Oxidation characteristics of calcium sulfite in sintering desulphurized ash

Considering the variation in dry heat oxidation of CaSO₃ in sintering semidry desulfurization ash under different reaction conditions, the effects of temperature, O₂ content and flow rate in gas, calcium compounds, iron oxide (Fe₂O₃), water vapor content, and flow rate on CaSO₃ oxidation were evaluated. It was determined that the reaction adheres to the Arrhenius equation. The oxidation rate of CaSO₃ increases from 380 ℃. Moreover, at 450 ℃, the oxidation rate of CaSO₃ exceeds 90%, and at 550 ℃, it is completely oxidized (98.2%). Under the condition of 10 ℃·min⁻¹ in the air atmosphere, the gas flow rate of 450 ℃ and 75 mL·min⁻¹ is the optimal process condition for economic dry heat oxidation. Water vapor is present on both sides of the CaSO₃ oxidation reaction. Moreover, the oxidation of CaSO₃ by calcium oxides was inhibited by inhibiting the generation of \textO_\text2^-\;\textand \;\textSO_\text3^- free radicals. The order of the inhibition of CaSO₃ oxidation by the three calcium oxides from weak to strong was CaCO₃ < Ca(OH) ₂ < CaCl ₂. The catalytic effect of Fe₂O₃ on the oxidation of CaSO₃ varies with temperature and concentration. When the temperature is less than 450 ℃ and the weight percentage of Fe₂O₃ is greater than 0.2%, it plays a certain catalytic role in the oxidation reaction. The doping of Fe₂O₃ accelerates the formation of \textO_\text2^- and \textS\textO_\text3^- free radicals. When the temperature exceeds 450 ℃ and the catalyst concentration is less than 0.2%, the catalyst concentration has no effect on the reaction process, and the temperature takes precedence. The microscopy analysis reveals that with the oxidation of CaSO₃ to CaSO₄, the morphology shifts from cluster to column. Furthermore, CaCl₂ inhibits not only the oxidation reaction but also the crystal form of CaSO₄. Fe₂O₃ aids the formation of CaSO₄ crystals. When the temperature exceeds 400 ℃, the internal temperature of desulfurized ash is higher than 500 ℃ for 5 min. Simultaneously, the conversion rate of CaSO₃ is greater than 85%, and the pilot test temperature is slower, which lacks this feature. The Gibbs free energy calculation results show that the most likely reaction is the oxidation of CaSO₃ and that decomposition of calcium below 600 ℃ is not feasible. The number of active sites in the process of CaSO₃ oxidation is proportional to temperature. Thus, when the temperature is between 623 and 723 K, the reaction is a first-order reaction. When the temperature exceeds 723 K, the reaction will be completed quickly in about 5 min, and the reaction order cannot be determined.