High Temperature Coal-Gas Desulfurization: Kinetic Evaluation of a Zinc Ferrite Sorbent.

Thermogravimetric techniques were used to monitor the weight of cylindrical zinc ferrite extrudes in order to examine the sulfidation and regeneration kinetics. The reaction of zinc ferrite with H$\sb2$S (sulfidation) was studied over a temperature range of 573-973 K in gas streams containing 0.6-3 mol% hydrogen sulfide. Reduced zinc ferrite, ZnO plus Fe$\sb3$O$\sb4$, is capable of complete sulfidation from 873-973 K except at conditions where the iron oxide in the sorbent is reduced to FeO, in which case sulfidation does not go to completion. The sulfidation reaction is controlled by internal diffusion and mass transfer from the bulk gas phase to the pellet surface at temperatures greater than or equal to 773 K. The chemical reaction resistance is significant at temperatures below 773 K. The sulfidation reaction can be modeled successfully over the temperature range of 773-973 K by the unreacted core model (except when reduction to FeO occurs and sulfidation is incomplete). Regeneration was studied over the temperature range of 523-1123 K in an oxygen, steam, and nitrogen atmosphere. Complete regeneration is possible over the temperature range of 823-1023 K. Regeneration at temperatures above 1023 K is inhibited by structural property changes and is incomplete. Zinc sulfate is formed under all regeneration conditions at 823 K and in steam-free atmospheres at 923 K. This zinc sulfate can be decomposed by heating to 923 K in nitrogen or by introducing reducing gases at lower temperatures. The regeneration reaction with oxygen is controlled by internal diffusion and mass transfer at temperatures above 823 K. The regeneration reaction with steam has a significant chemical reaction resistance at all temperatures. The unreacted core model was used to describe regeneration with oxygen at 923 K. Multiple cycle studies showed that the sulfidation reactivity declines following regeneration at 1023 K, but increases following regeneration at 823 K. The increase is temporary and the reactivity slowly declines in subsequent cycles.