Study of Calcium-Based Sorbents for High-Temperature H2S Removal. 1. Kinetics of H2S Sorption by Uncalcined Limestone

One of the most important needs in coal gasification is the development of reliable methods of removing solid and gaseous contaminants from coal gas at high temperatures and pressures. In about 160 experiments, +6 mesh (1--2 mm diameter) limestone samples were exposed to an atmosphere of CO{sub 2} (87--88%), H{sub 2} (1--2%), CO (4--5%), H{sub 2}O (4--5%), and H{sub 2}S (0.5--1.85%) for various temperatures (570--860 C) and durations (5--240 min). Limestone sulfidation was followed quantitatively as well as qualitatively to elucidate the reaction mechanism. The morphology of the inside and of the outside of reacted limestone samples was observed with a scanning electron microscope, and X-ray maps of the sulfur content of the reacted samples were generated. The reaction between H{sub 2}S and limestone is first-order with respect to the H{sub 2}S partial pressure in the 560--660 C range, where the kinetics is controlled by the chemical reaction with an activation energy of about 39 kcal/mol. Between 660 and 710 C the reaction rate decreases as the temperature increases and the apparent order of the reaction changes from first- to half-order. Finally, in the 710--860 C range, the reaction becomes controlled by solid-state diffusion through the CaS product layer withmore » an activation energy of 30--40 kcal/mol.« less