Impact of gas impurities on the Hg0 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.