Kinetics of the Reaction of Dense CaO Particles with SO2

Abstract

The kinetics of the reaction of dense CaO particles (2.5 mm dia.) with sulfur dioxide was studied using a thermogravimetric technique under the conditions of 750–950 °C, 1000–5000 ppm SO2, and 6 h reaction time. The growth of CaSO4 product layer was observed using a scanning electron microscope. The effect of internal structural change due to reaction, which complicates the kinetic analysis of porous particles, was eliminated in the present case. The thickening of the product layer behaved as the shrinking-core type reaction. The product layer was composed of many CaSO4 grains, the size of which increased singnificantly with increasing temperature. The long-term reaction kinetics was well described by the shrinking-core model for changing particle size by considering the product layer diffusion and intrinsic chemical reaction. The apparent reaction order of SO2 was 0.3 and close to 1.0 only at low CaO conversions. The values of chemical reaction rate constant and the activation energy for chemical reaction rate constant or product layer diffusivity were in the range reported by previous investigators, but the values of the product layer diffusivity were much higher. The model equations for the case involving the inward or outward solid-state diffusion of ions were derived. The reaction kinetics can be explained by considering both the diffusion of Ca2+ and O2– ions and the diffusion of gaseous reactants. The product layer tended to fracture continually and form more cracks at higher temperatures, thus resulting in higher values of product layer diffusivity and a high activation energy for diffusivity, and leading to an apparent SO2 reaction order smaller than first order.