Reaction of Ca(OH)(2) with SO2 at low temperature

Lime is widely utilized as a sorbent in dry and semidry sulfur removal processes. In this study limestone was calcined at high temperature and lime was produced by water vapor steam treatment at low temperature. Experiments were performed at conditions that also cover those usually present in coal-fired power plants. The investigation of the reaction of lime with synthetic flue gas showed that the relative humidity of the gas has a major impact on the reactivity. Also the presence of oxygen and/or carbon dioxide does not show a significant influence on the sulfur dioxide removal reaction though some intermediate formation of calcium carbonate cannot be excluded. Mainly calcium sulfite hemihydrate was found to be the reaction product, but some calcium sulfate hemihydrate was also formed when oxygen was present. Four regions with different prevailing reaction mechanisms can be defined: In the beginning the formation of a monolayer is mainly determined by the SO2 concentration and to some smaller extent also by the relative humidity. This period is followed by the formation of the consecutive product layers which can initially be determined by a dissolving process of SO2. Afterward this formation is only determined by the relative humidity and most likely by product layer diffusion. Above a conversion of around 9% reaction rate drops significantly which can be due to pore closure.