Discrete models are developed to simulate the reaction of solids with complex pore structures in the kinetic- and diffusion-controlled regimes. Simulations employ two- and three-dimensional computational grids, and model parameters are directly obtainable from measurable physical properties of the unreacted solid. Model predictions can be displayed pictorially, allowing for direct visualization of the simulated process, while image processing of the computational grids can provide detailed structural information not easily obtainable from other analytical models. The effects of various pore structural parameters on the predicted reactivity patterns are investigated. Several examples are presented to illustrate the use of the models and demonstrate the significance of the pore structure in determining the reactivity of coal-derived chars.