Performance of Natural Sorbents during Calcium Looping Cycles: A Comparison between Fluidized Bed and Thermo-Gravimetric Tests

Six European limestones and one European dolomite were tested for CO2 capture by simulating calcium looping cycles both in a lab-scale fluidized bed (FB) and in a thermo-gravimetric (TG) apparatus. The FB tests were carried out under severe conditions representative of a process with calcination in an oxy-firing environment (T = 940 degrees C, 70% CO2). The TG tests were performed with a somewhat milder calcination environment (T = 900 degrees C, 15% CO2). Carbonation conditions were the same for the two types of experiments (T = 650 degrees C, 15% CO2). The effect of the presence of SO2 (during both calcination and carbonation) at two different concentration levels was also studied in both devices. The aim of these experiments was to select the best sorbents for further larger scale testing and to establish if TG testing is representative of results obtained under more realistic FB conditions, with regards to CO2 capture capacity and overall ranking of the sorbents. Results showed that the CO2 capture capacity of the sorbents measured in TG tests was generally larger than that measured in FB experiments. This was attributed to the severer calcination conditions in the latter tests, implying an increased effect of sintering. A notable exception was the dolomite sorbent, which was less subject to sintering and more prone to fragmentation during the FB tests. The presence of SO2 depressed the CO2 capture capacity of all the sorbents in both types of experiments, due to the sulfate layer formation around the particles. SEM/EDX and porosimetric analyses of the spent sorbents confirmed the above findings. Interestingly, the ranking of the sorbents toward CO2 capture was similar under FB and TG conditions and was not significantly influenced by SO2.