Hydrodynamical model and experimental results of a calcium looping cycle for CO2 capture

High temperature looping cycles involving solid circulation, such as carbonation–calcination, play an essential role among the CO2 capture technologies under development. The low cost and high availability of Ca-based sorbents together with the feasibility of integration between these capture systems and existing power plants lead to very competitive potential costs of avoided CO2, below 20 €/tonne. Optimal configurations make use of several interconnected fluidized beds. One promising configuration for Ca-based sorbents looping systems relies on the use of two circulating beds (carbonator and calciner) and two bubbling beds acting as non-mechanical valves. Fluidized beds are well characterized when operating independently since they are extensively used in industrial applications, power and chemical plants. However, the operation when two or more fluidized beds exchange solid material through non-mechanical valves is still uncertain because of the more complex pressure balance of the system. Theoretical studies based on thermo-chemical simulations and experimental studies show that minimum CO2 capture cost is attained with large solid circulation flow between reactors. The challenge is to reach the required particle circulation in a system with a complex configuration and be able to control it. Solid internal recirculation in any of these fluidized beds would provide flexibility in its control but it will also make harder the characterization of the whole system. The aim of this work is to analyse the hydrodynamics of the system and to generate a comprehensive mathematical model to better understand the interaction between the elements of the system. Measurements of circulation rate, static pressure, voidage profiles and standpipe height of solids have been used to identify trends in the hydrodynamic behaviour of the whole system while varying fluidizing gas velocities in the risers, loop-seals, solid inventories in the reactors or size distribution of the particles. These measurements are also used to adjust and validate the mathematical model presented in this study.