https://doi.org/10.3390/nano11123188
“The different types of behavior of the CaTi-3 sorbent cyclic experiment under N2 flowing at low temperatures (700 °C for 30 min, mild condition) or under CO2 flowing at high temperatures (920 °C for 15 min, severe condition) may be related to calcium formation on the support surface based on the XRD results of sorbents collected after cyclic experiment (Figure 11). Under mild regeneration conditions, the cyclic experiment process mainly involves Ca(OH)2 and CaCO3 species transformation. Ca(OH)2 is under the molten state (melting point ~512 °C), while CaCO3 is in the solid state (melting point ~825 °C) during carbonation and regeneration. During carbonation, the ions diffuse in molten Ca(OH)2 layers more efficiently; thus, almost all calcium species can be utilized. When CaCO3 forms, it will precipitate due to its larger density (2.71 cm3/g) than Ca(OH)2 (2.21 cm3/g). During regeneration, the molten Ca(OH)2 layer permits the easy regeneration of CaCO3 located in the interior position. Therefore, nearly no capture capacity decay occurs. Under severe regeneration conditions, the cyclic experiment process mainly involves CaO and CaCO3 species transformation. CaCO3 is in the solid state during carbonation and under the molten state during regeneration; furthermore, CaO is in the solid state, and sintering cannot occur during carbonation and regeneration (melting point ~2613 °C, Tammann temperature ~1154 °C) [29]. During carbonation, external CaO can react with CO2 to form CaCO3, whereas CO32− and O2− exchange occurs at the internal CaCO3/CaO interface [4]. During regeneration, CaO forms and precipitates in molten CaCO3 (density of CaO is 3.34 cm3/g). The microstructure stacked by these aggregated precipitates will affect subsequent cycle carbonation reaction performance. More precipitants with densely stacked structures reduce the carbonation efficiency.”
“Figure 11. Schematic diagram of CaTi-3 under (a) mild regeneration conditions, and (b) severe regeneration of multicycle experiment.” https://doi.org/10.3390/nano11123188