Mayenite-supported CaO for thermochemical storage applications. Ageing time effect over conversion

Thermo Chemical Storage systems (TCS) are gaining an increasing interest in the field of long term-thermal energy storage, thanks to their potential applicability in the Concentrated Solar Power technology to increase the electricity generation flexibility, and in different energy sectors operating at medium/high temperature levels, including the industrial one, to optimize the heat recovery and accumulation. As for the high temperature (HT) applications, research is currently focused on oxides and hydroxides-based as well as carbonates-based ones. The latter have been mainly proposed in the literature as CaO/CaCO3 system, initially applied to CO2 capture technologies. However, in order to overcome the intrinsic ciclability limitation observed for natural dolomite and limestone, synthetic sorbents have been recently developed, like the calcium oxide supported on Mayenite (Ca12Al14O33), which has also been investigated by ENEA, showing a significant durability. Despite a fast reaction kinetics and a high conversion extent (up to 80-90% in long isothermal conditions), both in carbonation and calcination steps, this material has shown interaction with air moisture and ambient CO2, affecting the reproducibility of the tests. The objective of the present work is to assess the ambient air stability of the CaO/Mayenite system, to verify the need of pre-treatment processes. At this purpose, CaO/Mayenite powder was synthesized, morphologically characterized and tested thermogravimetrically at different ageing level, namely 30 days and 60 day. The experimental campaign confirms that carbonation conversion remarkably enhances after 30 days of ambient air exposure, while it remains unvaried for further ageing times as a consequence of material properties stabilization. This result indicates that an initial air exposure of about 1 month is sufficient to guarantee a substantial reproducibility of the material performances and no energy consuming pre-treatments are required to stabilize the material response.