Surface structure change and sulphur transformation of coal char under oxygen-steam atmosphere at high temperature

The global decarbonization trend has been prompting combustion technology integrated with carbon capture. Oxygen-steam combustion can achieve high carbon dioxide concentration in the dewatered flue gas, favourable for carbon capture. Compared to conventional combustion technology, the changes in the combustion atmosphere may influence the reaction pathway in the transformation of char structure and the sulphur species. In this study, experiments with steam and/or various oxygen concentrations were carried out in an entrained flow reactor at high temperature. Carbon, oxygen, and sulphur transformation were characterized with an elemental analyzer, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). The results showed that carbon, oxygen, and sulphur were mainly in the forms of hydrocarbons (C-C, C-H), carbon–oxygen single bonds, or carboxyl groups, thiophene sulphur, pyrrhotite, or troilite in the raw char surface, respectively. The presence of steam alone had little effect on the carbon-containing and oxygen-containing functional groups. Thiophene sulphur content on the surface decreased while the sulphate content increased. In the oxygen atmosphere, the content of C-C, C-H, and C-O functional groups on the surface decreased, with the formation of C-O or O-C-O occurring during char combustion. FeS in the char was sulphided first to form a large amount of sulphate, which was enriched on the surface of coal char, then the generated sulphate decomposed, or FeS was directly oxidized to form Fe₂O₃/Fe₃O₄. However, in the oxygen-steam atmosphere, the concentration of C-O and O-C-O functional groups reduced on the char surface. The conversion of sulphate to Fe₂O₃/Fe₃O₄ was promoted in the oxygen-steam atmosphere.