Chemical Properties of Superfine Pulverized Coal Particles. Part 4. Sulfur Speciation by X-ray Absorption Near-Edge Structure Spectroscopy

Exploring the transformation mechanisms of sulfur during the superfine comminution plays a crucial role in constructing a coal macromolecular model and developing clean coal technologies. In this work, the occurrences of sulfur in raw coal were quantitatively analyzed by X-ray absorption near-edge structure (XANES) and X-ray photoelectron spectroscopy (XPS). The sulfur forms in two typical rank coals were compared, and the influence of particle size on the transformation of sulfur was elucidated. Furthermore, the mechanochemical effect on sulfur speciation during the superfine comminution was focused on. The final results indicate that HN coal contains more thiophene with ring structures, while sulfate is the dominant sulfur form in NMG coal. In addition, pyrite can be easily oxidized to sulfate and elemental S, while sulfide, sulfoxide, and sulfone are susceptible to transformation to thiophene during the coalification. It is worth noticing that during the superfine comminution, the organic sulfur shows a decreasing trend, while the inorganic sulfur shows the opposite, suggesting that a conversion from organic to inorganic sulfur exists due to the mechanochemical effect. In addition, two conversion pathways of sulfur are concluded, i.e., sulfide -> sulfoxide -> sulfone and thiophene -> sulfoxide -> sulfone -> sulfate, which further elucidate the sulfur transformation mechanisms in coal. As the particle size decreases, there is an evident decline for sulfide and thiophene, while pyrite, elemental S, and sulfate generally increase. As for sulfoxide and sulfone, different tendencies are observed in HN and NMG coals due to the complex oxidation processes. The combined application of XANES and XPS is recommended for better characterization of the sulfur speciation and distribution due to the different probing depths in coal particles. This research provides a comprehensive understanding of the mutual transformation between organic and inorganic sulfur in coal. The elucidation of sulfur migration pathways can further promote the exploitation of more valuable desulfurization methods.