The large surface area and high pore volume will favor CO2 diffusion in the sorbent particles. However, a high specific surface area does not mean that CA-91 sorbent can have a good stability over the multiple carbonation and calcination cycles. With less Al2O3 additive in CA-91 sorbent, only a little amount of inert support substance Ca12Al14O33 can be formed, so the sintering problem of CaO grains cannot be significantly eliminated like CA-82 sorbent. For CA-82 sorbent with 18% Al2O3 additive, the BET special surface area is reduced to 11 m2/g, but it has a higher content of inert support substance Ca12Al14O33, the decay rate of CO2 capture capacity is very weak even over the long-term recycling. Koirala group reported similar results for the synthetic CaO–Al2O3 sorbent with 82% CaO [9].
The crystal structures of the synthetic CaO–Al2O3 sorbents are characterized by XRD, and the diffractograms are shown in Fig. 6. It is found that CaO–Al2O3 sorbent with 18% Al2O3 additive (CA-82) consists of active CaO and inert support substance Ca12Al14O33, as reported in literature [10], [12], [13] and [14]. CA-91 sorbent with 9% Al2O3 additive has the characteristic peaks of CaO, Ca12Al14O33 and Ca3Al2O6, so there are two inert support substances (Ca3Al2O6 and Ca12Al14O33) in CA-91 sorbent. Generally, Ca3Al2O6 is a more stable Ca–Al–O phase in the Ca–Al–O material with a high CaO/Al2O3 ratio at high temperature, and can be formed when the calcination temperature is beyond 1000 °C [19]. In this work, it is found that the inert support substance Ca3Al2O6 can be formed in CA-91 sorbent when the calcination temperature is 720 °C, which is due to the high reaction activity of nano-sized CaO grains with Al2O3 in the synthetic sorbent. After 16th carbonation (600 °C) and calcination (850 °C) cycle, as shown in Fig. 6, Ca12Al14O33 formed in CA-91 sorbent is completely transformed into the stable Ca3Al2O6.