Influence of alkali metal content for CaO-based CO2 capture

https://doi.org/10.1002/cssc.202002078

” The cyclic CO₂ uptake performance of alkali metal-modified CaO varies strongly depending on the nature of the alkali metal (i. e., Li, Na, K, Rb and Cs), the precursors used for synthesis (e. g., Na₂CO₃ vs. NaCl) and the content of alkali metal salt added.^11, 70, 88 However, a general trend of a decreased CO₂ uptake with increasing alkali metal content seems to hold for most studies. For high contents of Na and K (i. e., several mol %), independent of the precursor used, various authors have shown that the CO₂ uptake performance is decreased, possibly due to enhanced sintering effects.^22, 86 In such cases, the addition of alkali metals leads to an inferior performance over cycling showing CO₂ uptakes as low as 0.05 g /g_Sorbent (e. g., 20 wt % NaOH, wet-impregnated)⁸⁷ already during the first carbonation step (40 % CO₂, 600 °C, 5 h).

For low alkali metal contents, some studies have observed a strong promotional effect of alkali metal salts such as NaCl⁸⁶ (2 wt %, wet impregnation, uptake 10^th cycle 0.48 g /g_Sorbent), KCl (2 wt %, wet impregnation, uptake 10^th cycle 0.50 g /g_Sorbent) and NaCl from sea salt (ca. 0.13 wt % NaCl, wet impregnation, uptake 10^th cycle 0.33 g /g_Sorbent). It has been proposed that there is an appreciable effect of alkali metals on the kinetics of the carbonation and calcination reactions which may enhance the CO₂ uptake performance⁹ For example, Wieczorek-Ciurowa et al.⁸⁹ have shown that the decomposition temperature of calcite is reduced by about 10 °C by the addition of NaCl, i. e. alkali metals can shift the CaO/CaCO₃ equilibrium. A similar shift of the decomposition temperature by about 10 °C has been reported by Xu et al. for the addition of KCl and KOH.⁸⁶ Other authors have reported that alkali metals may also positively affect the diffusion process of CO₂ in the product layer of CaCO₃. For example, Gonzalez et al.⁹⁰ reported that for wet-impregnated limestone (0.05–0.5 m KCl/K₂CO₃ aqueous solution) the conversion rate of CaO in the diffusion-controlled carbonation regime was enhanced by the addition of either KCl or K₂CO₃ compared to the pristine limestone benchmark. The authors concluded that the incorporation of potassium impurities in the sorbent promotes the diffusional processes during carbonation (i. e., the diffusion of CO₂ through the CaCO₃ product layer). “