The influence of the physichemical behavior of carbon on the melting and crystallization of gasification slag

Residual carbon is an essential component in coal gasification slag, significantly influencing the gasification process. In this study, the impact of the physicochemical behavior of carbon on the melting and crystallization characteristics of gasification slag (GCS) was investigated. When carbon content increased from 2 to 20%, the softening temperature and hemisphere temperature of GCS rose by nearly 120 °C, and the flow temperature by approximately 170 °C. At high temperatures, carbon reduces Fe₂O₃ of slag to elemental Fe and further forms the refractory mineral Fe₃Si. Fe₃Si and residual carbon constitute the framework structure of the melt, delaying the collapse of aluminosilicate melt and thereby increasing the flow temperature of GCS. The increased carbon content in GCS facilitates the formation of Fe₃Si at high temperatures. During the cooling process, the liquid phase in the melts deposits to form a dense aluminosilicate matrix, while refractory phases (Fe₃Si and residual carbon) precipitate and coalesce into spherical aggregates. During the quenching process, the high viscosity of aluminosilicate inhibits the precipitation of refractory phases. As residual carbon content increases, the refractory skeleton disperses more in the high-viscosity melt, creating a denser structure that reduces mass transfer efficiency and promotes the formation of smaller, more dispersed particles instead of larger spheres during cooling.

Graphic abstract

During the coal gasification process, the physicochemical behavior of carbon significantly influences the transformation of minerals and the melting and crystallization characteristics of inorganic ash and slag. At high temperatures, carbon reduces Fe₂O₃ of slag to elemental Fe and further forms the high-melting-point mineral Fe₃Si. The increased carbon content in GCS facilitates the formation of Fe₃Si at high temperatures, while carbon evolves into a refractory framework within the slag, thereby enhancing the fluidity temperature of the GCS. A significant amount of organic carbon precipitates from the aluminosilicate melt in conjunction with iron-rich substances, forming spherical particles under natural cooling conditions. In contrast, during rapid cooling processes, the limited time available prevents extensive migration of carbon from the melt, leading to its partial dispersion within the dense sintered structure. As the carbon content increases, the viscosity of the melt rises, thereby slowing down the precipitation of spherical particles primarily composed of iron-bearing minerals and carbon.