Carbonate looping cycle for CO2 capture: Hydrodynamic of complex CFB systems

High temperature looping cycles, such as carbonation-calcination cycles based on calcium sorbents or chemical looping combustion are being developed and play an essential role in CO2 capture technologies. Among proposed configurations, outstanding schemes make use of a number of interconnected fluidized beds and may operate at bubbling or circulating regime. Fluidized bed behaviour is well-known since they are included in many industrial applications, such as power plants and chemical industries. However, there is a lack of knowledge about their operation when more than one fluidized bed are coupled in the same system.One promising configuration for Ca-based sorption looping systems relies on the use of two circulating beds as carbonator and calciner and two bubbling beds acting as loop-seal valves. Many theoretical and lab experimental studies point out the need of large solid circulation in the system to reach high carbonation efficiencies. The control of this flow in complex CFB looping systems, where also internal recirculation exists in the risers, becomes a difficult task and deserves further studies to characterize them. The challenge to solve, through experimental tests and mathematical modelling, is finding a comprehensive control method to operate two circulating beds in turbulent regime and two bubbling sealing devices. Experimental results supporting high carbonation efficiency or feasibility in scaling-up solid circulation rates and inventories are needed to make the system more reliable.A lab-scale cold flow facility has been designed based on Glicksman’s scaling rules and constructed in order to conduct experimental tests. The mechanical design of the facility and the choice of solid material, fluidizing gas and operating conditions should be such as to ensure the circulation of solids between reactors and the presence of solids inventory in the carbonator which are necessary to achieve high capture efficiencies.Operation of the system has been tested for a long number of hours under very different conditions. 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 gas velocities in the risers, loop-seals, inventories in the reactors or size distribution of the particles. The circulation rates attained in the cold flow plant are comparable, after scaling-up, to solid flows in the loop which lead to high enough carbonation efficiencies of the system.