Potential application for direct air capture and conversion

The following content is copied from this reference (https://doi.org/10.1016/j.ccst.2022.100052):

“Apart from the widely investigated ICCC for CO₂ at high concentration levels in industrially relevant processes as listed in Table 5, DMFs were also reported to be promising for direct air capture (CO₂ at ppm levels) and conversion by a few studies. It was demonstrated that the Ni/Na-γ-Al₂O₃ could convert >90% of the captured CO₂ from a gas stream containing 100–400 ppm CO₂ to more concentrated CH4 even in the presence of 20% O₂ as shown in Fig. 21a (Kosaka et al., 2021). Ru-based DFMs were also investigated for the CO₂ direct air capture and methanation. Jeong-Potter et al. used 0.5%Ru-6.1%Na₂O/γ-Al₂O₃ for the integrated capture of diluted CO₂ in a simulated air stream (400 ppm) and methanation at 320°C (Jeong-Potter and Farrauto, 2021). More than 82% of the absorbed CO₂ can be effectively converted to CH₄ with a yield of 0.172 mol/kg and selectivity of 100% under dry conditions. The developed DFMs show great potential for combining direct air capture and methanation in one process. Nonetheless, the sorption of CO₂ was conducted at 320°C meaning the energy consumption for heating up the air stream. The energy required for the temperature swing mode of the direct air capture and methanation was estimated to be 9 MJ/m³-CH₄ in another study, although with the CO₂ sorbent and catalyst deployed in two separate reactors (Veselovskaya et al., 2020). Lower operation temperatures need to be conducted to reduce the energy requirement (Jeong-Potter and Farrauto, 2021). In a further study, the 1%Ru-10%Na₂O/γ-Al₂O₃ was used for the direct air capture of CO₂ at ambient temperature and methanation at 300°C as shown in Fig. 21b (Jeong-Potter et al., 2022). The CH₄ yield of 1.067 mol/kg with 100% selectivity was achieved. It was found that with the exposure of 0.5%Ru-6.1%Na₂O/γ-Al₂O₃ to a high content of O₂ (∼21%) for a long time, the Ru in DFM can be completely reactivated in the methanation step (Jeong-Potter and Farrauto, 2021). In the presence of 2.0% H₂O in air, it can significantly improve both the sorption and methanation performances of 1%Ru-10%Na₂O/γ-Al₂O₃, which were 2.4 and 3.5 times greater respectively than that under dry condition. This was supposed to be caused by the formation of amorphous NaHCO₃ in the presence of H₂O. On Ru-CaO/γ-Al₂O₃ DFM, on the contrary, humid air was found to adversely affect the sorption of CO₂ (Jeong-Potter et al., 2022). Considering the lower operation temperature of Na₂O-DFM compared to CaO-DFM due to the higher stability of formed carbonates on CaO (Arellano-Treviño et al., 2019a; Bermejo-López et al., 2019a), it appears that Na₂O containing Ru-based DFMs is more promising for direct air capture/methanation under realistic conditions (Jeong-Potter et al., 2022).”

“Fig. 21. Schematic of diluted CO₂ (atmospheric level) capture and conversion using DFMs in (a) isothermal and (b) temperature swing operation mode (Jeong-Potter et al., 2022; Kosaka et al., 2021).”

“Nonetheless, intensive investigations are needed for integrated direct air capture and conversion. For example, more DFMs need to be developed and tested for DAC applicability, thus guiding the frame of promising materials combination. Detailed and extended effects of O₂, H₂O, and other prevalent components on the sorption and catalytic activity of DFMs remain to be tested. Prolonged experimental tests should be conducted in ambient air conditions to evaluate the robustness of the developed DFMs. Methane is currently the only conversion product for the integrated DAC process. Other possible products can be investigated to achieve the diversity of the process. Appropriate operation temperatures for the sorption and subsequent conversion should be studied for process optimization. Since the CO₂ concentration is much lower, thus the sorption duration of CO₂ capture was much longer than that for the industrial flue gases to reach reasonable CO₂ uptake. However, the time needed for the conversion was usually much lower than the sorption process (Kosaka et al., 2021). Therefore, the matching between these two consecutive processes should be properly designed and optimized for practical application.”