Micro-scale study of mineral transformation and coverability at single coal char particle surface during high-temperature gasification process

During the high-temperature coal gasification process, mineral transformation is essential to the formation of residual carbon and ash/slag discharge. The current work investigated the mineral transformation and coverability of the single coal char particle with various ash content during gasification, as well as established the correlation between the ash layer and carbon conversion. Three stages of the gasification process at 1350 degrees C were defined by the quadratic difference method based on the interaction between the char and the liquid slag, and the two critical points that distinguish the three stages were between 15%-20% and 71%-90% respectively. Further investigation showed that the melting behavior of coal char during gasification was affected by the inhomogeneous distribution of elements. Ca showed a tendency to migrate from the unmolten area to the molten area. The migration behavior was attributed to the eutectic formation between silica-aluminate in the molten area and calcium salts in the unmolten area, leading to a reduction in melting point and gradual melting of minerals in the unmolten area. Eventually, a homogeneous gehlenite phase was formed and covered on the residual char surface. Besides, the relationship between liquid slag coverage and carbon conversion was established, and for SF, YL, and XLT chars, the maximum coverage of liquid product occurs at the conversion of 0.86, 0.78, and 0.66, respectively. This paper provides an in-depth understanding of the slag melting behavior and residual carbon formation in the entrained gasifiers.