Reaction kinetic study of elemental mercury vapor oxidation with CuCl2

Abstract

In this study, the reaction kinetics for a heterogeneous oxidation reaction of elemental mercury (Hg(0)) vapor with CuCl2 was studied in a fixed-bed reactor using 2%(wt) CuCl2/α-Al2O3 between 100 and 180 °C for Hg(0) oxidation after air preheater at a typical coal-fired power plant. The reaction rate expression was first order with respect to Hg(0). However, between 100 and 180 °C, CuCl2 over α-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. This result corroborates that CuCl2 can enhance Hg(0) oxidation by lowering the activation energy barrier with the reduction of Cu(2+) to Cu(1+) and supplying thermally stable surface Cl sites following a Mars-Maessen mechanism. CuCl2-based catalyst has potential to be applied after air preheater for Hg(0) oxidation followed by the separation of Hg(2+) in wet flue gas desulfurization (FGD) system or by activated carbon (AC) injection.