CONTROL OF MERCURY EMISSIONS
FROM COAL-FIRED ELECTRIC UTILITY BOILERS
Air Pollution Prevention and Control Division
National Risk Management Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, NC
Introduction
During combustion, the mercury (Hg) in coal is volatilized and converted to elemental mercury
(Hg0) vapor in the high temperature regions of coal-fired boilers. As the flue gas is cooled, a
series of complex reactions begin to convert Hg0 to ionic mercury (Hg2+) 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,
Hg0 oxidation reactions are kinetically limited and, as a result, Hg enters the flue gas cleaning
device(s) as a mixture of Hg0, Hg 2+, and Hgp. This partitioning of Hg into Hg0, Hg 2+, 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 Hg0.
Control of mercury emissions from coal-fired boilers is currently achieved via existing controls
used to remove particulate matter (PM), sulfur dioxide (SO2), and nitrogen oxides (NOx). This
includes capture of Hgp in PM control equipment and soluble Hg 2+ compounds in wet flue gas
desulfurization (FGD) systems. Available data also reflect that use of selective catalytic
reduction (SCR) NOx control enhances oxidation of Hg0 in flue gas and results in increased
mercury removal in wet FGD.
Table 1 shows the average reduction in total mercury (HgT) emissions developed from EPA’s
Information Collection Request (ICR) data on U.S. coal-fired boilers. Plants that employ only PM
c