Numerical simulation of gasification of a shrinking char particle in supercritical water

The technology of supercritical water gasification provides a promising way for clean and efficient utilization of coal for hydrogen production. Char conversion is the rate-determining step of the gasification process, and the development of char particle size and shape directly affects the drag force, heat transfer and particle conversion characteristics in a supercritical water gasifier. Thus, a 2D transient CFD-based model was developed in this work to describe the conversion of a shrinking char particle in supercritical water, and the dynamic mesh method was used for particle surface tracking. 0.1 and 1 mm sized char particles were gasified at 873 to 1023 K, and the char conversion was obviously restrained by heat and mass transfer with the increase of particle size and temperature. The effect of convection was then studied with Reynolds number varying from 0 to 200. Convection effectively promoted the char conversion by enhancing the heat and mass transfer, and meanwhile, irregular development of particle shape was caused by the uneven temperature and supercritical water distribution on the particle surface. Owing to the special high concentration of supercritical water which ensured relative uniform distribution of supercritical water all the time, the irregular shape development was not obvious even at high Reynolds number. Moreover, the wake vortex formed at high Reynolds number promoted the fluid mixing, and an inflection point with the minimum carbon consumption rate appeared on the downstream side.