The effect of the pellet size on the effective CH4 formation rate, already obvious from experimental results displayed in figure , is analyzed in detail based on the simulation results. Therefore, the simulated spatial profiles of the CO and CO2 molar fractions and of the CO and CO2 hydrogenation rates at 556 K are shown for 175 and 800 µm pellet sizes in figure . For the small pellets, shown in a, c and e, the profiles of both the molar fraction and the reaction rate are very flat irrespective of the specific 𝑧 value. This means that no diffusion limitation is predicted for small pellets. Furthermore, the flat reaction rate profiles confirm absence of significant limitations by chemical equilibrium, which is verified by calculation of the equilibrium composition via Gibbs-energy minimization using a model previously published . In contrast, pronounced profiles are derived for large pellets for all CO/CO2 ratios, which indicate significant diffusion limitations. Specifically, for 𝑧 = 0 (figure ) the CO2 molar fraction and CO2 hydrogenation rate decreases from the external catalyst surface towards the center, while the reaction rate is linked to the molar fraction according to the kinetics. At 𝑧 = 0.2 (figure ), both CO and CO2 are affected by diffusion.