Origins, Distribution, and Migration of Alkali Metals in High-Sodium Naomaohu Coal During Fixed-Bed Gasification

Naomaohu coal from the Santanghu Basin, Xinjiang, is characterized by anomalously high Na and Ca contents, which strongly affect its gasification behavior and slagging tendency. However, the genetic linkage between geological alkali enrichment and their transformation during thermal processes remains insufficiently constrained. In this study, an integrated investigation combining coal seam profile analysis, coal petrography, mineralogical characterization, and fixed-bed gasification experiments was conducted to elucidate the enrichment mechanisms and transformation pathways of alkali and alkaline earth metals (AAEMs). A total of forty six samples were collected along a vertical seam profile to determine the depositional control of alkali and alkaline earth metals (AAEMs), and seven representative samples were further subjected to pressurized fixed-bed gasification. Alkali migration and mineral phase evolution were systematically analyzed using XRD, XRF, and SEM-EDS. The results indicate that Na enrichment is mainly controlled by groundwater infiltration and weak paleoweathering, while Ca accumulation reflects deposition in humid, Ca-rich mire environments. During gasification, Na volatilizes and recondenses as Na-feldspars (NaAlSi2O6) and NaCl, whereas Ca decomposes into gehlenite (Ca2Al2SiO7) and brownmillerite (Ca2AlFeO5). The formation of these low-melting Na–Al–Si phases and Ca–Fe–Al phases dominate the ash fusion and slagging behavior. This study establishes a coupled geological–thermal transformation model for AAEMs in high-Na coal, providing mechanistic insight into mineralogical inheritance and offering guidance for mitigating alkali-induced slagging during gasification.