The CO/CO2 ratio of reactions during char combustion was investigated numerically. The simulations used the pore model with fractal properties, a gas diffusion model suitable for fractal pores, and a carbon-oxygen reaction model to describe the char oxidation. The CO/CO2 ratio in the primary reactions was derived assuming Gaussian distributions of the activation energies and Maxwell distribution of the oxygen molecular velocity. The numerical simulations of the char combustion for various particle sizes, pore structures, and temperatures revealed that the effects of the secondary reactions and the pore structure on the CO/CO2 ratio are the main reasons why different researchers obtain different experimental results for of the CO/CO2 ratio.