Effects of Chlorine Addition on Nitrogen Oxide Reduction and Mercury Oxidation over Selective Catalytic Reduction Catalysts.

The effect of chlorine on mercury oxidation and nitrogen oxides (NO ) reduction over selective catalytic reduction (SCR) catalysts was investigated in this study. Commercial SCR catalysts achieved a high Hg oxidation efficiency when Cl was sprayed into the flue gas. Results indicated that an appropriate concentration of Cl was found to promote NO reduction and Hg oxidation significantly. An optimal concentration of Cl (25 ppm) was found to significantly promote NO reduction and Hg oxidation. Moreover, we studied the effects of Cl on NO reduction and Hg oxidation over SCR catalysts under different concentrations of SO. The SO poisoning effect was decreased by Cl when the SO concentration was low (below 1500 ppm). However, sulfate gradually covered the catalyst surface over time during the reaction, which limited the impact of Cl. Finally, different sulfur-poisoned catalysts were examined in the presence of Cl. The NO reduction and Hg oxidation performances of sulfate-poisoned catalysts improved when Cl was added to the flue gas. Mechanisms for NO reduction and Hg oxidation over fresh catalysts and sulfate-poisoned catalysts in the presence of Cl were proposed in this study. The mechanism of Cl-influenced NO reduction was similar to that for the NH-SCR process. With Cl in the flue gas, the number of Brønsted active sites increased, which improved catalytic activity. Furthermore, Cl reoxidized V-OH to V=O and caused the NH-SCR process to operate continuously. The Langmuir-Hinshelwood mechanism was followed for Hg oxidation by SCR catalysts when Cl was in the flue gas. Cl increased the number of Lewis active sites, and catalytic activity increased. Hg adsorbed on the surface of the catalysts and was then oxidized to HgCl. Adding Cl to the flue gas increased the strength and number of acid sites on sulfate-poisoned catalysts.