Effects of Fly Ash Transition Metal Content and Flue Gas HCl/SO2 Ratio on Mercury Speciation in Waste Combustion

Abstract

Understanding the transformation of mercury species (elemental and oxidized forms) in stationary combustion sources is of interest to both those responsible for developing control technologies and those responsible for setting regulations. Elemental mercury (Hg0) poses a challenging flue gas control issue, and oxidized forms of mercury (Hg2+) are of concern due to local deposition potential. In a previous study, mercury speciation, or more specifically oxidation of Hg0, was studied in simulated flue gases and in the presence of three-component model fly ashes. Gas-phase studies indicated that oxidation of Hg0 in the presence of hydrogen chloride (HCl) is rather slow, and proceeds at measurable rates only at temperatures >700 °C and HCl concentrations in the range of 100–200 ppm. The effect of fly ash composition was investigated using a fixed-bed reactor containing different synthetic model fly ash components such as alumina (Al2O3), silica (SiO2), ferric oxide (Fe2O3), copper oxide (CuO), and calcium oxide (CaO). Transition metal oxides, CuO and Fe2O3, exhibited significant catalytic activity in the surface-mediated oxidation of Hg0 in the presence of HCl. The observed Hg0 oxidation activities of the two oxides are possibly caused by the Deacon process in which chlorine gas (Cl2) is produced via catalytic oxidation of HCl over these two oxides. In the present follow-up study, the effect of sulfur dioxide (SO2) to HCl ratio on Hg0 oxidation was investigated. The addition of SO2 to the moist flue gas at high SO2:HCl ratios (10:1 to 4:1) caused a decrease in oxidation of Hg0. This is attributed to a scavenging effect of SO2 and H2O on Cl2. Addition of CaO to the synthetic fly ashes also caused a drop in Hg0 oxidation. It is possible that due to the partial removal of HCl by reaction with CaO less HCl is available for the catalytic Deacon reaction. The Hg0 oxidation activity of a cement kiln dust (CKD) sample collected from a full-scale hazardous waste incinerator was also studied. Qualitatively, it exhibited Hg0 oxidation catalytic behavior similar to that observed with CaO-containing model fly ashes. According to this study, only moderate oxidation of Hg0 (40%) can be expected in cement kilns burning hazardous waste. During hazardous and municipal waste combustion, the presence of sufficiently high HCl and/or Cl2 in the gas phase may cause the metallic oxide compounds in the fly ash to be converted into metal chlorides. Additional tests were performed using synthetic fly ashes containing cuprous chloride (CuCl). These results were compared to those obtained using CuO. It was found that CuCl is so reactive that it oxidizes Hg0 even in the absence of HCl in the simulated flue gas. This is contrary to the behavior shown by CuO, which requires the presence of HCl in the flue gas in order for Hg0 oxidation to occur.