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Controlling Power Plant Emissions: Controlling Mercury with Existing Controls
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Controlling Power Plant Emissions
Data from EPA's Mercury Information Collection Request (ICR) revealed that many power plants have existing mercury capture as a co-benefit of air pollution control technologies for NOx, SO2 and Particulate Matter (PM). This includes capture of particulate-bound mercury (Hgp) in PM control equipment and capture of soluble Hg2+ in wet flue gas desulfurization (FGD) systems. Additional data have also shown that the use of selective catalytic reduction (SCR) for NOx control enhances oxidation of Hg0 to the soluble ionic form, resulting in increased removal in the wet FGD system for units burning bituminous coal. A typical configuration of this air pollution control equipment is shown in Figure 1.
Figure 1. Typical Air Pollution Control Equipment Configuration
Figure 2 shows the range of mercury removal from data obtained in the EPA Information Collection Request. Plants that employed only PM controls experienced average Hg (total) emission reductions ranging from 0 to 90 percent. Units with fabric filters (FF) obtained the highest average levels of control while decreasing average levels of control were generally observed for units equipped with only electrostatic precipitators (ESP). For plants that used both PM and SO2 controls, the mercury emission reductions ranged from 0 to 98 percent, with those plants that were burning bituminous coal showing a much higher average removal and those plants that were burning subbituminous and lignite coal showing a much lower average removal. See http://www.epa.gov/airmarkets/progsregs/epa-ipm/index.html for more information on how EPA estimates mercury removal from existing controls.
Overall the ICR data revealed higher levels of Hg capture for bituminous coal-fired plants as compared to low-rank coal-fired plants, large ranges of Hg capture in existing plants, higher levels of Hg capture in fabric filters (FF) compared to electrostatic precipitators (ESPs), and a significant capture of Hg2+ in wet SO2 scrubbers. Based on the ICR data, it is estimated that existing controls remove about 36% of the mercury that is emitted from U.S. coal-fired boilers.
Figure 2. Mercury Removal using Existing Air Pollution Control Equipment (EPA ICR Data)
Mercury Capture in PM Controls
Mercury that is adsorbed (bound) to fly ash and injected sorbent or other particulate will likely be removed in a PM control device. Approximately 72% of U.S. coal-fired boilers use ESPs for PM control; approximately 14% use fabric filters (baghouses). In general, Hg2+ compounds are more readily adsorbed than Hg0. The composition and temperature of the flue gas can significantly affect the adsorption (binding) of mercury to flue gas solids. The presence of chlorine compounds tends to result in increased Hg2+. The composition of the fly ash is also significant. Fly ashes with higher amounts of unburned carbon (UBC) tend to have a higher natural mercury removal capacity. Additionally, the temperature and in-duct residence time also affect the formation and capture of Hgp. Research is ongoing to determine the flue gas conditions that most enhance the formation and capture of particulate-bound mercury.
Mercury Capture in Wet Scrubbers
Elemental mercury (Hg0) vapor is insoluble and is not captured in a wet SO2 scrubber. However, the soluble Hg2+ compounds can be absorbed and captured in the scrubbing solution. Currently wet SO2 scrubbers are installed at plants generating approximately 98 GW of power (33% of total capacity). This is projected to increase to over 185 GW in 2015 as a response to the proposed Clean Air Interstate Rule. SO2 scrubbers are typically installed in plants burning medium-to-high sulfur bituminous coals. These coals tend to naturally produce a greater amount of the ionic Hg2+ compounds. Research is ongoing to maximize mercury removal in wet scrubbers.
The amount of ionic Hg2+ in the flue gas may be enhanced for plants that use selective catalytic reduction (SCR) systems for NOx control. Under the proper conditions, a significant fraction of Hg0 may be catalytically oxidized to the soluble Hg2+ form as it passes through the SCR unit. Plants that burn bituminous coal and use a combination of SCR + PM control + wet SO2 scrubber have the potential to remove very high levels of total mercury. The performance and applicability of SCR-enhanced mercury oxidation is less certain for plants burning low-rank coals. Characterization of SCR impact is being pursued in on-going full-scale tests. Additional research on the effects of catalyst volume and aging is needed. Designing a catalyst specifically for mercury oxidation should also be possible.