Regional Haze Rule (NOx)

NGS is one of nearly 600 coal-fired power plants operating in the United States. Under the Clean Air Act, Congress established the Regional Haze Rule (RHR) to regulate emissions that may affect visibility, including sulfur dioxide (SO2) and nitrogen oxide (NOx), at national parks and protected wilderness areas.


The Environmental Protection Agency (EPA) issued a final Best Available Retrofit Technology (BART) rule for NOx emissions at NGS on July 28, 2014. This established a lifetime facility-wide cap in total NOx emissions from NGS between 2009 and 2044.

In compliance with the 2014 BART ruling, NGS' Annual Emission Report is available.


NOx is formed during the combustion of coal when nitrogen in the air and coal combine with oxygen. NOx can impact visibility in two ways:

  • NOx consists of nitrogen oxide and nitrogen dioxide (NO2). NO2 is a gas that absorbs light and is visible as a local brown plume.
  • Additional reactions in the atmosphere between NOx and background ammonia that may be present due to other sources create ammonium nitrate particles. These particles last longer and can impact visibility as a component of regional haze.

Point sources of NOx emissions, such as power plants, account for a minor portion of total NOx emissions. As illustrated by the Western Regional Air Partnership Electronic Data Management System (2005 Inventory), NOx emissions from NGS constitute only a fraction of the regional NOx emissions. Mobile sources, such as vehicles, are major contributors to total NOx emissions.

Two main technologies are currently available to control NOx emissions:

  • Low-NOx burners and separated overfire air systems (LNB/SOFA) are furnace modifications that control air-to-fuel ratios to reduce NOx formation during combustion. This type of control technology represents a cost-effective means of reducing NOx emissions by 40% without the need for costly chemicals and other potentially negative environmental consequences. LNB/SOFA technology was installed on all units at NGS between 2009 and 2011. Installation was completed at a cost of approximately $45 million.
  • Selective catalytic reduction (SCR) consists of a reactor in which ammonia is injected to combine with NOx to form nitrogen and water. The cost of installing SCR on all three units of NGS would be approximately $650 million. If baghouses were required to mitigate the increase in emissions of other pollutants created by the SCR, the total cost of controls would rise to approximately $1.3 billion.

While SCR may achieve additional NOx reductions, monitoring data collected near NGS indicates that air quality already meets all of the EPA's National Ambient Air Quality Standards, which are established and periodically reviewed by the EPA for the purpose of protecting public health within a margin of safety. Therefore, the current emission controls at the plant are already protective of public health.

Low-NOx burners modify the combustion process, so there are no significant adverse effects resulting from this method of NOx removal. SCR, however, creates sulfuric acid mist that also impairs visibility. Because sulfuric acid mist is a regulated pollutant, EPA's regulations could require the injection of sorbents to mitigate the mist emissions. The addition of sorbents would increase the particulate matter (PM) load and potentially require installation of baghouses at a significant additional cost to reduce PM emissions.