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By: Thomas Newhouse, Environmental Associate & Lucian Hill, Director, Environmental Services
On January 15, 2026, the U.S. Environmental Protection Agency (EPA) finalized a New Source Performance Standard (NSPS) for Stationary Combustion Turbines and Stationary Gas Turbines, under 40 CFR Part 60, Subpart KKKKa (KKKKa). The rule became effective upon publication in the Federal Register and applies to combustion turbines with a base load heat input capacity of 10 MMBtu/hr or greater that are newly constructed, modified, or reconstructed after December 13, 2024, and reflects EPA’s updated assessment of emission control technologies and operating practices within the power and industrial turbine sectors. Units subject to KKKKa are no longer regulated under NSPS Subpart GG or Subpart KKKK.
Subpart KKKKa restructures the former NSPS Subpart KKKK by setting NOx emission standards based on turbine size, utilization, and design efficiency. Turbines are categorized by size as Large (>850 MMBtu/hr), Medium (greater than 50 MMBtu/hr for new turbines and greater than 20 MMBtu/hr for modified or reconstructed turbines, up to 850 MMBtu/hr), and Small. Utilization is classified as high (>45 percent) or low (≤45 percent) based on a 12‑month capacity factor, generally distinguishing combined‑cycle from simple‑cycle operation. For certain large natural gas turbines, EPA further differentiates standards using manufacturer‑rated design efficiency thresholds of high (≥38 percent) and low (<38 percent) on a higher heating value (HHV) basis.
While Subpart KKKK generally relied on combustion controls to meet NOx standards, KKKKa establishes lower NOx limits for many turbines and, for certain large, high‑utilization units, identifies selective catalytic reduction (SCR) combined with combustion controls as the best system of emission reduction (BSER). KKKKa also applies emission standards at all times, including startup, shutdown, and malfunction conditions, providing greater regulatory clarity and consistency.
Sulfur dioxide (SO₂) requirements remain largely unchanged. EPA continues to rely on a fuel‑based compliance approach, with SO₂ compliance demonstrated through the use of low‑sulfur fuels and fuel records rather than continuous exhaust monitoring.
Subpart KKKKa requires initial performance testing to demonstrate compliance with applicable NOx standards, with limited exceptions for certain low-utilization or temporary units. After initial testing, owners and operators must demonstrate ongoing compliance through either continuous emissions or parameter monitoring systems, depending on turbine type. The rule also includes recordkeeping and reporting requirements, such as SO₂ fuel records and operating data used to calculate capacity factors. EPA also allows delayed testing for units with extended periods of non-operation.
EPA finalized a new temporary stationary combustion turbine subcategory under NSPS Subpart KKKKa for small and medium turbines (≤850 MMBtu/hr). Temporary status is limited to turbines located at a single site for no more than 24 consecutive months and cannot be extended by replacing one temporary unit with another. Temporary turbines may be used during outages or maintenance, and some portable turbines regulated as Title II nonroad engines may be excluded. These units are subject to a 25‑ppm NOx limit when firing natural gas and have reduced monitoring and reporting requirements if manufacturer certification and recent performance testing are documented. Subpart KKKKa creates a more tailored, and in some cases more stringent, regulatory framework for new combustion turbines, particularly with respect to NOx control. Early evaluation of turbine utilization, control technology, manufacturer efficiency, and monitoring strategies will be important to ensure compliance and avoid permitting delays. While existing units are generally unaffected unless modified or reconstructed, KKKKa will significantly influence new generation projects moving forward.