APPLIED CHEMISTRY Long-Term Behavior of CaO-Based Pellets Supported by Calcium Aluminate Cements in a Long Series of CO 2 Capture Cycles Vasilije Manovic and Edward J. Anthony* CanmetENERGY, Natural Resources Canada, 1 Haanel DriVe, Ottawa, Ontario, Canada K1A 1M1 A series of carbonation/calcination tests consisting of 1000 cycles was performed with CaO-based pellets prepared using hydrated lime and calcium aluminate cement. The change in CO 2 carrying capacity of the sorbent was investigated in a thermogravimetric analyzer (TGA) apparatus and the morphology of residues after those cycles in the TGA was examined by scanning electron microscopy (SEM). Larger quantities of sorbent pellets underwent 300 carbonation/calcination cycles in a tube furnace (TF), and their properties were examined by nitrogen physisorption tests (BET and BJH). The crushing strength of the pellets before and after the CO 2 cycles was determined by means of a custom-made strength testing apparatus. The results showed high CO 2 carrying capacity in long series of cycles with an extremely high residual activity of the order of 28%. This superior performance is a result of favorable morphology due to the existence of large numbers of nanosized pores suitable for carbonation. This morphology is relatively stable during cycles due to the presence of mayenite (Ca 12 Al 14 O 33 ) in the CaO structure. However, the crushing tests showed that pellets lost strength after 300 carbonation/calcination cycles, and this appears to be due to the cracks formed in the pellets. This effect was not observed in smaller particles suitable for use in fluidized bed (FBC) systems. 1. Introduction Carbon dioxide capture and sequestration (CCS) from flue and syngas obtained from the combustion/conversion of fossil fuels appears to be essential to mitigate global warming and climate change. 1 However, the CO 2 capture/separation step from large point sources is problematic due to issues relating to the technical feasibility and cost of the overall carbon sequestration process. In particular, CO 2 separation is the most technically challenging and energy intensive step for CCS; and hence, much research has been targeted at improving current technologies or developing new approaches for CO 2 separation and capture. Looping cycles for CO 2 capture, employing a solid CaO- based carrier, represent an important new type of technology, which has the potential to inexpensively and effectively remove CO 2 from combustion/gasification gases, allowing it to be regenerated as a pure CO 2 stream suitable for sequestration. 2,3 The process of CO 2 capture is based on the reversible carbonation/calcination reaction: Carbonation is an exothermic reaction and favored at lower temperatures; however, the reaction rate also falls with decreas- ing temperatures. An optimal temperature window from a practical point view is 650-700 °C, resulting in CO 2 concentra- tions e5% in the exhaust gas. 4,5 The reverse reaction, calcina- tion, is thermodynamically favored at higher temperatures, and in order to regenerate sorbent in an almost pure CO 2 stream, temperatures >900 °C are required at atmospheric pressure. 4,5 CaO-based CO 2 capture is designed as a cyclic process since the same amount of sorbent is used for CO 2 capture (carbon- ation) and regenerated (calcination) numerous times. The cyclical transport of large amounts of solids from one chemical and thermal environment to another appears to be best achieved using fluidized bed combustion (FBC) systems. 2 However, in a FBC environment significant attrition and elutriation of the CaO- based sorbents are expected. 5,6 This is more pronounced for Ca looping cycles because, apart from mechanical stresses, the sorbent is subjected to thermal stresses due to the fact that the adsorption and regeneration steps occur at different temperatures. As a result, a significant amount of sorbent is lost from the FBC system, which together with sulphation 7,8 and sintering phenomena 9,10 demands potentially large amounts of fresh sorbent makeup. The disposal of elutriated/spent sorbent and makeup of fresh sorbent diminish both the economic and environmental benefits of the technology. 11-13 Hydration of spent sorbent has been investigated as a possible method for reactivation of the spent sorbent. 7,14 While the * To whom correspondence should be addressed. E-mail: banthony@ nrcan.gc.ca. Tel.: (613) 996-2868. Fax: (613) 992-9335. CaO (s) + CO 2(g) ) CaCO 3(s) (1) Table 1. Elemental Composition of Cadomin Limestone Sample Used (0.25-1.4 mm) component content SiO 2 , wt % 5.47 Al 2 O 3 , wt % 1.54 Fe 2 O 3 , wt % 0.61 TiO 2 , wt % <0.03 P 2 O 5 , wt % <0.03 CaO, wt % 50.67 MgO, wt % 0.55 SO 3 , wt % <0.10 Na 2 O, wt % <0.20 K 2 O, wt % 0.35 Ba, ppm 618 Sr, ppm 272 V, ppm <50 Ni, ppm <50 Mn, ppm 1132 Cr, ppm <50 Cu, ppm 36 Zn, ppm 78 loss on fusion, wt % 40.48 sum, wt % 99.98 Ind. Eng. Chem. Res. 2009, 48, 8906–8912 8906 10.1021/ie9011529 CCC: $40.75 Published 2009 by the American Chemical Society Published on Web 09/18/2009