EPA Proposes NSPS KKKKa – Lowers the NOx Limits for Many New Combustion Turbines

On December 13, 2024, EPA published a proposed rule to revise the New Source Performance Standard (NSPS) for new, modified, or reconstructed 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 KKKKa is the requirement for most CTs constructed or reconstructed that operate above a certain capacity factor (either 20% or 40%) to operate with SCR for NO_X control.

EPA is proposing to require new or reconstructed large CTs (>850 MMBtu/hr) that have annual capacity factors above 20 percent to meet a limit of 3 ppm (natural gas) and/or 5 ppm (non-natural gas fuel), using a combination of combustion controls and SCR. EPA is proposing to require new or reconstructed medium CTs (between 250 MMBtu/hr and 850 MMBtu/hr) that have annual capacity factors above 20 percent to meet a limit of 3 ppm (natural gas) and/or 9 ppm (non-natural gas fuel), also using a combination of combustion controls and SCR. Finally, EPA is proposing to require new or reconstructed small CTs (< 250 MMBtu/hr) that have annual capacity factors above 40 percent to meet a limit of 3 ppm (natural gas) and/or 9 ppm (non-natural gas fuel), again using a combination of combustion controls and SCR. More stringent NO_X standards are also proposed for modified CTs of every size and capacity factor subcategory.

This proposed rule comes at a time when the combustion turbine market is experiencing substantial growth. The ongoing demand for renewable capacity drives the CT market as renewable projects need reliable backup power. The need for reliable power for database centers has also led to growth in the CT market. As CT projects are being designed and implemented, it is important for stakeholders to understand the changing regulations. Note that the determination of whether or not a CT is new, modified or reconstructed and subject to the requirements of any final NSPS KKKKa will depend on the date of the proposed rule’s publication in the Federal Register.

The EPA’s proposed NO_X limits based on the use of SCR are a significant change from what is currently required by NSPS KKKK. NSPS KKKK has NO_X limits based on the use of wet and dry combustor controls, specifically not mandating the use of SCR. In 2006, when EPA promulgated NSPS KKKK, these controls were finalized as the Best System of Emission Reduction (BSER). NSPS KKKK includes BSER determinations and corresponding NO_X emissions standards for 14 subcategories of CTs, ranging from 15 ppm for CTs firing natural gas with a design heat input rating of greater than 850 MMBtu/hr to 150 ppm for CTs firing non-natural gas fuels with a design heat input rating of less than or equal to 50 MMBtu/hr.

Currently, there are three size-based categories in NSPS KKKKa. Within each size-based subcategory, there are individual NO_X standards for both natural gas or non-natural gas fuels. There are also separate subcategories for modified and reconstructed stationary CTs, heat recovery units operating independent of the CT, CTs operating at partial operating loads or at low ambient temperatures (or north of the Arctic Circle); and offshore CTs.

In the proposal for NSPS KKKKa, EPA has changed the subcategories from NSPS KKKK to be able to make more appropriate BSER determinations. The proposed changes to the subcategories include:

1. Broadening the small CT category to encompass CTs up to 250 MMBtu/hr, rather than up to 50 MMBtu/hr, and adjusting the medium category to cover CTs between 250-850 MMBtu/hr;
2. Establishing subcategories based on the maximum annual capacity factor;
3. Using a different size threshold for subcategorizing the part-load emission standards (a heat input greater than 250 MMBtu/hr, as opposed to an output greater than 30 MW); and
4. Lowering the operating load that defines a “partial operating load” from 75% to 70% of base load.

There are a total of 21 proposed subcategories. Table 1 shows the proposed subcategories, the BSER determinations, and the corresponding NO_x limits for NSPS KKKKa. Note that EPA is proposing that CTs that co-fire hydrogen as a supplemental fuel should be subject to the same standards of performance for NO_X emissions as CTs firing natural gas or non-natural gas fuels. EPA is proposing that CTs that burn less than 30% hydrogen by volume (on a volumetric basis) be categorized as natural gas burning and subject to the applicable standards for natural gas based on size and capacity factor, and CTs that burn more than 30% hydrogen are categorized as non-natural gas burning and subject to the applicable standards for non-natural gas based on size and capacity factor.

Table 1. Summary of BSER Determinations and Emission Standards for NO_X from Combustion Turbines – Proposed NSPS KKKKa.

Proposed NSPS KKKKa

With respect to the information in Table 1, the EPA is requesting comment on alternative approaches to determining the BSER and appropriate NO_x emission standards (particularly for small CTs), alternative size and capacity factor thresholds for establishing the subcategories, and BSER for CTs that operate at partial operating loads.

The EPA recognizes that individual units have the potential to operate on different fuels; that units will vary their annual capacity factor over time, and that units may even operate over a range of loads. EPA is proposing that a NO_X limit be determined for each hour of operation, but compliance demonstration will be based on a 4-hour average using a blended standard approach similar to NSPS KKKK. The final emissions standard would be based on the heat input weighted average of the four applicable hourly emissions standards.

The EPA is not proposing any changes to the definition of the affected source, where the definition covers both the “combustion turbine engine” as well as the heat recovery steam generator for combined cycle units. However, the EPA is proposing a significant new requirement for how the definition should be considered in a reconstruction analysis for combined cycle units. EPA is proposing that a reconstruction analysis for combined cycle units only include the “combustion turbine engine” portion of the “stationary combustion turbine” affected source definition and not the heat recovery steam generator. This will ensure that if significant capital is spent to repair or replace the combustion turbine engine, the reconstruction analysis will exceed the 50 percent threshold, and the CT will be required to meet the emissions standards for new and reconstructed sources.

In conclusion, any new CT project should consider the proposed changes under NSPS KKKKa and should specifically determine whether an SCR is required. The only way for a new or reconstructed CT to avoid an SCR under the proposed rule would be to operate with an annual capacity factor less than 20 or 40%, depending on the size of the CT.

There are other considerations related to the capacity factor of the CT. For example, 40 CFR Part 60, Subpart TTTTa is the NSPS that regulates GHGs from new CTs. There are reduced capacity factors that impact the BSER determination for CO2 that may need to be considered.

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