DEVELOPMENT OF ZINC FERRITE SORBENTS FOR DESULFURIZATION OF HOT COAL-GAS IN A FLUID-BED REACTOR

Advanced integrated gasification combined cycle (IGCC) power generation systems require the development of high-temperature, regenerable, desulfurization sorbents capable of removing hydrogen sulfide from coal gasifier gas to very low levels. Fludized-bed desulfurization reactors offer significant potential advantages in IGCC systems compared to fixed- and moving-bed reactors because of their ability to control the highly exothermic regeneration involved. However, a durable, attrition-resistant sorbent in the 100- to 300-mu-m-size range is needed. Thus, the objective of this investigation was to identify and demonstrate methods for enhancing the long-term chemical reactivity and mechanical strength of zinc ferrite, a leading regenerable sorbent, for fluidized-bed applications. A bench-scale, high-temperature, high-pressure, fluidized-bed reactor (3-in. i.d.) system capable of operating up to 350 psig and 850-degrees-C was designed and built. A total of 175 sulfidation-regeneration cycles were carried out using KRW-type coal gas with various zinc ferrite formulations. A number of sorbent manufacturing techniques, including spray drying, impregnation, crushing and screening, and granulation, were investigated. While fluidizable sorbents prepared by crushing the durable pellets and screening had acceptable sulfur capacity, they underwent excessive attrition during multicycle testing. The sorbent formulations prepared by a proprietary granulation technique were found to have excellent attrition resistance and chemical reactivity during multicycle testing. However, application of zinc ferrite was limited to a maximum temperature of about 550-degrees-C, above which excessive sorbent weakening was observed possibly due to chemical transformations.