On December 13, 2024, EPA published a proposed rule to revise the New Source Performance Standard (NSPS) for new, modified, or reconstructed stationary combustion turbines (CTs) under Title 40 of the Code of Federal Regulations (40 CFR) Part 60, Subpart KKKKa. Under the rule, CTs constructed, modified, or reconstructed after December 13, 2024, must meet more stringent emission standards. The most significant proposed change for NSPS Subpart KKKKa is the requirement for most CTs constructed or reconstructed that operate above a certain annual capacity factor (either 20% or 40%) to operate with SCR or equivalent technology for NO_X control. The comment period closes on March 13, 2025.

Trinity is closely tracking this proposed rulemaking and hosted a complimentary webinar on February 4 that presented an overview of the proposed rule, including several topics for which EPA is soliciting public comment. Many of the topics are highly technical in nature and EPA would benefit from thoughtful engagement from various stakeholders. While the comment period may be extended consistent with the new administration’s overall agenda and Regulatory Freeze Pending Review memorandum, which directs agencies to consider postponing proposed rules for 60 days, the likelihood of an extension is unclear at this time.        

For those planning on submitting comments on the rule, Trinity has identified several key areas to consider addressing:

1. Compliance Schedule

a. EPA has determined that the best system of emission reduction (BSER) for stationary CTs is dependent upon several the annual capacity factor and size subcategory. More stringent emissions standards and testing and monitoring requirements may be triggered once a CT exceeds a capacity factor threshold. However, the proposed rule contains no specific timeline for installing and operating the required control technology and/or monitoring systems. For example, exceeding the 12-calendar month rolling average capacity factor threshold of 40% for small CTs (≤250 MMBtu/hr) would require the immediate installation of SCR and a NO_X continuous emissions monitoring system (CEMS). This type of project would require months of planning, significant unit outages, and air permitting.

2. Range and Format of Emission Standards

a. While an emission limit of 0.011 lb/MMBtu (equivalent to approximately 3 ppm) was proposed for several categories of CTs (e.g., new or reconstructed, >40% annual capacity factor, ≤250 MMBtu/hr base load rating), EPA is considering if emissions standards equivalent to a range of 2-5 ppm are more appropriate based on SCR as BSER.

b. EPA is requesting comment on several alternative approaches to determining the BSER and appropriate NO_X emission standards for small CTs. EPA acknowledged a lack of detailed data on the capital or operating and maintenance (O&M) costs for small natural gas-fired CTs with dry combustion controls and NOX emission rates of 15 ppm or less, relative to the costs of comparable CTs with 25 ppm emission rate guarantees. In the preamble to the proposed rule, EPA noted that if a 15 ppm emissions rate was achievable for natural gas-fired small CTs using only combustion controls (in lieu of 25 ppm), then the higher per-ton incremental costs of SCR compared to that baseline may no longer be viewed as justified.

c. EPA did not propose to establish BSER or standards of performance for ammonia emissions, but is requesting information on opportunities to reduce emissions from SCR operation.

3. Partial Load Considerations

a. NSPS Subpart KKKK includes a partial load standard for units operating at ≤75% of base load rating. EPA has proposed maintaining the partial load standard at a reduced threshold of ≤70% of base load rating, and suggested the final rule could limit the use of the partial load standard by:

i. Establishing a maximum limit to the number of hours per year that the partial load standard can be applied, 

ii. Limiting the partial load standard only to those hours when a CT is in startup or shutdown, or

iii. Defining longer averaging times for compliance determinations such that emissions standards can be complied with even during partial load operations.

a. As noted above, EPA is soliciting comment on SCR as appropriate BSER for the small CT subcategory. EPA acknowledged that if SCR is not an appropriate BSER for all CTs ≤250 MMBtu/hr base load rating, then it may be appropriate to adjust the size-based thresholds such that CTs of greater than 50, 100, or 150 MMBtu/h of heat input should be treated as medium CTs.

b. While not proposed, EPA noted that a 12-calendar month mass-based NO_X emission limit of 0.75 tons per MW of design capacity, applicable to new and reconstructed CTs at capacity factors greater than 20%, could be an appropriate alternative to annual capacity factor subcategories and corresponding heat input- or output-based emissions limits.

a. EPA explicitly noted in the preamble to the proposed rule that only the “combustion turbine engine” should be considered when determining whether a CT is new or reconstructed, which excludes the heat recovery steam generator (HRSG) and equipment associated with combined cycle and cogeneration units. Still, exactly which equipment included in the definition of “stationary combustion turbine,” but not “combustion turbine engine,” that may be considered in a reconstruction evaluation remains unclear. For example, equipment such as “control systems,” and the methodology to distinguish the fixed capital cost of control systems associated with the “combustion turbine engine” versus control systems for other equipment may be subject to interpretation by individual states and/or EPA regions.

a. Title V Permitting for Minor Source – EPA may determine that some low-emitting CTs subject to either NSPS Subparts GG, KKKK, or the proposed KKKKa may be exempted from Title V air permitting requirements, similar to the provisions of NSPS Subparts IIII and JJJJ for Reciprocating Internal Combustion Engines (RICE) at 40 CFR 60.4200(c) and 60.4230(c), respectively. This exemption was not proposed; however, EPA is soliciting comment on whether there are specific circumstances for which it is “impracticable, infeasible, or unnecessarily burdensome” for stationary CTs operated at facilities that are not major sources (as defined in Clean Air Act Section 502) to obtain and comply with a Title V air permit.

b. Temporary CT – EPA is soliciting comment on creating a subcategory for temporary CTs, defined as CTs operating in one location for less than 1 year, with combustion controls as BSER. The overall compliance approach may be similar to NSPS Subparts IIII and JJJJ, including documentation of manufacturer’s certification that emissions standards are met. Should a temporary CT remain in place for longer than 1 year, then it would not be considered temporary for any period of its operation and must immediately comply with all requirements applicable to a non-temporary CT.

As noted extensively in the public hearing held on January 8, EPA is overdue on the statutory requirement to review and, unless inappropriate, revise the NSPS at least every eight years per Section 111(b)(1)(B) of the Clean Air Act. Per the terms of a proposed consent decree, EPA plans to finalize either a revised NSPS Subpart KKKKa or formal determination not to revise the standard by November 12, 2025. Given the wide adoption of SCR over the last two decades, it is reasonable to conclude that emissions standards routinely associated with SCRs operated on medium and large CTs will be reflected in any final rule. However, a final rule’s impact on small CTs (≤250 MMBtu/hr base load rating), particularly the requirement to install SCR on small CTs if annual capacity factors exceed 40%, is less certain due to EPA’s questionable cost analyses supporting the rulemaking.

If you would like to discuss the revised NSPS for combustion turbines and how they may impact power generation projects, please email Kasi Dubbs or Chris Weber in Trinity’s Kansas City office or call 913.894.4500.