Accurate modeling of the reaction rate of char particles is decisive for precise computational fluid dynamics simulations of gasifiers. Pore diffusion limits the reaction rate at medium temperatures and strongly depends on the pore size. In most of the present research, pore diffusion is either completely neglected or modeled with a single mean pore size. This work introduces a particle model with two pore sizes. Large pores transport the reactant gases into the particle and distribute them to small pores, in which the heterogeneous reactions take place. This approach is compared with a simpler model with a single mean pore size. The particle and the small pores are discretized by an appropriate number of elements, which allows an exact numerical calculation of the concentration profile for nth-order reactions. Both models are implemented into a two-dimensional computational fluid dynamics simulation of a lab-scale fixed bed gasifier. Simulations and experiments are carried out for 1.362 mm, 0.815 mm, and 0.458 mm sized particles, which are gasified with carbon dioxide and steam at 1200 °C and 1.1 bar total pressure. The reactant gases can penetrate smaller particles more easily. However, the measured product gas flow is independent of the particle size although the reactions take place under pore diffusion limitation. Only the proposed model with two pore sizes is able to reproduce the experiments adequately. However, it causes a significant extra computational effort.