Bench- and Pilot-Scale Studies on Mercury Removal by Potassium Iodide in Coal-Fired Flue Gas

Mercury (Hg) is a toxic air pollutant with coal-fired utility plants being the largest anthropogenic Hg emission source in the United States. The extent that Hg can be removed from power plants using conventional air pollution control devices (APCDs) is significantly affected by Hg speciation in flue gas. Oxidized Hg (Hg) is preferable to elemental Hg (Hg) in that Hg is less volatile and soluble in water and thus is easier to be captured through wet scrubbers and/or electrostatic precipitators. Chlorine is the major halogen species in coal and it plays a very important role in both homogeneous and heterogeneous Hg oxidation in flue gas. 4 Other halogen species have also demonstrated their abilities to oxidize Hg. 6 It has also been reported that activated carbon impregnated with sulfur, chlorine, and iodine has greater ability of capturing Hg compared with untreated carbon. 7, 8 Particularly, iodine impregnated activated carbon showed superior capacity of Hg capture. 9 Another advantage of using iodine promoted sorbent is the lower volatility of HgI2 (higher boiling point) compared to other mercury halides, which enhances the stability of the spent sorbent. On the other hand, injection of carbon based sorbents can compromise fly ash properties due to the residual carbon content, and thus, the operation cost will be increased since the fly ash is no longer sellable. Therefore, non-carbon based sorbent with an iodine promoter is preferred.