Influence of calcination conditions on carrying capacity of CaO-based sorbent in CO2 looping cycles

This study examines the loss of sorbent activity caused by sintering under realistic CO2 capture cycle conditions. The samples tested here included two limestones: Havelock limestone from Canada (New Brunswick) and a Polish (Upper Silesia) limestone (Katowice). Samples were prepared both in a thermo-gravimetric analyzer (TGA) and a tube furnace (TF). Two calcination conditions were employed: in N-2 at lower temperature; and in CO2 at high temperature. The samples obtained were observed with a scanning electron microscope (SEM) and surface compositions of the resulting materials were analyzed by the energy dispersive X-ray (EDX) method. The quantitative influence of calcination conditions was examined by nitrogen adsorption/desorption tests, gas displacement pycnometry and powder displacement pycnometry; BET surface areas, BJH pore volume distributions, skeletal densities and envelope densities were determined. The SEM images showed noticeably larger CaO sub-grains were produced by calcination in CO2 during numerous cycles than those seen with calcination in nitrogen. The EDX elemental analyses showed a strong influence of impurities on local melting at the sorbent particle surface, which became more pronounced at higher temperature. Results of BET/BJH testing clearly support these findings on the effect of calcination/cycling conditions on sorbent morphology. Envelope density measurements showed that particles displayed densification upon cycling and that particles calcined under CO2 showed greater densification than those calcined under N-2. Interestingly, the Katowice limestone calcined/cycled at higher temperature in CO2 showed an increase of activity for cycles involving calcination under N-2 in the TGA. These results clearly demonstrate that, in future development of CaO-based CO2 looping cycle technology, more attention should be paid to loss of sorbent activity caused by realistic calcination conditions and the presence of impurities originating from fuel ash and/or limestone. (c) 2009 Elsevier Ltd. All rights reserved.