Interface-resolved direct numerical simulations of interphase momentum, heat, and mass transfer in supercritical water gasification of coal

Interactions between a reacting particle and the surrounding fluid are complex due to the interplay between flow dynamics, heat and mass transfer, and chemical reactions. In the present work, particle-fluid transport processes in supercritical water gasification of coal are studied using high-fidelity interface-resolved direct numerical simulations. The impact of different factors on the particle-fluid interactions are evaluated by performing simulations of the flow around two-dimensional particles considering different numerical configurations. The outgoing Stefan flow from the particle surface is found to cause expanded boundary layers for velocity, heat, and species. The temperature-induced changes in transport properties around a heated particle lead to a higher drag force and decreased heat/mass transport; those differences are further enlarged when taking into account the volumetric expansion of the fluid. Transport limitation for coal gasification in a realistic configuration is finally investigated. Temperature-induced fluid dilatation is then the major factor affecting drag force and heat transfer around the reacting particle, and mass transport is significantly impacted by species production or consumption in the boundary layer. Reaction heat release and variations in fluid composition within the thermal boundary layer lead to a slight enhancement of heat transfer. This work reveals and quantifies the main mechanisms affecting the exchanges between a reacting coal particle and surrounding supercritical water regarding both thermal and chemical aspects. It also provides high-fidelity data to later fit the reduced models needed for simulations of large-scale supercritical water gasification installations.