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Primary PM2.5

According to EPA estimates, primary PM2.5 emissions (fine particles emitted in solid or liquid form) from coal combustion at electric generating facilities constitute less than 2% of anthropogenic primary PM2.5 emissions nationwide.  Information collected under NETL’s Air Quality Research program also suggests that primary PM2.5 from coal plants generally accounts for less than 1% of ambient PM2.5 mass, even in locations such as Pittsburgh, where the impact of coal plants would be expected to be greatest.  Existing technologies for primary PM2.5 collection - electrostatic precipitators (ESPs) and fabric filters (FFs)  – can currently achieve efficiencies of 99.5% or greater, so additional improvements in these technologies is unlikely to result in a significant nationwide reduction in ambient PM2.5 mass concentrations. 

However, existing particulate collection devices, especially older ESPs, may experience decreased efficiencies when different fuel types are used in coal boilers.  For example, ESPs sometimes have difficulty collecting the high-resistivity fly ashes commonly produced by low-sulfur coals, resulting in excessive stack opacity.  In some cases, plants have been “derated” (operated at less-than-optimum power output), at considerable cost to the utility, solely to reduce the flue gas flow rate such that the ESP can perform efficiently enough to meet opacity requirements.  Although some plants have successfully added sulfur trioxide (SO₃) as a flue gas conditioner to decrease ash resistivity and improve ESP efficiency, this introduces other undesirable consequences such as the need to receive, store, handle, and inject a toxic substance (SO₃).  Replacing the older ESP with a FF is usually a cost-prohibitive option; therefore, DOE-NETL has sponsored several projects to develop cost-effective technologies for improving ESP efficiency when collecting high-resistivity fly ashes.

Acid Gases (Sulfuric Acid)

Even when the primary particulate collection device (ESP or FF) is operating at very high efficiency, stack opacity can be a problem when excessive SO₃ is present in the flue gas. About 1-2% of the sulfur in coal is converted to SO₃ within the coal boiler; although some SO₃ is desirable as a means of decreasing fly ash resistivity and improving ESP collection efficiency (see above), excess SO₃ can condense and combine with water to form sulfuric acid downstream of the primary particulate collection device.  In some cases, excess SO₃ can result in a “blue plume” of sulfuric acid that exceeds opacity limits.   Plants equipped with SCR for NOx removal and wet FGD scrubbers for SO₂ control have experienced the greatest problem in this regard because (1) the SCR catalyst promotes the formation of additional SO₃ in the flue gas; and (2) rapid cooling at the wet FGD inlet creates submicron-size sulfuric acid aerosols that cannot be removed efficiently by the wet FGD device.  Sulfuric acid controls are also of interest to the utility industry because sulfuric acid is a Toxic Release Inventory species and can cause a variety of plant operation problems such as air heater plugging and back-end corrosion.  DOE-NETL has therefore sponsored several projects to develop technology to address the problem of sulfuric acid releases from wet FGD scrubber systems.