Speciation and thermal transformation of sulfur forms in high-sulfur coal and its utilization in coal-blending coking process: A review

Abstract The utilization of high-sulfur coal is becoming more urgent due to the excessive utilization of low-sulfur, high-quality coal resources, and sulfur removal from high-sulfur coal is the most important issue. This paper reviews the speciation, forms and distribution of sulfur in coal, the sulfur removal from raw coal, the thermal transformationof sulfur during coal pyrolysis, and the sulfur regulation during coal-blending coking of high organic-sulfur coals. It was suggested that the proper characterization of sulfur in coal cannot be obtained only by either chemical method or instrumental characterization, which raises the need of a combination of current or newly adopted characterization methods. Different from the removal of inorganic sulfurfrom coal, the organic sulfurcan only be partly removed by chemical technologies; and the coal structure and property, particularly high-sulfur coking coals which have caking ability, may be altered and affected by the pretreatment processes. Based on the interactions among thesulfur radicals, sulfur-containing and hydrogen-containing fragments during coal pyrolysis and the reactions with minerals or nascent char, regulating the sulfur transformation behavior in the process of thermal conversion is the most effective way to utilize high organic-sulfur coals in the coke-making industry. An in-situ regulation approach of sulfur transformation during coal-blending coking has been suggested. That is, the high volatile coals with an appropriate releasing temperature range of CH4 overlapping well with that of H2S from high organic-sulfur coals is blended with high organic-sulfur coals, and the C–S/C–C bonds in some sulfur forms are catalytically broken and immediately hydrogenated by the hydrogen-containing radicals generated from high volatile coals. Wherein, the effect of mass transfer on sulfur regulation during the coking process should be considered for the larger-scale coking tests through optimizing the ratios of different coals in the coal blend.