Performance of sorbents during simulated calcium looping cycles: a comparison between fluidized bed and thermo-gravimetric tests

Six limestones and one dolomite from different European countries were tested for CO2 capture during simulated calcium looping cycles 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°C, 70% CO2). The TG tests were performed with a somewhat milder calcination environment (T = 900°C, 15% CO2). Carbonation conditions were the same for the two types of experiments (T = 650°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, as regards 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 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 towards CO2 capture was similar under FB and TG conditions and was not significantly influenced by the presence of SO2.