Experimental Validation of the Calcium Looping CO2 Capture Process with Two Circulating Fluidized Bed Carbonator Reactors

Postcombustion CO2 capture using CaO as a regenerable solid sorbent in a circulating fluidized bed (CFB) carbonator is emerging as a promising CO2 capture technology. Experimental validation of this concept is provided through a comparative analysis of the results obtained in two laboratory-scale dual fluidized bed (DFB) installations located at INCAR-CSIC (Spain) and IFK (Germany). The analysis is focused on the performance of the CFB carbonator reactors operated with continuous solid circulation of CaO. A reasonable closure of the carbon balances (i) between the CO2 that has disappeared from the gas phase, (ii) the CaCO3 circulating between the reactors, and (iii) the CaCO3 that is formed within the carbonator bed has been established. A necessary condition for the capture of a given molar flow of CO2 is experimentally demonstrated and requires that a slightly overstoichiometric molar flow of active CaO is supplied to the carbonator. The deactivation behavior of the sorbents during continuous looping conditions has been measured. The key parameter to interpret the carbonator reactor results has been the active space time, that is indicative of the CaO inventory per molar flow of CO2 participating in the carbonation reaction and of the reaction rate of the solid inventory in the reactor. Two different approaches have been utilized in order to find a suitable expression for this parameter, thus achieving its correlation with the CO2 capture efficiency. A simple model assuming instant mixing of solids and plug-flow of the gas has been tested. Based mainly on carbonator active space time variation, the CO2 capture efficiency are shown to lie between 30% and above 90%. These results confirm the technical viability of the calcium looping postcombustion CO2 capture process. They have been used for designing the current pilot-plant facilities which are scaled up 20-50 times in regard to the lab-scale units. Moreover, the lab-scale results obtained allow for simulation work to be initiated in regard to the full scale Ca looping application.