Effect of chemical structure and sulfur speciation of high-sulfur coking coals on sulfur transformation during pyrolysis

The chemical structure, content and distribution of sulfur forms in coal and coke of four high-sulfur coking coals were characterized by FT-IR, Raman, TG, and sulfur K-edge XANES technique, and effects on sulfur transformation during pyrolysis were also investigated. The results show that sulfur transformation behavior is related to the sulfur forms in coal as well as the release of volatile matters during pyrolysis. For lower rank coking coals, decomposition of unstable aliphatic structure releases plenty of volatiles with wider range. The interactions between sulfur radicals from cleavage of sulfur forms and hydrogen-rich radicals in volatiles promote release of sulfur into gas phase. This increases total sulfur removal and results in the higher content of thiophene in coke bulk than that on coke surface, while sulfide compounds have an opposite distribution. The degree of aromatization and relative content of thiophene increase with increasing coal rank, leading to lower desulfurization rate and unapparent difference of sulfur distribution between bulk and surface of coke. Inorganic sulfur removal is related to degree of decomposition of pyrite directly, and inter-conversions of sulfur species during pyrolysis process would generate new inorganic sulfur and retain in coke ultimately. Organic sulfur removal is determined by the coal structure and organic sulfur forms, and decreases obviously with increasing coal rank.