Carbonation of Limestone Derived CaO for Thermochemical Energy Storage: From Kinetics to Process Integration in Concentrating Solar Plants

Thermochemical energy storage (TCES) is considered as a promising technology to accomplish high energy storage efficiency in concentrating solar power (CSP) plants. Among the various possibilities, the calcium-looping (CaL) process, based on the reversible calcination-carbonation of CaCO3 stands as a main candidate due to the high energy density achievable and the extremely low price, nontoxicity, and wide availability of natural CaO precursors such as limestone. The CaL process is already widely studied for CO2 capture in fossil fuel power plants or to enhance H-2 production from methane reforming. Either one of these applications requires particular reaction conditions to which the sorbent performance (reaction kinetics and multicycle conversion) is extremely sensitive. Therefore, specific models based on the conditions of any particular application are needed. To get a grip on the optimum conditions for the carbonation of limestone derived CaO in the CaL-CSP integration, in the present work is pursued a multidisciplinary approach that combines theoretical modeling on reaction kinetics, lab-scale experimental tests at relevant CaL conditions for TCES, process modeling, and simulations. A new analytic equation to estimate the carbonation reaction rate as a function of CO2 partial pressure and temperature is proposed and validated with experimental data. Using the kinetics analysis, a carbonator model is proposed to assess the average carbonation degree of the solids. After that, the carbonator model is incorporated into an overall process integration scheme to address the optimum operation conditions from thermodynamic and kinetics considerations. Results from process simulations show that the highest efficiencies for the CaL-CSP integration are achieved at carbonator absolute pressures of similar to 3.5-4 bar, which leads to an overall plant efficiency (net electric power to net solar thermal power) around 41% when carbonation is carried out at 950 degrees C under pure CO2.