Kinetics of CO2/char gasification at elevated temperatures. Part II: Clarification of mechanism through modelling and char characterization

Char gasification experiments in a batch fluidized bed reactor at high temperatures revealed that for chars with low ash fusion temperatures, the reaction rate levelled off or even decreased a little as temperature increased. The mechanism was studied through modelling and char characterization. Investigation through modelling revealed that the increase in external mass transfer resistance with temperature was not the main factor accounting for the leveling off or decrease of gasification rate of a char at elevated temperatures. At high temperatures, the porosity and specific surface area increased with pyrolysis temperature for a char with a high ash fusion temperature, but decreased for a char with a low ash fusion temperature. For a char with a low ash fusion temperature, the surface became smooth and ash accumulation near the char surface was apparent after pyrolysis at an elevated temperature. However, for a char with a high ash fusion temperature, its surface became porous and no ash accumulation occurred after pyrolysis at an elevated temperature. According to the results of this work, ash fusion and ash accumulation near the char surface were very likely the main factors accounting for the leveling off, or decrease, of gasification rate at elevated temperatures.