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The homogeneous oxidation of elemental mercury (Hg⁰) can promote Hg pollution control in coal-fired power plants, while the mechanisms and quantitative contributions of homogeneous reactions in Hg⁰ oxidation, especially the reactions between Hg and chlorine (Cl), are still unclear. Here, a numerical study on the homogeneous reactions of Hg was conducted within a 600 MW tangentially fired boiler for the first time. A novel Hg sub-model was established by coupling the thermodynamics, reaction kinetics and fluid dynamics. The results showed that the higher Cl content in coal was beneficial to the oxidation of Hg⁰. The homogeneous reactions of Hg mainly occurred in the vertical flue pass at low temperature. Hg⁰ was still the dominant Hg-containing species at the boiler exit, and the concentration of mercury chloride (HgCl₂) was the highest among the oxidized mercury. When low-Cl coal was fired, the addition of a small amount of chlorine species into the boiler at the burnout area increased the ratio of HgCl₂ by over 16 times without causing serious chlorine corrosion problems.