Mercury control research: effects of fly ash and flue gas parameters on mercury speciation

Abstract

Mercury Control Research: Effects of Fly Ash and Flue Gas Parameters on Mercury Speciation Chun Wai Lee and James D. Kilgroe U.S. Environmental Protection Agency National Risk Management Research Laboratory Research Triangle Park. NC S. Behrooz Ghorishi ARCADIS Geraghty & Miller, Inc. Research Triangle Park, NC In flue gas from combustion systems, mercury (Hg) is typically in the vapor phase at flue gas cleaning temperatures, and the control of Hg emissions is dependent on the specific Hg compounds that are present (speciation) and the type of air pollution devices employed. In dry and semi-dry scrubbing systems, the control of Hg emissions is dependent on the sorption of Hg by particulate matter (PM) which can be subsequently collected in a PM control device. In wet scrubbing systems, the principal mechanisms of control are the removal of soluble forms of Hg and the collection of particle-bound Hg. At combustion temperatures, Hg is believed to be predominantly in the form of elemental mercury (Hgo). As the flue gas is cooled, thermochemical equilibrium calculations indicate that Hgo is converted primarily to ionic mercury (Hg++) in the form of mercuric oxide (HgO) or mercuric chloride (HgCl2). Hgo is insoluble in water, but HgO has a low solubility while HgCl2 is highly soluble. The oxidation of Hgo to an ionic form depends on the temperature, the time-temperature proflle, the flue gas composition, the reaction kinetics, and the presence of solids that may catalyze reactions. Bench-scale experiments were conducted to study the effects of flue gas and fly ash parameters on the oxidation of HgOin simulated flue gases containing hydrogen chloride (Hel). Gas-phase studies indicated that the in-flight post-combustion oxidation of Hgo in the presence of HCI is very slow and proceeds at measurable rates only at high temperatures (>700 °C) and high HCI concentrations (>200 ppm). The presence of sulfur dioxide (S02) and water vapor in the simulated flue gas significantly inhibited the gas-phase oxidation of Hgo. On the other hand, a preliminary investigation indicated that the gas-phase reaction of Hgo with chlorine (Cl:z) is fast At 40°C and in the presence of 50 ppm C12, 100% of the input Hgo was oxidized to HgCl2 in less than 2 seconds, indicating that Cl2 is a much more active chlorinating agent than HCl. The effects of fly ash composition were investigated using a fixed-bed reactor containing different model fly ashes (simulated fly ash) consisting of mixtures of some major components found in coal and municipal waste combustor (MWC) fly ashes. Work to date has focused on the