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 (Hg 0 ) poses a challenging flue gas control issue, and oxidized forms of mercury (Hg 2ϩ ) are of concern due to local deposition potential. In a previous study, mercury speciation, or more specifically oxidation of Hg 0 , was studied in simulated flue gases and in the presence of three-component model fly ashes. Gas-phase studies indicated that oxidation of Hg 0 in the presence of hydrogen chloride (HCl) is rather slow, and proceeds at measurable rates only at temperatures Ͼ700 °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 (Al 2 O 3 ), silica (SiO 2 ), ferric oxide (Fe 2 O 3 ), copper oxide (CuO), and calcium oxide (CaO). Transition metal oxides, CuO and Fe 2 O 3 , exhibited significant catalytic activity in the surface-mediated oxidation of Hg 0 in the presence of HCl. The observed Hg 0 oxidation activities of the two oxides are possibly caused by the Deacon process in which chlorine gas (Cl 2 ) is produced via catalytic oxidation of HCl over these two oxides. In the present follow-up study, the effect of sulfur dioxide (SO 2 ) to HCl ratio on Hg 0 oxidation was investigated. The addition of SO 2 to the moist flue gas at high SO 2 :HCl ratios (10:1 to 4:1) caused a decrease in oxidation of Hg 0 . This is attributed to a scavenging effect of SO 2 and H 2 O on Cl 2 . Addition of CaO to the synthetic fly ashes also caused a drop in Hg 0 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 Hg 0 oxidation activity of a cement kiln dust (CKD) sample collected from a full-scale hazardous waste incinerator was also studied. Qualitatively, it exhibited Hg 0 oxidation catalytic behavior sim-221

Impact of Fly Ash Composition on Mercury Speciation in Simulated Flue Gas

“…Such findings agree with results from Thorwarth et al 15 and Ghorishi et al, 25 but they do not necessarily mean that these oxides have no effects on mercury uptake or oxidation. They may inhibit mercury uptake or oxidation by consuming acid gases such as HCl, which is especially true for CaO.…”

“…Some of the fly ash components promote oxidation of Hg°, whereas others do not. Ghorishi et al 25 studied the effects of synthetic model fly ash components on mercury speciation. The results showed that transition metal oxides, ferric oxide (Fe 2 O 3 ) and copper oxide (CuO), exhibited significant catalytic activity in oxidation of Hg°in the presence of hydrochloric acid (HCl) in simulated flue gas.…”

High temperature behavior of electrostatic precipitator ash from municipal solid waste combustors

“…However, the high chlorine partial pressure prevailing during waste combustion may cause the oxidation of mercury, and chloride mercury 13 species may be the major form in the cooling process of flue gas, as shown by Ghorishi et al. [23], Chen et al [24]. Lead, cadmium and copper show similar evaporation curves, with the principal volatilization between 650 and 1000 °C.…”

Density functional theory study of mercury adsorption on metal surfaces

“…3 In order to ensure the efficiency of mercury removal via WFGD units, there is a need for effective and regenerable mercury oxidation catalyst materials, and noble metals such as copper, silver, gold, and palladium are being investigated for this application. [4][5][6] Noble metals including platinum, gold, palladium, and copper have been investigated for incorporation in novel mercury removal sorbents, both for conventional coal-fired power plants and in the context ͑i.e., different temperature and pressure͒ of new integrated gasification combined-cycle ͑IGCC͒ power generation plants. 7,8 Despite the interest in using metals in mercury removal, very little research has been carried out to determine basic information about the binding of mercury to metal surfaces.…”