CO2 Methanation over an Ni/YSZ Catalyst: Impact of Altering the Catalyst Bed Ratio in Two Reactors in Series
Authors: Omoregbe Osaze , Majewski Artur Jacek , Steinberger-Wilckens Robert 
Published: September 2023

CO2 methanation is a promising method of preventing the wastage of excess energy generated from renewable electricity sources such as wind and solar. The excess energy is used to power an electrolytic cell to produce H2 which is then reacted with CO2 to form CH4 (chemical energy). The stored CH4 can be converted back to electric energy and released into the power grid whenever energy demand exceeds production. To achieve the best result, a suitable catalyst and good reactor design are crucial. Therefore, this study investigated the effect of varying the catalyst ratio in two reactors in series, including water removal, for CO2 methanation over a 10%Ni/YSZ catalyst. The water removal was installed to prevent or minimise the interference effect of water formed in the reactor on the catalyst along the further reactor bed. The results revealed that the reactor configurations with less catalyst in the first reactor compared to the second reactor generally showed better performances. These results can be attributed to the fact that the majority of the water produced in the first reactor was trapped before entering the second reactor since the quantity was small while the reacting species entering reactor 2 had more active sites for the reaction to take place. This study has shown that the ratio of catalysts in multiple reactors connected in series considerably influences the performance of the system.
The hydrogenation of CO2 into CH4, also known as CO2 methanation, can help to resolve the problem of safe storage and transportation associated with H2, since CH4 has a higher volumetric energy density [C. Wang, Nano Res. (2023) 1–12, L. Shi, J. Energy Storage. 62 (2023) 106846]. In addition, CO2 methanation is a promising method of preventing the wastage of excess energy generated from renewable sources such as wind and solar. The excess energy is used to power an electrolytic cell to produce H2 which then is reacted with CO2 to form CH4 (chemical energy). The stored CH4 can be converted back to electric energy and released into the power grid whenever energy demand exceeds production, or more generally be used as a substitute for natural gas.
The activity of the Ni/YSZ catalyst was examined in two fixed-bed quartz reactors arranged in series with a water trap. The product stream was analysed by gas chromatography. We investigated the effect of varying the catalyst ratio in the two-reactor configuration in series with a water trap for CO2 methanation over the 10%Ni/YSZ catalyst. The water trap was installed to prevent or at least minimise the interference effect of water formed in the first reactor on the catalyst in reactor 2. Many studies have reported that water has a negative effect on the activity of catalysts during CO2 methanation. The water from reactor 1 will compete with other reacting species at the catalyst active sites if not removed before entering reactor 2. For example, Hernandez et al. [Chem. Eng. J. 390 (2020) 124629] reported that the addition of water in the feed caused a negative effect during CO2 methanation. Hashemi et al. [Energies. 14 (2021)] also found that the removal of water significantly improved the average reaction rate within the reactor, leading to an increase in CO2 conversion within a reactor operating at static conditions. It is also believed that the removal of water before the reactant stream enters the second reactor helps to overcome the thermodynamic limitations of conversion [S.E. Hashemi, Energies. 14 (2021)].
The effect of varying the catalyst ratios in two reactors in series with water removal for CO2 methanation over 10%Ni/YSZ catalyst was studied. The water trap was installed to prevent or minimise the interference effect of water formed in reactor 1 on the catalyst in reactor 2. The results revealed that the reactor configurations with less catalyst in the first reactor compared to the second reactor generally showed better performances. These interesting results can be attributed to the fact that as the majority of the water produced in the first reactor was trapped before entering the second reactor, the reacting species entering reactor 2 had more active sites for the reaction to take place, as they were not competing with the water molecules. This resulted in a better overall conversion rate. This study has shown that the ratio of catalysts in multiple reactors connected in series influences the performance of the system, along with the provision of product water removal along the reaction pathway.