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EFFECTS OF FLY ASH TRANSITION METAL CONTENT AND FLUE GAS HCL/SO2 RATIO ON MERCURY SPECIATION IN WASTE COMBUSTION
Citation:
Ghorishi, S. B., W. Jozewicz, C W. Lee*, AND J D. Kilgroe*. EFFECTS OF FLY ASH TRANSITION METAL CONTENT AND FLUE GAS HCL/SO2 RATIO ON MERCURY SPECIATION IN WASTE COMBUSTION. ENVIRONMENTAL ENGINEERING SCIENCE. Mary Ann Liebert, Inc., Larchmont, NY, 22(2):221-231, (2005).
Description:
The paper presents results of research on the effect of sulfur dioxide (SO2):HCI ratio on heterogeneous Hg0 oxidation. The addition of SO2 to moist flue gas at high SO2:HCI ratios (4:1 to 10:1) caused a decrease in oxidation of Hg0 relative to flur gas without SO2. This is attributed to the scavenging effect of SO2 and H2O on Cl2. Addition of CaO to model fly ashes containing three components from a group composed of alumina (AI2O3), silica (SiO2), ferric oxide (Fe2O3), copper oxide (CuO), and calcium oxide (CaO) also caused a drop in Hg0 oxidation compared to fly ashes without CaO. It is possible that, due to the partial removal of HCI by reaction with CaO, less HCI is available for the catalytic Deacon reaction in which chlorine gas (Cl2) is produced via catalytic oxidation of HCI over CuO and Fe2O3. 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 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 HCI 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 (CuCI), and it was found that CuCl is so reactive that it catalytically oxidizes Hg0 even in the absence of HCI in the simulated flue gas. This is contrary to the behavior shown by CuO, which requires the presence of HCI in the flue gas in order for Hg0 oxidation to occur.