The validated model was applied to investigate the pellet size effect on the reactant selectivity for methanation of CO/CO2 mixtures based on detailed simulations. Therefore, the spatial profiles of CO and CO2 consumption and effective CH4 formation rate are calculated for pellet sizes in the range between 0.1 to 4 mm and temperatures in the range between 513 to 573 K. From these results the reactant selectivity towards CO methanation is derived via equation . The respective results are shown in figure and figure for the example conditions of equimolar CO/CO2 ratio (𝑧 = 0.5) and for 𝑧 = 0.25 and 𝑧 = 0.75 in the appendix. All other variables are used as stated in table 1 for small pellets. Note that the extrapolation focuses on the trends with respect to pellet size effects, rather than on quantitative analysis. Therefore, specific values provided for CO reactant selectivity or CH4 formation rate for particle sizes exceed ca. 1 mm are discussed carefully.