EPA Proposes Stricter Standards for NOx Emissions at New Natural Gas Power Plants

 

     The EPA of Biden’s outgoing administration has proposed stricter nitrogen oxides (NOx) limits for new natural gas power plants. These standards were last updated in 2006. According to Utility Dive:

“The proposal would ensure that new turbines built at power plants or industrial facilities — especially large ones that could operate for decades — would be among the most efficient and lowest-emitting turbines ever built,” the EPA said. “The proposal provides regulatory certainty for the power sector, while supporting the continued delivery of reliable and affordable electricity.”

The new rules would apply to most new, modified, and reconstructed fossil fuel-fired stationary combustion turbines. NOx compounds are a major contributor to smog and reducing emissions of them could help nearby communities reduce asthma and respiratory infections for vulnerable people, including children the elderly, and those with pre-existing illnesses. NOx compounds react with volatile organic compounds (VOCs) to form ozone and particulate matter.  The EPA announcement notes:

“The proposed New Source Performance Standards (NSPS) are based on the application of combustion controls and selective catalytic reduction (SCR), a cost-reasonable and widely used add-on control technology that limits emissions of NOx. In addition, EPA is proposing to maintain the current limits for sulfur dioxide, which is well-controlled in this sector based on the long-term required use of low-sulfur natural gas and distillate fuels. The proposed stronger standards for NOx would apply to facilities that begin construction, reconstruction, or modification after the date of publication of the proposed standards in the Federal Register.”

“To strengthen the NOx performance standards for new stationary combustion turbines, EPA is proposing:

To determine that combustion controls with the addition of post-combustion SCR is the best system of emission reduction (BSER) for most combustion turbines.

To lower the NOx standards of performance for affected sources based on the application of the BSER.

To establish more protective NOx standards for affected new sources that plan to fire or co-fire hydrogen, ensuring that these units have the same level of control for NOx emissions as sources firing natural gas or non-natural gas fuels.”

     The EPA is basing the requirements on turbine sizes and capacity factors (utilization rates). This is due to the higher relative costs of using SCR technology to mitigate smaller emissions sources and less utilized sources.

     The Sierra Club urged the incoming Trump administration to keep the proposed stricter standards but it is unclear whether likely incoming EPA chief Lee Zeldin will agree to that, especially as the focus of the incoming Trump EPA will likely be on deregulation or looser regulations rather than stricter regulations.

 

 

Selective Catalytic Reduction (SCR) Technology

     The chemical reactions involved in SCR technology are given below. SCR utilizes anhydrous ammonia, aqueous ammonia, or dissolved urea as reductants along with a porous catalyst such as titanium oxide, vanadium oxide, or zeolite materials. The choice of reducing agent is typically based on the turbine size. Smaller units typically utilize urea in solution while larger units are operated with aqueous ammonia. 

     According to an article about SCR technology in Power Magazine:

“SCR selectively reduces NOx emissions by injecting ammonia (H3) into the exhaust gas upstream of a catalyst. The NOx reacts with NH3 and oxygen (O2) to form nitrogen (N2) and water (H2O), primarily according to the following equations:

4NH3 + 4NO + O2 ➝ 4N2 + 6H2O

2NH3 + NO + NO2 ➝ 2N2 + 3H2O

4NH3 + 2NO2 + O2 ➝ 3N2 + 6H2O

The catalyst’s active surface is usually a noble metal, base metal (titanium or vanadium) oxide, or a zeolite-based material. Metal-based catalysts are typically applied as a coating over a metal or ceramic substrate, according to NETL, while zeolite catalysts are typically a homogeneous material that forms both the active surface and the substrate.”

The performance of different catalysts is very dependent on temperatures. The presence of fly ash in coal plants complicates SCR configuration as the fine dust particulates can clog the small holes in the cells. The size of the openings or holes is known as the pitch. Thus, larger holes are used for coal-fired plants than for gas-fired plants which do not precipitate residual combustion materials or ash.

     Dan Johnson, VP at CORMETECH, an SCR system provider, makes the following comments in the Power Magazine article regarding SCR for gas units:

“Gas units have set formulas that are commonly used. “Those formulas change, not with varying SO2 oxidation, but with varying temperature,” Johnson said. “As you go higher in temperature, you have to change the mixture of the catalytic metals that you’re using, because some of them will not perform well at higher temperatures. At CORMETECH, we pride ourselves on custom engineering our catalysts for every application, and it really is necessary to meet the performance requirements.”

     In the future, SCR technology may very well be combined with carbon capture technology and carbon monoxide abatement in a single unit as shown in the CORMETECH conceptualized graphic below.

 

References:

 

EPA proposes tightening NOx limits for new gas-fired power plants. Ethan Howland. Utility Dive. November 25, 2024. EPA proposes tightening NOx limits for new gas-fired power plants | Utility Dive

Selective catalytic reduction. Wikipedia. Selective catalytic reduction - Wikipedia

EPA Proposes Tighter Limits on Harmful NOx Emissions from New Stationary Combustion Turbines to Better Protect Nearby Communities. U.S. EPA. November 22, 2024. EPA Proposes Tighter Limits on Harmful NOx Emissions from New Stationary Combustion Turbines to Better Protect Nearby Communities | US EPA

Understanding Selective Catalytic Reduction Systems and SCR Design Considerations. Aaron Larson. Power Magazine. November 1, 2024. Understanding Selective Catalytic Reduction Systems and SCR Design Considerations

Could SCR Catalyst Technology Adoption Be a Roadmap for Power Plants Seeking Economical and Efficient CO2 Point-Source Solutions? Sonal Patel. Power Magazine. August 10, 2024. Could SCR Catalyst Technology Adoption Be a Roadmap for Power Plants Seeking Economical and Efficient CO2 Point-Source Solutions?