Isotope study on diffusion in CaSO4 formed during sorbent‐flue‐gas reaction

Abstract

In sorbent-flue-gas reactions, porous CaO sorbent particles are used to capture SO{sub 2} by formation of CaSO{sub 4}. Because of the large molar volume of CaSO{sub 4}, the internal surface area which is originally available for reaction diminishes as CaSO{sub 4} forms. Once the CaSO{sub 4} layer forms, further sorbent sulfation is believed to be controlled by the product layer diffusion process. It has been suggested that the product layer diffusion occurs by gaseous diffusion (Simons and Garman, 1976) and by ionic diffusion (Bhatia and Perlmutter, 1981). In this work, a two-stage sulfation experiment using {sup 32}SO{sub 2} and {sup 34}SO{sub 2} was performed. For the first stage of sulfation, at 1,300 C, 5,000 ppm {sup 32}SO{sub 2}/air mixture was passed into the mullite tube and circulated out through the bubbler continuously. This stage lasted for 14 days. When the first stage was terminated, the tablets were removed from the furnace and examined. At the beginning of the second stage sulfation, 5,000 ppm {sup 32}SO{sub 2}/air mixture was first used during the heating period. As soon as the tube temperature reached 1,300 C, the mechanical pump was turned on and the pressure in the tube was reduced immediately. Upon themore » completion of the evacuation, isotope gas 75%{sup 34}SO{sub 2}-25%{sup 32}SO{sub 2} was introduced into the mullite tube. Appropriate amount of air was also introduced into the tube such that the total SO{sub 2} concentration was roughly 5,000 ppm. The second stage sulfation lasted for three days. The SIMS analysis was performed by Microelectronics Center in North Carolina.« less