This investigation provides a comprehensive analysis of entrained-flow biomass particle combustion processes. A single-particle reactor provided drying, pyrolysis, and reaction rate data from poplar particle samples with sizes ranging from 3 to 15 mm. A one-dimensional particle model simulates the drying, rapid pyrolysis, gasification, and char oxidation processes of particles with different shapes. The model characterizes particles in three basic shapes (sphere, cylinder, and flat plate). With the particle geometric information (particle aspect ratio, volume, and surface area) included, this model can be modified to simulate the combustion process of biomass particles of any shape. The model also predicts the surrounding flame combustion behaviors of a single particle. Model simulations of the three shapes agree nearly within experimental uncertainty with the data. Investigations show that spherical mathematical approximations for fuels that either originate in or form aspherical shapes during combustion poorly represent combustion behavior when particle size exceeds a few hundred microns. This includes a large fraction of the particles in both biomass and black liquor combustion. In particular, composition and temperature gradients in particles strongly influence the predicted and measured rates of temperature rise and combustion, with large particles reacting more slowly than is predicted from isothermal models.