Kinetic modeling of mercury oxidation by chlorine over CeO2-TiO2 catalysts

Hg0 oxidation by chlorine over CeO2-TiO2 (CeTi) catalysts with different CeO2/TiO2 ratios was investigated both in the absence and the presence of SO2. Experimental data were analyzed using a kinetic model incorporating mass transfer, adsorption equilibrium and mass balance with key variables of interest being residence time, catalyst type and HCl concentration in simulated coal combustion flue gas. The experimental data fitted the proposed kinetic model very well, indicating the validity of the model for describing the mechanism of Hg0 oxidation by chlorine over CeTi catalysts. Important kinetic parameters, i.e. the overall reaction rate constant and the Langmuir adsorption constant, were obtained by fitting the model with experimental data. Without SO2, pure CeO2 exhibited the best performance in catalyzing Hg0 oxidation by chlorine, which had a high reaction rate constant k of 130 ± 16.3 s−1. An addition of SO2 did not inhibit the adsorption of HCl on pure CeO2; however, it inhibited reactions between Hg0 and chlorine species over pure CeO2, hence resulting in a decrease of Hg0 oxidation efficiency. Combining CeO2 with TiO2 enhanced its resistance to SO2. Ce1.5Ti exhibited excellent resistance to SO2. Both the overall reaction rate constant and the Langmuir adsorption constant maintained at the same level when 400 ppm SO2 was introduced to the flue gas containing N2 and HCl. The identification of the kinetics of heterogeneous oxidation of Hg0 by chlorine in this study is of fundamental importance for scaling up mercury removal by selective catalytic reduction (SCR) plus wet flue gas desulfurization system (WFGD) from a laboratory system to a pilot or full scale.