EPA Tightens NOx Standards for New Stationary Combustion Turbines

On Jan. 9, the U.S. Environmental Protection Agency (EPA) announced a final rule to amend the New Source Performance Standards (NSPS) for Stationary Combustion Turbines and Stationary Gas Turbines, also known as the Rule. The Rule became effective on Jan. 15, and it comes nearly 20 years after the standards were last updated in 2006. According to EPA, the Rule is intended to reflect current control technology performance, reduce nitrogen oxide (NOx) pollution, and streamline compliance and permitting for certain units.

Specifically, the Rule includes the following key components:

Tightens and restructures NOx limits for gas turbines and combustion turbines constructed, modified, or reconstructed after Dec. 13, 2024;
Creates a new “temporary turbine” subcategory;
Leaves sulfur dioxide (SO₂) limits largely unchanged; and
Includes a new NSPS subpart—40 CFR part 60, subpart KKKKa—and amends existing turbine NSPS subparts GG and KKKK to address technical and editorial issues.

Notably, in its Economic Impact Analysis, EPA also changed how it evaluated potential public health benefits from the Rule, which was widely reported and discussed in a number of recent media stories.

Expected Emissions, Cost, and Industry Impacts

EPA projects that the NSPS will reduce annual NOx emissions by up to roughly a few hundred tons by 2032 compared with a no‑action baseline, although this is significantly less than reductions forecast under the more stringent 2024 proposal that required selective catalytic reduction (SCR) controls on more units. EPA also emphasizes that the Rule focuses SCR requirements on a narrow slice of large, high‑utilization turbines where control costs per ton are lowest, while allowing combustion‑controls‑only compliance for the majority of new, modified, or reconstructed turbines.

Of note, EPA did not quantify or monetize public health benefits for the Rule. Historically, EPA routinely estimated and monetized public health benefits from reduced air pollution under Clean Air Act rules, often finding that these benefits far exceed compliance costs. EPA previously analyzed the benefits of reducing pollution, including but not limited to improved health outcomes, avoided healthcare costs, fewer missed workdays, and the compliance costs for industry.

While commenters and public‑interest groups argued that omitting monetized health benefits in the Rule marks a departure from that long‑standing practice, and downplays the health value of NOx reductions that contribute to lower ozone and fine particulate levels, EPA stated that the health benefit estimates from changes in NOx (and resulting ozone and particulate matter) are too uncertain to rely on in a formal regulatory context, and that assigning dollar values to lives saved or health improvements from reduced air pollution can create a “false sense of precision.” See EPA’s Economic Impact Analysis for the New Source Performance Standards Review for Stationary Combustion Turbines: Final Rule, PDF p. 41. Ultimately, EPA limited its formal economic analysis to compliance costs and cost savings relative to a no‑action baseline, while qualitatively acknowledging that NOx reductions can improve air quality and health. EPA estimates that the Rule will save power plant owners approximately $87 million over eight years.

Subcategorization and BSER Determinations

The Rule establishes multiple subcategories based on turbine size (MMBtu/hr heat input), utilization (e.g., 12‑month capacity factor), fuel type, and design efficiency, with 38% efficiency on a higher heating value (HHV) basis used to distinguish “higher” from “lower” efficiency units. For the majority of turbine subcategories, EPA determined the best system of emission reduction (BSER) for NOx remains advanced combustion controls (e.g., dry low‑NOx burners), while for new large, high‑utilization units (capacity factor above roughly 45%) BSER is combustion controls plus post‑combustion SCR. The Rule maintains existing SO₂ limits while making limited clarifications and flexibilities in how SO₂ compliance is demonstrated, based on the determination that current SO₂ standards already reflect achievable control.

Amended Nox Standards and Performance Levels

The Rule revises NOx performance standards for selected subcategories to align with what EPA finds achievable using BSER, tightening the limit for some large high‑duty turbines and setting different limits for high‑ and low‑efficiency units. For new large turbines (greater than approximately 850 MMBtu/h) with high utilization and higher efficiency, the Rule includes a stringent NOx limit (on the order of single‑digit ppm when firing natural gas) based on combustion controls plus SCR. For lower‑efficiency and medium‑sized units relying on combustion controls alone, the Rule imposes less stringent Nox limits. The Rule, generally, specifies hourly NOx standards based on the assumption that natural gas firing and operation are above 70% of base load. There are separate provisions for non‑natural gas fuels and part‑load operation, and the Rule provides a table of size‑ and duty‑based ppm limits.

New Temporary Turbine Subcategory

The Rule creates a distinct subcategory for “stationary temporary combustion turbines,” covering small and medium turbines up to 850 MMBtu/h that operate in short‑term, temporary applications (e.g., to provide power during outages or maintenance at permanent units). Temporary turbines under this subcategory are subject to BSER of combustion controls and a NOx emission standard of 25 ppm when firing natural gas, reflecting achievable performance while recognizing their intermittent, short‑term role. The Rule restricts how long a unit can be treated as temporary turbine (typically up to 24 months on‑site) and prohibits serial replacement of one temporary turbine with another solely to extend “temporary” status beyond the intended period. However, temporary units can substitute for offline permanent generators.

Monitoring, Recordkeeping, and Permitting Flexibilities

The Rule substantially reduces monitoring, recordkeeping, and reporting requirements for temporary turbines, in recognition that full NSPS‑level compliance programs can exceed the duration of temporary deployments and discourage use of such units for short‑term reliability needs. For temporary units, owners and operators must maintain on‑site documentation that each turbine is manufacturer‑certified to meet a 25‑ppm NOx rate and underwent a performance test at least once in the prior five years (for units older than five years since initial sale), rather than conducting frequent stack tests. The Rule also exempts certain combustion turbine sources from title V operating permit requirements and clarifies that some portable turbines regulated as nonroad engines under Title II are outside the scope of this NSPS, thereby reducing permitting burdens for qualifying units.

Looking Ahead

Project developers planning new large, baseload‑type gas turbines should consider whether their turbines must treat combustion controls plus SCR as the default design basis where its units exceed EPA’s high‑utilization threshold and efficiency criteria and should consider whether the turbine must meet the tighter single‑digit‑ppm‑range NOx limits in subpart KKKKa.

Owners of smaller, peaking, or intermediate‑duty turbines will, generally, meet revised limits through advanced combustion controls, but should confirm subcategory assignments (size, capacity factor, efficiency, and fuel use) early in project design to ensure the correct NSPS standard is applied.

Utilities and developers relying on short‑term power solutions should consider the use of the new temporary turbine subcategory to deploy certified low‑NOx temporary units with streamlined monitoring and permitting, while carefully managing deployment duration and documentation to maintain compliance with the temporary status limitations.

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EPA Tightens NOx Standards for New Stationary Combustion Turbines

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