A TEM and XRD study of the structural changes involved in manganese-based regenerable sorbents for hot coal gas desulfurization

New technologies for power generation demand the development of regenerable sorbents for hot coal gas desulfurization down to very low levels of H2S- Unfortunately, long-term tests in different reactors show that usual regenerable sorbents exhibit such efficiency decay and degradation of mechanical properties that their use is impractical in the current state of the art. In a previous study, some long-term tests in a fixed bed reactor have shown that Zn- or Cu-doped manganese oxide sorbents exhibit good performance. The highlighting features in the behavior of these sorbents are a substantial change in sulfidation reactivity as they are doped with zinc or copper oxides and a pronounced efficiency increase during the first 1-5 sulfidation-regeneration cycles. To understand this apparent behavior, a detailed characterization study of fresh, sulfided, and regenerated sorbents has been carried out using X-ray diffraction and transmission electron microscopy techniques. This study shows that the structure and degree of dispersion of the active phases in the fresh and regenerated sorbents and those of sulfides in the sulfided sorbents are highly dependent on the chemical nature of the dopant used. Zn enhances the formation of mixed oxide or sulfide species, while Cu increases mostly the degree of dispersion. These structural and textural changes can be associated with the high sulfidation reactivity exhibited by the studied manganese-based sorbents.