Hg can exist in three major forms
Hg can exist in three major forms in coal-burning flue gas streams: elemental Hg (Hg0), oxidized Hg (Hg2+), and particle-bound Hg (Hgp) . Compared to Hg2+ and Hgp, Hg0 is much more difficult to remove with conventional air pollution control devices due to its high volatility and insolubility in water. Numerous technologies have been developed for effectively removing low-concentration Hg0, including adsorption using such as activated carbon and zeolite  and catalytic HATU using metal oxide composites , , , , , , , , , ,  and . Removal of Hg0 via catalytic oxidation of Hg0 into Hg2+ using metal oxide catalysts has been extensively examined recently because the converted Hg2+ is water-soluble and can be readily removed by a subsequent wet flue gas desulfurization (FGD) system. Conceptually, there is no need to install additional control devices for Hg0 control because DeNOx selective catalytic reduction (SCR) catalyst and FGD systems have been set in existing coal-fired power plants for NOx and SOx removal, respectively.