Ind. Eng. Chem. Res. 2008, 47, 9537–9543
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SEPARATIONS Parametric Study of the CaO-Ca12Al14O33 Synthesis with Respect to High CO2 Sorption Capacity and Stability on Multicycle Operation Christina S. Martavaltzi† and Angeliki A. Lemonidou*,†,‡ Department of Chemical Engineering Aristotle UniVersity of Thessaloniki, P.O. Box 1517, UniVersity Campus, GR-54124 Thessaloniki, Greece, and Chemical Process Engineering Research Institute CPERI/CERTH, P.O. Box 361, 6th km Charilaou-Thermi Road, 570 01 Thessaloniki, Greece
This study focuses on the effect of various parameters of the synthesis procedure of the CO2 sorbentsCaO-Ca12Al14O33 derived from calcium acetateson the CO2 sorption ability and lifetime. The synthesis procedure consists of the calcination of the acetate CaO precursor (step 1), the precipitation of the CaO with aluminum nitrates (step 2), followed by calcination at 500 °C for nitrates removal (step 3) and the calcination at 900 °C for the synthesis of Ca12Al14O33 (step 4). It has been shown, by employing TGA, that the applied difference in the calcination conditions (900 °C/2 h, 850 °C/1 h, 750 °C/0.5 h,) for the decomposition of the CaO acetate precursor (step 1) is not adequate to affect the sorption fixation and stability results. On the other hand calcination history of the precipitate and the addition of water in between the two calcination steps (nitrates removal and Ca12Al14O33 synthesis reaction) are of high importance for ensuring sorption capacities higher than 6.5 mol CO2/kg of sorbent in the first cycle. SEM observations showed that the addition of water after calcination at 500 °C (step 3) accounts for the generation of the regular hexagonal crystalloids of Ca(OH)2 which transforms in a porous network during calcination at 900 °C. It was also demonstrated that the quantity of CO2 molecules retained increases with decreasing aging time of the precipitate as a result of higher surface area (smaller crystal size) of the as-synthesized samples. The sorbent with the lower binder content (CaO/Ca12Al14O33 ) 85:15) showed the prospective higher capacity (45% weight increase) in the first cycle but also a fair stability on repetitive sorption-desorption cycles. Introduction Carbon dioxide capture and storage has emerged as a critical technology pathway to control the heat-trapping capability of the atmosphere (greenhouse effect).1 Currently, the only commercially available technology to separate CO2 is based on amine scrubbing systems which introduce severe technology penalties and high utility costs.2 Therein, the optimization of more energy and cost efficient systems is of high research importance. Selective sorption of CO2 by a naturally occurring or synthetic material has been identified as one of the most promising technologies to offer the solution for high temperature applications, >600 °C (hydrogen production via steam reforming, power generation, biomass gasifiers).3-7 However, the sorbent properties (heat tolerance, sorption capacity and stability in multicycle operation, fast kinetics) are very important for the economic viability of the process. According to literature, materials which are of interest for high temperature (>600 °C) applications are mostly calcium oxide and lithium oxides.8-19 Studies on lithium oxides proved that the reactivity of lithium orthosilicate (Li4SiO4) is better than that of lithium zirconate (Li2ZrO3), sorbing up to 8 mol CO2/kg of sorbent under 100% CO2 at 650 °C.13-19 However, the studies of Venegas et al.17 and Xiong et al.19 proved that both materials overcome the diffusion limitations and show accepted capacities when the size of the particle approximates the unrealistic for industrial * To whom correspondence should be addressed. E-mail: alemonidou@ cheng.auth.gr. Tel.: +30 2310 996273. Fax: +30 2310 996184. † University of Thessaloniki. ‡ Chemical Process Engineering Research Institute.
applications value of 1 µm. Calcium oxide has been thoroughly studied because of its high sorption ability. Gupta and Fan5 obtained a CO2 retention of 14 mol/kg of CaO under 100% CO2 flow at 650 °C using a mesoporous structured calcium oxide. Lu et al.,20 prepared calcium oxide sorbent from calcium acetate monohydrate and about 90 mol% (18mol/kg) of the sorbent carbonated with CO2 within the first 10 min at 700 °C. Even though there are studies on sorption ability of different materials, scarce publications are concerned with the lifetime of the sorbents. However, once the material loses its CO2 fixing ability, regeneration of the sorbent, in higher temperatures, is required. Therefore, development of a sorbent material which keeps its regeneration ability constant is of high importance for the economical and waste management efficiency of the process. Natural Ca-based sorbents, such as limestone and dolomite, are potentially ideal sorbents because of their wide availability and low cost. However, fast reactivity loss affects the cost of the CO2 separation system. Abanadez et al.21 modeled the deactivation of Ca-based sorbents and reported that the CO2 absorption capacity decays as a function of the number of calcinationcarbonation cycles. Abanadez8 collected experimental data from different authors and concluded that the highest carbonation capacity of CaO is 14mol/kg and it decreases to 3.78 mol/kg after 20 cycles and keeps decreasing. Replacing limestone and dolomite with a synthetic sorbent of higher lifetime in a carbonation-calcination cycle is economically feasible, provided its uptake is higher than that of limestone and dolomite for a large number of cycles.22 Li et al.23 used CaO-Ca12Al14O33 and succeeded at constant capacity of 5 mol/kg for 13 cycles. The stable performance of the material was attributed to the
10.1021/ie800882d CCC: $40.75 2008 American Chemical Society Published on Web 11/05/2008
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presence of Ca12Al14O33. The synthesis procedure ensures uniform dispersion of the tolerant in high temperatures Ca12Al14O33 particles among CaO particles, providing a stable framework thus inhibiting the sintering of CaO. Our group improved the performance of the mixed CaO-Ca12Al14O33 using CaO derived from calcined calcium acetate.24 The new material shows a low tortuosity in its pore system resulting in a decreased resistance in CO2 access to the active sites. The weight increase achieved approaches 40% (9 mol CO2/kg of material) in the first 5 cycles with a moderate loss in capacity (15%) after 45 sorption-desorption cycles.24 To clarify which of the synthesis parameters and to what extent they affect the sorption capacity and stability of the sorbent, CaO-Ca12Al14O33, we did a parametric study of its synthesis procedure. We synthesized, successfully, CaOCa12Al14O33 with various sorption capacities and stabilities by varying the synthesis parameters such as calcination conditions of the precursor, stirring time, aging time, synthesis steps, and CaO/Ca12Al14O33 ratio. The synthesized materials are characterized with respect to their surface area, crystal structure, and morphology while their sorbing capacity is tested under 15% CO2 flow in repetitive sorption-desorption cycles. Experimental Section Chemicals, Sorbents, and Gases. Calcium acetate (Panreac, 99%) was used as CaO precursor. Aluminum nitrate hydrate (Riedel-de Haen, 98%) and 2-propanol (Merck, 99.5%) were used in the synthesis of CaO-Ca12Al14O33. Nitrogen, used as purge gas during the calcination period and as dilution gas during the carbonation period, had a purity of 99.999%. The concentration of carbon dioxide used for sorption tests was 15% (85% N2). Sorbents Preparation. The precursor material (Ca(CH3COO)2) was calcined (900 °C/2 h, 850 °C/1 h, 750 °C/0.5 h) in order to decompose to pure CaO in a box-type furnace under air atmosphere. For the synthesis of CaOCa12Al14O33, aluminum nitrate enneahydrate (Al(NO3)3 9H2O) and calcium oxide derived from calcination of calcium acetate was added to a solution of 2-propanol and distilled water so that the weight ratios of CaO to Ca12Al14O33 were 65/35 or 75/ 25 or 85/15. The solution was stirred for 0.5 h or 1 h at 75 °C and then left at room temperature for 0 or 1 or 5 days. Drying at 120 °C overnight followed. The samples were calcined at 500 °C for 3 h in the presence of air to decompose the nitrates. Further treatment of the samples proceeded in one or two steps. A group of samples was calcined in one step at 900 °C for 1.5 h, while in the other group after calcination at 500 °C, water was added and the obtained kneaded paste was dried at 120 °C overnight and then calcined at 900 °C for 1.5 h. All the materials synthesized were ground and sieved and a 106-350 µm fraction was collected for the sorption tests. The denotations of the samples and the synthesis parameters varied for each one are summarized in Table 1. Experimental System. An SDT Q600 (TA Instrument) thermal gravimetric analysis (TGA) instrument was employed for the carbonation and calcination experiments. SDT Q600 works in conjunction with a controller and associated software to make up a thermal analysis system. The weight sensitivity of the balance is 0.1 µg. A small sample of the sorbent (15-20 mg) was placed in an alumina cup and its weight was continuously recorded. Experimental Procedure. A small quantity of the sorbent placed in an aluminum sample cup was initially heated at 850 °C in the presence of 100 cm3/min pure N2 for 10min (sufficient
Table 1. Sample Denotations and Synthesis Parameters Varied for Each Sample calcination stirring aging conditions of time time thermal sample denotation the precursor (h) (days) profile CaO/Ca12Al14O33 CaAl-9-75-10B CaAl-8-75-10B CaAl-8-75-0.51A CaAl-8-75-15A CaAl-8-75-0.55A CaAl-8-65-10B CaAl-8-85-10B
900 850 850 850 850 850 850
°C, °C, °C, °C, °C, °C, °C,
2 1 1 1 1 1 1
h h h h h h h
1 1 0.5 1 0.5 1 1
0 0 1 5 5 0 0
B B A A A B B
75:25 75:25 75:25 75:25 75:25 65:35 85:15
time to complete calcination). The temperature was then decreased at 690 °C and the valve was switched to 100 cm3/ min CO2 flow (15%). The low partial pressure of CO2 was selected to simulate the real composition of the reformer exit where the CO2 concentration does not surpass 15%. The sorption duration was 30 min. Desorption at 850 °C for 5 min under 100 cm3/min pure N2 flow was followed. Multiple cycles, 45, of sorption and desorption were repeated to test the ability of the sorbents to retain their CO2 sorption capacity. Sorbent Characterization. The crystalline structure of the sorbents was characterized by X-ray diffraction (XRD) on a Siemens D500 diffractometer using Cu Ka radiation (λ ) 1.5406 Å). Specific surface area and pore size distribution were measured on a Quantachrome Autosorb automated gas sorption system. The measurements were performed using nitrogen physisorption and desorption isotherms at -196 °C. The sorbents were degassed at 250 °C overnight in the degassing port before the actual measurements. BJH method was applied for the determination of pore size distribution. The actual atomic Ca and Al concentrations were measured by the inductively coupled plasma-atomic emission spectroscopy (ICP-AES) technique using a Perkin Elmer Plasma 40 instrument, equipped with CETAC6000AT and an ultrasonic nebulizer to determine elemental compositions. The morphologies of the sorbents before and after the experiments were observed using a scanning electron microscope JEOL 6300, coupled with X-ray energy dispersive spectroscopy (X-ray EDS, Oxford Link ISIS-2000) for local elemental composition determination. Results and Discussion The crystal structures of all the CaO-Ca12Al14O33 samples synthesized were similarly independent of the different synthesis variables applied. Representative diffractograms of the samples are presented in Figure 1. All samples showed characteristic peaks of calcium oxide (2θ ) 32.2, 37.35, 53.85, 64.15, 67.3) and Ca12Al14O33 (2θ ) 33.41, 41.21, 55.22, 57.52). Phases such as CaAl4O7 and CaAl12O19 or Ca3Al2O6 were not detected with the exception of the hydrated structure of Ca2Al2O5, which is present in the sample with CaO to Ca12Al14O33 nominal ratio equal to 65/35, probably due to the excess of Al. The low intensity of the main peak of Ca2Al2O5 6H2O implies its small concentration in the sample. Simple calculations, show that the ratio of the intensities of the main peak of CaO and Ca12Al14O33 increases, as expected, as the ratio of CaO over Ca12Al14O33 increases. All samples synthesized, attained an atomic Ca and Al concentration (as it was measured in ICP) very close to the nominal ones as it can be seen in Table 2. The sorption capacity of the final materials was tested in 45 repetitive cycles of sorption-desorption as it is illustrated in Figure 2 for the CaO-Ca12Al14O33 derived from calcium acetate calcined at 850 °C for 1 h and synthesized with water addition in between the two calcination steps of the synthesis procedure.
Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9539
Figure 3. Effect of different calcination conditions of calcium acetate (CaO precursor) on sorption capacity and stability of CaO-Ca12Al14O33 in the presence of 15% CO2 (sorption, 690 °C, 30 min; desorption, 850 °C, 5min).
Figure 1. XRD patterns of the synthesized CaO-Ca12Al14O33 samples with different CaO/Ca12Al14O33 ratios. Table 2. Atomic Ca and Al Concentrations (Nominal and ICP Results) nominal
ICP results
synthesized sorbent
Ca
Al
Ca
Al
CaAl-9-75-10B CaAl-8-75-10B CaAl-8-75-0.51A CaAl-8-75-15A CaAl-8-75-0.55A CaAl-8-65-10B CaAl-8-85-10B
62.15 62.15 62.15 62.15 62.15 58.50 65.90
6.80 6.80 6.80 6.80 6.80 9.50 4.00
59.3 62.00 57.78 58.3 61.60 58.50 65.00
6.45 6.40 7.00 7.14 7.20 9.50 3.80
The cyclic absorption capacity increases with cycle numbers for the first three cycles in agreement with Li et al.23 who also studied the CO2 sorption performance of CaO-Ca12Al14O33. The slight loss in capacity after the fourth cyclescompared to other CaO sorbents which show more drastic decrease of their fixing ability as reviewed by Abanades et al.21sis attributed to the uniform dispersion of Ca12Al14O33 among CaO particles which provides a stable framework inhibiting sintering of active CaO sites.23,24 Effect of the Calcination Conditions of the Precursor. Calcium acetate was used as the starting material for the preparation of the CaO because of its superior sorption capacity
Figure 2. Multiple cycles of carbonation-calcination of CaAl-8-75-10B at 690 °C and 15% CO2 for 30 min (sorption) and at 850 °C and 100% N2 for 10 min (desorption).
compared to other CaO precursors. This superiority may be attributed to the higher surface area20 and/or to a simpler tortuosity in its pore system which enhances the easy CO2 access to the active sites.24 Calcium acetate was calcined at different calcination conditions (900 °C for 2 h, 850 °C for 1 h and 750 °C for 0.5 h) to study the influence on the sorption capacity and stability of the final CO2 sorbent, CaO-Ca12Al14O33. Sorption-desorption experiments showed that differences of the calcination conditions of the precursor in the range of 850-900 °C do not induce significant variations in sorption capacity and stability. Both CO2 sorbents synthesized showed 35% weight increase in the first sorption cycle which remained almost constant during the 45 cycles of sorption-desorption (Figure 3). The sorption capacity was equilibrated at 30% during the last cycles. These results imply that the differences in the calcination conditions of the CaO precursor are not adequate to affect drastically the sorption fixation and stability results of the TGA experiments. Even though milder calcination conditions (750 °C, 0.5 h) could lead in higher surface areas and consequently in higher sorption capacities, it was not possible to succeed full decomposition of the required amount of calcium acetate for the synthesis of the final material. The calcined calcium acetate at 750 °C for 30 min showed nonuniform color (gray coloring) implying that the temperature and the duration of calcination were not quite high for the complete decomposition of the organic carbon. Effect of Water Addition in between the Two Calcination Steps. For the synthesis of CaO-Ca12Al14O33, CaO (derived from calcium acetate calcined at 850 °C, 1 h) and Al(NO3)2 · 9H2O were mixed in distilled water. The suspension was stirred at 75 °C for 0.5 or 1 h and dried at 120 °C overnight. Two groups of materials were synthesized and calcined using two different thermal profiles as shown in Figure 4. A group of materials (CaAl-8-75-0.51A, CaAl-8-75-15A, CaAl-8-75-0.55A) was heated with 10 °C/min at 500 °C for 3 h and then was heated at 900 °C for 1.5 h (thermal profile A). Another group (CaAl-9-75-10B, CaAl-8-75-10B, CaAl-8-65-10B, CaAl-8-8510B) was heated with 10 °C/min at 500 °C for 3 h, and after cooling at room temperature a small amount of water was added, and the obtained paste was dried at 120 °C overnight. Further calcination at 900 °C for 1.5 h followed (thermal profile B). The addition of water, shown in Figure 4, refers only to the group which follows the thermal profile B. The sorbing performance of the two different groups is depicted in Figure 5. It is clear that independent of the other synthesis variables, all the samples synthesized with intermediate water addition show higher sorption capacity than the samples synthesized without water addition.
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Figure 4. Thermal profiles of the two-synthesized group.
Figure 5. Effect of the addition of water in between the two calcination steps (500 and 900 °C) on sorption ability of CaO-Ca12Al14O33 in presence of 15% CO2 (sorption, 690 °C, 30 min; desorption, 850 °C, 5min).
Scanning electron microscopy (SEM) observations of the samples offer explanation for the importance of the two different calcination steps and the addition of water in between the two steps. It becomes evident from Figure 6a,b that the porosity of the fresh samples synthesized in one calcination step, without water addition, (a) is drastically reduced compared to the fresh samples synthesized in two calcination steps, with addition of water in between (b). The plethora of large voids instead of the existence of a uniform porous network and the larger particle size are responsible for the surface area reduction. These observations are in agreement with the pore volume measurements (Table 3). Indeed the materials CaAl-8-75-0.51A, CaAl8-75-15A, CaAl-8-75-0.55A, which have been prepared without water addition in between the two calcination steps show much less pore volume (0.008-0.026 cm3/gr) than the other samples (0.069-0.1 cm3/gr). To explain why the synthesis procedure induces this difference in the textural properties of the two groups of the sorbent materials, a more thorough study took place. The morphology of the intermediate samples after the calcination at 500 °C was examined with SEM. Figures 7 and 8 illustrate, respectively, the morphology of the samples after calcination at 500 °C and after calcination at 500 °C followed by the addition of water and drying. As seen in Figure 7, calcination at 500 °C induces the formation of large aggregates of a mixture of calcium oxide and aluminum oxide. According to EDS analysis the sintered crystals are almost pure CaO (98%) while the spherical crystals contain almost 30% of Al2O3. The formation of large particles during the first calcination step is the reason that the mixed CaO-Ca12Al14O33, which is synthesized, when following thermal profile A, acquires a surface with large voids instead of a porous network (Figure 6a). To avoid the undesired phenomenon, thermal profile B is suggested. Water added after
Figure 6. Fresh CaO-Ca12Al14O33 synthesized without addition of water in between two calcination steps (a) and with addition of water (b). Table 3. Physical Characteristics for the Synthesized Samples synthesized sorbent
surface area (m2/g)
pore volume (cm3/g)
CaAl-9-75-10B CaAl-8-75-10B CaAl-8-75-0.51A CaAl-8-75-15A CaAl-8-75-0.55A CaAl-8-65-10B CaAl-8-85-10B
10.56 9.64 3.18 3.49 1.36 8.80 11.68
0.100 0.086 0.021 0.026 0.008 0.069 0.098
calcination at 500 °C leads to the hydration of CaO to Ca(OH)2. Because of the volume increase from CaO to Ca(OH)2 and the expansion caused by the exothermic hydration of CaO (+67 kJ/mol), the aggregates crack and swell and the regular hexagonal crystalloids of Ca(OH)2 are generated25,26 (Figure 8). EDS analysis of the sample before the final calcination step (not shown) revealed that the addition of water after the calcination at 500 °C ensures uniform distribution of Ca/Al equal
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Figure 9. Cyclic sorption capacity of CaO-Ca12Al14O33 with different CaO/ Ca12Al14O33 ratios in the presence of 15% CO2 (sorption: 690 °C, 30 min, desorption: 850 °C, 5min).
Figure 7. Intermediate sample after calcination at 500 °C for 3 h.
Figure 8. Intermediate sample after calcination at 500 °C for 3 h, addition of water and drying overnight.
to 91.7/8.3 (atomic) very close to the theoretical value of 90.6/ 9.4 (refers to the sample CaO/Ca12Al14O33: 75/25). Calcination at 900 °C (where the reaction of CaO and Al2O3 takes place) alters this hexagonal plate structure in a porous network (Figure
6b) which enhances the CO2 sorption ability of the synthesized CaO-Ca12Al14O33. Effect of the Stirring and Aging Time. Despite the low sorption capacities of the materials synthesized following the thermal profile A, valuable remarks are drawn for the effect of the stirring and aging time on the sorption ability of CaOCa12Al14O33. Based on the “Ostwald ripening” rule, when solid structure units are dispersed in their “mother” liquid, there is a tendency for the smaller to dissolve and the solute to be deposited later on the larger units enforcing the growing of the size of the crystals. Several research groups showed that increased aging time leads to larger crystal size on the basis of the above-mentioned rule.27,28 Our group also confirmed the rule by varying the aging time between 0-5 days and subsequent crystal size calculations based on XRD patterns using the Debye-Sherrer equation and considering the basal reflections (111), (200), and (220). The results are summarized in Table 4. The sample with the lower stirring time (0.5 h) and higher aging time (5 days) has the bigger crystal size of the active CaO (58.7 nm), thus the lower surface area and consequently the lower sorption capacity. Therefore, it is clearly demonstrated that the number of CO2 molecules absorbed increase with increasing stirring time and decreasing aging time as a result of higher surface area of the as-synthesized samples. Effect of the CaO/Ca12Al14O33 Ratio. The CaO-Ca12Al14O33 mixed oxide derived from calcium acetate has been proven as an effective CO2 sorbent with adequate cyclic stability.24 However, to try to further improve either the absorption capacity or the stability in repetitive sorption-desorption cycles, various sorbents were synthesized with different CaO/Ca12Al14O33 weight ratios (65:35, 75:25, 85:15). With an increase in the percentage of the inert Ca12Al14O33,we expect two opposite effects: a decrease in CO2 sorption ability due to the decreased content of the active CaO and a more stable performance on repetitive sorption-desorption cycles due to the higher content of Ca12Al14O33 which provides a stable framework inhibiting deactivation of CaO.23,24 The experimental results in CO2 sorption experiments are shown in Figure 9 and Table 5. It is clear that the sorbent with the higher Ca12Al14O33 (CaAl-8-6510B) content has remarkable stability but very low sorption
Table 4. Effect of Stirring and Aging Time on the Crystal Size and Surface Area of the Samples effect of stirring time
effect of aging time
sample
CaAl-8-75-0.55A
CaAl-8-75-15A
CaAl-8-75-0.51A
CaAl-8-75-0.55A
stirring time (h) aging time (days) CaO average crystal size (by XRD) (nm) surface area (m2/g)
0.5 5 58.70 1.36
1 5 37.20 3.49
0.5 1 42.00 3.18
0.5 5 58.70 1.36
9542 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 Table 5. Effect of CaO Weight Content on the Sorption Capacity and Stability of the Samples CaO/Ca12Al14O33 max capacity (% wt increase) min capacity (% wt increase) % declination from max to min capacity % wt increase (max capacity based on free CaO)
85/15 75/25 65/35 45.41 35.74 21.29 51.12
35.78 30.03 16.07 47.71
24.86 22.01 11.46 38.25
Figure 10. Dependence of CO2-fixation ability on the surface area (SA) of the samples.
capacity (22%) as expected. On the other hand the sorbent with the lower binder (CaAl-8-85-10B) content showed the prospective higher capacity (45.41%) in the first cycle, and even after 45 cycles the weight increase (35.5%) of the sorbent was higher than the weight increase of the first cycle (34.5%) of the material with the 75:25 ratio. Moreover, the trend of the curve of the 85:15 sample after the 20th cycle shows almost the same slope as the one of the material with the 75:25 ratio, anticipating that the sorption ability of the material will remain at high levels retaining its advancement even after more than 45 cycles. This will be confirmed with further study on the samples performing more than 45 cycles of sorption-desorption. The sorption capacity of the material with the higher CaO content (85:15) is also superior when compared on the basis of free CaO in the sorbent (Table 5). The reason might be that the CO2 molecules find easier access to the active CaO sites through the inactive Ca12Al14O33. A large amount of inactive CaAl in the materials makes the gaseous molecules inaccessible to the active CaO phase. Sorption Capacity Dependence on Surface Area. Previous study of our group showed that the sorption capacity of CO2sorbent materials is affected not only by the surface area and the pore volume but also by the tortuosity of the pore network which may differ for different CaO precursors.24 In this study, where only one CaO precursor (calcium acetate) was examined the sorption capacity depends only on the surface area of the material. The sorption capacity of the first cycle of all the materials synthesized as a function of their surface area is illustrated in Figure 10. The value of the regression factor R2 (0.94) confirms the linear affinity between CO2 sorption capacity and surface area of the sorbent material. It is clear that all the materials synthesized without intermediate water addition (profile A) have lower surface area and thus lower sorption capacity than the materials synthesized with water addition (thermal profile B). Moreover, decreasing aging time and increasing CaO content lead to a material with higher surface area and sorption capacity. Conclusions In this work a parametric study of the synthesis procedure of CaO-Ca12Al14O33 was performed to elucidate the effect of
synthesis parameters such as calcination conditions of the precursor, stirring time, aging time, synthesis steps, and CaO/ Ca12Al14O33 ratio on the CO2 sorption capacity and stability. Sorption-desorption experiments showed that differences of the calcination conditions of the precursor in the range of 850-900 °C do not induce significant variations in sorption capacity and stability. Both CO2 sorbents synthesized showed 35% weight increase in the first sorption cycle which remained almost constant during the 45 cycles of sorption-desorption. On the other hand, the gray coloring of the calcined calcium acetate at 750 °C and 0.5 h implied that the temperature and the duration of calcination were not quite high for the complete decomposition of the organic carbon. Independent of the other synthesis variables, all the samples synthesized with water addition in between calcination at 500 °C and calcination at 900 °C show higher sorption capacity than the samples synthesized without water addition. SEM/EDS observations of the samples showed that the addition of water leads to the cracking of the aggregates formed after calcination at 500 °C and to the generation of a hexagonal plate structure with uniform Ca/Al distribution. This structure is transformed in a porous network which enhances the CO2 sorption ability of the material. The number of CO2 molecules retained by the synthesized sorbent increases with increasing stirring time and decreasing aging time as a result of smaller crystal size (as calculated from XRD patterns) and thus higher surface area. The sample with CaO to Ca12Al14O33 ratio equal to 85:15 showed superior sorption capacity, compared to samples with ratios 75:25 and 65:35, and retained its superiority even after more than 45 cycles. Moreover, it is important to note that the sorption capacity of the 85:15 sample is higher even when this capacity is calculated on the basis of active CaO in the material. The reason might lie in the fact that Ca12Al14O33 obstructs the pathways to the active CaO sites. The smaller is the amount of inactive Ca12Al14O33 in the crystal network of the sorbent, the easier is the access of the CO2 molecules toward CaO. Acknowledgment The authors are grateful to Ms. Sophia-Irini Chatoutsidou for her help in the synthesis of the materials, Mrs. Olga Orfanou and Thalia Vavaleskou from the Analytical Services Unit CPERI/CERTH for their help in sorbents characterization, and Dr. Lori Nalbandian and Dr. Eleni Heracleous for the fruitful discussions. Literature Cited (1) Aresta, M. Carbon Dioxide RecoVery and Utilization; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2003. (2) 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. (3) Florin, H. N.; Harris, A. T. Hydrogen Production from Biomass Coupled with Carbon Dioxide Capture: The Implications of Thermodynamic Equilibrium. Int. J. Hydrogen Energy 2007, 32, 4119. (4) Yi, K. B.; Harrison, D. P. Low Pressure Sorption-Enhanced Hydrogen Production. Ind. Eng. Chem. Res. 2005, 44, 1665. (5) 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. (6) Xiu, G.; Li, P.; Rodrigues, A. E. Sorption-Enhanced Reaction Process with Reactive Regeneration. Chem. Eng. Sci. 2002, 57, 3893. (7) Johnsen, K.; Ryu, H. J.; Grace, J. R.; Lim, C. J. Sorption-Enhanced Steam Reforming of Methane in a Fluidized Bed Reactor with Dolomite as CO2-Acceptor. Chem. Eng. Sci. 2006, 61, 1195.
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ReceiVed for reView June 4, 2008 ReVised manuscript receiVed September 29, 2008 Accepted September 30, 2008 IE800882D
