Crystallization and Fracture:  Product Layer Diffusion in Sulfation of Calcined Limestone

Sintered samples of calcined limestone were sulfated for extended times in a differential reactor to study the reaction kinetics after formation of a product layer. The reaction rate was controlled by product layer diffusion, except during the initial period. The product layer diffusivity increased with temperature, but decreased with increasing SO 2 concentration. The diffusivity not only depends on current conditions, but also on previous conditions that led to the formation of the layer. The results support a crystallization and fracture model and a hypothesis that the rate-limiting mechanism changes from inward gas diffusion control in the early stages to outward ionic diffusion control after formation of a continuous product layer, a change attributed to the need for the reaction to do mechanical work to displace the product layer and make room for increased solid volume at the CaO/CaSO 4 interface. A criterion is established to determine whether a product layer can be fractured. A gas containing 2250 ppm SO 2 at 900 °C cannot fracture a product layer as thick as 233 nm, while such a layer can be fractured by steam at 250 °C and a partial pressure of 1 bar.