Decoupling Mineral Shielding and Carbon-Structure Inertness in Pressurized Coal Gasification Fine Slag Combustion via Sequential Demineralization

Abstract Pressurized coal gasification fine slag exhibits poor combustion reactivity due to the coupled effects of mineral encapsulation and highly ordered residual carbon. In this study, sequential HCl → HF → HNO3 demineralization was employed as a mechanistic probe to distinguish the predominant contributions of mineral shielding and carbon-structure inertness to combustion behavior. The ash content decreased from 66.27 wt % to 0.83 wt %, while the carbon content increased to 93.82 wt %. HCl and HF primarily removed mineral barriers and opened oxygen-accessible pore networks, increasing the specific surface area and pore volume. In contrast, HNO3 oxidation increased carbon disorder and redistributed surface oxygen species toward a higher relative abundance of carbonyl/quinone-like structures. The comprehensive combustion index increased from 0.09 × 10–8 to a maximum of 13.95 × 10–8 %2·min–2·°C–3 after HCl → HF treatment. Subsequent HNO3 oxidation increased the maximum mass-loss rate from 5.42 to 6.21%·min–1 but decreased the mean mass-loss rate from 3.64 to 2.95%·min–1, resulting in a slightly lower S value of 13.20 × 10–8 %2·min–2·°C–3. These results demonstrate that mineral shielding removal and pore accessibility dominate the overall combustion improvement, whereas HNO3-induced defects and surface-oxygen redistribution mainly enhance localized oxidation reactivity.