10 CaO-based sorbents were synthesized by a sol–gel process supported with various materials, and their cyclic behavior was investigated under the same reaction conditions.

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A novel cost-effective method was applied to modified calcium-based sorbents for cyclic high-temperature CO 2 capture. The sorbents were derived from cheap limestone and sea salt, and the main processes of the preparation involved two simple steps: hydration of CaO and impregnation with sea salt in CaO. Results indicated that the simultaneous hydration− impregnation (SHI) method contributed to the formation of highly improved calcium-based sorbents during cyclic calcination/ carbonation reactions. After 40 cycles, the SHI limestone doped with 3.0 wt % sea salt achieved a CO 2 capture capacity of 0.31 g of CO 2 /g of sorbent, which was 126% higher than that of natural limestone. Moreover, the SHI limestone sorbent contained numerous macropores after several cycles. Further investigation on microstructure changes of the sorbents showed that the macropores were relatively stable during cyclic reactions, which can be attributed to the stable behavior of the sorbent. In contrast, the natural limestone lost its micro-and macropores during initial reactions, thereby rapidly losing its sorption capacity during calcium looping cycles. Furthermore, the SHI limestone sorbents showed slightly better mechanical strength than the natural limestone sorbent demonstrated by attrition tests.

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The calcium looping process is well regarded as one of the most prospective technologies for trapping CO2 from flue gas. However, the carbonation conversion of CaO derived from natural Ca precursors decayed drastically during the long-term cycles. Doping has been considered as a promising method to improve the cyclic carbonation performance. However, alkali salt, which derives from abundant natural sustainable resources such as seawater, saline, bittern deposit, and so on, is regarded as a low melting material that can hardly enhance the cyclic carbonation performance of the CaO-based sorbents. In this study, two common alkali salts, sodium and potassium, were doped to investigate the potential enhancing effect of alkali salt on CaO-based sorbents. The cyclic carbonation performance of those sorbents was tested in a simultaneous thermal analyzer (STA), and the enhancing mechanisms were illustrated by scanning electron microscope (SEM), X-ray powder diffraction (XRD), and N2 physical absorption methods. The results illustrated that KCl, NaCl, and K2CO3 boosted the carbonation property of CaO markedly, while KOH, NaOH, and Na2CO3 were detrimental to the sorbents. The synergistic effect of K+, Na+, and Cl– can improve the cyclic carbonation performance of CaO. After 50 cycles, the carbonation conversions of CaO modified by KCl or NaCl were about twice that of unmodified CaO. In general, the KCl and NaCl are promising dopants that can markedly enhance the carbonation property of CaO-based sorbents.

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