Particulate Matter

The first standards for PM were established by the U.S. Environmental Protection Agency (EPA) in 1971 under the Clean Air Act.  The EPA's Maximum Achievable Control Technology (MACT) rules, which were released Feb. 23, introduced additional, more stringent requirements to new and existing boiler PM and other emission limits and requirements. Utility MACT specifically limits Hazardous Air Pollutants (HAPs). 

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.

EPA revised the PM NAAQS in 1987 to regulate particles smaller than 10 micrometers in diameter (PM10). In 1997, EPA revised the NAAQS to also regulate particles smaller than 2.5 microns in diameter (PM2.5). In 2006, EPA reduced the PM2.5 NAAQS from the 1997 level of 65 µg/m3 to 35 µg/m3. At this point, the PM NAAQS are so low that stationary sources may require installation of more efficient baghouses or control the PM that forms as gases condense after emission from a stack. 

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 NO_x chemically interact in the atmosphere. EPA created the CATR to force utilities to limit their PM2.5 precursor emissions (that is, SO₂ and NO_x) 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. 

Two main drivers influence the amount of PM emitted from coal-fired boilers in power generation: the ash content in the coal and the type of boiler used. 

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.

Fabric filters can drive down PM levels more than dry ESP because the technology is not as affected by particulate characteristics.  A fabric filter typically has a lower capital cost than an ESP, but a higher maintenance cost. You trade initial capital investment cost for maintenance costs. A fabric filter collects PM on the surfaces of fabric bags. Most of the particles are captured on already collected particles that have formed a dust layer. According to EPA, a fabric filter on a coal-fired power plant can capture up to 99.9 percent of total particulate emissions and 99.0 to 99.8 percent of PM2.5. Thirty-five percent of coal-fired power plants in the U.S. have already installed fabric filters, according to Environmental Health and Engineering. (Power Engineering, August 2011)