Simultaneous Removal of SO₂ and Hg⁰ by Composite Oxidant NaClO/NaClO₂ in a Packed Tower

Based on the implementation of the global Minamata Convention, developing an efficient and economical technology for mercury reduction in coal-fired flue gas becomes a hotspot in the field of air pollution control. The composite oxidant NaClO/NaClO₂ combined with limestone was used in the simultaneous removal of SO₂ and Hg⁰ in this study, and the three-factor and four-level orthogonal experiments were performed in a packed tower. The influential sequence of various factors on SO₂ and Hg⁰ removals was investigated through range analysis of the orthogonal experiments. Results showed that factors affecting desulfurization was C > A > B (liquid-gas ratio > oxidant concentration ratio > initial pH of absorption liquid), while factors affecting Hg⁰ removal was A > C > B (oxidant concentration ratio > liquid-gas ratio > initial pH of absorption liquid). Optimum conditions of simultaneous desulfurization and demercuration by NaClO/NaClO₂ were A4B1C4; that is, the oxidant concentration ratio was 10/4 (mmol/L:mmol/L), the initial pH was 5, and the liquid-gas ratio was 18 (L/m³). The simultaneous removal efficiencies of SO₂ and Hg⁰ reached 99.5 and 85.4% under these optimum conditions, respectively. Analysis of the characteristics of the solid products showed that the main products of the wet oxidation were CaSO₄ and CaSO₃. Analysis of the existing form of oxidized mercury showed that 23% of mercury was in the gypsum, while 77% was in the supernatant. Results of this research would provide a practical reference for promoting the simultaneous removal of SO₂ and Hg⁰ by NaClO/NaClO₂ with limestone in industrial application.