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.