Advanced CO2 capture systems based on Calcium Looping for deep decarbonization of flue gases

Abstract

CO2 capture efficiencies in post-combustion Calcium looping (CaL) systems are usually reported to be around 0.90, mainly due to the operation of the carbonator at temperatures around 650 ºC and the restrictions imposed by the CO2-CaO equilibrium. This work analyzes a solution to increase the CO2 capture efficiencies, by reducing the temperature in the solid entrainment upper part of the circulating fluidized carbonator. To overcome known CaO conversion limits in such region, an additional small flow of Ca(OH)2 is fed to the cooled solid entrainment region of the carbonator. Thus, the dense bed, where most of the CO2 is captured, is operated at standard conditions but the full CaL process includes a small hydrator to treat a small fraction of the purge of CaO-rich solids and to provide the necessary flow of Ca(OH)2. A second approach also discussed in this work involves similar actions but carried out after the carbonator cyclone, where the cooling is facilitated by the absence of the circulating solids. In this case, the flue gas leaving the CFB carbonator is put in contact with Ca(OH)2 in a second carbonation entrainment zone. These two CaL configurations can reach CO2 capture efficiencies of 0.99. Experimental work characterizing the fast carbonation kinetics of Ca(OH)2 powders in the entrainment zones has been completed. Mass and energy balances have been solved to discuss feasible process configurations. Net energy efficiencies of 0.342 and 0.345 have been estimated for the two CaL configurations, which result into a decrease of only 0.021 and 0.018 net points respect to a standard CaL system with a CO2 capture efficiency of 0.90.