A simplified model for the combustion of solid fuel particles is derived, relevant for particle sizes and shapes used in fluidized and fixed-bed combustors and gasifiers. The model operates with a small number of variables and treats the most essential features of the conversion of solid fuel particles, such as temperature gradients inside the particle, the release of volatiles, shrinkage, and swelling. Typical shapes (spheres, finite cylinders, and parallelepipeds) are also considered. The model treats the particle in one dimension, and to describe the conversion inside a fuel particle the model only needs the transfer of heat and mass to an element of its external surface. When modeling a large combustion system, it is a great advantage that the conversion is related to the external surface, because the model does not have to be limited to just a single particle. In fact, it can handle the conversion of a solid phase in a computational cell, where the conversion is related to surface area per unit volume, instead of the surface area of a single particle. The model divides the particle into four layers: moist (virgin) wood, dry wood, char residue, and ash. The development of these layers is computed as function of time. The model shows satisfactory agreement with measurements performed on more than 60 samples of particles of different sizes, wood species, and moisture contents. Comparison with the experiments shows that the simplifications made do not significantly influence the overall accuracy of the model. The model also demonstrates the great influence of shrinkage on the times of devolatilization and char combustion.