8906
Ind. Eng. Chem. Res. 2009, 48, 8906–8912
APPLIED CHEMISTRY Long-Term Behavior of CaO-Based Pellets Supported by Calcium Aluminate Cements in a Long Series of CO2 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 CO2 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 CO2 cycles was determined by means of a custom-made strength testing apparatus. The results showed high CO2 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 (Ca12Al14O33) 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 CO2 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, CO2 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 CO2 separation and capture. Looping cycles for CO2 capture, employing a solid CaObased carrier, represent an important new type of technology, which has the potential to inexpensively and effectively remove CO2 from combustion/gasification gases, allowing it to be regenerated as a pure CO2 stream suitable for sequestration.2,3 The process of CO2 capture is based on the reversible carbonation/calcination reaction: CaO(s) + CO2(g) ) CaCO3(s)
(1)
Carbonation is an exothermic reaction and favored at lower temperatures; however, the reaction rate also falls with decreasing temperatures. An optimal temperature window from a practical point view is 650-700 °C, resulting in CO2 concentrations e5% in the exhaust gas.4,5 The reverse reaction, calcination, is thermodynamically favored at higher temperatures, and in order to regenerate sorbent in an almost pure CO2 stream, temperatures >900 °C are required at atmospheric pressure.4,5 CaO-based CO2 capture is designed as a cyclic process since the same amount of sorbent is used for CO2 capture (carbonation) and regenerated (calcination) numerous times. The cyclical transport of large amounts of solids from one chemical * To whom correspondence should be addressed. E-mail: banthony@ nrcan.gc.ca. Tel.: (613) 996-2868. Fax: (613) 992-9335.
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 CaObased 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 sulphation7,8 and sintering phenomena9,10 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 Table 1. Elemental Composition of Cadomin Limestone Sample Used (0.25-1.4 mm) component
content
SiO2, wt % Al2O3, wt % Fe2O3, wt % TiO2, wt % P2O5, wt % CaO, wt % MgO, wt % SO3, wt % Na2O, wt % K2O, wt % Ba, ppm Sr, ppm V, ppm Ni, ppm Mn, ppm Cr, ppm Cu, ppm Zn, ppm loss on fusion, wt % sum, wt %
5.47 1.54 0.61
