Stability of CaO-based Sorbents under Realistic Calcination Conditions: Effect of Metal Oxide Supports

Solid metal oxides with high Tammann temperatures have been investigated as stabilizers to remedy the sintering of CaO sorbents during the calcium looping process. A methodical comparison between different metal stabilizers must take into account two factors: (1) equal molar ratio of metal stabilizer to active sorbent and (2) homogenous dispersion of the stabilizer in the sorbent. The negligence of one of these two factors has caused a discrepancy among the current work in the literature with respect to the optimum metal stabilizer for CaO sorbents. In this work, seven metal promoters (Mg, Y, Ce, Nd, Zr, La, and Al) were studied to assess their stabilization effect on CaO sorbents used in cyclic carbonation and calcination experiments. Scanning electron microscopy/energy dispersive X-ray analysis elemental mapping was performed on all sorbents to ensure uniform dispersion of stabilizers in the sorbents. Results indicate that among the metal supports tested in this study, Mg-stabilized sorbents are most efficient in maintaining a high uptake capacity through cycles, under both mild and harsh calcination conditions. This is partly attributed to the high Tammann temperature of MgO stabilizer in comparison to other promoters, resulting in efficient separation of CaO grains and inhibition of sintering and agglomeration. Mg-stabilized sorbents (Mg/Ca = 1:14) showed a first-cycle uptake capacity of 0.68 and 0.63 g/g under mild and harsh calcination conditions, respectively. Sorbents with higher stabilizer ratio (as high as Mg/Ca = 1:2) were tested in 100 cycles and showed a residual uptake capacity of similar to 0.15 g/g. In contrast to previously reported results in the literature, the stabilization effect of Mg promoter was observed even at Mg/Ca molar ratios as low as 1:14.