Experimental Characterization of the Calcium Looping Process for CO₂ Capture

A competitive post-combustion CO₂ capture process is Calcium looping. It requires a Dual Fluidized Bed (DFB) with continuous looping of CaO between the carbonator, where flue gas CO₂ is removed, and the regenerator, where highly concentrated CO₂ is released. A 10 kW_th DFB has been built and operated at the Institute of Combustion and Power Plant Technology (IFK) of the University of Stuttgart, consisting of a riser and a Bubbling Fluidized Bed (BFB) to provide proof of principle and characterize fluid-dynamic and reactor performance interactions. Latter interactions have been studied in cooperation with the Instituto Nacional del Carbón (INCAR) of the Spanish Research Council (CSIC).

Fluid-dynamic interactions were mainly studied with use of a scaled cold model of the 10 kW_th IFK DFB. The independent variables of carbonator superficial velocity, Total Solid Inventory, reactor overpressure, loop seal aeration, mechanical valve opening for solid looping control and the Particle Size Distribution were varied. A stable operating region, bordered by two unstable regions has been identified for the carbonator riser. Moreover, the influence of above independent variables on carbonator riser dependent variables, i.e. pressure drop, inventory, flow structure, entrainment and solid flow between the two beds has been assessed. The study of reactor performance interactions has taken place with use of the 10 kW_th IFK DFB facility, utilizing its riser or BFB as the carbonator, and the 30 kW_th INCAR-CSIC DFB facility. The design and results of the latter facility have been generated outside the thesis scope. Result analysis examines the closure of the CO₂ carbonator mass balance expressions between (i) the CO₂ that has disappeared from the gas phase (ii) the CaCO₃ that is circulating between reactors and (iii) the CaCO₃ that is formed within the carbonator bed. Through the first mass balance expression, the quality of measurements is confirmed. Through the second mass balance expression, results indicate that a slightly overstoichiometric flow of active Ca is needed for a given CO₂ capture efficiency to be achieved. The third expression requires utilization of a carbonation rate term, prior to fitting to experimental data. Two theoretical approaches (A & B) utilized for this matter, lead to different carbonator models and active space time expressions. The active space time is the key parameter in both carbonator models since it is indicative of the CaO inventory per molar flow of CO₂ participating in the carbonation reaction and of its reaction rate. Under the light of these approaches, the behaviour of dependent variables included in the models is analyzed i.e. that of the decay of the maximum carbonation conversion, the actual carbonation conversion in/after the regenerator and the form of the axial vol.-% CO₂ & pressure drop profiles. In addition, the models explain the effect of further dependent variables, i.e. carbonator temperature & space time, the Ca looping ratio on the carbonator CO₂ capture efficiency. Approach B, utilizing a reaction rate independent of the difference of the maximum and actual particle carbonation conversion fits all experimental data sets well contrary to approach A. Hence approach B, with its respective model and active space time expression, is considered as generic. Respective active space time variation leads to a CO₂ capture efficiency variation between 30 % and greater than 90 %. The lab-scale unit results presented here confirm the technical viability of the Calcium looping process and have been used for the design of 20-50 times higher capacity pilot-scale units.