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

Abstract

Combustion and gasification of pulverized char often occur under zone II conditions, in which the rate of conversion depends on both heterogeneous reaction and gas transport within the particle's porous structure. 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 irregularly shaped and contain pores and voids which can approach the size of the particles themselves, conventional models based on spherical symmetry and coarse-grained, upscaled, effective continuum conservation equations are not applicable or appropriate. A recent 3-D, pore-resolving CFD simulation approach based on real char particle geometries obtained from X-ray micro-computed tomography (micro-CT) obviates the 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 char particles enhance reactant transport throughout the particles, even within the micro- and meso-porous carbon surrounding the large pores. This is particularly the case for network-type particle structures, due to the prominence of channels that extend from the particle surface. Because reactor-scale codes often employ one-dimensional models to calculate the reaction rates of tracked particles, pore-resolving simulations are used to assess the accuracy of existing effectiveness factor models for real char. Cenospherical particles can be reasonably modeled using an effectiveness factor solution for hollow spheres, but the behavior of more complex network morphologies is not well-predicted by any of the effectiveness factor models examined.