Type of metal oxides stabilizers for CaO

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

“Generally, metal oxide stabilizers can be classified into reactive metal oxides and inert metal oxides,¹² whereby the latter do not form mixed oxides with CaO (such as MgO) under CL conditions. The formation of mixed oxides has been reported for a variety of stabilizers including Al₂O₃, ZrO₂ and SiO_2,^{17, 57, 59, 64, 65, 71} see Table 1, and has in some cases been linked to the deactivation of the sorbent. In general, the incorporation of calcium into a CO₂-inert mixed phase leads to a loss in CO₂-reactive CaO, which decreases the maximum CO₂ uptake of the sorbent.⁹ Since in many cases metal oxide stabilizers have been added in comparatively high quantities (5–20 wt %), the formation of CO₂-capture-inactive mixed oxides can reduce significantly the maximal theoretical CO₂ uptake of the material. For example, Al₂O₃-stabilized CaO can form at least two Ca-Al mixed oxides under reactive conditions, viz. tricalcium aluminate (Ca₃Al₂O₆) and mayenite (Ca₁₂Al₁₄O₃₃).^17, 23 For ZrO₂– and SiO₂-stabilized CaO, the formation of CaZrO₃ and CaSiO₃ as well as Ca₂SiO₄ has been observed, respectively.^{57, 64, 66, 71} These high-Tammann temperature [T_T(CaZrO₃)=1280 °C; T_T(CaSiO₃)=770 °C; T_T(Ca₂SiO₄)=1070 °C] mixed phases typically form during the heat treatment (i. e. initial calcination) of the sorbent. Hence, for reactive stabilizers, the stabilization effect is linked to the presence of the mixed phase (e. g., Ca₃Al₂O₆) rather than the initially added simple oxide (e. g., Al₂O₃). Therefore, an important aspect when choosing a stabilizer is a careful consideration of the phase diagram and the chemical and structural stability of the metal oxide under the temperature swing between CO₂ capture and calcination conditions.”