Effect of pre-desulfurization process on the sulfur forms and their transformations during pyrolysis of Yanzhou high sulfur coal

To achieve the utilization of high sulfur coals efficiently and economically, the effects of different pre-desulfurization processes including flotation, acid washing, ultrasonication and stepwise treatment on sulfur removal and changes of sulfur forms in a high sulfur coal, sulfur transformation behavior during pyrolysis and sulfur distribution in char were investigated. Flotation removed almost all the sulfate and partial pyrite from the raw coal, while ultrasonication was more effective in removing some low molecular-weight organic sulfur together with weakening the C-S bond in some sulfur species. The process of HCl-HF-HNO3 and stepwise treatment reduced the ash content and sulfur content to a larger extent, with ultrasonication after flotation and acid treatment having the highest sulfur removal from raw coal. The sulfur forms on coal surface were more modified after different pre-desulfurization processes, together with different sulfur transformations during pyrolysis. The accumulated amount of sulfur-containing gases during pyrolysis of coals after stepwise treatment, in particular of coals treated by the combination of flotation, acid washing and ultrasonication, generated the highest amount of sulfur-containing gases. For the utilization of high sulfur coals, in particular, of high organic-sulfur coals, the above-mentioned treatment of combination of traditional flotation technique and organic sulfur removal process should be considered, which could remove the inorganic sulfur and organic sulfur simultaneously, and release the most sulfur-containing gases during pyrolysis. Besides, it would be helpful to choose the utilization methods based on the sulfur species in raw coal, and the effect of pre-desulfurization process on sulfur transformation behavior.