In Situ Analysis and Mechanism Study of Char-Ash/Slag Transition in Pulverized Coal Gasification

The behaviors of char-ash/slag transition of three different rank coals during char-CO2 gasification were studied by the combination of thermogravimetric analyzer and an in situ heating stage microscope. The effects of coal ranks and gasification temperatures on the interactions of char-ash/slag were analyzed. The results show two reaction forms, which relate to the reaction conditions and coal properties in the chat-ash/slag transition processes. (1) When the temperature was lower than the ash fusion temperature of coal, the char particles exhibited the shrinking particle mode in the initial and midterm reaction stages, and then showed the shrinking core mode at the late stage. The accumulated ash might block some pore structure of semicoke, which was unfavorable for the char-CO2 reaction proceeding at high carbon conversions. (2) When the gasification reaction was conducted above the ash fusion temperature of coal, the liquid,film formed because the ash melted with the shrinkage of char particles. Moreover, the molten slag enclosed residual carbon at high carbon conversions. These two forms of the char-ash/slag transition can well explain the variation of the reactivity of coal char at different reaction temperatures. A threshold value of gasification conversion (x = 0.9) exists during the char-ash/slag transition processes of both Yunnan lignite coal and Shenfu bituminous coal, while the threshold did not appear in the gasification processes of Zunyi anthracitic coal for its high ash fusion temperature. The present work shows that the coal ranks, coal ash properties, and reaction conditions have great effects on the whole char-ash/slag transition processes.