A MS, SEM-EDX and XRD study of Ti or Cu-doped zinc ferrites as regenerablesorbents for hot coal gas desulfurization

Kinetic studies in thermobalance carried out by several authors previously have shown that small concentrations of TiO2 or CuO can increase substantia lly the overall sulfidation rate of zinc ferrites, as regenerable sorbents for hot coal gas desulfurization. These oxides modify the textural properti es of both the fresh or regenerated and the sulfided sorbent, modifying con sequently the sulfidation rate because it is a partially diffusion-controll ed process. However, by using grain models it is shown that most of the obs erved changes are due to changes in the intrinsic reactivity of the sorbent . Detailed studies of characterization in previous papers using different t echniques have failed in revealing differential chemical changes that could be associated with a different behavior. In fact, the only significant cha nges observed in these studies were an apparent disappearance in fresh sorb ents calcined at very high temperatures of the Raman effect, and a slight s hift of the XPS binding energy of Fe levels, indicating a probable site mig ration and/or a change of the oxidation state. These characterization resul ts, however, were not completely conclusive and additional efforts should b e undertaken. In this paper more sensitive techniques such as Mossbauer spectroscopy (MS) , powder X-ray diffraction (XRD) and scanning electron microscopy (SEM-EDX) , have been used for the characterization of fresh, regenerated and sulfide d sorbents. The study shows that the addition of TiO2 or CuO induce substan tial structural changes in zinc ferrites that can explain their apparent en hancing effect on the overall sulfidation reactivity. Additionally, this ef fect is decreased as the number of sulfidation-regeneration cycles increase s, probably explaining the performance decay exhibited by these sorbents in multicycle tests in a fixed bed reactor. (C) 2000 Elsevier Science B.V. Al l rights reserved.