Experimental study on the evolution of inorganic composition during purification of high-alkali coal at different temperatures

The Purification-combustion concept has gained significant attention in recent years. This study investigates the purification of high-alkali coal (HAC) using a high temperature purification drop-tube furnace (DTF), with a focus on the mass distribution of gas–solid products and the migration and transformation of inorganic components under varying purification temperatures (T_p). Results demonstrate that high temperatures and reducing atmosphere offer significant advantages during purification. Over 78% of the HAC is converted into syngas, while remaining char exhibits an enhanced porous structure. High-temperature solid-phase reactions leading to the aggregation of inorganic components and the formation of stable eutectics. When T_p exceeds the fusion temperature (FT) of raw coal, white molten spheres rich in Na, Si, Al, and O form on the char surface, reflecting the migration behavior of inorganic elements. Alkali and alkaline earth metals (AAEMs) predominantly remain in the solid phase as amorphous forms. Although higher T_p promotes the volatilization of Na, this effect plateaus beyond 1100℃. The presence of high-melting-point minerals such as CaSO₄ and MgFe₂O₄ in char raises the FT by 90℃-102℃. FactSage simulations confirm a reduction in low-melting Na-K-Si-Al eutectics, elevatting melting points. The optimal T_p is identified as 1200℃-1300℃. Within this range, char can achieve efficient liquid slag discharge with minimal surface area shrinkage, while maximizing the retention of AAEMs in the ash phase, thereby mitigating slagging risks during subsequent combustion. This study highlights the potential of the purification in enabling the clean, efficient, and low-carbon utilization of HAC, paving the way for advanced thermal energy applications.