Improving the performance of calcium looping for solar thermochemical energy storage and CO2 capture

Concentrating solar thermal (CST) technologies for power production can play a major role in the future portfolio of renewable energies. Limestone calcination/carbonation (Calcium Looping (CaL)), is an appealing reaction whose integration with CST is widely investigated for thermochemical energy storage (TCES) and carbon capture and storage/utilization (CCSU). Experimental data under realistic CST conditions/reactors currently lacks, since most of the experimental activities have been performed in thermogravimetric analyzers. In this study, CaL-CST integration was investigated in a lab-scale directly irradiated fluidized bed reactor, able to mimic the operating conditions required for industrial implementation of the technology. Three different techniques to improve the performance of CaL-CST for TCES and CCSU were investigated: i) lowering of calcination temperature; ii) precalcination; iii) use of dolomite instead of limestone. Experimental results revealed that all the strategies moderately improve system performance. After 20 cycles, depending on the technique applied, the mean carbonation degree ranges within 28.1-37.1% (TCES) and 15.3-18.7% (CCSU) with limestone, and values 61.5% (TCES) and 36.7% (CCSU) with dolomite. Figures of energy storage density are less sensitive to the different techniques, as pay for the lower calcination temperature (limestone), or for the presence of an inert MgO fraction (dolomite). Corresponding values range within 941-1065 MJ m (TCES) and 777-872 MJ m (CCSU), for loose-packed conditions. N-physisorption analyses revealed that the increased reactivity arises from better microstructural properties in terms of specific surface. Optimal choice among the different strategies should consider the intrinsic peculiarities of each investigated technique.