On January 9, 2026, and in accordance with the terms of a consent decree, EPA Administrator Lee Zeldin signed a prepublication version of a final rule to revise the New Source Performance Standard (NSPS) for new, modified, or reconstructed combustion turbines (CTs) under Title 40 of the Code of Federal Regulations (40 CFR) Part 60, Subpart KKKKa. The final rule impacting CTs constructed, modified, or reconstructed after December 13, 2024, is significantly less stringent than the version proposed in 2024, and includes an important new source category for temporary CTs. The most significant change for NSPS KKKKa is the requirement for large new CTs (>850 MMBtu/hr base load rating) to operate with SCR for NOX control if the 12-month capacity factor exceeds 45 percent.
Timing Considerations
This final rule comes at a time when the CT market is experiencing substantial growth. As CT projects are being designed and implemented, it is important for stakeholders to understand the changing regulations. Recently permitted projects and particularly those that began construction after December 13, 2024, may need to re-evaluate performance limits, Best Achievable Control Technology (BACT) determinations, Continuous Emissions Monitoring Systems (CEMS) requirements including emissions data processing and handling, as well as longer-term permitting strategies. Be sure to attend Trinity’s upcoming complimentary webinar on February 3, 2026, for more in-depth analysis of NSPS Subpart KKKKa and amendments to NSPS Subparts GG and KKKK. Some of the most impactful provisions of the final rule are discussed below.
Revised BSER
In 2006, when EPA promulgated NSPS Subpart KKKK, the use of wet and dry combustor controls were finalized as the Best System of Emission Reduction (BSER). NSPS Subpart KKKK includes BSER determinations and corresponding NOX emissions standards for 14 subcategories of CTs, ranging from 15 ppm for CTs firing natural gas with a design heat input rating of greater than 850 MMBtu/hr to 150 ppm for CTs firing non-natural gas fuels with a design heat input rating of less than or equal to 50 MMBtu/hr.
The final NSPS Subpart KKKKa retains combustion controls as BSER for all but one subcategory of CT. For new large CTs with high rates of utilization (i.e., 12-calendar-month capacity factors greater than 45 percent), the BSER is combustion controls with SCR and a performance standard of 5 ppm corrected to 15 percent O2 when firing natural gas. In some cases, a CT may be able to meet the 5 ppm standard without a SCR (e.g., using advanced combustion controls), and the rule does not mandate the use of a particular control technology, only compliance with the applicable performance standard. In a departure from the proposed rule, SCR was not determined as BSER for any other subcategory of CT. EPA also determined the final NOX emission standards for modified and reconstructed CTs should be less stringent than for new sources as retrofitting units to include a SCR can dramatically increase costs.
New Subcategories
In the final rule, EPA abandoned the proposed 250 MMBtu/hr threshold for small CTs and established 18 subcategories that are generally more consistent with NSPS Subpart KKKK. Within each size-based subcategory, there are individual NOx standards for both natural gas and non-natural gas fuels. There are also separate subcategories for modified and reconstructed CTs, heat recovery units operating independent of the CT, CTs operating at partial operating loads or at low ambient temperatures (or north of the Arctic Circle), offshore and temporary CTs, as well as combined cycle CTs that bypass a heat recovery unit. Table 1 below presents the final rule subcategories and corresponding NOX limits.
CT Size Subcategories
Large: >850 MMBtu/hr
Medium: >50 MMBtu/hr (or >20 MMBtu/hr for modified or reconstructed CTs) and ≤850 MMBtu/hr
Small: ≤50 MMBtu/hr (or ≤20 MMBtu/hr for modified or reconstructed CTs)
New large or medium CTs with a 12-month capacity factor greater than 45 percent are now subcategorized as high-utilization sources with more stringent NOX emissions standards. Two new subcategories for large low-utilization CTs are now based on the design efficiency of the turbine, which accounts for different levels of emissions performance that can be achieved without SCR. For new large low-utilization CTs with design efficiencies greater than or equal to 38 percent on a HHV basis, the EPA is finalizing BSER as the use of combustion controls alone. For new large low-utilization CTs with design efficiencies less than 38 percent, BSER is the use of combustion controls with NOX emissions rate guarantees based on the use of technologies such as lean premix combustion and dry low-NOX (DLN) or ultra DLN burners.
Note that EPA added a provision to the final rule clarifying that operation during system emergencies would not be included when determining the utilization-based subcategorization.
Temporary Combustion Turbines
EPA has finalized a new subcategory for small and medium CTs used in temporary applications at a single location for up to 24 months. BSER for temporary CTs is combustion controls with an associated standard of 25 ppm NOX when combusting natural gas and 74 ppm NOX when burning non-natural gas fuels, along with a simplified compliance demonstration based on manufacturer certification. It is important to note that these temporary CTs do not have to be portable.
Should a temporary CT 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.
In the preamble to the final rule, EPA acknowledged some stakeholders’ interest in treating CTs used in stationary temporary applications as nonroad engines, subject to regulation under title II of the Clean Air Act. While an exemption was included in the final NSPS Subpart KKKKa for portable CTs that meet the definition of “nonroad engine,” it will not be available until EPA in the future adopts nonroad emissions standards and certification requirements specific to portable CTs.
NSPS Subpart KKKK was also amended to include identical temporary CT standards and an exemption for nonroad engines.
Title V Operating Permit Exemption
EPA is finalizing a Title V Operating Permit exemption for combustion turbines that are not major sources or located at major sources for Title V within the small and medium subcategories and the large low-utilization subcategory under NSPS Subpart KKKKa and for all non-major CTs under NSPS Subparts GG and KKKK. This exemption is not available to the large high-utilization CT subcategory in NSPS Subpart KKKKa. In the preamble to the final rule, EPA noted that state permitting authorities may find it efficient to pursue further streamlining of minor-source permitting for such sources such as a general permit or permit-by-rule approach, or alternatively, subject all stationary CTs to Title V Operating Permit programs.
Partial Load Emission Standard
Similar to NSPS Subpart KKKK and the proposed Subpart KKKKa, EPA established a separate subcategory, BSER, and NOX standard for turbines operating at partial loads. In Subpart KKKK, a partial-load hour is defined as any hour when the heat input rate is less than 75 percent of the base load rating of the CT. If the heat input rate drops below 75 percent at any point during the hour, the entire hour is considered a partial-load hour, and the partial-load standard is applicable during that hour. However, for the final NSPS Subpart KKKKa, EPA lowered the partial load threshold to 70 percent of the base load rating.
Alternate Mass-Based NOX Emission Standard
In the proposed rule, EPA solicited public comments on the benefits and drawbacks of mass-based NOX emissions standards as an alternative to subcategorizing CT based on capacity factor or partial load operation. New alternate mass-based standards were included in the final NSPS Subpart KKKKa and may be an effective way to simplify compliance and recognize the environmental benefit of efficient generation. Affected sources under NSPS Subpart KKKKa can now choose between the heat input-based or output-based standards listed in Table 1 or the mass-based standards below:
- Natural gas:
- 0.83 lb/MW-rated output on a 4-operating hour rolling average, and
- 0.48 ton/MW-rated output on a 12-calendar month rolling average.
- Non-natural gas:
- 1.8 lb/MW-rated output on a 4-operating hour average, and
- 0.81 ton/MW-rated output on a 12-calendar month average.
These mass-based standards apply irrespective of the actual utilization (i.e., capacity factor) of the CT, and there is no consideration of separate standards for full- and partial-load hours. These standards are likely most relevant to CTs where the low-utilization and high-utilization emission standards vary significantly. According to EPA, more efficient CTs achieving the same input-based emissions rate (e.g., lb NOX would be able to operate at higher capacity factors while still maintaining emissions below the annual standard.)
Definition of Affected Source
In 2024, EPA proposed a significant new requirement for how the definition of the affected source should be considered in a reconstruction analysis for combined cycle units. Specifically, EPA proposed that a reconstruction analysis for combined cycle units only include the “combustion turbine engine” portion of the “stationary combustion turbine” affected source definition and not the heat recovery steam generator. EPA is not finalizing the proposed, category-specific definition of “reconstruction” for CTs under NSPS Subpart KKKKa.
Compliance Demonstration
Below are some highlights of ongoing compliance demonstration requirements in the final rule:
- For CTs using SCR or other post-combustion controls, the final Subpart KKKKa requires that continuous compliance with the applicable NOX standard must be demonstrated with a Continuous Emission Monitoring System (CEMS).
- CTs that have been out of operation for 60 days or longer at the time of a required performance test are not required to conduct the performance test until 45 days after the facility is brought back into operation, or until after 10 operating days, whichever is longer.
- CTs that have operated 50 hours or less since the previous performance test was required to be conducted can request a compliance extension until the unit has operated more than 50 hours.
- For both NSPS Subparts KKKKa and KKKK, a streamlined emissions test procedure is available for groups of no more than five similar CTs at a single source under common ownership. Such units may not be equipped with SCR.
- CTs subject to either NSPS Subparts GG, KKKK, or KKKKa must now submit electronic copies of initial and periodic performance test reports (including relative accuracy test audits (RATAs)), and compliance reports through the EPA’s Central Data Exchange (CDX) using the Compliance and Emissions Data Reporting Interface (CEDRI).
