Pore-resolving simulations to study the impacts of char morphology on zone Ⅱ combustion and effectiveness factor models

摘要

Combustion and gasification of pulverized char often occur under zone II conditions, in which the rateof conversion depends on both heterogeneous reaction and gas transport within the particle’s porousstructure. The morphology of porous char has a strong influence on intra-particle diffusion, and thus,on the overall conversion rate. Because pulverized coal and biomass char particles are often irregularlyshaped and contain pores and voids which can approach the size of the particles themselves, conventionalmodels based on spherical symmetry and coarse-grained, upscaled, effective continuum conservationequations are not applicable or appropriate. A recent 3-D, pore-resolving CFD simulation approachbased on real char particle geometries obtained from X-ray micro-computed tomography (micro-CT) obviatesthe need to upscale over large heterogeneities and to make oversimplifying geometric assumptions.The micro-CT-based pore-resolving approach is employed here to study zone Ⅱ combustion for fifty pulverized,porous coal char particles produced at a high heating rate. The large pores often present in charparticles enhance reactant transport throughout the particles, even within the micro- and meso-porouscarbon surrounding the large pores. This is particularly the case for network-type particle structures, dueto the prominence of channels that extend from the particle surface. Because reactor-scale codes oftenemploy one-dimensional models to calculate the reaction rates of tracked particles, pore-resolving simulationsare used to assess the accuracy of existing effectiveness factor models for real char. Cenosphericalparticles can be reasonably modeled using an effectiveness factor solution for hollow spheres, but thebehavior of more complex network morphologies is not well-predicted by any of the effectiveness factormodels examined.