Sorbents during Calcium Looping Cycles fo - American Chemical

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Ind. Eng. Chem. Res. 2010, 49, 11778–11784

Development and Performance of CaO/La2O3 Sorbents during Calcium Looping Cycles for CO2 Capture Cong Luo, Ying Zheng,* Ning Ding, Qilong Wu, Guan Bian, and Chuguang Zheng State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong UniVersity of Science and Technology, Wuhan 430074, China

The calcium looping cycles method has been identified as an attractive method for CO2 capture during coal combustion and gasification processes. However, it is well-known that the capture capacity of CaO undergoes a rapid decrease after mutiple cycles. In order to improve the stability of CO2 capture capacity in CaO, this paper focuses on the development and performance of the synthetic CaO/La2O3 sorbents for calcium looping cycles.The sorbents were synthesized by three different methods: dry physical mixing, wet chemistry, and sol-gel combustion synthesis (SGCS). Their multicyclic CO2 capture capacity and the effect of the additive La2O3 were investigated in a fixed bed reactor system. The results indicate that the additive of La2O3 plays a positive role in the carbonation/calcination reactions, and the SGCS-made synthetic sorbent is composed of ultrafine well-dispersed hollow structured particles which are beneficial to the gas-phase diffusion on the surface area and can prevent small CaO particles from agglomeration effectively. As a result, the novel synthetic sorbent with the molar ratio of Ca to La of 10:1 made by the SGCS method provides the best performance of a carbonation conversion of 72% under mild calcination conditions and a carbonation conversion of 36% under severe calcination conditions (high temperature and high CO2 concentration) after 20 cycles. 1. Introduction The calcium looping cycles method has been identified as one of the best candidates for CO2 capture during coal combustion and gasification processes.1-6 The reversible carbonation of CaO-based CO2 sorbents has the potential to be used in a clean, economically feasible CO2 separation process under moderate conditions, for generation of electricity.1 However, it is widely accepted that the capture capacity of CaObased CO2 sorbents decreases when the cycles extend.2,3 The carbonation conversion of natural limestone declines to about 20% after 20 cycles.3 This problem, an indication of poor reversibility of the carbonation/calcination cycles of CaO/ CaCO3, is a major challenge for the future application of CaObased technology for capturing CO2 and needs to be solved. Therefore, it is necessary to improve the reversibility of CaObased sorbents for extended cycles. A large number of methods have been carried out to increase the life cycle performance of CaO-based sorbents. Some researchers improved the durability of CaO-based sorbents without any doped materials. Many researchers increased the pore volume and pore surface area of different limestone samples by a process of steam hydration.7-10 They found that the steam hydration/activation method could improve the longterm behavior of the sorbents. A lot of researchers synthesized high surface area CaO sorbents from precipitated calcium carbonate.2,11-13 Their studies suggested that a mesoporous structure with porosity in the small pore size range would be less susceptible to pore blockage and, thus, provided higher CO2 capture capacity and better stability of CaO. Lu et al. have investigated the performance of CaO sorbents derived from different precursors.14,15 The best performing calcium oxides were obtained from calcination of calcium propionate and calcium acetate, a fact attributed to networks of meso- and macropores (ranging from 10 to 100 nm), present to lesser extents in CaO formed by the other precursors. * To whom correspondence should be addressed. Tel.: 86-2763120550. Fax: 86-27-87545526. E-mail: [email protected].

Other researchers have developed durable doped CaO-based sorbents by different methods. Various materials were doped in CaO to improve its cyclic capture capacity for repetitive carbonation/calcination reactions. Wu et al. reported a CaObased sorbent coated with CaTiO3 to prevent the nano-CaO particles from coming into contact with each other under high temperature.16 Li et al. investigated the effect of adding an inert phase consisting of Ca12Al14O33 to CaO.17,18 They showed that a CaO/Ca12Al14O33 (75/25 wt %) was optimal for preparing this sorbent. Manovic et al. investigated the performance of calcium aluminate pellets in depth.19-21 The pellets prepared from hydrated lime and commercial CA-14 calcium aluminate cement had a good performance after a large number of cycles in high temperature/high CO2 concentration calcination conditions. Recently, the MgO doped CaO-based sorbents have also been widely investigated by many researchers.22-24 Since the MgO has high stability and lack of reaction with CaO or CaCO3 during calcium looping cycles, it can highly improve the stability of the CaO-based sorbents. In this paper, we have synthesized a few novel CaO/La2O3 sorbents. In the earlier study, Albrecht et al. reported that the additive material of La2O3 had a better performance than MgO for the CaO-based sorbents in the long term cycles when MgO and La2O3 were doped with the identical weight ratio by the same method, respectively.22 However, there are few reports about the CaO/La2O3 sorbent, so it is of great significance to conduct further research on the functions of the CaO/La2O3 sorbent. This paper attempts to study the cyclic characterization of CaO/La2O3 sorbent and to produce it, which has the optimal concentration of La2O3 needed in CaO and retains its reactivity over many calcium looping cycles. 2. Experimental Section 2.1. Preparation of CaO/La2O3 Sorbents. All of the precursors used in this paper were analytical reagent. The CaO/ La2O3 sorbents were synthesized with the molar ratio of Ca to La of 10:1 by three different methods as follows.

10.1021/ie1012745  2010 American Chemical Society Published on Web 09/15/2010

Ind. Eng. Chem. Res., Vol. 49, No. 22, 2010

Figure 1. The main technological process of the sol-gel combustion synthesis (SGCS) method.

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reactor and to react for 15 min at 650 °C in 15 vol % CO2/85 vol % N2 in the carbonation reactor. The actual temperature of the sample is measured by two thermocouples placed in the center of the two compartments, respectively. The reaction gas flow of CO2 and N2 is controlled by the three way valve and flow meter controller. The calcined or carbonated sample can be moved to the left or right side of the tube for cooling in N2 and then can be taken out to be weighed with a delicate electronic balance at room temperature. The microstructure of calcined samples was investigated by field emission scanning electron microscopy (FSEM; SIRION200, FEI Inc.) with 20 kV of accelerating voltage under high vacuum. The microscope was equipped with energy dispersive X-ray (EDX) analyzers, which enabled the determination of elemental compositions at points of interest on the sorbent surface. The phase composition of samples was determined by X-ray diffraction (XRD; PANalytical B.V.) with Cu KR radiation, λ ) 0.1542 nm in the 2θ range of 20°-70° with a scanning step of 0.02°. The pore structure parameters of the calcined sample were examined from N2 adsorption and desorption isotherms, measured at the temperature of liquid N2 (Micromeritics ASAP 2020-M). The surface area and pore volume were calculated from the Brunauer Emmet Teller (BET) equations and Barrett-Joyner-Halenda (BJH) method, respectively. 2.3. Experimental Data Analysis Method. The carbonation conversion (Xn) and the capture capacity (Cn) are calculated by the equations widely used before as follows:

Figure 2. Schematic diagram of a fixed bed reactor system.

2.1.1. Dry Physical Mixing. Calculated amounts of La2O3 and CaCO3 were mechanically mixed together before being calcined at 800 °C in pure N2 for 2 h. 2.1.2. Wet Chemistry. First, CaCO3 was calcined in pure N2 at 800 °C for 2 h before added into distilled water with La(NO3)3 · 6H2O in calculated amounts. Then, the solution was stirred for 1 h at 80 °C and was dried at 120 °C for another 2 h before it was calcined at 800 °C for 2 h in a muffle furnace. 2.1.3. Sol-Gel Combustion Synthesis (SGCS). Figure 1 shows the main technological process of the SGCS method. Step 1: Calculated amounts of La(NO3)3 · 6H2O, Ca(NO3)3 · 4H2O, and C6H8O7 · H2O (citric acid) were added into distilled water with the molar ratio of water to metal ions of 40:1 and the molar ratio of citric acid to metal ions of 1:1 at room temperature. Then, the mixture was continuously stirred and kept at 80 °C in an electric-heated thermostatic water bath for a sufficient period of 7 h which resulted in the well-dispersed sol being formed. Step 2: The sol was placed at room temperature for 18 h to form a wet gelatin. Step 3: The wet gelatin was put into a drying oven at 80 °C for 5 h and then dried at 110 °C for another 12 h until the dry gelatin was formed. Step 4: The dry gelatin was put into the muffle furnace quickly at 600 °C, and it was burned off within a few minutes. Step 5: When the combustion process was over, the sample was calcined at 700 °C for 2 h in a muffle furnace. 2.2. Experimental System. Figure 2 shows a fixed bed reactor system, the furnace in this system has two compartments including a carbonation reactor and a calcination reactor designed at atmospheric pressure. Both of the two reactor rooms can be heated separately. The reacting gases which injected from the left side of the tube are forced to pass through the sample boat and leave the furnace from the right side of the tube. The sample boat can be shifted between two reactors. Samples were allowed to react for 10 min at 850 °C in 100% N2 (typically investigated laboratory-scale conditions) or at 950 °C in 100% CO2 (realistic conditions in the calciner) in the calcination

Xn )

mn - m0 WCaO · m0 · b WCO2

(1)

mn - m0 m0

(2)

Cn )

Where mn is the mass of the carbonated sample after n cycle(s), m0 is the initial mass of the calcined sample, b is the content of CaO in the initial calcined sample. WCaO and WCO2 are mole mass of CaO and CO2, respectively. As a result, Xn reflects only the reversibility of CaO in the sorbent, while Cn reflects the overall performance of sorbent. 3. Results and Discussion 3.1. Evaluation of Different Preparation Methods. Here, analytical reagent CaCO3 (>99.5 wt %) is used as a reference for further comparison because it is the convenient source of CaO, whose property is similar to lime used in the power generation systems. Figure 3 shows that the carbonation conversions of CaO/La2O3 sorbents made by the different methods are higher in different degrees than those of the pure CaCO3 sorbent from cycle 3 to cycle 20. It is also obvious that SGCS-made sorbent does much better than other sorbents on cyclic CO2 capture. After 20 cycles, the carbonation conversion of SGCS-made sorbent is 72%, whereas wet chemistry-made sorbent provides a carbonation conversion of 43% which is still much higher than that of dry physical mixing-made sorbent (28%) and pure CaCO3 sorbent (21%). Previous researchers indicated that doped sorbents often show slower decay in the initial cycles in that the doped inert materials can prevent small CaO particles from agglomeration by physical separation of small CaO grains.16-24 Unlike MgO, which is considered as inert material, and Al2O3, which reacts with CaO into inert material of Ca12Al14O33, La2O3 plays an active role in the calcium looping cycles and cannot be considered as inert

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Figure 3. Carbonation conversions of the Ca/La ) 10:1 sorbents made from different preparation methods under laboratory-scale conditions (carbonated at 650 °C in 15% CO2 for 15 min and calcined at 850 °C in 100% N2 for 10 min).

Figure 4. Equilibrium pressure of CO2 over CaCO3 and La2O2CO3.

material. During the calcium looping cycles of the CaO/La2O3 sorbent, there are two reversible reactions as follows: CaCO3 h CaO + CO2

(3)

La2O2CO3 h La2O3 + CO2

(4)

The thermal equilibrium (Figure 4) of reaction 3 is described by eqs 525 and 626 regarding different temperature ranges, and the thermal equilibrium of reaction 4 is described by eq 7.27 log10 p(atm) ) 7.5214 - 8792.3/T(K)

(723 e T(K) e 1177)

(5) log10 p(atm) ) 7.079 - 8307.8/T(K)

(1177 e T(K) e 1513)

(6) lnp(pa) ) 25.87 - 17502.2/T(K)

(773 e T(K) e 1223) (7)

In the process of carbonation of 15% CO2 at 650 °C under atmospheric pressure, La2O3 adsorbs CO2 into La2O2CO3, while in the process of calcination of 100% N2 at 850 °C under atmospheric pressure, La2O2CO3 decomposes into La2O3 again. In the case of the dry physical mixing method, the carbonation conversion of CaO/La2O3 sorbent is only 7% higher than that

of pure CaCO3 sorbent after 20 cycles. The molar ratio of CaO to La2O3 is 20:1 in the synthetic sorbents, so the La2O3 can enhance a maximum 5% higher carbonation conversion for the novel sorbent theoretically. As a result, the adsorbing CO2 of La2O3 should partly be responsible for the enhanced performance of CaO/La2O3 sorbents, especially for the dry physical mixingmade sorbent. Since the CaO and La2O3 in the synthetic sorbent are mixed at grain level and the molar ratio of La2O3 is very low in the sorbent, La2O3 cannot be able to prevent small CaO particles from agglomeration effectively by the dry physical mixing method. In the case of the wet chemistry method, the carbonation conversion of CaO/La2O3 sorbent is 22% higher than that of pure CaCO3 sorbent after 20 cycles, so the main reason for highly improved carbonation conversion of wet chemistry-made sorbent must be the physical separation of small CaO particles by La2O3. During the preparation process, ultrafine Ca(OH)2 particles can be generated from the cracking and refining of CaO particles suspended during hydration and penetration of nitrate into particles, enabling more porosity during the decomposition of nitrate; thus, the CaO can be mixed with La2O3 at microscopic level, and the wet chemistry-made sorbent can get a higher carbonation conversion. In this study, SGCS-made CaO/La2O3 sorbent provides the highest carbonation conversion compared with the other sorbents. This sorbent has stable carbonation conversions of about 85% in the initial 10 cycles with slow decrease in the following 10 cycles. The CaO can also be mixed with La2O3 at microsopic level, and a hollow structured CaO-based sorbent can be formed by the SGCS method (disscussed later) which is different from the wet chemistry-made sorbent. Because the SGCS-made sorbent displayed best CO2 capture performance, further investigation was undertaken to study the performance of it in depth and find out the optimal concentration of La2O3 needed in CaO. 3.2. Evaluation of SGCS-Made Sorbents under Realistic Conditions. Manovic et al. have reported that high temperature and high CO2 concentration during calcination greatly affect the sorbent carrying capacities with CO2 capture cycles.21 Once the SGCS method was selected as the best way to produce the CaO/La2O3 sorbent, further research in this paper was tested under the realistic conditions of high temperature and high CO2 concentration in the calcination process. The curves of carbonation conversions of the SGCS-made sorbents and pure CaCO3 under realistic conditions are presented in Figure 5. During the initial calcium looping cycles, the smaller the amount of La2O3 used, the higher carbonation conversion the synthetic sorbent achieves. The result is obvious that more amount of CaO is present with a lesser amount of dopant-incorporated sorbent. However, although pure CaCO3 contains the most amount of CaO, all of the SGCS-made sorbents have higher carbonation conversions than pure CaCO3 during the initial cycles. This can be attributed to the hollow structured CaO-based sorbent made during the SGCS preparation (compared in Figures 6 and 7). When the sorbents are made by the SGCS method, the calcination of metal nitrates consists of many physical processes and chemical reactions. In the process of Step 4 in Figure 2, the dry gelatin of lanthanum nitrate, calcium nitrate, and citric acid can be ignited at 600 °C in the muffle furnace. Metal nitrates work as oxidant, while citric acts out fuel: La(NO3)3 · 6H2O + (5/6)C6H8O7 · H2O f (1/2)La2O3 + 5CO2 + (61/6)H2O + (3/2)N2

(8)

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Table 1. Capture Capacity of SGCS-Made Sorbents and Pure CaCO3 after Mutiple Cycles sample

Cn [g CO2/g sorbent] a

pure CaCO3 after 10 cycles Ca/La ) 10:1 after 10 cyclesa Ca/La ) 10:1 after 10 cyclesb Ca/La ) 10:1 after 20 cyclesb Ca/La ) 5:1 after 20 cyclesb Ca/La ) 20:1 after 20 cyclesb pure CaCO3 after 20 cyclesb

0.22 0.51 0.25 0.22 0.21 0.16 0.05

a Carbonated at 650 °C in 15% CO2 for 15 min and calcined at 850 °C in 100% N2 for 10 min (lab conditions). b Carbonated at 650 °C in 15% CO2 for 15 min and calcined at 950 °C in 100% CO2 for 10 min (real conditions).

Figure 5. Carbonation conversions of the SGCS-made sorbents with different molar ratios of Ca/La under realistic conditions (carbonated at 650 °C in 15% CO2 for 15 min and calcined at 950 °C in 100% CO2 for 10 min).

Figure 6. FSEM image of pure CaCO3 after initial calcination at 850 °C in 100% N2 for 10 min.

Figure 7. FSEM image of SGCS-made Ca/La ) 10:1 sorbent after initial calcination at 850 °C in 100% N2 for 10 min.

Ca(NO3)2 · 4H2O + (5/6)C6H8O7 · H2O f CaO + 5CO2 + (49/6)H2O + N2 (9) The reactions 8 and 9 release a large amount of gas and heat, leading to an intense combustion of the dry gelatin. The particles

of the product can be homogeneously dispersed and sintered in the microstructure, and the CaO and La2O3 can be mixed at microscopic level. Finally, ultrafine hollow well-dispersed powders are produced. These hollow structure particles are beneficial to the gas-phase diffusion on the surface area and well-dispersed La2O3 in CaO can prevent small CaO particles from agglomeration effectively. In contrast, after 20 calcium looping cycles, the larger the amount of La2O3 used, the higher carbonation conversion the synthetic sorbent achieves. This is in agreement with the previous research22,23 that larger amount of dopant-incorporated sorbents usually give better stability and higher CaO utilization after mutiple cycles. However, the presence of a large amount of doped material in an industrial sorbent is undesirable since it may decrease the capture capacity of the sorbents. Although the carbonation conversion of the Ca/La ) 5:1 sorbent is higher than that of the Ca/La ) 10:1 sorbent, Ca/La ) 10:1 sorbent still provides a higher capture capacity (0.22 g of CO2/g of sorbent) after 20 calcium looping cycles as shown in Table 1. As a result, in the present investigation, consideration is given to the use of Ca/La ) 10:1 sorbent made by the SGCS method. 3.3. Cyclic Characterization of SGCS-Made Ca/La ) 10:1 Sorbent. Elemental FSEM-EDX analysis of the surface shows the influence of chemical composition on particle morphology. Figure 8 shows the surface of calcined SGCSmade Ca/La ) 10:1 sorbent after 10 cycles under realistic conditions. There are two different surface morphologies: one (spectrum 1) is a rough appearance with a lower weight ratio concentration of La (3.6 wt %), and the other (spectrum 2) is a smooth appearance with a higher weight ratio concentration of La (15.8 wt %). Usually, CaO grains that have not been sintered show a rough porous appearance, while sintered material appears agglomerated and has a smooth appearance, which also correlates well with high content of impurity. The doped La2O3 does not have a uniform distribution on the surface of the synthetic sorbent after many cycles. Figure 9 shows the XRD spectra of the novel sorbent after 10 calcium looping cycles under lab conditions and real conditions. During the laboratory calcination conditions of 100% N2 at 850 °C under atmospheric pressure, La2O2CO3 can be completely decomposed into La2O3 as the discussion before; but during the realistic calcination conditions of 100% CO2 at 950 °C under atmospheric pressure, reaction 4 achieves a chemistry equilibrium state (as shown in Figure 4), so the La2O2CO3 cannot be completely decomposed. It is obvious that the remainder La2O2CO3 cannot absorb CO2 in the process of carbonation, leading to the lower capture capacity for the novel sorbent compared with the case of calcined in lab conditions. Furthermore, the grain size of CaO can be determined from the main peak breadth in the XRD spectra. The mean grain size

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D)

Figure 8. FSEM-EDX elemental analysis of the surface of calcined SGCSmade Ca/La ) 10:1 sorbent after 10 cycles under realistic conditions: (a) FESM image, (b) spectrum 1, (c) spectrum 2.

Figure 9. XRD spectra of calcined SGCS-made Ca/La ) 10:1 sorbent after 10 calcium looping cycles.

based on the XRD line breadth from the Scherrer equation has been determined as follows:

kλ βcos θ

(10)

Where D is the grain size of CaO derived from different sorbents, β is main peak breadth in XRD spectra, λ is X-ray wavelength (λ ) 0.154056 nm), θ is Bragg angle, and k is the Scherrer constant. The main peaks of CaO are located at 2θ of 32.2, 37.4, 53.9, 64.2, 67.4 degrees (Figure 9). It is calculated that the main peaks of CaO under lab conditions are broader than that under real conditions. As a result, from the eq 10, the main grain size of CaO under real conditions is bigger. The bigger the grain size of CaO, the smaller is the space between two CaO grains, and the greater CO2 diffusion resistance becomes.28 As the discussion above, the SGCS-made Ca/La ) 10:1 sorbent has a much lower capture capacity during calcium looping cycles under the realistic calcination conditions of high temperature/high CO2 concentration. As shown in Table 1, the capture capacity of this novel sorbent under laboratory-scale conditions is twice as much as that of it under realistic conditions after 10 cycles. The BET surface area presented in Table 2 shows that surface area of the initial SGCS-made Ca/La ) 10:1 sorbent is 1.4 times as large as that of the initial pure CaCO3, resulting in superior performance of carbonation conversions during initial calcium looping cycles according to Figures 3 and 5. Moreover, the surface area of novel sorbent is 4 times that of pure CaCO3 after 10 calcium looping cycles under lab conditions. It means that the novel sorbent undergoes less severe sintering during the looping cycles. It is also observed from Table 2 that novel sorbent suffers from slower surface area decrease even under the real-condition looping cycles. The surface area of the novel sorbent after 10 cycles under real conditions is still more than half of that of its initial sorbent. This may be due to the hollow structured CaO-based sorbent formed during the SGCS preparation as described before, and this morphology can alleviate the sintering and agglomeration phenomena for the sorbent during the looping cycles. Figure 10a shows two peaks of ∼4 and ∼50 nm in a logarithmic scale of the BJH pore volume distributions of different calcined samples. It is found that there is a certain loss in the small pores (∼4 nm) and a certain increase in large pores (∼50 nm) for SGCS-made CaO/La2O3 sorbent after 10 cycles under lab conditions compared with its initial sample. However, both of its small pores and large pores decrease under real conditions under identical operation cycles. Therefore, the high temperature and high CO2 concentration atmosphere should accelerate the sintering of this sorbent. Nevertheless, the SGCSmade CaO/La2O3 sorbent under severe conditions still has a better pore volume distribution than pure CaCO3 sorbent even under mild conditions after the same cycles. Figure 10b shows the volume of pores over the entire pore size range measured of pure CaCO3 decreases sharply with the cycle number from 1 to 10 even under mild conditions. Figure 11 shows the curves of carbonation conversions of the CaO-based sorbents tested under the same conditions as those in Figures 3 and 5 from cycle 1 to cycle 20, but carbonation conditions were changed from cycle 21 to cycle 25. The aim of this experiment is not to use the sorbents in 50 min carbonation in industrial application but to show some interesting characterization of the differences between novel sorbent and commercial pure CaCO3. Because longer carbonation time can increase the CO2 capture capacity during the gasphase diffusion controlled phase and higher CO2 concentration during the carbonation process can provide a stronger driving

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Table 2. BET Surface Area of Calcined SGCS-Made Ca/La ) 10:1 Sorbent and Pure CaCO3 sample a 3

initial pure CaCO pure CaCO3 after 10 cycles (lab conditions) initial Ca/Laa Ca/La after 10 cycles (lab conditions) Ca/La after 10 cycles (real conditions) a

SBET [m2/g] 12.36 2.98 17.49 13.75 10.02

Calcined at 850 °C in 100% N2 for 10 min before the BET test.

Figure 11. Carbonation conversions of the sorbents with only carbonation conditions change during 21st-25th cycles. (From cycle 1 to cycle 20, the samples were carbonated at 650 °C in 15% CO2 for 15 min; from cycle 21 to cycle 25, the samples were carbonated at 650 °C in 100% CO2 for 50 min.)

Figure 12. Carbonation conversions of the novel sorbent during calcium looping cycles.

Figure 10. BJH pore volume distributions of different calcined samples: (a) SGCS-made CaO/La2O3 and (b) pure CaCO3 (initial samples were calcined at 850 °C in 100% N2 for 10 min before the BJH test).

force for gas-phase diffusion to the least accessible reactive surface area,13 the carbonation conversion of the sorbents should be highly improved. However, the carbonation conversion of pure CaCO3 fails to be enhanced after 20 cycles under favorable carbonation conditions even with the mild calcination conditions. This may be ascribed to the severe sintering-agglomeration during the looping cycles which resulted in a loss of activity of pure CaCO3. On the contrary, SGCS-made CaO/La2O3 sorbent has superior performance in long-run calcium looping cycles, it has a large increase in carbonation conversion from the 21st cycles, no matter if under lab conditions (from 72% to about 90%) or under real conditions (from 36% to nearly 50%). Because SGCS-made sorbent exhibits a better pore volume distribution than pure CaCO3 after many cycles, as shown in Figure 10, a stronger driving force for gas-phase diffusion to the least accessible reactive surface area can enhance the capture capacity of the novel sorbent. In an early study, Bhatia et al.

demonstrated that the maximum carbonation conversion is dependent only on the fraction of porosity associated with relatively small pores of smaller than 100 nm.29 As a result, although the carbonarion rate of SGCS-made sorbent decreases after mutiple CO2 capture cycles, favorable carbonation conditions can lead to higher carbonation conversions for this sorbent. Finally, Figure 12 presents the performance of SGCS-made CaO/La2O3 sorbent during 50 calcium looping cycles under realistic calcination conditions and laboratory calcination conditions. After 50 cycles, the carbonation conversion of the novel sorbent achieves 65% under mild conditions, whereas the carbonation conversion of it still decreases to 28% while under severe conditions. Therefore, the further investigation on calcination conditions of the novel CaO-based sorbent should be meaningful. 4. Conclusions Three methods were used to make the synthetic CaO/La2O3 sorbent for testing their reversibility during calcium looping cycles. The novel synthetic sorbent with the molar ratio of Ca to La of 10:1 made by the SGCS method has excellent

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regeneration capacity compared with the other preparation methods and the other additive concentrations. It is shown that La2O3 has the ability to capture CO2 during calcium looping cycles and ultrafine hollow well-dispersed powders are produced during the SGCS preparation for the novel sorbent. Since these hollow structured particles are beneficial to the gas-phase diffusion on the surface area and well-dispersed La2O3 in CaO can prevent small CaO particles from agglomeration effectively, its carbonation conversion maintains 72% and 36% after 20 cycles under laboratory-scale conditions and realistic conditions (temperature and CO2 concentration), respectively. Furthermore, SGCS-made synthetic CaO/La2O3 sorbent has a slower decrease of surface area and pore volume after mutiple cycles compared with pure CaCO3, and this stable porosity of the novel sorbent during calcium looping cycles can lead to much higher carbonation conversions for it, when it is tested under more favorable carbonation conditions of prolonged carbonation time and higher CO2 concentration after many calcium looping cycles. Acknowledgment This work is supported by the National Basic Research Program of China (No. 2006CB705807) and the National Natural Science Foundation (Nos. 50936001, 50721005, 50676038). Literature Cited (1) 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. (2) 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. (3) Abanades, J. C.; Alvarez, D. Conversion Limits in the Reaction of CO2 with Lime. Energy Fuels 2003, 17, 308–315. (4) 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. (5) Hughes, R. W.; Lu, D. Y.; Anthony, E. J.; Macchi, A. Design, Process Simulation and Construction of an Atmospheric Dual Fluidized Bed Combustion System for in Situ CO2 Capture Using High-Temperature Sorbents. Fuel. Process. Technol. 2005, 86, 1523–1531. (6) Feng, B.; Liu, W.-q. Overcoming The Problem of Loss-in-Capacity of Calcium Oxide in CO2 Capture. Energy Fuels 2006, 20, 2417–2420. (7) Hughes, R. W.; Lu, D.; Anthony, E. J.; Wu, Y. Improved Longterm Conversion of Limestone-Derived Sorbents for in Situ Capture of CO2 in a Fluidized Bed Combustor. Ind. Eng. Chem. Res. 2004, 43, 5529–5539. (8) Zeman, F. Effect of Steam Hydration on Performance of Lime Sorbent for CO2 Capture. Int. J. Greenhouse Gas Control 2008, 2, 203– 209. (9) Manovic, V.; Anthony, E. J. Steam Reactivation of Spent CaO-based Sorbent for Multiple CO2 Capture Cycles. EnViron. Sci. Technol. 2007, 41, 1420–1425. (10) Fennell, P. S.; Davidson, J. F.; Dennis, J. S.; Hayhurst, A. N. Regeneration of sintered limestone sorbents for the sequestration of CO2 from combustion and other systems. J. Energy Inst. 2007, 80, 116–119.

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ReceiVed for reView June 12, 2010 ReVised manuscript receiVed September 1, 2010 Accepted September 1, 2010 IE1012745