A plausible model for the sulfidation of a calcium-based core-in-shell sorbent

Abstract

A promising reusable sorbent for desulfurizing hot coal gas is being developed in the form of core-in-shell pellets which consist of a highly reactive CaO core encased in a porous protective shell made largely of an inert material. The spherical pellets are made by pelletizing plaster of Paris and then applying a coating of alumina (80%) and limestone (20%) particles. Subsequent high temperature treatment converts the cores to CaO and sinters the coating material to form the porous shells. The pellets absorb and react with H₂S at high temperatures (e.g., 800-900°C) to form CaS and are regenerated by applying a cyclic oxidation/reduction method. The rate of sulfidation does not appear to be controlled by chemical reaction but, instead, seems to be controlled by one or more of the following diffusion resistances: external gas film, porous shell, micropores between CaO grains in the core, and product layer surrounding CaO within each grain. After considering various models, a brief review of which is included in the paper, a semiempirical model was chosen which represents the process well over a limited range of conversion (approximately 0-85%). This is not a serious limitation for the model because above this range the rate of conversion becomes too slow for most applications. The model assumes that within this range the rate of conversion is controlled by the resistance to diffusion offered by the porous outside shell together with the resistance to diffusion presented by a porous layer of reacted material. The model is in effect an extension of the well-known shrinking, unreacted core model for gas-solid reactions. As more basic diffusion data become available, a more rigorous model, such as a modified grain model, is expected to be developed.