Kinetics of H2S reaction with calcined calcium-based sorbents

Sulfidation experiments at atmospheric pressure in a thermogravimetric analyzer were performed with two calcined limestones and a fully calcined dolomite at temperatures between 450 and 700 degrees C and with sorbent particle sizes between 0.4 and 1.6 mm. The effects of reaction temperature, sorbent particle size, and H2S concentration were analyzed. The sulfidation rate of the dolomite and a limestone increased with temperature in all of the ranges tested. However, the sulfidation rate of the other limestone increased with temperature until 600 degrees C and was nearly constant from this temperature. The temperature effect on the conversion versus time curves diminished with increasing particle size. At low temperatures (<500-550 degrees C) the sulfidation rate did not depend on sorbent particle size. On the other hand, at temperatures above 500-550 degrees C the sulfidation rate increased with decreasing sorbent particle size due to the effect of the intraparticle diffusion. This finding was in agreement with the SEM-EDX sulfur distribution profiles measured in partially sulfided sorbent particles. The sulfidation rate increased when the H2S pressure was raised, and a reaction order in H2S of 1 could adequately describe the sulfidation reaction of the sorbents. The changing grain size model proposed by Georgakis et al. together with the molecular scale hypotheses of Attar and Dupuis was used to determine the kinetic parameters of the reaction and to tests its ability to predict the data for the sulfidation reaction over a broad range of particle sizes and temperatures. Good agreement between measured and predicted conversion time curves and sulfur distributions inside the particles was observed. The values of the activation energies varied from 29.1 to 56.5 kJ mol(-1) to calculate the chemical reaction rate constant, k(s), and from 154.6 to 217.5 kJ mol(-1) to calculate the product layer diffusion coefficient, D-s. Analysis of the kinetic parameters showed that the resistance due to the chemical reaction on the overall reaction rate increased in relation to the resistance of the H2S diffusion through the product layer with increasing the reaction temperature. This effect was very similar for the two limestones and higher than for the dolomite.