Intermediate temperature of supercritical water enhances the dispersion of cohesive particles

Abstract

Supercritical water gasification represents a promising technology for the clean conversion of coal to hydrogen, so that it is expected to replace the traditional use of coal by burning. However, the flocculation of cohesive coal particles significantly reduces the contact area between particles and supercritical water, which limits the chemical reaction that produces hydrogen. Based on Lifshitz theory, we propose a cellular flow model for numerically reproducing the flocculation of cohesive particles in supercritical fluid turbulence. Cohesion, lubrication, and direct contact forces between particles are fully accounted for, so that the model allows us to conduct a detailed analysis of the translation, rotation, growth, and breakage of flocs in supercritical fluid turbulence. The results show a transient flocculation phase of cohesive particles characterized by an increasing average floc size, followed by a statistically steady equilibrium phase. Increasing the density ratio between particles and supercritical water significantly accelerates the flocculation of dispersed cohesive particles. Higher temperature of the fluid yields faster flocculation and larger flocs when the temperature is below the critical value of 660 K. Upon raising the temperature above 660 K, the flocculation rate and floc size first decrease, and then increase again, yielding an optimal temperature around 750 K where the primary particles are most effectively dispersed by the supercritical water turbulence.