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On April 12, 2019, EPA published in the Federal Register, at 84 Fed. Reg. 15,046, a proposed rule, titled “Stationary Combustion Turbines Residual Risk and Technology Review” (RTR), to address the results of the review for stationary combustion turbines that EPA is required to conduct in accordance with the Clean Air Act.

Trinity is providing this update and insight to the rulemaking process, based on a review of comments that were submitted in response to the proposed rule, as well as discussions with industry and EPA. The public comment period closed on May 28, 2019 and notable comments were received from industry, as well as the Electric Power Research Institute (EPRI). The rule is expected to be finalized on March 13, 2020 and, depending on the outcome, affected sources could become subject to the rule and have 180 days from the effective date to complete initial performance testing.

 Trinity notes that the majority of industry's comments and concerns were with respect to the following issues:

  • Finalize the RTR Determination for Stationary Combustion Turbines as Proposed

  • Stay of the Combustion Turbine Standards

  • Proposed Operational Standard During Periods of Startup

  • Proposed Operational Standard for Periods of Shutdown

  • Compliance Reporting

  • Definition of Lean Premix Oil-Fired Stationary Combustion Turbine

EPA Should Finalize Its Proposed RTR Determination for Stationary Combustion Turbines

EPA is proposing that the residual HAP risks from the stationary combustion turbine source category are acceptable, and that the existing NESHAP provides an ample margin of safety to protect public health. EPA's results are conservative, especially when compared with the independent risk assessment recently performed by EPRI. The major difference between the two studies is that EPRI expended considerable effort updating model input data, such as stack parameters, locations, and emissions data. The EPRI analysis concludes that risks from HAP emissions from the source category are even lower than estimated by EPA. EPRI's two published technical reports were submitted during the public comment period and can be accessed through the rulemaking docket (EPA Docket No. EPA-HQ-OAR-2017-0688):

  • Inhalation Human Health Risk Assessment for U.S. Stationary Combustion Turbines.

    EPRI, Palo Alto, CA: 2019. Report #3002016528.

  • Multi-Pathway Human Health Risk Assessment for U.S. Stationary Combustion Turbines.

    EPRI, Palo Alto, CA: 2019. Report #3002016745. 

Industry commenters suggested that, since both the EPA and the EPRI analysis have shown that stationary combustion turbines present negligible health and environmental risks and that EPA has found no technology developments which necessitate regulatory revisions, EPA should therefore finalize its proposed RTR determination.

Stay of the Combustion Turbine Standards

In 2004, EPA temporarily stayed the obligation of new lean premix and diffusion flame gas-fired turbines to comply with Subpart YYYY's limitations, pending EPA's proposal to delist these subcategories from the source category list.  Subsequently, the D.C. Circuit decided that EPA cannot delist subcategories. According to the DC Circuit decision, delisting applies only to entire source categories. Based on the proposed results of the residual risk analysis conducted as part of this current rulemaking, EPA has indicated that it does not have sufficient information to delist the entire source category at this time. Therefore, the lean pre-mix and diffusion flame gas turbine subcategories must remain subject to the rule. EPA now proposes to lift the stay for new lean premix and diffusion flame gas-fired turbines, requiring these affected units at major HAP sources to comply with all applicable regulatory requirements upon EPA's final action. As a result, initial performance tests would be required within 180 calendar days of the effective date of the final rule. 

When EPA established the 91 ppbvd formaldehyde emission limit, it acknowledged that the standard was based on limited data and might require revision. This is still the current proposed emission limitation and the industry has a legitimate concern over the availability of qualified emissions test companies capable of accurately and reliably determining compliance at a 91-ppb formaldehyde concentration. The proposed method under Subpart YYYY of Part 63, Test Method 320 uses a FTIR spectrometry, which has a detection limit of approximately 100 ppbvd or higher (or effectively 91 ppbvd when corrected to 15% O2). A recent study from EPRI also concluded that because of the complexity of the method and the potential interferences for quantitation of target compounds, the ability of FTIR methods to achieve accuracy and precision at the concentration level of 91 ppb is uncertain. Some industry comments implied that fewer than five vendors in the country can provide testing with detection levels below 91 ppbvd. These vendors would be in high demand as, under the Proposed Rule, many sources would need to be seeking to conduct testing simultaneously.

Interestingly, one utility supplied a comprehensive data set for six combustion turbines. This utility also commented that the only robust and well established method identified that could effectively measure formaldehyde concentrations below 91 ppbvd was through use of EPA Method 18 (Measurement of Gaseous Organic Compound Emissions by Gas Chromatography). The detection limit of this method, using the identified analytical equipment, is between 10 ppbvd and 15 ppbvd for formaldehyde, with a cost of approximately $26,000 per test.

If compliance testing is required, many industry commenters requested flexibility in when they conduct their compliance tests. It was recommended that EPA at least synchronize the testing requirements of Subpart YYYY with the CEMS annual RATA scheduling provisions of 40 CFR Part 75 Appendix B in the event that the stay is lifted. Appendix B allows for an extension of time between RATA tests for what are considered "non-QA quarters" where a unit operates less than 168 hours.  This change would help avoid situations where a unit is forced to operate just to conduct emissions testing.

Based on the recent EPRI studies, as well as additional testing data submitted during the comment period, several industry commenters believe that EPA now possesses sufficient information to support the conclusion that the entire Stationary Combustion Turbine source category should be delisted in its entirety. Since EPA is not required to lift the stay on the subcategory at this time, one option would be for EPA to sever from this rulemaking the proposal to lift the stay for certain new turbines, and instead evaluate the newly available data and assessments to pursue a formal delisting of the entire stationary combustion turbine category as provided under section 112.

EPA's Proposed Operational Standard During Periods of Startup

During startup events, emissions from stationary combustion turbines are significantly different from emissions during normal operation, primarily due to the nature of the startup operation where a turbine must go through a variety of operational modes before it can achieve optimal combustion. The EPA determined that no practicable measurement methodology exists because all test methods in place assume stable operation. Accordingly, rather than a numerical emission limit, EPA proposes an operational standard, that owners and operators minimize the time the turbine is spent at idle or low load operations, “not to exceed 1 hour for simple cycle stationary combustion turbines and 3 hours for combined cycle stationary combustion turbines.” As currently proposed, lean pre-mix and diffusion flame gas turbines would be subject to this work practice standard during startup. Combustion turbines, other than these two types, would not have an hour limitation in the rule as currently proposed. 

Industry commenters are supportive of the one-hour timeframe proposed for a simple cycle turbine startup. However, the three-hour timeframe EPA has proposed for combined cycle turbines is problematic for certain events (e.g., a startup with a cold steam turbine and a cold combustion turbine). The firing rate of a combined cycle turbine must be increased much more slowly than a simple cycle and is not able to reach the dry low NOx (DLN) mode as quickly. Commenters suggest that the operational standard require that units minimize the time the turbine is spent under startup conditions and operate the equipment in a manner consistent with good air pollution control practices for minimizing emissions. This approach would address the longer startup times that may be required depending on the startup conditions and recognize that a one-size-fits-all strategy is unworkable. Notably, it is in the facility's financial interest to not linger in a startup mode, but rather bring these units up as quickly and efficiently as possible to accommodate changing power demands, such as when renewable generation intermittently wanes or peak demand periods occur, because turbines are not efficient during startup. This approach also would address situations in which malfunction events occur during startup periods.

EPA Should Implement an Operational Standard for Periods of Shutdown

Based on the lack of such data showing that turbine emissions would be higher during periods of shutdown than during normal operation, EPA is not proposing a different standard during the shutdown process. As with startups, the testing methodologies in place all assume continuous operation and do not account for the changing conditions during shutdown periods. However it is appropriate to conclude that emissions of organic-based HAPs are different during shutdown periods for the same reasons that they are higher during startup periods. EPA also has ample data from Continuous Emissions Monitoring Systems (CEMS) showing that emissions of CO (which can be an indicator of other organic HAP emissions) during shutdown are higher than normal operation.

Since operating parameter tolerances are based on the assumption that units are continuously operating, it could be problematic to comply with such parameters during shutdown. For example, in the event of a requirement to maintain a certain catalyst inlet temperature on a four-hour rolling average, the inlet temperatures could drop below the allowed range during a shutdown, which could lead to the source being in violation. Therefore, several industry commenters suggested a separate operational standard for shutdown should be established as best operation and maintenance practices during the shutdown period (e.g., a 30-minute period for simple cycle turbines and a 90-minute period for combined cycle turbines).

Compliance Reporting

As previously stated, both EPA's and EPRI's analysis demonstrate that the risks from stationary combustion turbines present negligible health and environmental risks. If EPA lifts the stay for new lean premix and diffusion flame gas-fired turbines, many industry commenters suggested that it should revise the subcategories' monitoring and reporting requirements to be less burdensome. In addition to the work practice and duration requirements, there are proposed changes to recordkeeping and reporting during SSM. These include maintaining records of startup conditions and excess emissions events. Owners and operators will be required to record the date, time, and duration of startup periods. For excess emissions events, owners/operators will be required to record actions taken to minimize excess emissions, to estimate the amount of emissions over the standard, and to describe the method used to estimate the excess emissions. Electronic reporting via the Compliance and Emissions Data Reporting Interface (CEDRI) is proposed for routine compliance reports and for annual emissions test reports.

Definition of Lean Premix Oil-Fired Stationary Combustion Turbine

EPA has defined lean premix oil-fired stationary combustion turbines in the rule as follows:

(ii) Each stationary combustion turbine which is equipped both to fire oil using lean premix technology and to fire gas, and is located at a major source where all new, reconstructed, and existing stationary combustion turbines fire oil more than an aggregate total of 1,000 hours during the calendar year, during any period when it is firing oil.

Several industry commenters stated that the 1,000 hour oil-firing threshold would be detrimental for facilities that have many turbines on site and that, whether a specific unit is a gas-fired unit or an oil-fired unit should be a determination based solely on the affected unit's operation, not its operation in conjunction with all other turbines onsite, including older turbines that would not be subject to the rule requirements.

An option would be for EPA to revise the definition of gas-fired and oil-fired stationary combustion turbines to include thresholds based on the fuel usage for each turbine and not all the turbines at the entire site. These units could be defined similar to the Boiler MACT or MATS rule based on their capacity factor or hours of operation on fuel for the turbine in question (not the entire site). For example, an oil fired turbine can be defined as a turbine that burns fuel oil equivalent to 10 percent or more of the gross heat input on an annual basis.


EPA estimates that 182 new lean pre-mix and diffusion flame gas-fired turbines were constructed or reconstructed between 2003 and 2017, with 39 of those potentially becoming subject to the rule. Analysis by EPA and EPRI have both shown that stationary combustion turbines present negligible health and environmental risks. In addition, EPA has found no technology developments which necessitate regulatory revisions. The rule is expected to be finalized on March 13, 2020 and based on language in the proposed rule, as well as consideration of the comments received, a logical outcome might be that EPA separates the rulemaking. Specifically, EPA could finalize its residual risk and technology review (RTR) as proposed and consider lifting the stay in a few years. At that time, depending on available data and other considerations, it's possible that delisting of the entire stationary combustion turbine category could occur, as provided under section 112.

Comments on this proposed rule can be found at and EPA Docket No. EPA-HQ-OAR-2017-0688.