We develop a transient multiscale model to describe the combustion of chars derived from plastic coals. This model visualizes the pore structure as an interconnected network of large cavities separated by walls consisting of microporous grains containing carbon and ash. The diffusion-reaction problems formulated for the particle and grain scales are solved numerically, and a systematic comparison of model predictions and experimental combustion data is carried out. The main conclusion from this analysis is that the porosity and surface area associated with large cavities can drastically affect the reactivity and ignition behavior of char particles in the regime of pore diffusional limitations. This effect diminishes when combustion takes place in the regimes of kinetic or external mass-transfer control.