Particle-wall interaction in entrained-flow slagging coal gasifiers: Granular flow simulation and experiments in a cold flow model reactor

Entrained-flow slagging coal gasifiers display large conversion efficiencies and small levels of unconverted carbon at the exhaust. Both features are apparently at odds with the fairly small "space-time" of the particle-laden gas feeding, as compared with the time scale of heterogeneous gasification of carbon. This apparent inconsistency can be explained by considering that fuel residence times are longer than the "space-time" due to segregation of fuel particles in the near-wall region of the gasifier. Segregation is promoted by swirl flow, by particle-wall interaction as the wall is covered by a molten layer of slag and by the establishment of a dense-dispersed flow of granular solids in the proximity of the wall. This study presents results of granular flow simulations of the interaction of a dense-dispersed particle flow with the confining wall. Simulations consider that both the particles and the wall may be either "sticky" or "non sticky", based on the prevailing elastic vs plastic behaviour upon collision. The effect of drag forces exerted on particles by swirled gas flow is simulated in a simplified manner. Particle-particle collisions are modelled with a Hertzian approach that includes torque and cohesion. The extent and time scale of segregation of a lump of particles loaded into a cylindrical vessel are assessed. Results clearly indicate the different structure of the layer of particles establishing at the wall surface in the different interaction regimes. Results of simulations are qualitatively compared with results of an experimental campaign performed in a reactor representing a cold flow model of the entrained-flow gasifier, where solid, molten or semi-molten particles have been simulated by atomized wax as surrogate material. Altogether, the results confirm the importance of the particle-particle and particle-wall micromechanical interactions for a correct prevision of the segregation of fuel particles in entrained-flow slagging gasifiers.