Effect of the CO2 adsorption temperature

“It was previously recalled that the CO2 adsorption on pre-reduced alkali-based DFMs is an exothermic process that is thermodynamically favoured at lower temperatures where it can still occur with fast kinetics (Jeong-Potter et al., 2022). At variance, the catalytic methanation of CO2 over Ru-alumina catalysts proceeds at a low rate below 200 °C but is more strongly activated by the temperature (Fig. 2).

To investigate the potential of operating the two half-cycles at different temperatures we lowered the temperature during the isothermal CO2 capture stage (down to 180 °C in 20 °C intervals) while the subsequent hydrogenation was performed in a temperature-programmed mode by heating the reactor at ca 10 °C/min up to 300 °C (keeping constant the total duration of 14 min). Results presented in Fig. 6 confirm that the CO2 capture capacity of the Li-RuA DFM almost linearly increased from 330 μmol/gDFM at 300 °C up to 390 μmol/gDFM at 180 °C. As a consequence, methane production was enhanced with respect to the reference case of isothermal operation at 300 °C (243 μmol/gDFM) reaching a maximum of 285 μmol/gDFM when CO2 capture was performed at 240 °C. The amount of CO2 lost during the methanation phase increased progressively when the initial temperature was below 240 °C, due to competition between thermal desorption and (initially slow) surface reaction (Jeong-Potter et al., 2022), which penalizes the CO2 conversion and limits methane yield. As such, higher concentrations of hydrogen and larger amounts of DFM can help to speed up and light off the methanation reaction also taking into account the internal heating effect due to the exothermal release.

Fig 6

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Fig. 6. Effect of the adsorption temperature on the CO2 capture as well as the CH4 and CO2 released during the subsequent temperature programmed methanation (10 °C min−1 up to 300 °C, then hold) with Li-RuA DFM. Adsorption conditions (18 min): 5% CO2, 1.5% H2O, 0.25% O2 in N2. Experimental details can be found in Section 2.6.

While fast external heating of a full-scale fixed bed reactor unit in between the capture and hydrogenation steps might be problematic (Jeong-Potter et al., 2022), running the integrated process with two interconnected fluidized bed reactors with circulating DFM particles (Kosaka et al., 2022) could allow to independently optimize the operating temperature of each reactor.

Overall, the same batch of Li-RuA DFM performed 81 cycles of integrated CO2 capture and methanation for the parametric study without any significant sign of degradation of its initial performance.”

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