https://doi.org/10.1002/cssc.202002078
“Turning to SiO2-stabilized CaO-based sorbents, several mixed oxide phases and polymorphs exist in the Ca-Si-O system, depending on the Ca/Si ratio and the respective pre-treatment and operating temperatures. Wang et al.79 showed that the calcination temperature and time influence the formation of mixed Ca-Si oxides for a given sorbent composition, i. e. the solid-state reaction to the thermodynamically favorable mixed oxide is slow. For example, after calcination at 900 °C for 1 h four Si-containing phases were detected: Ca2SiO4, CaSiO3 (calcium silicate and wollastonite polymorphs) and SiO2. However, based on the CaO-SiO2 phase diagram, only Ca2SiO4 should be observed (8 wt % SiO2, i. e. Ca/Si=12, 900 °C), indicating that the transformation to the thermodynamically favorable phase was not completed. In addition, both α- and γ- phases of Ca2SiO4 exist, and the transition between these two polymorphs is observed at around 700 °C. Therefore, the stabilizing phase in SiO2-CaO sorbents may transform between different phases owing to the temperature swing between calcination (typically around 900 °C) and carbonation (typically around 650 °C) as well as the incomplete transformation to the thermodynamically most favorable phase during the first heat treatment. Yet, the influence of these phase transformations during cycling has not been investigated in detail.”