Effect of H2 concentration during methanation

“A further set of experiments at 300 °C were run to investigate the effect of the H2 concentration during the methanation phase following a CO2 capture stage (9 mins) from a humid feed gas also containing O2. As shown in Fig. 5, at increasing the H2 concentration from 10 to 15 and 20% the methane production became faster and the methane peak sharper. In particular, the slopes of the initial methanation rate profiles scaled proportionally to the concentration of hydrogen (0.41, 0.63 and 0.84 mmol s−2). In each case, the peak production rate reached after ca 20 s corresponded to the complete consumption of all the H2 fed to the reactor. It can be argued that the methanation of the CO2 preadsorbed on the Li-RuA DFM initially proceeded with an apparent first-order dependence on the partial pressure of H2 and probably under mass transfer control. The fast initial methanation rate can be explained by the fast reaction of highly reactive CO2 ad-species/carbonates stored onto the Li-Al sites, which can easily spill-over onto vicinal Ru sites that are reduced back to the metal form as soon as exposed to H2 (Arellano-Treviño et al., 2019Cimino et al., 2022).

Fig 5

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Fig. 5. Methane production rates from CO2 preadsorbed over Li-RuA DFM as a function of the inlet partial pressure of H2 at 300 °C. Adsorption conditions: 5% CO2, 1.5% H2O, 0.25% O2 in N2 for 9 min. Dashed horizontal lines correspond to the maximum theoretical CH4 flow depending on the H2 feed rate. Inset: CO2 conversion at the end of the methanation phase (7 min).

After the methane peak, the formation rate became limited by the availability of residual CO2 on the DFM, thus showing a faster drop for larger concentrations of H2. The faster methanation rate achieved at higher H2 concentration promoted the overall CO2 conversion (inset in Fig. 5) since less CO2 was thermally desorbed from the DFM and lost before it could react with the incoming H2 flow.

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