Oxidation characteristics of calcium sulfite in sintering desulphurized ash

Abstract

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 \begin{document}$ {\text{O}}_{\text{2}}^{-}\;\text{and }\;{\text{SO}}_{\text{3}}^{-} $\end{document} free radicals. The order of the inhibition of CaSO3 oxidation by the three calcium oxides from weak to strong was CaCO3 < Ca(OH)2 < CaCl2. The catalytic effect of Fe2O3 on the oxidation of CaSO3 varies with temperature and concentration. When the temperature is less than 450 ℃ and the weight percentage of Fe2O3 is greater than 0.2%, it plays a certain catalytic role in the oxidation reaction. The doping of Fe2O3 accelerates the formation of \begin{document}$ {\text{O}}_{\text{2}}^{-} $\end{document} and \begin{document}$ \text{S}{\text{O}}_{\text{3}}^{-} $\end{document} 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.

Keywords

• semi-dry desulfurization
• desulfurization ash
• calcium sulfite
• dry method
• oxidation characteristics