Control of Mercury Emissions from Coal-fired Electric Utility Boilers
During combustion, the mercury (Hg) in coal is volatilized and converted to elemental mercury (Hg°) vapor in the high temperature regions of coal-fired boilers. As the flue gas is cooled, a series of complex reactions begin to convert Hg° to ionic mercury (Hg2T) compounds and/or Hg compounds (Hgp) that are in a solid-phase at flue gas cleaning temperatures or Hg that is adsorbed onto the surface of other particles. The presence of chlorine gas-phase equilibrium favors the formation of mercuric chloride (HgCl2) at flue gas cleaning temperatures. However, Hg° oxidation reactions are kinetically limited and. as a result. Hg enters the flue gas cleaning device(s) as a mixture of Hg°, Hg 2~, and Hgp. This partitioning of Hg into Hg°, Hg ~+, and Hgp is known as mercury speciation, which can have considerable influence on selection of mercury control approaches. In general, the majority of gaseous mercury in bituminous coal-fired boilers is Hg2*. On the other hand, the majority of gaseous mercury in subbituminous- and lignite-fired boilers is Hg°.
Control of mercury emissions from coal-fired boilers is currently achieved via existing controls used to remove particulate matter (PM), sulfur dioxide (S02), and nitrogen oxides (NOx). This includes capture of Hgp in PM control equipment and soluble Hg'' compounds in wet flue gas desulfurization (FGD) systems. Available data also reflect that use of selective catalytic reduction (SCR) NOx control enhances oxidation of Hg° in flue gas and results in increased mercury removal in wet FGD.
Table 1 shows the average reduction in total mercury (Hgx) emissions developed from EPA's Information Collection Request (ICR) data on U.S. coal-fired boilers. Plants that employ only PM controls experienced average HgT emission reductions ranging from 0 to 90 percent. Units with fabric filters (FFs) obtained the highest average levels of control. Decreasing average levels of control were generally observed for units equipped with a cold-side electrostatic precipitator (CS-ESP), hot-side ESP (HS-ESP), and particle scrubber (PS). For units equipped with dry scrubbers, the average HgT emission reductions ranged from 0 to 98 percent. The estimated average reductions for wet flue gas desulfruization (FGD) scrubbers were similar and ranged from 0 to 98 percent.
As seen in Table 1, in general, the amount of Hg captured by a given control technology is greater for bituminous coal than for either subbituminous coal or lignite. For example, the average capture of Hg in plants equipped with a CS-ESP is 36 percent for bituminous coal, 3 percent for subbituminous coal, and 0 percent for lignite. Based on ICR data, it is estimated that existing controls remove about 36% of the 75 tons of mercury input with coal in U.S. coal-fired boilers. This results in current emissions of 48 tons of mercury.
There are two broad approaches to mercury control: (1) activated carbon injection (ACI), and (2) multipollutant control in which Hg capture is enhanced in existing/new S02, NOx, and PM control devices. Relative to these two approaches, this paper describes currently available data, limitations, estimated potential, and Research Development and Demonstration (RD&D) needs. Depending on levels appropriated by congress, EPA may not be able to continue it's review of mercury removal technologies in fiscal year 2004.