Structural changes in zinc ferrites as regenerable sorbents for hot coal gas desulfurization

It is well known that zinc ferrites can be used as regenerable sorbents for hot coal gas desulfurization in IGCC processes. The H2S concentration in the outlet gas from a fixed-bed reactor can be as low as a few ppmv and the sorbent conversion at breakthrough may be above 70%. indicating that the sorbent reactivity in the sulfidation process is high. Experiments show that the reactivity is highly dependent on the calcination temperature used for spinel formation, the concentration of component oxides above or below stoichiometry and the possible use of gamma -alumina as an inert diluent or support. These features are frequently associated with structural changes occurring both in the fresh or regenerated and in the sulfided sorbent, but, unfortunately, these changes are not detected by the usual characterization techniques. In this study, the structural changes occurring in zinc ferrites during calcination at high temperature or regeneration in oxidant atmosphere and those in iron sulfides during the sulfidation process have been investigated in detail by Mossbauer spectroscopy (MS) and powder X-ray diffraction (XRD) using Rietveld methods for structure refinement. The study reveals that fresh sorbents prepared by calcination at low temperature consist of lattice-strained small domain-size zinc ferrites that relax if the calcination temperature is high enough. Structural changes are also observed through the formation of different types of iron sulfides in the sulfidation process. These apparent structural changes can explain the changes exhibited in the sulfidation reactivity of the sorbent. (C) 2000 Elsevier Science B.V. All rights reserved.