U.S. EPA Releases Proposed Updates to Stationary Combustion Turbines NSPS

On November 22, 2024, U.S. Environmental Protection Agency (EPA) proposed the first of what is expected to be three new or revised rules related to the emissions of criteria pollutants, air toxics, and greenhouse gasses (GHG) for stationary combustion turbines. The proposed update to the New Source Performance Standards (NSPS) for Stationary Gas and Combustion Turbines (40 CFR Part 60, Subparts GG and KKKK and the proposed new KKKKa) lowers the standards for the emissions of nitrogen oxides (NO_X) for most new, modified, and reconstructed stationary combustion turbines, while keeping the current standards for sulfur dioxide (SO₂) emissions the same.

What Sources are Subject to the Proposed Rules?

The new and revised Part 60 rules would apply to all stationary combustion turbines with a base load rating of greater than 10 million British thermal units per hour (MMBtu/hr) heat input, except for those classified as emergency combustion turbines, including emergency-standby combustion turbines, military combustion turbines, and firefighting combustion turbines. The standards of performance codified in 40 CFR Part 60, Subpart KKKKa, once promulgated, would be directly applicable to affected sources that begin construction, modification, or reconstruction after the date of publication of the proposed standards in the Federal Register. The proposed updates to Subparts GG and KKKK would apply to the sources covered by those rules immediately upon promulgation.

Proposed Changes to Subparts GG and KKKK

U.S. EPA is proposing two changes to Subparts GG and KKKK:

• Exemptions for turbines subject to more stringent standards (e.g., stationary combustion turbines at refineries subject to Subpart J or Ja).
• Allowing turbines subject to Subpart GG or KKKK to petition the Administrator to comply with Subpart KKKKa.

Proposed New Subpart KKKKa

The proposed new Subpart KKKKa reflects the following:

• Development of a new structure for subcategories, some of which mirror the subcategories for combustion turbines in Subpart TTTTa.
• Determination that the use of combustion controls with the addition of post-combustion selective catalytic reduction (SCR) is the best system of emissions reduction (BSER) for most combustion turbines.
• Lower the NO_X standards of performance for several subcategories based on the application of SCR as BSER.
• More protective NOx standards for affected new sources that plan to fire or cofire hydrogen, ensuring that these units have the same level of control for NOx emissions as sources firing natural gas or non-natural gas fuels.

U.S. EPA is also proposing to revise the definition of stationary combustion turbine. This change would affect the determination of whether a unit is new or reconstructed. The new language would clarify that the test for determining if an affected facility is a new source would be based on whether the combustion turbine portion of the affected facility is entirely replaced. The reconstruction determination would be based on whether the fixed capital cost of the replacement of the combustion turbine engine portion exceeds 50% of the fixed capital cost that would be required to install only a comparable new combustion turbine engine portion of the affected facility. This change excludes the heat recovery steam generator (HSRG) and duct burner at combined cycle (CC) and combined heat and power (CHP) facilities. U.S. EPA’s reasoning is that the combustion turbine engine itself is the primary source of emissions and that it is appropriate that when CC and CHP facilities undertake major capital investments in the combustion turbine engine portion of the facility, they should invest in emissions control equipment as well.

Subcategorization of Gas-Fired Stationary Combustion Turbines

U.S. EPA is proposing revised NO_X emissions limits based on three size-based subcategories that reflect consideration of the performance of different combustion turbine designs and current NOx control technologies. Those categories are:

• Large combustion turbines: units with a base load heat input rating > 850 MMBtu (approximately 85+ megawatt or MW)
• Medium combustion turbines: units with a base load heat input rating > 250 MMBtu and < 850 MMBtu (approx. 25 to 85 MW)
• Small combustion turbines: units with a base load heat input rating < 250 MMBtu (less than 25 MW)

These sources would then be further subcategorized by low, intermediate, or high loads, based on a 12-month capacity factor, similar to the approach taken in 40 CFR Part 60, Subpart TTTTa (standards of performance for greenhouse gas emissions for new and reconstructed combustion turbine electric generating units):

• High load: capacity factor > 40 percent
• Intermediate load: capacity factor > 20 percent and < 40 percent
• Low load: capacity factor of < 20 percent

While U.S. EPA is proposing to determine that SCR is BSER for most turbines, the proposed rule recognizes that for smaller turbines and those turbines that operate at lower capacities, SCR may not be cost effective and thus proposes non-SCR based emissions rates for certain turbine subcategories, including:

• Small combustion turbines that operate at low and intermediate loads
• Medium combustion turbines that operate at low loads
• Large combustion turbines that operate at low loads

There are also additional subcategories for units running alternative fuel mixes including hydrogen, operating above the Arctic Circle (primarily related to Alaskan oilfield operations), operating at an ambient temperature below 0^o F, and other special operating conditions.

Revised NO_X Emission Limits for Gas-Fired Turbines

U.S. EPA is proposing input-based NO_X standards in the form of pounds per MMBtu (lb/MMBtu) for the new Subpart KKKKa, with compliance determined based on a four-hour rolling average. No changes to the SO₂ standards are proposed and no PM or CO standards were deemed necessary. The following are the proposed BSER and NO_X emissions standards for Subpart KKKKa:

  Table 1—Proposed BSER and NO_X Emissions Standards*

*Review of New Source Performance Standards for Stationary Combustion Turbines and Stationary Gas Turbines – Table 1, Page 66

Note that although U.S. EPA has retained the low load limit of 96 ppm, they are taking comment on whether they should limit the part load standard only to those hours when a combustion turbine is in startup or shutdown. This requirement would impact turbines that are turned down to low loads to either accommodate facility standby conditions or to accommodate an influx of solar energy to the grid.

What Are the Implications of the Proposed Rule?

Based on the lowered emissions rates presented in Table 1 (especially for turbines with a capacity factor greater than 20%), and the establishment of SCR as BSER for several subcategories, in most cases if you are planning a project that will involve the construction, modification, or reconstruction of a combustion turbine, you should expect to either deploy SCR for NO_X control or take permit limits that will significantly limit the amount of operation of the turbine. Either path may significantly increase the capital and/or operating costs of your project above and beyond what would have been necessary in the past.

What about the Administration Change?

The public comment period will begin once the proposed rule is published in the Federal Register and last 90 days, so at this point it will extend into the second Trump administration prior to being finalized. Given that timing, the final rule could be significantly altered to be less stringent, though at this point it could be hard to argue that SCR is not BSER for most large gas-fired turbines with high capacity factors, given its widespread use. The other two gas-turbine rules that are not expected to be proposed until 2025 involve emissions of toxics and GHG.  The GHG proposal is on much more uncertain ground and may never be proposed given the expected deregulatory position of the next U.S. EPA, but some proposed changes to the Part 63 toxics rule are likely, given the data that U.S. EPA has collected recently.

Summary

U.S. EPA believes that the proposed changes to Subparts GG and KKKK and new Subpart KKKKa would result in a reduction of 198 tons of NO_X in 2027 and 2659 tons in 2032, equating to net benefits to society of an estimated $340 million. However, with the administration change, the final rule may look significantly different than the version that has just been proposed.

ALL4 will continue to track this and all regulatory developments closely as the new administration takes over in 2025. If you have questions or concerns about implications of this rule to your project or fleet of stationary combustion turbines, feel free to contact your ALL4 Project Manager or Rich Hamel. We can also help you develop comments on the proposed rule, evaluate permitting risks, and develop a strategy to make the permitting of your project as efficient as possible.