Methanation of CO/CO2 mixtures: Evaluation of pellet size effect on methane formation rate and reactant selectivity

In the present contribution, numerical experiments are used to interpret results obtained from physical diffusion-influenced experiments for the CO and CO2 co-methanation. Physical and numerical experiments are conducted in the temperature range from 513 to 573 K and different CO/CO2 ratios. It is revealed that CO and CO2 behave very differently when both are simultaneously converted into CH4, which is mainly due to the competing reaction kinetics. Since CO inhibits the methanation of both CO2 and itself, large pellets and the associated diffusion limitation can be used to reduce the concentration of CO inside the pellets and hence its overall inhibiting effect. This catalyst design aspect can be used to increase the effective methane formation rate up to 35 % for a co-methanation of CO and CO2, while larger pellets allow to reduce the pressure drop in the reactor at the same time, which provides further advantages for reactor operation. Moreover, it is found that a selective CO methanation can be conducted with low-loading (nickel) catalysts, as long as the catalyst is operated in the low-temperature region (T << 300 °C) with small particle sizes below 1.5 mm. In addition, it is shown that industrially relevant catalysts significantly affect the reactant selectivity.