MgAl2O4 Spinel-Stabilized Calcium Oxide Absorbents with Improved

May 18, 2010 - Liyu Li,* David L. King, Zimin Nie, Xiaohong Shari Li, and Chris Howard. Institute for Interfacial Catalysis, Pacific Northwest Nationa...
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Energy Fuels 2010, 24, 3698–3703 Published on Web 05/18/2010

: DOI:10.1021/ef100245q

MgAl2O4 Spinel-Stabilized Calcium Oxide Absorbents with Improved Durability for High-Temperature CO2 Capture Liyu Li,* David L. King, Zimin Nie, Xiaohong Shari Li, and Chris Howard Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, Post Office Box 999, Richland, Washington 99354 Received March 3, 2010. Revised Manuscript Received May 5, 2010

With efficient energy recovery, calcium-oxide-based absorbents that operate at elevated temperatures have an advantage over absorbents that operate at lower temperatures for CO2 capture from coal power plants. The major limitation of these absorbents is that the carbonation and decarbonation reactions of CaO and CaCO3 are far from complete or reversible. Rapid loss of CO2 capacity over many carbonation/ decarbonation cycles is always observed because of severe absorbent sintering. We have found that this sintering effect can be effectively mitigated by properly mixing calcium oxide precursors with small rod-like MgAl2O4 spinel nanoparticles. A new class of CaO-based absorbents with much improved hightemperature durability was developed by wet physical mixing of calcium acetate with nano MgAl2O4 spinel particles followed by high-temperature calcination. CaO-MgAl2O4 (32 wt % spinel content) material provides 34 wt % CO2 capacity after 65 carbonation-decarbonation cycles (650 and 850 °C, respectively), corresponding to 63% CaO use. Under the same test conditions, the CO2 capacity of natural dolomite (35 wt % MgO and 65 wt % CaO) decreases rapidly from 25 wt % for the 1st cycle to less than 5 wt % for the 50th cycle.

Table 1 compares the heat of reaction, theoretical capacity, CO2-capture temperature, and CO2-release temperature (on the basis of a temperature swing process) of some promising regenerable CO2 absorbents. Given the large heat of reaction for all of these systems, effective energy recovery is required for an efficient process. With efficient energy recovery, CaO-based absorbents have great advantages over other absorbents that operate at low temperatures. Recently, Romano reported that, for air-fired coal power plants, if CO2 is captured by natural CaO minerals from postcombustion flue gas and CaCO3 is regenerated via oxyfuel combustion of coal, a net lower heating value (LHV) efficiency of 37.4% could be achieved with 97% CO2 capture.18 As a comparison, for the full oxy-combustion and

1. Introduction To mitigate global climate change, large-scale CO2 capture and sequestration (CCS) has been proposed and widely studied.1,2 Calcium-oxide-containing materials are good candidate absorbents for large-scale CO2 capture because of their high reactivity toward CO2 at elevated temperature (600700 °C), high CO2 capacity, and low material cost.3 The high carbonation temperature makes it possible to efficiently recover as high-quality heat the large amount of energy released during the conversion of calcium oxide to calcium carbonate.4 *To whom correspondence should be addressed. E-mail: liyu.li@ pnl.gov. (1) Plasynski, S. I.; Litynski, J. T.; McIlvried, H. G.; Srivastava, R. D. Progress and new developments in carbon capture and storage. Crit. Rev. Plant Sci. 2009, 28 (3), 123–138. (2) Intergovernmental Panel on Climate Change (IPCC). IPCC Special Report: Carbon Dioxide Capture and Storage; Metz, B., Davidson, O., Coninck, H., Loos, M., Meyer, L., Eds.; Cambridge University Press: New York, 2005. (3) Abanades, J. C.; Rubin, E. S.; Anthony, E. J. Sorbent cost and performance in CO2 capture systems. Ind. Eng. Chem. Res. 2004, 43, 3462–3466. (4) Shimizu, T.; Hirama, T.; Hosoda, H.; Kitano, K.; Inagaki, M.; Tejima, K. A twin fluid-bed reactor for removal of CO2 from combustion processes. Chem. Eng. Res. Des. 1999, 77, 62–68. (5) Alie, C. F. CO2 capture with MEA: Integrating the absorption process and steam cycle of an existing coal-fired power plant. M.Sc. Thesis. University of Waterloo, Waterloo, Ontario, Canada, 2004. (6) Rao, A. B.; Rubin, E. S. A technical, economic, and environmental assessment of amine-based CO2 capture technology for power plant greenhouse gas control. Environ. Sci. Technol. 2002, 36, 4467–4475. (7) Lee, S. C.; Chae, H. J.; Lee, S. J.; Choi, B. Y.; Yi, C. K.; Lee, J. B.; Ryu, C. K.; Kim, J. C. Development of regenerable MgO-based sorbent promoted with K2CO3 for CO2 capture at low temperatures. Environ. Sci. Technol. 2008, 42, 2736–2741. (8) Mayorga, S. G.; Weigel, S. J.; Gaffney, T. R.; Brzozowski, J. R. Carbon dioxide adsorbents containing magnesium oxide suitable for use at high temperatures. U.S. Patent 6,280,503 B1, 2001. (9) Lee, J. B.; Ryu, C. K.; Baek, J. I.; Lee, J. H.; Eom, T. H.; Kim, S. H. Sodium-based dry regenerable sorbent for carbon dioxide capture from power plant flue gas. Ind. Eng. Chem. Res. 2008, 47, 4465–4472. r 2010 American Chemical Society

(10) Knuutila, H.; Svendsen, H. F.; Anttila, M. CO2 capture from coal-fired power plants based on sodium carbonate slurry: A systems feasibility and sensitivity study. Int. J. Greenhouse Gas Control 2009, 3, 143–151. (11) Siriwardane, R. V.; Stevens, R. W., Jr. Novel regenerable magnesium hydroxide sorbents for CO2 capture at warm gas temperatures. Ind. Eng. Chem. Res. 2009, 48, 2135–2141. (12) Siriwardane, R. V.; Robinson, C.; Shen, M.; Simony, T. Novel regenerable sodium-based sorbents for CO2 capture at warm gas temperatures. Energy Fuels 2007, 21, 2088–2097. (13) Siriwardane, R. V. Regenerable sorbents for CO2 capture from moderate and high temperature gas streams. U.S. Patent 7,314,847 B1, 2008. (14) Ida, J. I.; Lin, Y. S. Mechanism of high-temperature CO2 sorption on lithium zirconate. Environ. Sci. Technol. 2003, 37, 1999–2004. (15) Kato, M.; Yoshikawa, S.; Nakagawa, K. Carbon dioxide absorption by lithium orthosilicate in a wide range of temperature and carbon dioxide concentrations. J. Mater. Sci. Lett. 2002, 21, 485–487. (16) Kato, M.; Nakagawa, K.; Essaki, K.; Maezawa, Y.; Takeda, S.; Kogo, R.; Hagiwara, Y. Novel CO2 absorbents using lithium-containing oxide. Int. J. Appl. Ceram. Technol. 2005, 2 (6), 467–475. (17) Nakagawa, K.; Ohashi, T. A Novel method of CO2 capture from high temperature gases. J. Electrochem. Soc. 1998, 145 (4), 1344–1346. (18) Romano, M. Coal-fired power plant with calcium oxide carbonation for post-combustion CO2 capture. Energy Procedia 2009, 1, 1099–1106.

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Table 1. Comparison of Some Candidate Absorbents for Regenerable Flue Gas CO2 Capture absorbent

reaction for regenerable CO2 capture

ΔH o (kJ/mol CO2)

temperature of carbonation (°C)

temperature of decarbonation (°C)

theoretical CO2 capacity (mmol of CO2/g of absorbent)

MEA2,5,6 K2CO37,8 Na2CO38-10 MgO11 NaOH12,13 Li2ZrO314-16 Li4SiO416,17 CaO4

2MEA þ CO2 T MEACOO- þ MEAHþ K2CO3 þ CO2 þ H2O T 2KHCO3 Na2CO3 þ CO2 þ H2O T 2NaHCO3 MgO þ CO2 T MgCO3 2NaOH þ CO2 T Na2CO3 þ H2O Li2ZrO3 þ CO2 T Li2CO3 þ ZrO2 Li4SiO4 þ CO2 T Li2CO3 þ Li2SiO3 CaO þ CO2 T CaCO3

-167.0a -142.8 -135.5 -100.9 -127.5 -162.6 -142.0 -178.2

40-65 ∼50 60-70 200-300 315 600 600 600-700

100-150 350-400 100-200 500-600 700 850 850 800-900

8.2 7.2 9.4 25.0 12.5 6.5 8.3 17.9

a This number is from the literature on system analysis of large-scale CO2 capture from flue gas using the conventional monoethanolamine (MEA) process.

employed: (1) incorporation of inert materials, such as ZrO2,24 MgO,25 La2O3,25 Al2O3,26-28 TiO2,29 and SiO2,30,31 and (2) modification of the stability32 and structure33-36 of pure CaO. On the basis of these two approaches, several promising absorbents have been developed, including the Al2O3CaO system by Li et al.26 and the special pore-structured CaO absorbent [calcined precipitated calcium carbonate (PCC) absorbent] by Fan et al.33-35 Recently, we reported that the method of admixing inert materials with CaO has a critical effect on long-term stability.37 A wet physical mixing method was developed, and a class of stable MgO-doped CaO absorbents was produced that showed stability better than previously reported candidate CaO-based absorbents. In this paper, using this recently developed wet physical mixing method, a new class of stable CaO-based absorbents was prepared on the basis of admixing a magnesium aluminate spinel phase with CaO. We report on the superior performance of this spinel-doped material.

the amine-based plants, net LHV efficiency of 36.3 and 32.6% were predicted, respectively. Similar configurations were proposed in the earlier reports by Abanades et al., who estimated the cost of per ton of CO2 avoided at $15.5 if oxyfuel combustion were used just to regenerate CaCO3, which is much lower than the $23.8 per ton of CO2 avoided if full oxycombustion was used.19,20 The major limitation of using CaO-based CO2 absorbents is that the carbonation and decarbonation reactions of CaO and CaCO3 are far from complete or reversible.21 The rapid loss of CO2 capacity over many carbonation/decarbonation cycles is always observed because of severe absorbent sintering. Therefore, enhancing the durability of high-temperature CaObased CO2 absorbents has been an active research topic for many years. In comparison to natural CaO minerals, CaObased absorbents with improved durability can further improve the energy efficiency of the coal power plants with a CO2-capture system and can largely lower the cost of unit CO2 avoided.22 Besides optimizing carbonation and calcination conditions,23 two primary material approaches have been

2. Experimental Section 2.1. Materials and Preparation Methods. Reagent-grade calcium acetate, Ca(CH3COO)2 3 0.4H2O, magnesium oxalate, MgC2O4 3 2H2O, and dolomite natural mineral were obtained from Alfa Aesar. Boehmite aqueous solution (Dispal 18N4-20, ∼25 wt % Al2O3) was provided by CONDEA Chemie GmbH. 2-Propanol was obtained from Fisher Scientific. All chemicals were used as received. Small MgO particles were synthesized by thermal decomposition of MgC2O4 3 2H2O at 700 °C for 2 h in air. Spinel MgAl2O4 was prepared by mixing an aqueous solution of Mg(CH3COO)2 with a Dispal 18N4-20 slurry, followed by drying and calcination at 1000 °C for 4 h in air. MgO- and spinel-stabilized CaO were prepared by overnight ball-milling of a 2-propanol slurry containing Ca(CH3COO)2 and MgO or MgAl2O4 particles, followed by drying and calcining in air at 800 °C for 2 h.

(19) Abanades, J. C.; Anthony, E. J.; Wang, J. S.; Oakey, J. E. Fluidized bed combustion systems integrating CO2 capture with CaO. Environ. Sci. Technool. 2005, 39, 2861–2866. (20) Abanades, J. C.; Grasa, G.; Alonso, M.; Rodriguez, N.; Anthony, E. J.; Romeo, L. M. Cost structure of a post-combustion CO2 capture system using CaO. Environ. Sci. Technol. 2007, 41, 5523– 5527. (21) Grasa, G. S.; Abanades, J. C. CO2 capture capacity of CaO in long series of carbonation/calcination cycles. Ind. Eng. Chem. Res. 2006, 45, 8846–8851. (22) Romeo, L. M.; Lara, Y.; Lisbona, P.; Martinez, A. Economical assessment of competitive enhanced limestones for CO2 capture cycles in power plants. Fuel Process. Technol. 2009, 90, 803–811. (23) Hughes, R. W.; Lu, D.; Anthony, E. J. Improved long-term conversion of limestone-derived sorbents for in situ capture of CO2 in a fluidized bed combustor. Ind. Eng. Chem. Res. 2004, 43, 5529–5539. (24) Lu, H.; Khan, A.; Pratsinis, S. E.; Smirniotis, P. G. Flame-made durable doped-CaO nanosorbents for CO2 capture. Energy Fuels 2009, 23, 1093–1100. (25) Albrecht, K. O.; Wagenbach, K. S.; Satrio, J. A.; Shanks, B. H.; Wheelock, T. D. Development of a CaO-based CO2 sorbent with improved cyclic stability. Ind. Eng. Chem. Res. 2008, 47, 7841–7848. (26) Li, Z. S.; Cai, N. S.; Huang, Y. Y. Effect of preparation temperature on cyclic CO2 capture and multiple carbonation-calcination cycles for a new Ca-based CO2 sorbent. Ind. Eng. Chem. Res. 2006, 45, 1911–1917. (27) Martavaltzi, C. S.; Lemonidou, A. A. Development of new CaO based sorbent materials for CO2 removal at high temperature. Ind. Eng. Chem. Res. 2008, 47, 9537–9543. (28) Feng, B.; Liu, W. Q.; Li, X.; An, H. Overcoming the problem of loss-in-capacity of calcium oxide in CO2 capture. Energy Fuels 2006, 20, 2417–2420. (29) Aihara, M.; Nagai, T.; Matsushita, J.; Negishi, Y.; Ohya, H. Development of porous solid reactant for thermal-energy storage and temperature upgrade using carbonation/decarbonation reaction. Appl. Energy 2001, 69, 225–238. (30) Lu, H.; Reddy, E. P.; Smirniotis, P. G. Calcium oxide based sorbents for capture of carbon dioxide at high temperatures. Ind. Eng. Chem. Res. 2006, 45, 3944–3949.

(31) Wang, M. H.; Lee, C. G.; Ryu, C. K. CO2 sorption and desorption efficiency of Ca2SiO4. Int. J. Hydrogen Energy 2008, 33, 6368–6372. (32) Lu, H.; Smirniotis, P. G.; Ernst, F. O. Nanostructured Ca-based sorbents with high CO2 uptake efficiency. Chem. Eng. Sci. 2009, 64, 1936–1943. (33) Gupta, H.; Fan, L.-S. Carbonation-calcination cycle using high reactivity calcium oxide for carbon dioxide separation from flue gas. Ind. Eng. Chem. Res. 2002, 41, 4035–4042. (34) Iyer, M.; Gupta, H.; Sakadjian, B. B.; Fan, L.-S. Multicyclic study on the simultaneous carbonation and sulfation of high-reactivity CaO. Ind. Eng. Chem. Res. 2004, 43, 3939–3947. (35) Fan, L. S.; Gupta, H. Sorbent for separation of carbon dioxide (CO2) from gas mixtures. U.S. Patent 7,067,456 B2, 2006. (36) Yang, Z. H.; Zhao, M.; Florin, N. H.; Harris, A. T. Synthesis and characterization of CaO nanopods for high temperature CO2 capture. Ind. Eng. Chem. Res. 2009, 48, 10765–10770. (37) Li, L. Y.; King, D. L.; Nie, Z. M.; Howard, C. Magnesiastabilized calcium oxide absorbents with improved durability for high temperature CO2 capture. Ind. Eng. Chem. Res. 2009, 48, 10604–10613.

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2.2. Carbonation-Decarbonation Performance Measurement. A Netzsch 409C thermogravimetric analyzer (TGA) was used to screen the performance of the absorbents. A typical experiment employed ∼20 mg of powder, and the carbonation-decarbonation test was carried out at a fixed temperature, 758 °C. At this temperature, the CO2 reaction with the CaO-based absorbents is rapid and full decarbonation is thermodynamically possible in a gas stream free of CO2. During each test, 70 mL/min 100% CO2 (for carbonation) and 130 mL/min pure He (for decarbonation) were introduced into the system alternatively via an automated switch valve every 30 min. This protocol allowed multiple CO2 uptake and regeneration cycles to be carried out in an acceptable time period. This simple test procedure has been employed by others as an effective and discriminating method for CaO-based absorbent screening.33-35,37 In this study, more than 100 carbonation-decarbonation cycles were carried out for each sample. The CO2 absorption capacity was calculated using the total weight gain during each carbonation cycle divided by the total weight of absorbent in the oxide form. CaO use was calculated as the percentage of CaO converted to CaCO3 on the basis of CO2 capacity and the CaO concentration in the absorbent. It is recognized that actual decarbonation of CaCO3 in power plant operation would be carried out at temperatures well above 758 °C (as high as 900 °C or greater), because of the presence of CO2 in the effluent stream affecting thermodynamic equilibrium conversion. Therefore, the best performing absorbent (32 wt % spinel-68 wt % CaO, 177-250 μm particles) was also evaluated in a small fixed-bed reactor (Inconel 625 tube reactor, 6.6 mm inner diameter) under more realistic carbonation-decarbonation temperatures (650 and 850 °C, respectively). The adsorbent granules were prepared by pressing an absorbent tablet at 20 000 psi for 5 min followed by crushing and screening. For the carbonation cycle, a gas composition of 30% CO2, 10% N2, 50% Ar, and 10% H2O was provided to the absorbent at a gas hourly space velocity (GHSV) of 550 h-1 for 60 min. This gas composition was to simulate fuel gas after complete watergas-shift reaction of CO.38 N2 and Ar were used to simulate H2. For the subsequent decarbonation cycle, a gas stream comprising 95% Ar, 2.5% N2, and 2.5% H2O was fed at 2200 h-1 GHSV using a protocol involving heating the absorbent from 650 to 850 °C at 5 °C/min and holding at 850 °C for 34 min. The sample was then cooled to 650 °C at 5 °C/min and held at that temperature for 10 min prior to beginning the next carbonation cycle. Two automated switch valves were employed. Dry gas flows were metered using MKS mass flow controllers. Steam was generated using a small cartridge vaporizer, and steam flow was controlled by a high-performance liquid chromatography (HPLC) pump. Downstream of the absorption bed, water was removed with a condenser followed by a 50-tube Nafion membrane dryer (Perma Pure LLC, Toms River, NJ). During the test, the exit gas flow rate was continuously measured using a volumetric digital flow meter (DryCal Definer 220, Bios International, Butler, NY). An Agilent 3000A micro gas chromatography (micro GC) was also used to analyze the gas composition. 2.3. Characterization. Scanning electron microscopy (SEM) analysis was carried out with a JEOL JSM-5900LV microscope. Transmission electron microscopy (TEM) analysis was carried out with a JEOL JEM 2010F microscope. Powder X-ray diffraction (XRD) measurement and analysis were conducted with a Philips PW3050 diffractometer using Cu KR radiation and JADE, a commercial software package. The nitrogen BrunauerEmmett-Teller (BET) surface area was measured with a Quantachrome AUTOSORB 6-B gas sorption system with 200 °C degassed samples.

Figure 1. Long-term CO2-capture performances of MgO-doped CaO absorbents prepared by wet physical mixing. Carbonation at 758 °C in 100% CO2 for 30 min and decarbonation at 758 °C in 100% He for 30 min.

Figure 2. SEM (top, left) and TEM (top, right) images and XRD pattern (bottom) of the spinel MgAl2O4 powders.

3. Results and Discussion Previously, we reported on the favorable stability of MgOstabilized CaO prepared by wet ball-milling of MgO with calcium acetate as described above.37 However, extending absorption-desorption studies to as many as 100 cycles reveals the loss of performance of MgO-stabilized CaO. Figure 1 shows an extended test of the CO2-capture performance of three MgO-doped CaO absorbents measured in a TGA unit at 758 °C. At 26, 32, and 42 wt % MgO content, a CO2 capacity decrease was observed after 25, 40, and 80 cycles, respectively. Clearly, a better material was required, leading to investigation of MgAl2O4 spinel as a stabilizing material for CaO. Figure 2 provides SEM and TEM images and the XRD pattern of the spinel sample prepared by mixing an aqueous solution of Mg(CH3COO)2 with an aqueous slurry of boehmite

(38) Wei, L.; King, D. L.; Liu, J.; Johnson, B.; Wang, Y.; Yang, Z. G. Critical material and process issues for CO2 separation from coalpowered plants. JOM 2009, 61 (4), 36–44.

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Table 2. BET Surface Area and Pore Volume of Spinel-Doped CaOBased Absorbents absorbent composition

BET surface area (m2/g)

pore volume (cm3/g)

100 wt % CaO 10 wt % spinel and 90 wt % CaO 18 wt % spinel and 82 wt % CaO 26 wt % spinel and 74 wt % CaO 32 wt % spinel and 68 wt % CaO 42 wt % spinel and 58 wt % CaO 100 wt % spinel

9.5 13.1 (16.6)a 17.4 (22.3)a 20.5 (28.0)a 21.8 (32.3)a 24.7 (39.4)a 80.8

0.06 0.18 (0.11)a 0.22 (0.16)a 0.28 (0.20)a 0.30 (0.23)a 0.32 (0.29)a 0.60

a Numbers in parentheses are calculated values assuming inert mixing of spinel and CaO particles.

AlO(OH), followed by calcination at 1000 °C for 4 h in air. Small rod-like spinel crystallites about 20 nm long and 10 nm in diameter were produced using this method. In addition, a small amount of MgO (periclase) is seen in the XRD. The BET surface area and pore volume of this sample were 80.8 m2/g and 0.60 cm3/g, respectively. Spinel-stabilized CaO absorbents were prepared by the wet ball-milling procedure, as described above. Table 2 provides the BET surface area and pore volume of these CaO-based absorbents prepared with different spinel content. The numbers in parentheses are calculated values assuming simple mixing of inert spinel and CaO particles. The measured BET surface is less than this calculated value, indicating a strong interaction between spinel and CaO particles. On the other hand, the measured pore volume is more than calculated from simple mixing, suggesting that a porous structure was developed during absorbent synthesis. Figure 3a gives the CO2-capture capacity of CaO-based absorbents with different MgAl2O4 spinel loading obtained using isothermal carbonation-decarbonation cycle tests in a TGA unit at 758 °C. In comparison to the pure CaO sample, spinel-doped absorbents showed much better long-term stability. It appears that approximately 26 wt % spinel is required to maintain stability to 100 cycles or greater. The CaO utilization is provided in Figure 3b. The CaO utilization is very similar for all of the materials up to approximately 80 cycles, indicating that high spinel loading has no adverse effect on CO2 capture by the CaO particles. Figure 4 compares SEM images of the fresh and used CaO and 42% spinel-doped CaO absorbent. Without spinel doping, large CaO particles were developed after 115 cycles of carbonation and decarbonation at 758 °C. A minimal morphology change was observed with the 42 wt % spinel-doped sample after 132 cycles, which is consistent with the TGA test result shown in Figure 3. The SEM-EDS analysis (not shown) indicated that the small MgAl2O4 particles were uniformly distributed in both the fresh and used absorbents. Figure 5 gives the average CaO single-crystal size for all of the fresh and used samples evaluated, estimated from the XRD pattern using Scherrer’s equation. For the fresh samples, the single-crystal size of CaO decreases as the content of spinel increases, indicating that mixing with small spinel crystals can effectively prevent the formation of large CaO crystallites. Surprisingly, the average size of the CaO single crystal in the used absorbent is smaller than that of the corresponding fresh sample, indicating that the carbonation-decarbonation cycles can disperse the larger CaO single crystals. Clearly, the addition of spinel nano crystallites effectively prevented the formation of large CaO single crystals during multiple carbonation-decarbonation cycles. As can be seen from the SEM images of the fresh and used 100% CaO absorbents in

Figure 3. Effect of spinel doping on the CO2-capture performances of CaO-based absorbents prepared by the wet physical mixing method. (a) CO2 capacity and (b) CaO utilization. Carbonation at 758 °C in 100% CO2 for 30 min and decarbonation at 758 °C in 100% He for 30 min. (Top) CO2 capacity versus the cycle number. (Bottom) CaO use versus the cycle number.

Figure 4, although the single-crystal size of CaO did not change much after 115 cycles of carbonation and calcination tests, severe agglomeration of small CaO crystals developed, which largely decreased the CO2-capture performance of the absorbent. The sintering of CaO-based CO2 absorbents is caused by three major factors: (1) The carbonation process is highly exothermic (CaO þ CO2=CaCO3, ΔHo=-178 kJ/mol). (2) There is a large volume increase from CaO to CaCO3 (from 16.9 to 34.1 cm3/mol), which greatly decreases the distance between absorbent particles in the carbonated state. (3) CaCO3 has a Tammann temperature (i.e., the highest treatment temperature before the sintering of a material becomes significant) of 533 °C, lower than normal carbonation temperatures.32 A long-term stable CO2 absorbent not only requires the size of starting CaO absorbents to be small39 but also requires these small CaO particles to be able to remain highly dispersed after multiple carbonation-decarbonation cycles.40,41 Adding spinel nanoparticles can effectively prevent small CaO particles from agglomeration by physically separating small CaO particles. (39) Barker, R. The reversibility of the reaction CaCO3 = CaO þ CO2. J. Appl. Chem. Biotechnol. 1973, 23, 733–742. (40) Blamey, J.; Anthony, E. J.; Wang, J.; Fennell, P. S. The calcium looping cycle for large-scale CO2 capture. Prog. Energy Combust. Sci. 2010, 36, 260–279. (41) Lysikov, A. I.; Salanov, A. N.; Okunev, A. G. Change of CO2 carrying capacity of CaO in isothermal recarbonation-decarbonation cycles. Ind. Eng. Chem. Res. 2007, 46, 4633–4638.

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Figure 4. SEM images of fresh and used CaO and 42 wt % spinel-doped CaO particles. Cycle numbers of used samples are the same as shown in Figure 3.

Figure 7. Performances of natural dolomite, 32 wt % MgO-doped CaO absorbent, and 32 wt % spinel-doped CaO absorbent in a fixed-bed reactor. Carbonation: 650 °C, 60 min, 30% CO2, 10% H2O, 50% Ar, 10% N2, 550 h-1 GHSV; decarbonation: 850 °C, 34 min, 2.5% H2O, 2.5% N2, 95% Ar, 2200 h-1 GHSV. The 10% CO2 breakthrough capacity is defined as the amount of CO2 absorbed with at least 90% of the CO2 in the feed gas captured by the absorbent during each cycle.

Figure 5. Estimated CaO single-crystal size in each fresh and used spinel-doped CaO absorbent from the XRD pattern using Scherrer’s equation. Cycle numbers of used samples are the same as shown in Figure 3.

at the 5th and 100th cycles. More than 80% of total CO2 capture was achieved within the first 4 min, indicating that fast absorption rates are achieved. On the other hand, it usually took about 15-20 min to completely regenerate the CO2loaded absorbent. It can be seen from the figure that minimal change in the carbonation-decarbonation curve was observed after 100 cycles. In an effort to more closely simulate the temperatures employed in actual plant operation, a fixed-bed test using 177-250 μm (60-80 mesh) particles of 32 wt % MgAl2O4doped CaO absorbent particles was carried out. The carbonation cycle was carried out at 650 °C and 550 h-1 GHSV, and the regeneration was carried out at 850 °C and 2200 h-1 GHSV. Gas compositions were described above in the Experimental Section. The test results are shown in Figure 7.

Figure 6. Details of the 5th cycle (left) and 100th cycle (right) of carbonation and decarbonation reactions of 32 wt % spinel-doped CaO absorbents. TGA test conditions: carbonation at 758 °C in 100% CO2 for 30 min and decarbonation at 758 °C in 100% He for 30 min.

Besides the excellent thermal and chemical stability of MgAl2O4 spinel, it is possible that its rod-like particle shape, as shown in Figure 2, also plays a role in retarding sintering. Figure 6 shows in greater detail the carbonation and decarbonation reactions of the 32 wt % spinel-doped absorbent 3702

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The 10% CO2 breakthrough capacity is defined as the amount of CO2 absorbed with at least 90% of CO2 in the feed gas captured by the absorbent during each cycle. This capacity under rapid switching relates more closely to the “fast absorption capacity”, which, according to Figure 6, is about 80% of the total capacity. Generally, this fixed-bed test result is consistent with the result obtained from the TGA test (see Figure 3), validating the effectiveness of the TGA screening test protocol used in this work. For comparison, the performances of natural dolomite (35 wt % MgO, and 65 wt % CaO) and 32 wt % MgO-doped CaO with the same particle size range are also given in Figure 7. Improved durability is clearly observed with the spinel-doped CaO absorbent. Recently, Romeo et al. carried out a detailed economic assessment to evaluate the competitiveness of improved CaObased absorbents over natural CaO minerals.42 Using a process simulation that includes thermal, chemical, and economic performances of two interconnected circulating fluidized beds integrated within a fossil fuel power plant with CO2 capture, the maximum cost for an economically feasible improved CaO-based absorbent can be quantified. When stable CaO-based absorbents are used, special attention should be given to the absorbent cost increase over the natural CaO mineral, which contributes significantly to the overall CO2-capture cost. However, a detailed economic assessment

on the new spinel-doped CaO absorbents is beyond the scope of the current work. 4. Conclusion The widely observed sintering effect of CaO-based absorbents during high-temperature carbonation and decarbonation cycles can be effectively mitigated through the use of MgAl2O4 spinel nanoparticles as stabilizers for CaO. A new class of CaO-based absorbents with much improved hightemperature durability was developed by wet physical mixing of Ca(CH3COO)2 with MgAl2O4 spinel nanoparticles followed by high-temperature calcination. With 32 wt % MgAl2O4 content, a CaO-MgAl2O4 mixture gives 34 wt % CO2 capacity after 65 carbonation-decarbonation cycles, corresponding to 63% CaO use. This new class of absorbent demonstrated better long-term CO2-capture performance than the previously reported MgO-doped absorbents, indicating the spinel crystallites are more effective for stabilizing CaO particles. Further work is being carried out to determine the source of this superior performance, including examination of the particle shape of the stabilizing material. Acknowledgment. Financial support from the Battelle Memorial Institute (BMI) is gratefully acknowledged. This work was performed in part at the Interfacial and Nano Science Facility in the William R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Office of Biological and Environmental Research of the U.S. Department of Energy and located at the Pacific Northwest National Laboratory (PNNL). PNNL is operated for the U.S. Department of Energy by Battelle.

(42) Lisbona, P.; Martinez, A.; Lara, Y.; Romeo, L. M. Integration of carbonate CO2 capture cycle and coal-fired power plants. A comparative study for different sorbents. Energy Fuels 2010, 24, 728–736.

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