Improved Sorbent for High-Temperature Production of Oxygen

Jul 18, 2007 - A CO2-Stable K2NiF4-Type Oxide (Nd0.9La0.1)2(Ni0.74Cu0.21Al0.05)O4+δ for Oxygen Separation. Yan Chen , Qing Liao , Yanying Wei ... Cha...
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Ind. Eng. Chem. Res. 2007, 46, 6025-6031

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SEPARATIONS Improved Sorbent for High-Temperature Production of Oxygen-Enriched Carbon Dioxide Stream Qing Yang and Y. S. Lin* Department of Chemical Engineering, Arizona State UniVersity, Tempe, Arizona 85287-6006

Perovskite-type La-Sr-Co-Fe metal oxides have been demonstrated as sorbents in a high-temperature sorption separation process for production of an oxygen-enriched gas stream for oxycombustion application. This paper reports on characteristics and fixed-bed performance of improved perovskite-type sorbents, Sr-CaCo-Fe oxides, in comparison with a reference La-Sr-Co-Fe oxide sorbent for high-temperature production of an oxygen-enriched carbon dioxide stream. The optimum composition of Sr0.5Ca0.5Co0.5Fe0.5O2.47 (SCCF) is identified through study of carbonation (oxygen desorption) kinetics of Sr-Ca-Co-Fe oxides with different Ca and Fe concentrations. Carbonation kinetics of SCCF were studied at different temperatures from 700 to 900 °C. Below 750 °C, interaction of SCCF with carbon dioxide results in formation of Sr and Ca carbonates. At higher temperature, SCCF turns to form oxides of the respective metals under a stream of carbon dioxide. The improved sorbent has a faster oxygen desorption kinetic rate and higher oxygen storage capacity than the reference material. Effects of the operation temperature on the fixed-bed desorption/adsorption process for production of an oxygen-enriched carbon dioxide stream were investigated. Optimal temperatures for adsorption and desorption processes are determined to be 700 and 850 °C, respectively. The fixed-bed process can produce an oxygen-enriched stream at an oxygen concentration of 50%. The improved sorbent exhibits a gradually decreased carbonation (oxygen desorption) kinetics in the first few cycles of oxygen sorption and desorption and then stable kinetics in cycles afterward. Introduction Perovskite-type metal oxides have a general formula of ABO3 with a cubic lattice structure. One new application of the perovskite metal oxides is as sorbents in high-temperature sorption processes for air separation to produce pure oxygen/ nitrogen or oxygen-enriched gas streams.1 The processes are based on the fact that some doped perovskite-type metal oxides can take large amounts of oxygen through a gas-solid defect reaction2 due to their high oxygen vacancy concentration. When exposed to oxygen-containing gas (such as air), the perovskitetype metal oxide can adsorb oxygen by the following defect reaction:

1 O (g) + Vo·· S Oo× + 2h· 2 2

(A)

where Vo··, Oo×, and h· denote positive oxygen vacancy, neutralized lattice oxygen, and mobile electronic-hole, respectively. The perovskite-type metal oxide sorbents exhibit extremely high selectivity for oxygen.2 Lin and co-workers have systematically studied oxygen sorption equilibrium,3 heat of sorption and heat management,4 and kinetics and fixed-bed performance5 for the lanthanum cobaltite sorbents for air separation by either temperature or pressure swing adsorption processes.1,6 Most power stations use air as the oxidant for fuel combustion. In addition to N2 and H2O, the flue gas typically comprises * To whom correspondence should be addressed. E-mail: Jerry.Lin@ ASU.edu. Phone: (480) 965-7769.

a large amount of greenhouse gas CO2 and acid gas NOx, both of which cause serious environmental pollution problems. Flue gas treatments to separate CO2 and NOx from the rest are usually complicated and expensive. The oxycombusition process, which uses oxidant stream with higher oxygen concentration (>30%), such as an oxygen-enriched carbon dioxide stream, can improve combustion efficiency and simplify the flue gas treatment process.7 Production of the oxygen-enriched stream for oxycombustion application by the energy-intensive cryogenic air separation method is not cost-effective. The high-temperature sorption process with perovskite-type metal oxide sorbent can be extended to produce an oxygenenriched carbon dioxide stream for oxycombustion application with carbon dioxide as purge gas, as described in ref 7. Carbon dioxide from flue gas, as the sweep gas, is fed to the fixed-bed presaturated with oxygen to produce an oxygen-enriched carbon dioxide stream, which is used as the oxidant for fuel combustion in the boiler. This combined high-temperature separation and oxycombustion process will provide substantial saving in energy and costs for power generation and flue gas treatment. It should be noted that perovskite-type oxygen transport membranes, which are oxygen semipermeable, can also be used to produce the oxygen-enriched carbon dixode stream for oxycombustion.8,9 The process faces major challenges with respect to membrane stability and scale-up and design of large plants (including the sealing problem).2,7 The sorption process utilizes conventional fixed-bed configuration and perovskite metal oxides in the conventional particulate form (as sorbent), which are easy to fabricate and readily available. Since there is significant industrial experience in operating large cyclic systems

10.1021/ie0703235 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/18/2007

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Figure 1. Carbonation uptake (oxygen desorption kinetic) curves of SrCa-Fe-Fe oxides with different Ca concentrations (PCO2 ) 1 atm, T ) 750 °C).

Figure 3. XRD patterns of fresh Sr0.5Ca0.5Co0.5Fe0.5O2.47 (SCCF) and La0.1Sr0.9Co0.5Fe0.5O2.6 (LSCF).

(carbonation) kinetics of LSCF. It is still difficult to achieve a sufficiently high kinetic rate to reach satisfactory separation efficiency. The problem can only be solved by identifying improved sorbents with faster oxygen desorption (carbonation) kinetics. Nomura et al.13-15 reported faster carbonation reaction kinetics for perovskite-type metal oxides containing alkaline earth elements like Ba, Sr, or Ca in the A site.13-15 The present study was focused on modifying LSCF sorbents by replacing La with Ca. The improved sorbents have general formula of Sr1-xCaxCo1-yFeyO3-δ. This paper reports oxygen desorption (carbonation) kinetics, fixed-bed performances, and reversibility of the improved sorbents in comparison with the reference sorbent reported in the previous study, La0.1Sr0.9Co0.5Fe0.5O2.6 (LSCF). Figure 2. Carbonation uptake (oxygen desorption kinetic) curves of SrCa-Fe-Fe oxides with different Fe concentrations (PCO2 ) 1 atm, T ) 750 °C).

such as a pressure swing adsorption unit, the sorption process offers an attractive alternative for producing an oxygen-enriched stream from an industrial viewpoint. Lin and co-workers10,11 recently demonstrated the production of an oxygen-enriched carbon dioxide stream by a fixed-bed packed with La0.1Sr0.9Co0.5Fe0.5O2.6 (LSCF) sorbent at 600 °C.5 The following reaction describes the oxygen desorption and sorption processes for LSCF:

La0.1Sr0.9Co0.5Fe0.5O2.6 + 0.9CO2 S 0.9SrCO3 + 0.05La2O3 +0.5CoO + 0.25Fe2O3 + 0.15O2 (B) The forward reaction is the carbonation reaction step corresponding to oxygen desorption with sorbent gaining weight. The reversed reaction is the oxygen sorption step with the sorbent losing weight. Clearly, with carbon dioxide as the sweep gas, the oxygen sorption and desorption processes involve a reversible chemical reaction. The main problem with LSCF as the sorbent is its slow oxygen desorption (or carbonation) rate. This has limited oxygen concentration and productivity of the separation process. Several methods, such as optimization of the reaction temperature, modification of the pore structure, and particle size of sorbents,11,12 have been studied to improve the oxygen desorption

Experimental Section The liquid citrate method followed by high-temperature sinteringwasusedtopreparethenewsorbents,Sr1-xCaxCo1-yFeyO3-δ, and the reference sorbent La0.1Sr0.9Co0.5Fe0.5O2.6 (LSCF). In synthesis, metal nitrate precursors were dissolved in deionized water at a certain molar ratio, according to the stoichiometry of the final product. The system was under heating and stirring during the polymerization and condensation processes. Water was gradually evaporated during the condensation process to facilitate gelation. Self-ignition occurred at 450 °C to burn organics from products after they had been dried at 110 °C for 24 h. Finally, as-prepared powders were sintered at 900 °C for 20 h with a ramping rate of 60 °C/hr. As-prepared samples were ground into fine powders for characterization and TGA measurements. LSCF power has a brown color, and SCCF materials all exhibit a black color. The average aggregate diameter of these materials was ∼10 µm. XRD analysis (Siemens D-50, Cu KR1 radiation) was performed to examine the crystalline structure of as-prepared perovskite-type ceramics. Kinetics of the carbonation reaction (oxygen desorption) and its reverse reaction (oxygen sorption) were studied by a TGA (TA Instrument, SDT 2960). About a 30-mg sample was placed into the sample pan of the TGA setup. The powder sample was uniformly distributed and formed a thin layer at the bottom of the sample holder. During the measurement, the sample was heated to a desired temperature in a flow of air in 1 atm at flow rate of 100 mL/min. Once the equilibrium was achieved, the air feed was switched to CO2 flow at the same flow rate, and

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Figure 4. XRD pattern of Sr0.5Ca0.5Co0.5Fe0.5O2.47 (SCCF) after complete carbonation at 700 °C.

the sample weight gains (carbonation uptake or oxygen desorption curves) were recorded until the reaction reached equilibrium. The reversed reaction was followed by a switch of the flow gas back to air, and the sample weight was recorded to obtain oxygen sorption kinetic curves. The experimental setup of the fixed-bed system includes a gas delivery system, an adsorber column, an oxygen analyzer (Ceramatec, model 1100) with a data acquisition system, and a thermal mass flow meter (McMillan, 50D-2) with another data acquisition system. The perovskite-type metal oxide sorbent of 3.60 g was packed in the middle of a dense aluminum oxide ceramic tube (6-mm i.d., 9-mm o.d., 100 cm long). The sorbent packing length was ∼10 cm, and it was supported by quartz particles (∼0.5 mm in diameter, with packing length of ∼45 cm on each end). The oxygen concentration and flow rate of the effluent from the fixed-bed were respectively measured by an oxygen analyzer (Ceramatec, model 1100) and a thermal mass flow rate meter (McMillan, 50D-2) with digital data acquisition systems. Prior to oxygen sorption, the sorbent was regenerated at 800 °C with CO2 feed at a flow rate of 5 mL/min for a time period until the O2 concentration in the effluent from the fixed bed dropped to 30%. Conditions for this prior process were fixed the same for all the experiments in order to ensure the same starting point of the sorption step. In the oxygen sorption step, the feeding gas was switched from CO2 to air at 5 mL/min for a fixed time period. The oxygen desorption (or carbonation) step was then followed with a switch of the feeding gas from air back to CO2, during which the oxygen concentration and flow rate of the effluent were measured to give the oxygen desorption breakthrough curves and flow rate curves, respectively. The desorption process was stopped when the O2 concentration in the effluent dropped to 30%. Results and Discussion Sorption and Structure Properties of Improved Sorbents. Figure 1 shows carbonation uptake (oxygen desorption) curves of Sr1-xCaxCo0.5Fe0.5O2.47 with different Ca concentration. As seen in Figure 1, carbonation rate increases as the Ca concentration, x, increases from 0.2 to 0.5 and then decreases as the Ca

concentration increases further. A complete substitution by the Ca component results in extremely slow carbonation kinetics. Figure 2 shows the effect of Fe concentration on carbonation kinetics of the series of materials Sr0.5Ca0.5Co1-yFeyO2.47. Carbonation kinetics is fastest when Fe is doped at a concentration of y ) 0.5. Clearly, the optimum composition with the fastest carbonation kinetics is Sr0.5Ca0.5Co0.5Fe0.5O2.47. Figure 3 compares XRD pattern of fresh Sr0.5Ca0.5Co0.5Fe0.5O2.47 (SCCF) with that of the previously studied reference material, La0.1Sr0.9Co0.5Fe0.5O2.6 (LSCF). Their major deflection peaks indicate distorted an orthorhombic perovskite-type structure for SCCF in comparison with the cubic perovskite-type structure for LSCF. Figure 4 shows the XRD pattern of SCCF after a complete carbonation reaction in pure CO2 at 700 °C in 1 atm (and quenched down for XRD measurement). SrCO3, CaCO3, Fe2O3, and CoO are identified from the XRD patterns (2000 JCPDS-International Center for Diffraction Data. v.2.1). The XRD patterns also show some impurity phases, possibly of metal oxides of strontium and calcium. The TGA experimental results show that the complete carbonation reaction on SCCF at 700 °C gives an equilibrium weight increase of 24.9 wt %. Considering the phases identified in Figure 5, the carbonation reaction on SCCF follows the reaction

Sr0.5Ca0.5Co0.5Fe0.5O2.47 + CO2 S 0.5SrCO3 + 0.5CaCO3 +0.5CoO + 0.25Fe2O3 + 0.11O2 (C) In reaction C, except for CO2 and O2, all other compounds are in the solid phase under the reaction conditions. The stoichiometric weight increase for reaction C is 25.2%, very closed to the experimentally measured value (24.9%). Compared with reaction B for LSCF, the main difference is that the carbonation reaction for SCCF results in formation of CaCO3, in comparison with La2O3 for LSCF. Figure 5 shows the comparison of TGA weight uptakes for the carbonation reaction (at 750 °C in 1 atm CO2) and reversed oxygen sorption reaction (at 800 °C in 1 atm air) between SCCF and LSCF. The carbonation reaction results in much more weight gain for SCCF than LSCF. Furthermore, SCCF exhibits much faster carbonation reaction

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Figure 7. Oxygen desorption breakthrough curves from fixed bed packed with Sr0.5Ca0.5Co0.5Fe0.5O2.47 (SCCF) and La0.1Sr0.9Co0.5Fe0.5O2.6 (LSCF) exposed to air for different periods of time.

Figure 5. Comparison of carbonation uptake (oxygen desorption) curves (at 750 °C) (upper) and oxygen sorption curves (at 800 °C) (bottom) between (Sr0.5Ca0.5Co0.5Fe0.5O2.47 (SCCF) and La0.1Sr0.9Co0.5Fe0.5O2.6 (LSCF). Figure 8. Effluent flow rate curves from fixed bed packed with Sr0.5Ca0.5Co0.5Fe0.5O2.47 (SCCF) in oxygen desorption step (same conditions as in Figure 7).

Figure 6. Carbonation uptake (oxygen desorption kinetic at initial 60 min) curves for Sr0.5Ca0.5Co0.5Fe0.5O2.47 (SCCF) at different temperatures.

(oxygen desorption) kinetics than LSCF. Both sorbents have a comparable oxygen sorption rate. Figure 6 shows carbonation uptake curves of SCCF at temperatures from 700 to 900 °C. The carbonation rate decreases with increasing temperature. This temperature dependency is different from the well-known kinetics for a gas-solid reaction for which the rate increases with temperature and the equilibrium weight is temperature independent.17 The unusual temperature dependency indicates that the reaction changes as temperature increases. At higher temperatures, SrCO3 and CaCO3 are not stable and can be decomposed to metal oxides and carbon dioxide. In other words, at higher temperatures, Sr0.5Ca0.5Co0.5-

Fe0.5O2.47 is more likely to be decomposed to the corresponding metal oxides, which is slower and takes less carbon dioxide than reaction C.12 Therefore, the optimum temperature for oxygen desorption (or carbonation) should be in 700-750 °C. Fixed-bed Performance and Reversibility. Figure 7 shows oxygen concentration in the effluent from the fixed bed packed with SCCF or LSCF in the oxygen desorption (carbonation) step after the feed was switched from air to CO2. The sorbent in the fixed bed had been exposed to air flow for various periods of time (0.5, 1, and 2 h). As shown in Figure 8, for SCCF the oxygen concentration in the effluent, starting at a value of ∼21%, increases to as high as 85% and decreases with time. For a sufficiently long time, the oxygen concentration in the effluent would reach zero. Figure 8 shows effluent flow rate for the SCCF fixed bed after the air to CO2 switch of the feed. The flow rate, after an initial quick decrease to a low value (e0.1 mL/min) (not visible in the figure), increases gradually to a value still lower than that of the feed in the time period shown in the figure. These results are typical for desorption of gas from a solid involving a chemical reaction.18 The oxygen desorption breakthrough curves for LSCF are also compared in Figure 8 with those for SCCF. SCCF gives oxygen desorption breakthrough curves with higher oxygen concentration and larger desorption amount than LSCF. More quantitative comparison of the fixed-bed performance for these two sorbents can be made with definitions of average oxygen

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Figure 9. Oxygen desorption breakthrough curves from fixed bed packed with Sr0.5Ca0.5Co0.5Fe0.5O2.47 (SCCF) at different desorption temperatures (air adsorption for 0.5 h at 850 °C. Flow rate 5 mL/min).

Figure 11. Comparison of the carbonation uptake (oxygen desorption) curves for various cycles shown in Figure 10.

Figure 10. Carbonation uptake (oxygen desorption) (weight increase) and oxygen sorption (weight decrease) curves for Sr0.5Ca0.5Co0.5Fe0.5O2.47 (SCCF) under cyclic flow of CO2 (750 °C) and air (850 °C). Table 1. Comparison of Oxygen Concentration and Productivity for Production of Oxygen-Enriched Carbon Dioxide Stream by Fixed Bed Packed with Sr0.5Ca0.5Co0.5Fe0.5O2.47 (SCCF) and La0.1Sr0.9Co0.5Fe0.5O2.6 (LSCF)

sorbent

adsorption time (h)

time range to collect the product in desorption (t1-t2) (s)

average O2 concn (%)

productivity (mL/min)

LSCF LSCF SCCF SCCF SCCF

0.5 1.5 0.5 1 2

282-418 420-603 720-1300 790-1790 900-2900

30.7 39.6 50.1 55.0 61.2

0.210 0.113 0.282 0.239 0.234

concentration of desorption effluent, C h , and productivity of the separation process P:

∫t t Fd,outCO dt C h) ∫t t Fd,out dt 2

2

1

2

(1)

1

and

∫t t

2

P)

1

Fd,out dt tcycle

(2)

where Fd,out is the flow rate of desorption effluent, CO2 the oxygen concentration of desorption effluent, t1 the starting time for collecting the desorption product when the oxygen concentration in the effluent reaches 21%, t2 the ending time for the desorption process when the oxygen concentration of effluents

Figure 12. Comparison of oxygen sorption curves for various cycles shown in Figure 10.

drops to 30%, and tcycle the total time of adsorption and desorption steps. The average oxygen concentration of the product and productivity of each sorption process shown in Figure 7 with SCCF and LSCF as sorbents are given in Table 1. Compared with LSCF, the new material SCCF provides higher quality of the product with a higher oxygen concentration and also higher productivity of the separation process. For both sorbents, the longer oxygen adsorption time leads to a higher average oxygen concentration of the product, but a lower productivity of the separation process because prolonging the adsorption time increases the cycle time. An optimal oxygen sorption time therefore exists with a compromise between the oxygen concentration of product and the productivity of process. For SCCF, the oxygen sorption process at the adsorption time of 0.5 h provides the highest productivity, and it produces an O2enriched CO2 stream with an average oxygen concentration of 50%, which can well satisfy the requirement for the fuel oxidant (30%) for the oxycombustion process. Figure 9 compares the oxygen desorption breakthrough curves at different desorption temperatures from 700 to 900 °C (at fixed oxygen sorption temperature of 850 °C). Consistent with the TGA kinetic curves in Figure 5, oxygen desorption at 700 °C provides the product with the highest oxygen concentration. The reversibility of reaction C was also studied by TGA. SCCF sample in the TGA system was exposed sequentially to the flow of CO2 and air at 1 atm. During the carbonation step

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Figure 13. Oxygen desorption breakthrough curves from fixed-bed desorption packed with Sr0.5Ca0.5Co0.5Fe0.5O2.47 (SCCF) for 14 cycles of sorption and desorption (sorption at 850 °C, desorption at 700 °C).

(oxygen desorption) the sample weight increases and during oxygen sorption the sample weight decreases. Figure 10 shows the weight change of the samples for 11 cycles of oxygen sorption and desorption (carbonation). The sample returns to

the original weight after each cycle. Kinetics of the carbonation reaction and oxygen deposition for each cycle is compared in Figures 11 and 12, respectively. In Figure 11, carbonation (oxygen desorption) kinetics of the first four cycles are very similar. After an obvious decrease in the fifth cycles, kinetics of cycles from the fifth to the 11th turn similar again. It is not clear why there was an obvious decrease in carbonation kinetics between the first four cycles and cycles afterward. Figure 12 shows that the oxygen sorption kinetics (reverse reaction) for each cycle is essentially the same. Reversibility of the oxygen sorption and desorption processes was also investigated by continuous cycles of fixed-bed sorption processes. For each cycle, the fixed bed was exposed to air flow at 850 °C for 0.5 h, and then oxygen desorption breakthrough curves were measured with carbon dioxide feed at 700 °C until the oxygen concentration in the effluents dropped to 30%. Then the feed was switched to air to start the next cycle of oxygen sorption and desorption. Figure 13 compares the oxygen desorption breakthrough curves of continuous 14 cycles. The first cycle exhibits the largest oxygen desorption area (which is proportional to the amount of oxygen desorbed). The amount

Figure 14. SEM images of (a) fresh Sr0.5Ca0.5Co0.5Fe0.5O2.47 (SCCF) sample and (b) SCCF sample after 6 cycles of oxygen sorption and adsorption.

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of oxygen desorbed decreases in the second and third cycles and then maintains essentially a constant value during cycles 3-14 with some random increase or decrease within the range of experimental error. These fixed-bed results are consistent with TGA cycles, as shown in Figure 11. Figure 14 compares the SEM images of a fresh SCCF sample (sintered at 950 °C) and a SCCF sample after six cycles of oxygen desorption and sorption. As seen in the SEM images, the sample after sorption and desorption exhibit a denser microstructure with smaller pores (∼0.2 µm) than the fresh sample (∼1.2-2.4 µm). The decrease in the carbonation kinetics after four cycles of oxygen sorption and desorption could be caused by the pore and surface structural changes of the SCCF sample. The pore structure appears to be stabilized after the first few cycles of sorption and desorption as indicated by constant carbonation kinetics for the additional cycles afterward, as seen in Figure 11 and Figure 13. Conclusions A new perovskite-type material, Sr0.5Ca0.5Co0.5Fe0.5O2.47 (SCCF), was identified as an improved sorbent for hightemperature separation to produce an oxygen-enriched carbon dioxide stream. The improved sorbent exhibits much faster carbonation (oxygen desorption) kinetics and much improved fixed-bed performance than the original perovskite-type sorbent La0.1Sr0.9Co0.5Fe0.5O2.6. Study of carbonation (oxygen desorption) kinetics on the improved sorbent shows the optimum oxygen desorption temperature of 700 °C, above which the carbonation rate decreases with increasing temperature due to formation of metal oxides. The optimum temperature for oxygen sorption should be at 850 °C. The fixed bed packed with SCCF can produce an oxygen-enriched carbon dioxide stream with oxygen concentration above 50%. The SCCF sorbent exhibits a gradual decrease in carbonation kinetic rate and separation efficiency in the first few cycles of oxygen sorption and desorption and stable operation afterward. Acknowledgment The work was supported by National Science Foundation (CTS-0132694) and Department of Energy (DE-FG2600NT4081). Literature Cited (1) Lin, Y. S.; Maclean, D. L.; Zeng, Y. High Temperature Adsorption Process. US Patent 6,059,858, 2000.

(2) Yang, Z.; Lin, Y. S.; Zeng, Y. High-Temperature Sorption Process for Air Separation and Oxygen Removal. Ind. Eng. Chem. Res. 2002, 41, 2775-2784. (3) Yang, Z.; Lin, Y. S. Equilibrium of Oxygen Sorption on Perovskite Type Ceramic Sorbents. AIChE J. 2003, 49, 793-798. (4) Yang, Z.; Lin, Y. S. Synergetic Thermal Effects for Oxygen Sorption and Order-Disorder Transition on Perovskite-Type Oxides. Solid State Ionics 2005, 176, 89-96. (5) Yang, Z. H.; Lin, Y. S. High Temperature Oxygen Sorption in FixedBed Packed With Perovskite-Type Ceramic Sorbents. Ind. Eng. Chem. Res. 2003, 42, 4376-4381. (6) Stevens, W. C.; Cummings, D.; Chen, P. High Temperature Pressure Swing Adsorption System for Separation of Oxygen-Containing Gas Mixtures. US Patent 6,361,584, 2002. (7) Acharya, D.; Krishnamurthy, K. R.; Leison, M.; Macadam, S.; Sethi, V. K.; Anheden, M.; Jordal, K.; Yan, J. Development of a High Temperature Oxygen Generation Process and Its Application to Oxycombstion Power Plants with Carbon Dioxide Capture, Proceedings, Pittsburgh Coal Conference, Pittsburgh, PA, September 12-15, 2005. (8) Dyer, P. N.; Richards, R. E.; Russek, S. L.; Taylor, D. M. Ion transport membrane technology for oxygen separation and syngas production. Solid State Ionics 2000, 134, 21-33. (9) Lin, Y. S. Microporous and Dense Inorganic Membranes, Current Status and Prospective. Sep. Purif. Technol. 2001, 25, 39-55. (10) Yang, Q.; Lin, Y. S.; Bulow, M. High Temperature Sorption Separation of Air for Producing Oxygen-Enriched Carbon Dioxide Stream. AIChE J. 2006, 52, 574-581. (11) Yang, Q.; Lin, Y. S. Fixed-bed Performance for Production of Oxygen Enriched Carbon Dioxide Stream by Perovskite-Type Ceramic Sorbent. Sep. Purif. Technol. 2006, 49, 27-35. (12) Yang, Q.; Lin, Y. S. Kinetics of Carbon Dioxide Sorption on Perovskite Type Metal Oxides. Ind. Eng. Chem. Res. 2006, 45, 63026310. (13) Nomura, K.; Ujihira, Y.; Hayakawa, T.; Takehira, K. CO2 Absorption Properties and Characterization of Perovskite Oxides, (Ba,Ca)(Co,Fe)O3-δ. Appl. Catal., A 1996, 137, 25-36. (14) Normura, K.; Homonnay, Z.; Juhasz, G.; Vertes, A.; Donen, H.; Sawada, Ts. Mossbauer Study of (Sr, Ca)(Fe, Co)O3-δ Applied to CO2 Absorption At High Temperatures. Hyperfine Interact. 2002, 139/140, 297305. (15) Nomura, K.; Kobayashi, S.; Jashimoto, K.; Sawada, Ts.; Homonnay, Z.; Vertes, A. Microstructure Analysis of (Ba,Ca)(Fe,Mg)O3-δ For Rapid CO2 Absorption By Mossbauer Spectroscopy. J. Radioanal. Nucl. Chem. 2003, 255/3, 513-518. (16) Lin, Y. S.; Deng, S. G. Removal of Trace Sulfur Dioxide from Gas Stream By Regenerative Sorption Processes. Sep. Purif. Technol. 1998, 13, 65-77. (17) Buelna, G.; Lin, Y. S. Characteristics and DesulfurizationRegeneration Properties of Sol-gel-derived Copper Oxide on Alumina Sorbents. Sep. Purif. Technol. 2004, 39, 167-179.

ReceiVed for reView March 2, 2007 ReVised manuscript receiVed May 23, 2007 Accepted May 31, 2007 IE0703235