https://doi.org/10.1002/cssc.202002078
“Alkali metals can react with CO2 to form carbonates of the form A2CO3 (A=Na, K, Li, Cs). If present in a CaO environment, some alkali metals may form mixed bicarbonates of the structural formula A2Ca(CO3)2 during carbonation. Similar to other alkali metal salts, these mixed bicarbonates have a low melting point (e. g., TM[Na2Ca(CO3)2]=810 °C). Recently, using in-situ XRD, Lee et al.22 have shown that the mixed bicarbonate Na2Ca(CO3)2 forms in Na2CO3-impregnated CaO (see Figure 11a) during carbonation (i. e., 600–700 °C in 15 vol % CO2). The authors provide a formation mechanism for the mixed bicarbonate and propose that it forms during the initial carbonation step, see Figure 11b. Hence, the study concludes that the mixed bicarbonate may form at the surface of the CaO particles and proposes the formation mechanism shown in Figure 11 b. Currently, we have no in-depth understanding of how these mixed phases may affect the performance of alkali-metal promoted CaO.”
“Figure 11 (a) In-situ XRD of Na2CO3-wet-impregnated CaO between 25 °C and 800 °C (b) Proposed formation mechanism of Na2Ca(CO3)2. Adapted with permission from Ref. [22]; copyright Elsevier, 2018.”