Heterogeneous modeling is used to investigate the significance of intraparticle diffusion, temperature fluctuations, and dispersion On predicting fixed-bed chemical-looping combustion (CLC) reactors. The model is validated against chemical-looping reduction experiments from the literature that utilize methane as fuel and a Ni-based oxygen carrier. The effect of particle size on CLC reduction reactions is studied by examination of the Weisz-Prater and Mears criteria for internal and external mass transfer limitations. Homogeneous and heterogeneous models are applied for two particle sizes and it is shown that diffusion limitations are significant for the larger particle. Furthermore, non-ideal flow patterns are evaluated, where superior predictions are obtained by considering the effects of dispersion and convection. Radial dispersion in each experiment is analyzed in detail and the predicted temperature fluctuations inside the reactor and particle are presented. Lastly, the heterogeneous model is applied to design a particle that maximizes chemical-looping efficiency, measured as fuel conversion and product selectivity. Enhancements to the overall reaction rate and temperature profile are observed for the optimal particle size as a result of reducing the intraparticle resistance. (C) 2013 Elsevier Ltd. All rights reserved.