In entrained-flow gasification process, minerals in coal are transformed into amorphous coal gasification fly ash (CGFA) with diverse chemical compositions, leading to serious ash deposition issues in syngas cooler. However, most existing studies focused on the coal gasification slag crystallization mechanism or the coal ash fusion mechanism, the fusion mechanism of CGFA remains poorly understood. In this study, fusion mechanism of typical CGFAs was revealed from the crystal evolution behavior. Results show that fusion of all CGFAs follows a “melting-dissolution” mechanism characterized by a narrow temperature interval (FT-DT < 36 ℃) and is predominantly governed by the nucleation, growth, and dissolution of anorthite formed through the rearrangement of short-range ordered structures Si-O-Al and Si-O-Ca. Notably, similar high flow temperatures arise from two distinct crystal-evolution pathways: high SiO₂+Al₂O₃ content inhibits anorthite precipitation and yields growth-dominated crystallization with large but sparsely distributed crystals (FT=1465 ℃), whereas a low Si/Al ratio promotes nucleation-dominated crystallization with numerous small, high-aspect-ratio crystals (up to 6.8) that persist to high temperatures (FT = 1466 ℃). In contrast, high CaO content facilitates anorthite nucleation into small, abundant crystals that dissolve at relatively low temperatures, yielding the lowest FT of 1324 ℃. Fe₂O₃ exerts a dual effect: it promotes anorthite nucleation and melting at relative low temperature while simultaneously generating high-melting-point spinel through reactions between Si-O-Fe and Si-O-Al. This study demonstrates that regulating the Al2O3 content in coal can modulate the crystallization behavior of CGFA, offering a practical strategy to mitigate ash deposition issues in syngas coolers.
 

Entrained-flow coal gasification; Fusion mechanism; Coal gasification fly ash; Short-range ordered structure; Anorthite crystallization.