International Journal of Greenhouse Gas Control 6 (2012) 164–170 Contents lists available at SciVerse ScienceDirect International Journal of Greenhouse Gas Control j our na l ho me p age: www.elsevier.com/locate/ijggc High-temperature CO 2 capture cycles for CaO-based pellets with kaolin-based binders Firas N. Ridha a,b , Vasilije Manovic a , Arturo Macchi b , Edward J. Anthony a,∗ a CanmetENERGY, Natural Resources Canada, 1 Haanel Drive, Ottawa, Ontario K1A 1M1, Canada b Department of Chemical and Biological Engineering, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada a r t i c l e i n f o Article history: Received 5 July 2011 Received in revised form 30 October 2011 Accepted 8 November 2011 Available online 30 December 2011 Keywords: CO2 capture CaO-based pellets Ca-looping Limestone Acetification a b s t r a c t Pellets from natural and acetified Havelock limestone were synthesized and tested for in situ post- combustion CO 2 capture in Ca-looping cycles. Natural kaolin and Al(OH) 3 obtained from acid leaching of kaolin were used as binders. The results showed that after 30 cycles, pellets prepared from acetified lime- stone with 10 vol.% acetic acid and CaO/Al(OH) 3 mass ratio of 5.5 had a somewhat higher CO 2 uptake of 0.13 g/g (g CO 2 /g sorbent), compared to 0.12 g/g for pellets prepared from natural limestone and Al(OH) 3 . These uptakes corresponded to 56% and 36.7% of their initial capture capacities, respectively. By contrast, natural sorbent and pellets prepared with kaolin demonstrated a significant decay in their CO 2 uptake down to 21% and 21.7% of their initial capacities, respectively. The superiority of acetified sorbents with Al(OH) 3 binders appears to be due to the creation of a highly developed porous structure. In particular, -Al 2 O 3 generated from Al(OH) 3 appears to create a stable framework dominated by mesopores, which also inhibits sintering over multiple cycles. However, acetification itself widens pores and alters pore surface area and volume, providing more space for CaCO 3 growth. Unfortunately, water addition dur- ing pellet synthesis appears to compromise this porous structure, thus reducing the original benefits of acetification. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction Calcium looping is a new emerging technology for CO 2 sepa- ration and capture from flue gas (Yongping et al., 2010; Alonso et al., 2010). In this approach, a CaO-based sorbent is circulated between two different reactors, i.e., carbonation and calcination, to achieve cyclic capture of CO 2 , as shown schematically in Fig. 1. The attractiveness of the Ca looping process arises from the fact that it uses industrially available circulating fluidized bed technology, and appears to be economically competitive with the commercially available amine-based absorption scrubbing (MacKenzie et al., 2007; Romeo et al., 2009). The circulation of sorbent between the carbonator and the calciner reactors requires, among other things, that the sorbent has good resistance to attrition in order to avoid excessive elutriation (Lu et al., 2008; Blamey et al., 2011). Also, as sorbent particles with diameters as large as 0.25 mm can still elutri- ate from a fluidized bed (Kunii and Levenspiel, 1991), there is some argument for studying particles of up to 1 mm or larger, which are outside of the typical circulating fluidized bed range. CaO-based sorbents from natural limestone show a fast decline in their CO 2 capture capacity over the few first ∗ Corresponding author. Tel.: +1 613 996 2868; fax: +1 613 992 9335. E-mail address: banthony@nrcan.gc.ca (E.J. Anthony). carbonation–calcination cycles (Salvador et al., 2003; Grasa and Abanades, 2006; Manovic and Anthony, 2008a; Chen et al., 2011). In order to improve the performance of CaO-based sorbents, a range of modifications have been explored. One of these modifica- tions explored here, was doping of CaO-based sorbents with Al 2 O 3 , which can lead to a significant improvement in sorbent stability and durability, and thus increased CO 2 uptake over an extended number of reaction cycles. More details about this approach can be found elsewhere (Wu et al., 2008; Martavaltzi and Lemonidou, 2008; Gruene et al., 2011). Another method to improve limestone performance is by thermal pretreatment of limestone at high tem- peratures, which stabilizes the structure of the sorbent prior to CO 2 capture and causes some limestone derived sorbents to display a “self-reactivation” behavior with enhanced conversion as a result (Manovic and Anthony, 2008b). Kaolin is an attractive source for Al 2 O 3 ; it is a natural and widely available material, which makes it a potentially cost-effective choice as a binder. Acid leaching of metakaolin (the product from kaolin calcination) also produces an Al(OH) 3 -rich phase (Tang et al., 2010). Wang et al. (2010) showed that CaO doped with Al(OH) 3 extracted from kaolin exhibited a significant improvement in sorbent CO 2 cyclic capture capacity. Under severe calcination conditions (950 ◦ C, pure CO 2 ), and carbonation at 700 ◦ C in 15% CO 2 (N 2 balance), the doped CaO maintained a conversion of 40%, compared to 10% for pure CaO after 25 cycles. However, no pellets 1750-5836/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijggc.2011.11.006