Tuesday, November 22, 2011
One might not think of PM being a driver initially for Utility MACT, but it is. That's because PM is not a type of HAP, but it is a surrogate pollutant for controlling HAPs. The surrogate comes into play for the 10 metallic HAPs the EPA has identified to control under Utility MACT.
Another regulatory program aimed at controlling PM emissions stems from the National Ambient Air Quality Standards (NAAQS). NAAQS define the concentration of a pollutant in ambient air that EPA deems to be protective of human health and the environment.
Many parts of the United States have current ambient air quality that does not meet the new PM2.5 NAAQS. To address this situation, EPA promulgated the Clean Air Transport Rule (CATR) to reduce the level of PM2.5 pollution across large regions.
PM2.5 is a rather unique pollutant in the sense that it can be emitted directly from a source, but can also be formed miles away when pollutants like SO2 and NOx chemically interact in the atmosphere. EPA created the CATR to force utilities to limit their PM2.5 precursor emissions (that is, SO2 and NOx) to reduce the downwind secondary formation of PM2.5. This is designed to help many parts of the country, primarily in the eastern U.S., achieve air quality that meets the NAAQS.
As far as control technologies go, the two main offerings targeting PM are electrostatic precipitators (ESP) and fabric filters, or baghouses.
An ESP uses high-voltage fields to apply electrical charges to particles moving through the field. This causes the charged particles to move toward an oppositely charged collection surface where they accumulate. ESPs are available in both dry and wet options. Most ESPs are dry, but special circumstances can require wet ESP installations. More than 70 percent of existing coal-fired power plants have installed ESPs, according to a report by Environmental Health and Engineering. According to the EPA, an ESP can capture more than 99 percent of total PM and 80 to 95 percent of PM2.5.
Power Engineering, August 2011)