expected to operate at the high utilization
levels that would trigger the SCR-based BSER subcategory.
There are several indicators that broadly support the cost-
reasonableness of SCR as part of the BSER for new large combined cycle
turbines that plan to operate at high rates of utilization. The cost of
SCR as a percentage of the capital costs associated with constructing a
new combined cycle turbine is estimated to be approximately 1 percent.
The estimation of spent capital cost for SCR is approximately $3
million to $7 million (2024$) depending on the size of the combined
cycle turbine. The capital costs of SCR on a capacity basis range from
$10 per kilowatt (kW) to $20/kW, depending on the size of the combined
cycle turbine. These costs translate into a relatively low cost per
unit of energy output, and their effects on prices or costs to the
consumer are relatively small and manageable. Total SCR cost
(annualized capital costs, fixed costs, and operating costs) per unit
of production (i.e., electricity generation) is approximately $0.66/
MWh, which represents a 2 percent increase in the LCOE for a new 370 MW
combined cycle combustion turbine operating at a 12-calendar-month
capacity factor of 51 percent for 30 years. This effect on the cost of
electricity generation compares favorably with cost analyses that have
been conducted in the past.\105\
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\105\ See, e.g., 80 FR 64510, 64565, tbl. 9 (Oct. 23, 2015).
While this comparison is useful to illustrate in a relative sense
this cost metric as used in prior EPA analyses, reference to this
prior rulemaking notice should not be understood as endorsing any
legal of factual determinations made at that time.
---------------------------------------------------------------------------

Turning to the $/ton cost-effectiveness metric: In the final cost
analysis for this rule, the EPA finds that the cost effectiveness on a
$/ton of NO<INF>X</INF> controlled basis varies significantly based on
the percent reduction and the size of the combined cycle turbine. SCR
costs decrease with economies of scale and there is no single $/ton
figure that can be used to broadly represent SCR costs.
For combined cycle turbines with combustion controls guaranteed at
25 ppm NO<INF>X</INF>, the incremental costs to reduce NO<INF>X</INF>
concentrations to 3 ppm range from $3,200/ton to $4,600/ton.\106\ For
combined cycle turbines with combustion controls guaranteed at 15 ppm
NO<INF>X</INF>, the incremental costs to reduce NO<INF>X</INF>
concentrations to 3 ppm range from $4,400/ton to $6,800/ton.\107\ For
combined cycle turbines with combustion controls guaranteed at 9 ppm
NO<INF>X</INF>, the incremental costs to reduce NO<INF>X</INF>
concentrations to 3 ppm range from $7,300/ton to $12,000/ton.\108\ For
combined cycle turbines with combustion controls guaranteed at 5 ppm
NO<INF>X</INF>, the incremental costs to reduce the NO<INF>X</INF>
concentration to 3 ppm range from $13,000/ton to $22,000/ton.\109\
---------------------------------------------------------------------------

\106\ The EPA reviewed the previous 5 years of emissions data to
determine long-term emission rates of turbines. A long-term
emissions rate of 3 ppm NO<INF>X</INF> was used for a turbine
complying with a short-term emissions rate of 5 ppm NO<INF>X</INF>.
The long-term emissions rate of a turbine with a 25 ppm
NO<INF>X</INF> guarantee is 20 ppm NO<INF>X</INF>. Using a long-term
emissions rate of 2 ppm or 4 ppm as representative for a combustion
turbine with SCR would not change the BSER determinations.
\107\ The long-term emissions rate of a turbine with a 15 ppm
NO<INF>X</INF> guarantee is 14 ppm NO<INF>X</INF>.
\108\ The long-term emissions rate of a turbine with a 9 ppm
NO<INF>X</INF> guarantee is 7 ppm NO<INF>X</INF>. The SCR costs are
estimated by assuming the SCR uses two catalyst layers instead of
three.
\109\ The EPA assumed the long-term emissions rate of a turbine
with a 5 ppm NO<INF>X</INF> guarantee is 5 ppm NO<INF>X</INF>. The
SCR costs are estimated by assuming the SCR uses two catalyst layers
instead of three.
---------------------------------------------------------------------------

SCR costs decrease with economies of scale, and the low end of each
range is more representative of the typical size of new combined cycle
turbines. The EPA has concluded that the costs of SCR for large high-
utilization turbines with combustion controls and guaranteed
NO<INF>X</INF> emission rates of 9 ppm or greater are reasonable.
Therefore, for these types of turbines, the EPA finds SCR to be cost-
effective. While the Agency finds the incremental costs of SCR from

[[Page 1933]]

a 5-ppm baseline would not be considered cost-effective, the large
high-utilization turbines for which the EPA is including SCR in the
BSER do not achieve an emissions rate this low with combustion controls
alone. (Further, as discussed in more detail below, the EPA is setting
the standard of performance associated with SCR at 5 ppm, meaning that
to the extent large, high-utilization combustion turbines are, or come
to be, capable of achieving 5 ppm with combustion controls alone, SCR
would not need to be installed to meet the emissions standard.)
The costs of SCR for new large high-utilization combustion turbines
on a per-ton of NO<INF>X</INF> abated basis (i.e., $/ton) compare
favorably with prior EPA rulemakings that regulate NO<INF>X</INF>
emissions. Although determinations concerning cost reasonableness in
one statutory or programmatic context may not necessarily translate to
another, these regulatory precedents offer points of comparison with
respect to the same pollutant that can be informative in evaluating the
most cost-effective opportunities for abatement of a common pollutant
across multiple program arenas and therefore are relevant to the BSER
analysis. That is particularly true when the relevant statutory
provisions involve cost considerations similar to CAA section
111(a)(1).
In prior NSPS and CAA rules, the EPA generally found incremental
costs in the range of $7,400/ton of NO<INF>X</INF> abated to be cost
effective (escalated to 2024$).\110\ The EPA has also recognized that
an SCR with incremental costs of approximately $12,000/ton of
NO<INF>X</INF> abated may be justifiably rejected as not cost-
reasonable (escalated to 2024$).\111\
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\110\ See, e.g., 71 FR 9866, 9870 (Feb. 27, 2006) (finding an
incremental cost for SCR on boilers of approximately $5,000/ton to
be reasonable).
\111\ See, e.g., 77 FR 20894, 20929 (Apr. 6, 2012) (approving
State determination rejecting SCR where incremental cost was
estimated at $8,845).
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In the proposed rule, the EPA cited the Federal Implementation Plan
Addressing Regional Ozone Transport for the 2015 Ozone National Ambient
Air Quality Standard rulemaking (commonly known as the Good Neighbor
Plan), as a comparison point. In that rule, the EPA estimated SCR costs
for retrofit applications of $14,000/ton of NO<INF>X</INF> abated
(escalated to 2024$) as the appropriate representative cost threshold
for defining ``significant contribution'' under CAA section
110(a)(2)(D)(i)(I).\112\ However, upon further review and taking into
account comments with respect to this particular rule comparison, the
EPA no longer believes the Good Neighbor Plan is an appropriate
comparator. First, we did not grapple at proposal with the Supreme
Court's decision to stay enforcement of the Good Neighbor Plan as
likely arbitrary and capricious.\113\ Although the Court addressed the
Agency's failure to consider a different aspect of the problem, its
opinion raised significant doubts about the adequacy of the EPA's
analysis and engagement with comments received. Because the Good
Neighbor Plan was never implemented and its assumptions about cost
reasonableness were not tested in the real world, we do not believe the
cost analysis in that rule is entitled to significant weight as a
regulatory precedent. Second, the cost analysis in the Good Neighbor
Plan assessed retrofit costs for coal units for the purpose of
promoting attainment of the NAAQS and therefore does not directly
translate to the situation here. As noted elsewhere in this preamble,
more stringent standards may be appropriate under the specific set of
facts presented in an individual permitting context than would be
appropriate for a NSPS. Similarly, more stringent standards, and
greater associated costs, may be appropriate when necessary to meet
statutory requirements for nonattainment areas. Finally, the EPA is in
the process of reconsidering the Good Neighbor Plan, and as such, no
longer believes this cost-per-ton figure should serve as an appropriate
comparison point. Although that process is not yet complete, its
initiation reflects the Agency's significant concerns with the analysis
and justifications underlying the Good Neighbor Plan.
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\112\ See 88 FR 36654 and 36746 (June 5, 2023).
\113\ Ohio v. EPA, 603 U.S. 279, 292-94 (2024).
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Turning to simple cycle turbines: The costs of SCR for simple cycle
combustion turbines are higher, especially for frame type turbines. SCR
catalysts require specific operating temperatures to control
NO<INF>X</INF> effectively, and the exhaust temperatures of simple
cycle turbines are generally too high to be used directly in the SCR.
The exhaust gases need to be cooled, generally through injecting
tempering air to cool the exhaust to avoid damaging the SCR catalyst.
Frame turbines require higher amounts of air tempering than
aeroderivative turbines because the exhaust temperature of the most
efficient frame-type combustion turbine is approximately 200[deg]C
higher than the most efficient aeroderivative combustion turbines. For
utility units at high utilization rates, it is generally more cost
effective to cool the exhaust prior to the SCR using the HRSG instead
of tempering air. Since a HRSG does not increase the volume of exhaust
gas entering the SCR, the SCR can be smaller and less costly, and the
recovered thermal energy can be used to generate additional useful
output. The EPA notes that there are technologies other than air
tempering and a traditional HRSG that can be used to cool the exhaust
gas prior to the SCR reactor. For example, a new combined cycle turbine
could be designed with a relatively simple, lower cost HRSG and the
recovered thermal energy (i.e., steam) could be used in a relatively
simple, lower cost steam turbine or injected into the combustion
turbine itself (i.e., a steam injection combustion turbine). These
technologies have efficiencies and costs that range between more
standard simple and combined cycle turbine configurations.
To estimate the costs of SCR on large simple cycle turbines, the
EPA scaled costs based on the NETL 50 MW simple cycle turbine using dry
combustion controls. These costs incorporate tempering air and are more
representative of the SCR costs for large simple cycle turbines than
the 100 MW simple cycle model plant the EPA used at proposal. The 100
MW aeroderivative model plant is a simple cycle turbine that uses
compressor intercooling and wet combustion controls--both of which
lower the exhaust temperature and reduce the need for tempering air. In
response to specific concerns raised by commenters, the EPA
incorporated several of the suggested adjustments to the SCR costing
equations.\114\ However, for simple cycle turbines, even with these
adjustments the EPA's estimated costs are significantly less than the
example costs provided by other commenters. Because the EPA finds
commenters' information credible and representative, this suggests that
actual costs could be as high as twice the EPA's derived costs.
Consequently, the EPA's cost analysis for simple cycle turbines likely
represents best-case scenario costs.
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\114\ The EPA continues to primarily use SCR costs derived from
the NETL Flexible Generation Report. Differences in the final rule
include using SCR fixed costs dervied from the EPA's pollution
Control Manual, accounting for capacity payments, using the base
cost of the combustion turbine without SCR when determining the
value of the lost electric sales, and using the six-tenths rule when
estimating the capital costs of SCR for different combustion turbine
sizes.
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The cost of SCR as a percentage of the capital costs associated
with constructing a new simple cycle turbine is estimated to be
approximately 5 percent. The estimation of spent capital cost of the
SCR reactor is approximately $8 million to $18 million (2024$),
depending on the size of the turbine.

[[Page 1934]]

The capital costs on a capacity basis range from $45/kW to $80/kW,
depending on the size of the simp

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