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Ind. Eng. Chem. Res. 2008, 47, 434-442
Compositional Effects of Nanocrystalline Lithium Zirconate on Its CO2 Capture Properties Esther Ochoa-Ferna´ ndez,† Magnus Rønning,† Xiaofeng Yu,‡ Tor Grande,§ and De Chen*,† Departments of Chemical Engineering, Physics, and Materials Science and Engineering, Norwegian UniVersity of Science and Technology, Sem Sælands Vei 4, NO-7491 Trondheim, Norway
A soft-chemistry route has been used for preparation of pure and promoted nanocrystalline lithium zirconate with different stoichiometries. The objective of this investigation has been to study the effect of different compositions on the acceptor and optimize the working properties of lithium zirconate. Special attention has been given to study the effect of different Li2O-ZrO2 stoichiometries on the CO2 capture rates. In addition, the partial substitution of Li2O with K2O as a promoter has been addressed. It has been found that both the capture rate and capacity of lithium zirconate depend considerably on the Li2O to ZrO2 ratio. Enhanced capture rates are observed when a deficiency of Li2O is introduced. It is believed that the excess ZrO2 might act as a dispersant and introduce more reactive boundaries. Moreover, the addition of K2O results also in improved capture rates due to the presence of molten carbonates, but lower capacities and poorer stability due to particle coarsening. The present of free ZrO2 seems also beneficial for the stability of K2O-doped acceptors. Therefore, controlling the K:Li:Zr ratio has been found to be crucial for tailoring the properties of lithium zirconate. An optimized composition can result in an acceptor with enhanced capture rates, stability, and higher degree of utilization. 1. Introduction The use of regenerable solid acceptors for removal of CO2 at high temperatures has attracted increasing interest in past years, with application in both pre- and postcombustion systems.1 For instance, CO2 sorption enhanced steam methane reforming (SESMR) is an application of CO2 acceptors for hydrogen production.2-4 This process is an alternative to conventional steam reforming where a CO2 acceptor is installed together with the catalyst in the reactor. In this way, the reaction equilibrium is shifted toward H2 production and higher H2 yields are obtained at lower temperatures (723-873 K) with a simpler process layout. The main challenge is, however, to find an acceptor with fast kinetics, easy regeneration, and high stability under the working conditions. Different candidates have been reported in past years. Reijers et al. have recently shown the workability of SESMR using hydrotalcite-like compounds as acceptors.3 The study shows that modified hydrotalcite compounds can adsorb CO2 efficiently at temperatures between 673 and 773 K. However, the main limitation is its poor capacity and stability. On the other hand, the use of calcium-based acceptors has also been proposed, resulting in high H2 yields due to the very favorable thermodynamics for CO2 removal.2 However, the main challenge again is to improve its stability. The regeneration of Ca-based acceptors is highly energy demanding. The use of high temperatures for the regeneration leads to sintering and subsequent pore blockage and losses in the porosity.5 In addition, CaO can react with steam under the reaction conditions leading to the formation of Ca(OH)2 and, thus, lowering the methane conversion and hydrogen yield.6 Nowadays, several efforts are underway in order to overcome the losses in capacity of calciumbased acceptors.7,8 * To whom correspondence should be addressed. Tel.: +47 73 59 31 49. Fax: +47 73 59 40 80. E-mail:
[email protected]. † Department of Chemical Engineering. ‡ Department of Physics. § Department of Materials Science and Engineering.
Also, it has been reported that Li-containing materials are promising candidates with high CO2 capture capacity and high stability.9-20 Nakagawa et al. reported that lithium zirconate can theoretically hold CO2 in amounts up to 28 acceptor weight percent at high temperatures according to the following reaction: 16
Li2ZrO3 + CO2 T Li2CO3 + ZrO2 ∆H298 K ) -160 kJ/mol (1) The high capture capacity and stability at 723-873 K of lithium zirconate make it promising for application in both pre- and postcombustion systems. However, kinetic limitations during the CO2 capture are the main obstacle. Pure lithium zirconate powders have been conventionally fabricated by solid state reaction processes.16,21 In these processes, two types of powders are mechanically mixed and treated at high temperatures for several hours. The final particle size of the powder prepared by solid-state methods is normally large, partially due to sintering during the high-temperature treatment. Despite the good stability of the resulting powders during successive CO2 capture cycles, the capture kinetics are very slow, making difficult its application in processes such as SESMR.10 There have been several efforts to reduce the starting size of Li2ZrO3.19,20,22 A novel soft-chemistry preparation route has been recently reported by Ochoa-Ferna´ndez et al.19,22 This method yields pure nanocrystals of Li2ZrO3 with tetragonal phase using relatively low calcination temperatures. As a result, the properties of the powders such as capture rate of CO2 and regeneration have been significantly improved compared to Li2ZrO3 prepared by solid-state reactions. The hydrogen production by SESMR using as acceptor pure Li2ZrO3 prepared by this soft-chemistry route has been simulated by a dynamic one-dimensional pseudohomogeneous model of a fixed-bed reactor.4 Simulation results show that the process is able to directly produce concentrations of H2 larger than 90 mol %. However, the working capacity of the process is very low and long residence times are needed. The CO2 capture rate
10.1021/ie0705150 CCC: $40.75 © 2008 American Chemical Society Published on Web 12/13/2007
Ind. Eng. Chem. Res., Vol. 47, No. 2, 2008 435 Table 1. Composition and Main Physical Properties of the Prepared Samples Calcined at 873 K sample
(Li + K)/Zr
K/(Li + K)
capacitya (wt %)
crystal sizeb (nm)
BET surface areac (m2/g)
pore volumec (cm3/g)
Li2ZrO3 Li1.8ZrO2.9 Li2.2ZrO3.1 K0.2Li1.6ZrO2.9 K0.2Li2ZrO3.1 K0.6Li2.2ZrO3.4 K0.8Li2.2ZrO3.7
2.0 1.8 2.2 1.8 2.2 2.8 3.0
0 0 0 11.1 9.1 21.4 26.7
28.7 26.4 28.2 25.3 27.1 23.9 22.7
14 14 13 15 17 17 17
2 1 1
