Emissions

EPA guidance will result in more PM2.5 dispersion modeling

EPA’s new modeling guidance makes a complicated situation more likely to occur at your plant.
EPA guidance will result in more PM2.5 dispersion modeling
(Photo credit: EPA.)

By Robynn Andracsek, PE; Providence Engineering and contributing editor

The U.S., Environmental Protection Agency’s new modeling guidance makes a complicated situation more likely to occur at your plant.

Bottom line: Under EPA’s new guidance, a project that is significant only for NOx or SO2 will also likely trigger PM2.5 dispersion modeling.

After reading a little background science, the impacts on your plant will become clear.

An air dispersion model predicts how emissions from a facility will dilute in the atmosphere and eventually settle in the ambient air that the public breathes. Air dispersion models are complex computer simulations that use real world meteorological data and terrain to simulate how pollution affects compliance with the National Ambient Air Quality Standards (NAAQS). 

Particulate matter less than 2.5 microns in diameter (PM2.5) is the smallest type of particulate pollution regulated by EPA. These fine, inhalable particles have low and strict NAAQS that make modeling compliance difficult.

PM2.5 can be directly emitted from smoke, dust and combustion. Indirect PM2.5 is harder to control since it comes from NOx and SO2 (and other chemicals) that are emitted as a gas and form PM2.5 in the atmosphere through oxidation.

You can use cleaner fuels or fabric filters to control direct PM2.5. The only way to reduce indirect PM2.5 is by addressing its precursors (NOx and SO2).

Dispersion modeling is most commonly required when applying for a Prevention of Significant Deterioration (PSD) permit. When a project’s emissions exceed the PSD threshold for that pollutant, a dispersion modeling analysis is conducted and modeling results are submitted along with the PSD application. Table 1 compares these pollutants and shows how much stricter the levels are for PM2.5.

Most states follow EPA’s guidance documents on how to conduct dispersion modeling. That guidance is continually revised as more research is conducted into the science behind air pollution.

EPA’s most recently issued modeling guidance, “Guidance for Ozone and Fine Particulate Matter Permit Modeling,” recommends that any project which triggers NOx or SO2 modeling also be required to model primary PM2.5 emissions. The recommendation holds even if PM2.5 emissions are below the PSD threshold.

PM2.5 emissions are abundant at coal-fired power plants, steel mills, and other sites with numerous haul roads and storage piles. This means that adding or changing a combustion source with NOx or SO2 emissions can require PM2.5 modeling even if the project has negligible PM2.5 emissions. Not every state may adopt this Guidance, but many will. That likely will make obtaining an air permit at existing power plants even more difficult.

Here are some ways your facility can prepare and mitigate the risk:

  • Run a preemptive dispersion model to understand potential PM2.5 concentration issues at your fence line.
  • Under attorney-client privilege, calculate the net emissions change before every outage for comparison to the PSD thresholds.
  • Change the project’s design to keep NOx and SO2 emissions increases below PSD thresholds (for example, by specifying low NOx burners).

Particulate modeling often identifies trouble spots at the fence line locations caused by haul roads and equipment with short stacks.

The bottom line is this: Under EPA’s new guidance, a project that is significant only for NOx emissions (such a boiler) or SO2 will also likely trigger PM2.5 dispersion modeling.



About the Author: Robynn Andracsek, PE, is a Senior Air Quality Engineer at Providence Engineering and Environmental Group LLC with 26 years of experience in air permitting for utilities and district energy facilities.  Providence is an employee-owned, multidisciplinary engineering and environmental consulting firm. Our work has taken us across the United States and beyond in support of our governmental and industrial clients’ goals and challenges all the while holding an unwavering dedication to our founding principles – to take care of our clients, make a little money, and have fun while doing it. Her email address is robynnandracsek@providenceeng.com.