As a core process in the coal chemical industry, entrained-flow gasification technology promotes clean energy conversion by efficiently producing syngas. In industrial applications, radiant syngas cooler (RSC) is usually adopted to recover high-temperature syngas and slag waste heat from the gasifier outlet to improve system heat utilization. However, the ash deposition and slagging behavior in RSC can lead to a decrease in heat transfer efficiency and limited production capacity, becoming a key bottleneck, restricting technological development. In this work, the ash slags in the critical areas (radiation screen top 25 m, 25/20/15 m around spray water, floating ash on water-cooled wall, and boiler bottom 0-1 m) in the RSC were taken as the research object, and its elemental composition, ash chemical composition, mineral evolution, and high-temperature melting, and crystallization behavior were systematically analyzed. The results showed that after rapid cooling by RSC, some carbon particles in the fly ash carried by high-temperature syngas terminated the gasification reaction due to insufficient temperature, forming residual carbon, resulting in a higher carbon content in the floating ash of the water-cooled wall. The sulfur element was primarily enriched in the middle area of the RSC (floating ash on the water-cooled wall and 15 m around spray water). Mineral evolution analysis showed that alkaline oxides (CaO, Fe2O3, Na2O) and acidic oxides (SiO2, Al2O3) in the ash slags inside the RSC underwent low-temperature eutectic reactions to form low melting point minerals, such as nepheline, anorthite, augite, and melilite. The higher content of ZnO in the ash slag at 15 m around the spray water reacted with CaO and SiO2 to generate the refractory mineral Ca2ZnSi2O7, resulting in an increase in its melting temperature. In situ observation showed that all the ash slag samples displayed three stages of "shrinkage-melting-flow". Due to the formation of the refractory substance Ca2ZnSi2O7 in the ash slag at 15 m around the spray water during heating, its melting temperature was high and required a higher temperature to complete the flow. During the cooling and crystallization process, some interlaced rod-like crystals and a few square crystals were mainly precipitated in the slag.
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