The reaction mechanism of Hg(0) vapor oxidation over cupric chloride (CuCl2) was investigated using 10%(wt) CuCl2/a-Al2O3 for mercury emissions control from coal-fired power plants. The CuCl2/a-Al2O3 sample showed &gt;90% Hg(0) oxidation with excellent resistance to SO2 at 140 °C in a simulated flue gas containing 10 ppmv HCl, 2,000 ppmv SO2, and 6% O2 gases balanced with N2 gas. In the absence of HCl and O2 gases, Hg(0) vapor was found to be oxidized by consuming atomic Cl of CuCl2 and reducing it to CuCl, following a Mars-Maessen mechanism. The reduced CuCl could be re-chlorinated back to CuCl2 by replenishing empty Cl atoms under 10 ppmv HCl and 6% O2 gases readily present in coal combustion flue gases even under 2,000 ppmv SO2 concentration for continuous Hg(0) oxidation. Cl2 gas generation was not observed over CuCl2 at 140° C, indicating a heterogeneous catalytic reaction. Then the reaction kinetics was expressed by a first-order reaction with respect to Hg(0). However, between 100-180° C, CuCl2 over a-Al2O3 agglomerates and sinters. This sintering effect added significant mass-transfer resistance to the diffusion of Hg(0) vapor, and thus made the conversion of CuCl2 incomplete. Therefore, a grain model was formulated to determine the rate constant by taking into account the mass-transfer resistance. The model constituted a two-parameter estimation problem for the determination of the rate constant and product layer diffusivity. The model predictions with the two optimum parameters were in good agreement with the experimental data. The activation energy value determined from the rate constant values was significantly lower than those for other Hg(0) oxidation catalysts under HCl and O2 gases reported in the literature.

Oxidation reaction mechanism and kinetics of elemental mercury vapor over CuCl2

256th National Meeting (2018)

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