Impact of gas impurities on the Hg 0 oxidation on high iron and calcium coal ash for chemical looping combustion

Coal-based mercury pollution from power plants has received increasing attention. In a previous study, high iron and calcium coal ash (HICCA) was found as a promising oxygen carrier (OC) for chemical looping combustion (CLC). The purpose of this study was to investigate the catalytic effect of HICCA on Hg0 removal as well as the impacts of several gas impurities, such as HCl, SO2, and NO. Experiments on Hg0 removal efficiencies for different atmospheres were performed in the fixed-bed reactor at 850 °C. Based upon the characterization of BET, SEM, XRD, XPS, and EDS of reaction products, the reaction mechanisms of different gases with the HICCA samples were established. The mechanisms were further explained using the thermodynamic equilibrium calculations. The experimental results showed that the Hg0 removal efficiency using HICCA was 11.60%, while the corresponding value in the presence of 50 ppm HCl was 90.46%. Hg0 removal by HICCA involving HCl is mainly attributed to homogeneous reaction between Hg0 and HCl as well as the formation of reactive species (Cl, Cl2, Cl2O, O, S, and SCl2) through the reactions of HCl with Fe2O3 and CaSO4 in HICCA. The formation of C-Cl bond is not the main pathway for the promotional effect of HCl on Hg0 removal. SO2 played a negative role in Hg0 removal by HICCA. The inhibition of SO2 may be attributed to its effect on the reduction of Fe2O3 and its bonding with C-O, COOH, and C(O)-O-C. NO enhanced Hg0 removal by HICCA primarily through the homogeneous reactions of Hg0 with N2O and O. In addition, NO also interacted with HICCA and promoted the heterogeneous oxidation of Hg0 by producing more C-O, C=O, and COOH/C(O)-O-C on HICCA surface. This study proved the effectiveness of HICCA on Hg0 removal in iG-CLC and revealed the mechanisms of the interaction between HCl/SO2/NO and MxOy/CaSO4 as well as carbon-oxygen groups.