Carbon structural characteristics and apparent combustion behavior of pressurized coal gasification fine slag

Pressurized coal gasification fine slag (PGFS) contains residual carbon and mineral-rich ash, but its combustion utilization is limited by poor apparent reactivity. In this study, raw coal (RC), PGFS, and acid-leached PGFS-CFN were investigated using sequential acid leaching and multi-scale characterization, including SEM–EDS, TEM, N₂ adsorption, XRD, Raman, XPS, and TG/DTG analysis. The results show that pressurized gasification forms a carbon–ash composite structure with high ash content, mineral-rich surface coverage, and a more ordered residual carbon skeleton. These structural features are associated with delayed ignition, a broader combustion temperature range, and a lower maximum mass-loss rate of PGFS. After sequential HCl–HF–HNO₃ acid leaching, the ash content decreased from 66.27 wt% to 0.83 wt%, while the specific surface area and total pore volume increased from 134.90 m²·g⁻¹ and 0.140 cm³·g⁻¹ to 412.56 m²·g⁻¹ and 0.263 cm³·g⁻¹, respectively. The acid-leached sample exhibited lower ignition and burnout temperatures, a higher maximum mass-loss rate, and a markedly increased comprehensive combustion index under the same TG conditions. The combined structural and combustion results suggest that ash-related shielding, pore accessibility, residual carbon ordering, and surface chemistry jointly regulate the apparent combustion behavior of PGFS. This study provides structural insight into the combustion inertness of PGFS and may support future optimization of combustion-oriented utilization strategies.