Gasification and combustion modeling for porous char particles

Abstract

Gasification and combustion of porous char particles occurs in many industrial applications. Reactor-scale outputs of importance depend critically on processes that occur at the particle-scale. Because char particles often possess a wide range of pore sizes and react under varying operating conditions, predictive models which can account for the numerous physical and chemical processes and time-dependent boundary conditions to which a particle is subjected are necessary. A comprehensive, transient, spherically symmetric model of a reacting, porous char particle and its surrounding boundary layer has been developed and validated. The model accounts for heterogeneous and homogeneous reactions, pore structure evolution, gas transport in and around the porous particle, thermal annealing, fragmentation and ash behavior. To model the pore structure evolution, an extension of the random pore model has been developed which allows different pore sizes to grow at different rates, depending on the instantaneous pore-scale reactant penetration at a given location within the particle. This is accomplished by incorporating pore-scale effectiveness factors, consistent with the random pore geometry, into equations for the growth of individual pore sizes. This framework allows the evolution of the char with local conversion to adapt to changes in boundary conditions (reactants, temperature) and the development of intra-particle gradients, rather than being pre-determined by the initial pore structure. The effects of char gasification reactions during oxy-combustion of pulverized coal are not fully understood. The single particle char consumption model is used with output from CFD simulations of high-volatile oxy-coal combustion to analyze representative regions and trajectories along which char particle burning occurs. These realistic, time-dependent boundary conditions are used to assess the importance of the gasification reactions to the overall rate of char consumption. As conversion proceeds, gasification reactions, when significant, can alter the location within the particle where char consumption occurs, further affecting the rate of conversion by inducing structural changes that can accelerate peripheral fragmentation.