U.S. EPA Finalizes Combustion Turbine NSPS Revisions

Less than one week after being signed by U.S. Environmental Protection Agency (U.S. EPA) Administrator Lee Zeldin, the Combustion Turbine New Source Performance Standards (NSPS) revisions were published in the Federal Register on January 15, 2026 and thus, are now effective.

This final rulemaking establishes emissions standards and compliance schedules under 40 CFR Part 60, Subpart KKKKa for stationary combustion turbines that are constructed, modified, or reconstructed after December 13, 2024. The final rule publication also includes revisions to 40 CFR Part 60, Subparts GG and KKKK. Specifically, this rule:

• Establishes nitrogen oxides (NO_X) emissions standards for various combustion turbine size, utilization, and design efficiency subcategories;
• Determines that selective catalytic reduction (SCR) is the best system of emissions reduction (BSER) for new, large combustion turbines with a high utilization rate (> 45 percent);
• Concludes that continued use of combustion controls is the BSER for limiting NO_X emissions from all other combustion turbine subcategories;
• Retains the sulfur dioxide (SO₂) standards from Subpart KKKK; and
• Does not establish standards for any new criteria pollutants, including particulate matter (PM/PM₁₀ /PM₅) or carbon monoxide (CO).

As rapid expansion in the power generation and data center sectors continues to fuel growth in the combustion turbine market, the much-anticipated final rule is, as expected, less stringent than the proposed rule issued at the end of the previous Administration. The rulemaking notice also states that the action is considered a deregulatory action under Executive Order 14192: Unleashing Prosperity Through Deregulation and is generally aligned with the current Administration’s “Unleashing American Energy” initiative.

NO_X Emissions Standards

As originally proposed, U.S. EPA sought to not only change the size-based subcategories under Subpart KKKK but also further subcategorize sources by capacity utilization (i.e., low, intermediate, or high load), consistent with 40 CFR Part 60, Subpart TTTTa (which U.S. EPA has since proposed to repeal on June 1, 2025).

After considering comments that criticized both the size-based subcategory revisions and the addition of utilization-based subcategories, U.S EPA decided to:

• Retain the size-based subcategories from Subpart KKKK;
• Shift to just two utilization-based subcategories for large and medium turbines aimed at distinguishing between simple cycle and combined cycle turbines: high-utilization (> 45%) and low-utilization (≤ 45%); and
• Introduce new subcategories based on the manufacturer’s design efficiency for large, low-utilization combustion turbines.

The following table summarizes the Subpart KKKKa NO_X emissions standards for the different subcategories established in the final rule.

(a)Utilization rate under Subpart KKKKa is determined based on 12-calendar-month annual capacity factor, defined as the ratio between the actual heat input to a stationary combustion turbine during a calendar year and the potential heat input to the stationary combustion turbine had it been operated for 8,760 hours during a calendar year at the base load rating.

(b) 0.30 lb/MW-net is listed in the final regulatory text in the docket, but is an apparent typo in the conversion of 0.29 kg/MWh-net to pounds.

Large Combustion Turbines

Large combustion turbines are defined as having a base load rated heat input greater than 850 million British thermal units per hour (MMBtu/h) on a higher heating value (HHV) basis. In a change from the proposed rule, U.S. EPA decided that there was no difference in combustion control reasonableness between “low” and “intermediate” load turbines and decided to change the three proposed utilization subcategories into just two. Generally, large, natural gas-fired, combustion turbines are now subcategorized as high-utilization or low-utilization based on whether the 12-calendar-month capacity factor exceeds 45 percent. Although not specifically defined as simple cycle versus combined cycle, the 45 percent cutoff was roughly based on the 99 percent confidence, maximum 12-calendar-month capacity factor for recently constructed simple cycle turbines. U.S. EPA also included provisions in Subpart KKKKa to exclude operation during system emergencies (i.e., Reliability Coordinator has declared an Energy Emergency Alert level 1, 2, or 3) from the utilization subcategorization.

U.S. EPA  has reversed its proposal that SCR is cost-effective for combustion turbines larger than 250 MMBtu/h and now has determined that combustion controls is BSER for most turbine subcategories. SCR is now only considered BSER for large, high-utilization combustion turbines. U.S. EPA also relaxed the proposed standard of 3 parts per million (ppm) for these turbines to 5 ppm, noting that the 5 ppm emissions standard will allow facilities to use higher efficiency (and therefore higher NO_X-emitting) turbines that will result in less environmental impact due to less fuel use and less ammonia slip. Although some combustion turbines  are capable of meeting 2 to 3 ppm [new sources may still be subject to a more stringent Best Achievable Control Technology (BACT) limit], U.S. EPA determined that that the 5 ppm standard  is achievable for all turbines in this subcategory.

In another change from the proposed rule, U.S. EPA has further subcategorized large, low-utilization turbines as high-efficiency and low-efficiency, based on a manufacturer-provided design efficiency cutoff of 38 percent, on a high heating value (HHV) basis. This subcategorization accounts for the tradeoff that exists for turbine manufacturers: design choices that improve efficiency also create conditions that favor NO_X formation (e.g., higher temperatures and pressures).

Medium Combustion Turbines

A medium combustion turbine is defined under Subpart KKKKa as a stationary combustion turbine with a base load rating greater than 50 MMBtu/h and less than or equal to 850 MMBtu/h of heat input. However, the rule also establishes a lower cutoff of 20 MMBtu/hr for modified or reconstruction sources because combustion controls that achieve the 25 ppm NO_X standard may not be available for all existing small combustion turbines that are modified or reconstructed. U.S. EPA did not further subcategorize based on design efficiency as there generally aren’t high efficiency (> 38 percent) turbines of this size.

Small Combustion Turbines

U.S. EPA removed any utilization-based subcategories for small combustion turbines and set the natural gas-fired standard for new sources at 25 ppm, based on the common manufacturer-guaranteed value for turbines in this size category using dry combustion controls.

Partial Load Standards and Alternative Mass-Based Standards

As  provided in Table 1, Subpart KKKKa establishes less stringent NO_X standards for turbines that are operating at 70 percent or less of their base load rating. The applicable NO_X standard is determined on an operating-hour basis, and if the turbine is operated at partial load at any point during the hour, then the partial load standard applies for the entire hour.

Short-term (4-hour) and long-term (12-month) mass-based NO_X standards (mass NO_X per MW-rated output) are added as optional alternatives to the input- and output-based NO_X standards. The mass-based standards are favorable in scenarios where high-efficiency, simple cycle turbines with SCR want to operate in excess of the 45 percent utilization rate and maintain compliance with the applicable standards.

Proposed Reconstruction Definition Not Finalized

In the December 2024 version, U.S. EPA proposed that only the  combustion turbine portion of the affected source would be considered when evaluating whether the affected source is reconstructed.  The comprehensive definition of “stationary combustion turbine” under 40 CFR §60.4420 (and now 40 CFR §60.4420a) includes the heat recovery steam generator (HRSG) and associated duct burners at combined cycle and combined heat and power CHP facilities. Those definitions are used when determining if an affected facility is reconstructed. Under U.S. EPA’s originally proposed approach, if the fixed capital cost of just the new combustion turbine engine components for a combined cycle turbine exceeded 50% of the cost of just a new combustion turbine engine, then the entire affected source (i.e., the entire combined cycle system) would have met the proposed reconstruction criteria and would have been subject to Subpart KKKKa. This was also, notably, in conjunction with U.S. EPA proposing to set the NO_X emissions standards for reconstructed sources equal to the standards for new sources, believing that reconstructed turbines could likely incorporate the same NO_X reduction technologies at similar costs compared to a greenfield facility.

ALL4 assisted several clients and industry associations in developing comments on the originally proposed more-restrictive reconstruction definition and emissions standards, which primarily took issue with the proposal’s intent to classify refurbishments to a portion of the affected source as substantially equivalent to constructing an entirely new  affected source. With renewed emphasis on the underlying statutory language behind Federal agencies’ interpretations stemming from  Loper Bright Enterprises v. Raimondo, 144 S. Ct. 2244 (2024), some industry commenters also pointed out that Section 111 of the Clean Air Act (CAA) only authorizes U.S. EPA to establish NSPS for newly constructed and modified sources (in contrast with CAA Section 112 which  includes reconstruction in the definition of a new source]). Commenters also argued that the NO_X standards for reconstructed units should be equal to the standards for modified sources because the same retrofit technology limitations and costs for modifications apply to reconstruction.

Ultimately, U.S. EPA  excluded the proposed reconstruction definition from the final Subpart KKKKa rulemaking. U.S. EPA also agreed with commenters that “the costs of retrofitting combustion turbines with SCR is significantly higher than for new sources” and aligned the standards for modified and reconstructed sources as  provided in Table 1.

New Subcategory for Temporary Turbines

After soliciting comments on creating either a subcategory or an exemption for temporary turbines, U.S. EPA  has finalized a new subcategory in Subpart KKKKa for small and medium stationary temporary combustion turbines that meet the following criteria:

• The turbine must have a base load rating less than or equal to 850 MMBtu/hr (i.e., a small or medium turbine).
• The turbine may only be located at the same stationary source (or group of stationary sources located within a contiguous area and under common control) for a total period of 24 consecutive months. The clock starts after the turbine commences operation at a location and continues regardless of whether the turbine operates for the entire 24-month period.
• Any temporary combustion turbine that replaces a temporary combustion turbine at a location and performs the same or similar function will be included in calculating the consecutive time period.
• The relocation of a stationary temporary combustion turbine within a single stationary source (or group of stationary sources located within a contiguous area and under common control) while performing the same or similar function (e., serving the same electric, mechanical, or thermal load) does not restart the 24-calendar month residence time period.

Stationary temporary combustion turbines must comply with the following requirements:

• Each temporary turbine must have a manufacturer’s emissions guarantee to meet a NO_X emissions standard of 25 ppm at 15% O₂ or 0.092 lb/MMBtu.
• Each temporary turbine must have been performance tested at least once in the prior 5 years meeting this NO_X emissions standard.
• The otherwise applicable SO₂ standard and fuel sulfur content recordkeeping will apply.

U.S. EPA  initially considered a 12-month period for the temporary turbine subcategory but finalized with the 24-month period, agreeing with commenters recommendations to account for situations requiring temporary power generation lasting longer than 12-months. U.S. EPA also noted that a 24-month period is consistent with Prevention of Significant Deterioration (PSD) policy that recognizes that emissions occuring for less than 24-months are temporary and excluded from some aspects of PSD review since their ambient air impacts are short-lived.

In the preamble to the final rule, U.S. EPA explained that the creation of the temporary turbine subcategory facilitates reducing the permitting burden for these types of turbines, for both minor NSR permitting – through States potentially establishing a general permit or a permit by rule (PBR) for temporary turbines – as well as major NSR permitting, by assisting States in clearly identifying temporary emissions that may be excluded from certain air quality modeling demonstrations  under 40 CFR 52.21(i)(3)(ii).

Based on ALL4’s review of the regulatory language posted to the docket, U.S EPA’s introduction of the temporary generator subcategory, although well-intended, may have created some confusion due to inconsistencies between the preamble and the regulatory language. In the preamble, U.S. EPA states that a 74 ppm NO_X emissions standard would apply to temporary turbines burning non-natural gas fuels, but the regulatory text for Subpart KKKKa provided in the docket does not appear to establish a non-natural gas NO_X standard for temporary turbines. Similarly, U.S. EPA added the temporary combustion turbine requirements under Subpart KKKK but did not explicitly add the temporary combustion turbine NO_X emissions standard to Table 1 to Subpart KKKK. ALL4 will be monitoring the docket closely as more files are added for any administrative updates addressing these inconsistencies, as well as the apparent typo in Table 1 to Subpart KKKKa footnoted in the table above.

Finally, U.S. EPA included language in Subparts GG, KKKK, and KKKKa that excludes portable combustion turbines from the definition of the affected source if portable combustion turbines were to be regulated as a “nonroad engine” under Title II of the CAA.

Conclusion

As U.S. is expecting a drastic step-change in electricity demand over the next five years and natural gas-fired generation is being developed as quickly as the supply chain allows, Subpart KKKKa will be impactful to multiple industries evaluating new stationary combustion turbines, including utilities and data centers. ALL4 is continuing to help our clients evaluate compliance strategies for Subpart KKKKa and the entire lifecycle of greenfield generation projects, including siting, permitting, commissioning, and ongoing compliance. Please reach out to me or ALL4’s Power Sector Lead Rich Hamel to learn more.