Effects of Fly Ash Transition Metal Content and Flue Gas HCl/SO 2 Ratio on Mercury Speciation in Waste Combustion

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.