Pilot-Plant Investigation of High-Temperature Thermochemical Energy Storage Based on the Material System CaO/Ca(OH)2 in a Bubbling Fluidized Bed

Thermochemical energy storage based on the material system CaO/Ca(OH)₂ is currently investigated at the Chair of Energy Systems using fluidized bed technology aiming at storage temperatures of 400 °C to 600 °C for industrial applications. Besides many advantages such as high storage density, broad abundance, low costs, nontoxicity and chemical stability, one of the challenges to face is the mechanical particle stability. It restricts the number of overall storage cycles to be performed. Particle breakage with increasing number of storage cycles leads to a rising share of fines in the fluidized bed leading to defluidization eventually.

Previous studies in laboratory scale facilitated the understanding on storage material behavior in the fluidized bed. Hereby both, the hydration (discharging) and dehydration (charging) step are best performed in pure steam atmosphere close to equilibrium conditions. Charging and discharging reaction are controlled by temperature change. This knowledge is now transferred to a technical scale.

The reactor utilized here has a cylindrical fluidized bed volume of up to 30 L (overall volume of 100 L), a maximum operation temperature of 700 °C, a pressure rating up to 6 bar(g) and can be operated with pure air, nitrogen, steam or mixtures thereof. It is designed for particle collectives with d₅₀ = 20 µm and higher.

First experimental results with calcination of the raw CaCO₃ (Sauter mean diameter is 219 µm) at 700 °C in pure steam atmosphere and subsequent cyclisation (successive charging and discharging) at a temperature difference of 50 K to the apparent reaction equilibrium are performed successfully. Results show promising scalability of the storage reaction in terms of material stability.