https://doi.org/10.1016/j.ccst.2021.100011
“Although the carbonation mechanisms have been extensively investigated, there remains disputations concerning the carbonation pathways of potassium-based adsorbents. As opinions vary, it is necessary to provide accurate and in-depth understanding of their carbonation mechanisms by combining some in-situ characterization techniques and DFT calculations. Efforts have been devoted to improving the CO2 capture capacity by materials design and process optimization. Yet, the currently reported CO2 capture capacities are still far from expectation. Significant loss in CO2 capture capacity is observed when dry potassium-based adsorbents are made into DFMs for ICCU application, while the yields of the desired synthetic fuels (syngas or SNG) in catalytic CO2 conversion are highly depended on their CO2 capture capacities. There is still much room for the improvement of their CO2 capture capacities by materials design to increase K2CO3 loading, promote K2CO3 dispersion and improve textural properties. Effects of processing parameters such as carbonation conditions, regeneration conditions, bed parameters and gas impurities have been given full consideration, and the carbonation reactivity has been further improved by optimizing these operating parameters. Despite that the deactivation pathways of the adsorbents in flue gas containing acid impurities have been well understood, more effective strategies for mitigating sorbent deactivation or recovering carbonation reactivity are still on the way, and this certainly deserves more attention in future work. Besides, when made into DFMs, both the adsorption and catalytic active sites might be covered by the gas impurities. The deactivation of DFMs in multiple gas impurities is worth in-depth study.”
“Another major issue that needs to be considered is the increasing pressure drop or elutriation problem when dry potassium-based adsorbents or DFMs are packed as powder state in a dual fixed-bed reactor system or a twin circulating fluidized-bed configuration. This will result in the frequent replenishment of fresh samples in order to retain stabilized CO2 capture and catalytic performance, and this will make the CO2 capture or CCR processes less cost-effective. Pelletization has been proposed and demonstrated as an effective strategy to overcome these issues. Up to now, only Qin et al. and Wang et al. reported the synthesis of cylindrical and spherical potassium-based adsorbent pellets via the extrusion and extrusion-spheronization methods (Qin et al., 2014; Wang et al., 2019). In practical terms, the extrusion-spheronization method might be suitable for manufacturing spherical potassium-based adsorbent pellets that can achieve steady fluidization in a twin circulating fluidized-bed configuration for continuous operation. Practical applications of potassium-based adsorbent pellets would require the pellets exhibit both high CO2 capture capacity and good mechanical properties (abrasion resistance). The reality is that their CO2 capture capacities have been adversely affected, while improving the mechanical strength of potassium-based adsorbent pellets. A perfect tradeoff should be gained between CO2 capture capacity and mechanical properties. Otherwise, more efforts should be made to increase K2CO3 loading and tune pore structures and K2CO3 dispersion for improved CO2 capture performance while maintaining good mechanical properties. Besides, more effective approaches including but not limited to the agar-assisted moulding technique, graphite-casting method, and fluidized bed spray granulation might be employed to fabricate potassium-based adsorbent pellets with desirable attributes. Even the emerging 3D printing technique can be employed to produce potassium-based composite monoliths as adsorbents or catalysts for CO2 capture and conversion. Furthermore, it is more challenging to manufacture K2CO3-containing DFMs pellets, since full consideration should be given to CO2 capture capacity, hydrogenation activity, and abrasion resistance of the DFMs pellets in CCR process.”