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 t
he 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 so
rbent 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 stoi
chiometry and the possible use of gamma -alumina as an inert diluent or sup
port. These features are frequently associated with structural changes occu
rring both in the fresh or regenerated and in the sulfided sorbent, but, un
fortunately, these changes are not detected by the usual characterization t
echniques. In this study, the structural changes occurring in zinc ferrites
during calcination at high temperature or regeneration in oxidant atmosphe
re and those in iron sulfides during the sulfidation process have been inve
stigated in detail by Mossbauer spectroscopy (MS) and powder X-ray diffract
ion (XRD) using Rietveld methods for structure refinement. The study reveal
s that fresh sorbents prepared by calcination at low temperature consist of
lattice-strained small domain-size zinc ferrites that relax if the calcina
tion temperature is high enough. Structural changes are also observed throu
gh the formation of different types of iron sulfides in the sulfidation pro
cess. These apparent structural changes can explain the changes exhibited i
n the sulfidation reactivity of the sorbent. (C) 2000 Elsevier Science B.V.
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