Mechanism of CaO reaction with H2S:: Diffusion through CaS product layer

Heterogeneous non-catalytic reaction between CaO and H2S to capture sulfur in coal-fired gasifier is studied in a differential bed flow-through reactor. Particles of CaO (<10 mu m) are exposed to simulated fuel gas containing up to 5000 ppm H2S in the temperature range 700-900 degrees C. The effects of reaction temperature, H2S concentration, and sorbent surface area on the extent of sulfur capture and conversion of sorbent are determined. The sulfidation reaction is analyzed using grain model with overall reaction rate controlled by chemical reaction at the interface and by diffusion through the product layer. At the later stages of the reaction, diffusion through the product layer is found to be controlling and involves migration of ions through the product layer. Direct experimental evidence is obtained to determine the migratory nature of ions and identify the mobile ionic species, by performing inert marker experiments. Scanning electron micrographs and sulfur mapping of the partially sulfided CaO tablets suggest that S2- diffuse in towards the CaO/product layer interface with a counter-diffusion of O2- rewards the product layer/gas interface. (C) 1999 Elsevier Science Ltd. All rights reserved.