Acid Removal for High Temperature Reduction of Divalent Mercury in Hg-CEMS

High-temperature reduction (HTR) is a vital technique in the mercury continuous emission monitoring system (Hg-CEMS) for converting mercuric chloride (HgCl2) into elemental mercury (Hg0) in flue gas. However, the presence of HCl and O2 components in the flue gas poses a significant challenge by promoting the oxidation of Hg0, severely inhibiting the efficiency of HTR. To address this challenge, alkaline acid remover with calcium oxide (CaO) as active component was selected as an effective method for mitigating the impact of acidic gases. In this study, experimental research focusing on the HTR of HgCl2 in an atmosphere containing HCl was conducted. The acid remover activated by CaO was prepared by chemical impregnation. The selection of the optimal precursor, carrier, and loading ratio were investigated through fixed-bed experiments with the characterization techniques such as XRD, BET, SEM and CO2-TPD. The mechanism of acid removing in HTR was deduced that CaO not only captures HCl to prevent O2 from activating it into Cl2 but also adsorbs Cl in HgCl2 to prevent O2 from activating it into ClO. By adsorbing HCl and Cl, the acid remover effectively prevents Cl2 and ClO from oxidizing Hg0, ensuring a high reduction efficiency in HTR. The acid removing effectively solves the Hg0 reoxidation problem in the HTR of HgCl2, providing a solid technical support to application of Hg-CEMS in coal-fired power plants.