Ind. Eng. Chem. Res. 2006, 45, 1911-1917
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Effect of Preparation Temperature on Cyclic CO2 Capture and Multiple Carbonation-Calcination Cycles for a New Ca-Based CO2 Sorbent Zhen-shan Li, Ning-sheng Cai,* and Yu-yu Huang Key Laboratory for Thermal Science and Power Engineering of the Ministry of Education, Department of Thermal Engineering, Tsinghua UniVersity, Beijing 100084, China
The cyclic CO2 capture, transient phases change, and microstructure appearance of a new kind of Ca-based regenerable CO2 sorbent, CaO/Ca12Al14O33, obtained by the integration of CaO as solid reactant with a composite metal oxide of Ca12Al14O33 as a binder, were investigated by thermogravimetric analysis, XRD, and SEM at different preparation calcination temperatures. When the calcination temperature in the preparation stage is higher than 1000 °C, the cyclic CO2 capture of this new sorbent declines. The lowered CO2 capture may mainly be attributed to the formation of Ca3Al2O6, which decreases the ratio of CaO to binder in sorbent, and the severe sintering of sorbent occurs when calcined at such high temperatures in the preparation processes. These results suggest that the calcination temperature for this new sorbent should not be higher than 1000 °C in order to obtain its high reactivity. The performance of the new sorbent over 50 cycles was evaluated under mild and severe regeneration conditions, respectively. CaO/Ca12Al14O33 attained 41 wt % CO2 capture after 50 carbonation-calcination cycles under mild calcination conditions (850 °C, 100% N2), and the results obtained here indicate that the new sorbent, CaO/Ca12Al14O33, has significantly improved CO2 capture and cyclic reaction stability over multiple carbonation-calcination cycles compared with limestone and dolomite under mild calcination conditions. When more severe calcination conditions (980 °C, 100% CO2) were used, the capture of CaO/Ca12Al14O33 decreased from 52 wt % in the first cycle to about 22 wt % in the 56th cycle; however, the capture of CaO/Ca12Al14O33 sorbent over 56 cycles is still higher than that of dolomite and limestone under the same severe calcination conditions. 1. Introduction Through addition of Ca-based CO2 sorbents into conventional methane steam re-forming reactor, the sorption-enhanced reforming process (SERP) can remove CO2 in situ as soon as it is formed and then the re-forming and water-gas shift reactions can proceed beyond the conventional thermodynamic limits and more methane will be converted. SERP combines reaction and separation, and thus can result in process simplification, improved energy efficiency, and increased reactant conversion and product yield.1-9 In the processes of SERP, the CO2 sorbents are used repeatedly, and the cyclic reaction/regeneration characteristics of these sorbents are very important for practical application. Most naturally occurring Ca-containing sorbents, such as limestone and dolomite suitable for the CO2 separation process, undergo a decay of CO2 capture during multiple carbonation-calcination reaction cycles.10-12 Harrison and coworkers found that the CO2 capture of sorbents decreased rapidly with the increasing of cyclic carbonation number.13,14 The CO2 capture of CaO showed also a rapid drop in the application of Ca-based chemical heat pump process.15 The decline of CO2 capture of CaO over multiple carbonation-calcination cycles was attributed to the changing microporosity within grains and the mesoporosity surrounding them due to sintering.16 Researchers at Toshiba Corp. founded that a lithium zirconate (Li2ZrO3) based sorbent provided 20 wt % capture over two cycles,17 and they later observed that the reactivity of lithium orthosilicate (Li4SiO4) was better than that of lithium zirconate.18-20 Iyer et al.21 obtained a micron-sized PCC-CaO by calcination of mesoporous CaCO3 that attained 36 wt % CO2 capture after 100 cycles of carbonation and calcination reactions at 700 °C. In the earlier study, a new kind of Ca-based regenerable CO2 * To whom correspondence should be addressed. Telephone: 8610-62789955. Fax: 86-10-62789955. E-mail address: cains@ tsinghua.edu.cn.
sorbent, CaO/Ca12Al14O33, was synthesized and significantly improved CO2 capture and cyclic reaction stability compared with other natural Ca-based CO2 sorbents after 13 carbonationcalcination cycles under mild calcination conditions.22 Hughes et al.23 found that the steam hydration/activation method can improve the long-term performance of the sorbent, resulting in directly measured conversions as high as 52% and estimated conversions as high as 59% after up to 20 cycles. It is obvious that improved CO2 capture with synthetic or reactivated sorbents would be highly beneficial for the SERP system. However, all sorbents will have to be tested for CO2 capture and cyclic stability at a much higher number of cycles at conditions closer to that of a real SERP system using these solids. For a SERP option of the direct production of H2 regardless of CO2 sequestration,3 the regeneration process of Ca-based sorbents can be operated under mild calcination conditions. If the SERP is operated in pursuit of as large as possible CO2 sequestration and hydrogen production,14 more severe conditions of higher concentrations of CO2 and higher calcination temperatures in every cycle during regeneration must be used in order to produce a pure CO2 stream. In this study, we have studied the effect of the calcination temperature in the preparation process on the cyclic CO2 capture of this new kind of Ca-based regenerable CO2 sorbent, CaO/ Ca12Al14O33, obtained by the integration of CaO as solid reactant with a composite metal oxide of Ca12Al14O33 as a binder during carbonation-calcinations cycles. Transient phase changes and microstructure appearances of the new sorbent at different calcination temperatures in the preparation process were given. Furthermore, the extended cyclic reactivity of the new sorbent CaO/Ca12Al14O33 was investigated. It has high reactivity and stability under the mild calcination conditions up to 50 carbonation-calcination cycles. Finally, the multicycle perfor-
10.1021/ie051211l CCC: $33.50 © 2006 American Chemical Society Published on Web 02/21/2006
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mance of the new sorbent was evaluated and compared with that of dolomite and limestone under severe regeneration conditions. 2. Experiments 2.1. Experimental System. A Dupont 951 TGA (TA Instrument 1200) was used to study the carbonation reaction of Cabased CO2 sorbents. Additional details can be found in the literature.22 The following operating conditions were used for carbonation-calcination tests: (i) carbonation temperature, 650 and 700 °C; calcination temperature, 850 and 980 °C; (ii) gas flow rate, 150 mL/min; (iii) gas compositions, CO2, 16%, 20%, 100%; N2, 84%, 80%; (iv) solid amounts, 10-15 mg. A threeway valve was used to switch between the pure nitrogen stream and the reaction gas mixture at certain carbonation and calcination time intervals. The time for carbonation and calcination is 30 and 5 min, respectively. The microstructure of samples was investigated by scanning electron microscopy (SEM, KYKY2000). The sample phase composition was determined by X-ray diffraction (XRD, D8, Advance, BRUKER). Specific surface areas of sorbent were measured by a gas physisorption and chemisorption analyzer (Micrometritics ASAP 2010). 2.2. Preparation of High Reactivity Calcium Oxide. Aluminum nitrate enneahydrate [Al(NO3)3‚9H2O] (28.4 g) and the powdered calcium oxide (26.2 g) were added into the mixture of 2-propanol (130 mL) and distilled water (760 mL) so that the weight ratio of calcium oxide to newly formed materials (Ca12Al14O33) would become 75:25 wt %. The weight ratio of CaO to Ca12Al14O33 can be adjusted by the relative amounts of CaO and Al2O3. If the cyclic stability or degradation rate is the same for two sorbents, the required quantity of sorbent with high CaO content to capture a certain amount of CO2 is smaller than that of sorbent with low CaO content; therefore, CaO/Ca12Al14O33 ) 75/25 (wt %) was chosen in this study. This solution was stirred for 1 h at 75 °C and was dried at 120 °C for another 18 h before being calcined at 500 °C for 3 h in air. By this method, 2-propanol, water, and nitric acid in the solution can be evaporated off in different stages, thereby offering the production of a fine and porous powder. Spherical particles were made from the paste by adding water to the produced powder. These particles were dried at 120 °C for 2 h and calcined in air at 800 °C (CaO-CA-0), 900 °C (CaOCA-1), 1000 °C (CaO-CA-2), 1100 °C (CaO-CA-3), 1200 °C (CaO-CA-4), and 1300 °C (CaO-CA-5) for 1.5 h, respectively. The mixed powder was ground and sieved to 1040 µm. 3. Results and Discussion 3.1. Effect of Preparation Temperature on Cyclic CO2 Capture. To understand the reaction mechanism during the high-temperature synthesis of this new sorbent made from a CaO-Al(NO3)3 mixture, samples treated from calcination temperatures ranging from 800 to 1300 °C were also studied by X-ray diffraction, as shown in Figure 1. In some of these samples, the transient phases, Ca12Al14O33, which is also named mayenite, were present. A small amount of Ca12Al14O33 will be formed as the intermediate phases when Ca3Al2O6, one of the minor (5-10%) components of portland cement, is to be synthesized as a pure phase. There are many publications on the synthesis of Ca3Al2O6. Williamson and Glasser24 reported Ca12Al14O33 as the principal nonequilibrium phase, and Singh et al.25 also mentioned the presence of several aluminate phases such as CaAl4O7, CaAl2O4, and Ca12Al14O33. Mohamed and
Figure 1. X-ray diffraction patterns obtained after the calcination under the temperatures shown at the right side. C stands for CaO, C12A7 for Ca12Al14O33, and C3A for Ca3Al2O6.
Figure 2. Cyclic CO2 capture at different calcination temperatures in the preparation stage. (The carbonation-calcination temperatures were 700 °C/ 850 °C for CaO-CA-1 and 650 °C/850 °C for other four sorbents.)
Sharp26 observed the formation of Ca12Al14O33 and CaAl2O4 as transient phases, and the experimental results of Mercury et al.27 also indicated that Ca12Al14O33 is one of the intermediate phases. When calcination temperatures in the preparation stage are in the range of 800-1000 °C, the intermediate phase is Ca12Al14O33. Beyond 1000 °C, Ca12Al14O33 begins to disappear and Ca3Al2O6 begins to form. The samples treated at 1200 and 1300 °C do not show any trace of other crystal phases apart from Ca3Al2O6 itself. However, phases such as CaAl4O7 and CaAl12O19 have not been detected during the reaction process in any of the studied samples. Figure 2 shows the effect of calcination temperature in the preparation stage on cyclic CO2 capture. CO2 (20%) was mixed
Ind. Eng. Chem. Res., Vol. 45, No. 6, 2006 1913 Table 1. Specific Surface Area of Three Sorbents sorbent name
specific surface area (m2/g)
CaO-CA-2 CaO-CA-3 CaO-CA-5
18.53 14.62 9.75
into the gas mixture for the carbonation of sorbent CaO-CA1, and the carbonation of other four sorbents proceeded with 16% CO2. Because of the introduction of 20% CO2 into the calcination process at the 11th and 17th cycle for CaO-CA-1, the subsequent cyclic CO2 capacities for the 12th and 18th cycle began to decline. CaO-CA-2, which was calcined at 1000 °C in the preparation process, shows the same high CO2 capture and cyclic stability as that of CaO-CA-1. However, upon increasing the calcination temperature in the preparation stage, the CO2 capture declines. When calcined at 1100 °C in the preparation stage, the CO2 capture of CaO-CA-3 declines to about 33 wt %(g of CO2/g of sorbent %) after 15 cycles, and when the calcination temperature increases to 1200 and 1300 °C, the CO2 capacities of CaO-CA-4 and CaO-CA-5 even decrease to about 20 wt % and 11 wt % after the 15th cycle, which was much lower than those of CaO-CA-1 and CaOCA-2. From the results shown in Figure 2, when the calcination temperature in the preparation stage was in the range of 8001000 °C, the transient phases Ca12Al14O33 were formed and detected. We assumed that all Al2O3 coming from Al(NO3)3‚ 9H2O reacted completely with CaO to form Ca12Al14O33. So the mass ratio of CaO to Ca12Al14O33 in the new sorbents, 75/ 25 wt %, can be calculated and adjusted according to the original mass of CaO and Al(NO3)3‚9H2O. In the same way, if the newly formed product is only Ca3Al2O6, the mass ratio of CaO to Ca3Al2O6 in the new sorbent will be 66/34 wt %. The theoretical maximum CO2 capacities for CaO/Ca12Al14O33 (75/25 wt %) and CaO/Ca3Al2O6 (66/34 wt %) sorbents are 0.786(0.75) ) 0.589 g of CO2/g of sorbent and 0.786(0.66) ) 0.519 g of CO2/g of sorbent at 100% CaO conversion. For 45 wt % CO2 capture, CaO conversion for CaO/Ca12Al14O33 (75/25 wt %) sorbent is 76.4%. Both mayenite and Ca3Al2O6 are included in the XRD result of CaO-CA-3 (1100 °C), and the relative amounts of Ca12Al14O33 and Ca3Al2O6 cannot be specified by XRD analysis. Therefore, CaO conversion for CaO-CA-3 (1100 °C) sorbent, which achieved 33 wt % CO2 capture, will be in the range of 55.1-63.6%. However, the CO2 capture abilities for CaO-CA-4 and CaO-CA-5 are about 20% and 11%, only 38.5% and 21.2% CaO conversion being achieved. From Table 1, it can be found that the sintering process would take place at temperatures higher than 1100 °C, resulting in the decline of sorbent specific surface area with the increase of calcination temperature in the preparation stage. Compared with CaO-CA-1 or CaO-CA-2, the decline of CO2 capture for CaO-CA-3, CaO-CA-4, and CaOCA-5 may mainly be attributed to two factors. One is the formation of Ca3Al2O6, which decrease the ratio of CaO to binder in sorbents, and the other is the severe sintering of sorbents when higher calcination temperatures are used in the preparation processes. When the Ca-based CO2 sorbent is calcined at higher temperature, CaO sintering results in the decrease of specific surface area and porosity, which are very important for the reaction of CaO with CO2.28,29 Through the observations with SEM, Abanades et al.16 found that the pore distribution of the calcines was continuously changing with cyclic numbers and they attributed the decay of CaO conversion to a certain loss of small pores and an increase of large pores. The microstructure
evolution with temperature and cyclic numbers was studied by scanning electron microscopy for the samples treated at 1000, 1100, and 1300 °C during 1.5-h calcination in the preparation stage. From the SEM image, the phenomena of particle growth and pore space decline have not been found when comparing the microgranular appearance of particles after the original 15th reaction (Figure 3b,d) and the microgranular appearance of original particles (Figure 3a,c). The reason for the cyclic stability of CaO/Ca12Al14O33 may be that, via introducing binder and the CaO hydration procedures during its preparation stage, the particles become ultrafine, and in the preparation calcination stage, the Al(NO3)3‚9H2O and CaO form Ca12Al14O33, a more stable material that is distributed uniformly among CaO crystallites and does not take part in reaction.22 Hence, the sintering of CaO particles in the calcination stage is effectively prevented and this results in the high CaO cyclic stability. However, the sample treated at 1300 °C shows significant sintering and considerable increment in grain size (Figure 3e,f). The severe sintering of sorbents in the preparation processes leads to the sharp decline of specific surface area (Table 1) and CO2 capture (Figure 2). 3.2. Mechanism of Preparation and Reaction Process of High-Reactivity Ca-Based CO2 Sorbent. The reaction pathway and microstructure change mechanism for the high-reactivity Ca-based CO2 sorbent is similar to that of Ca3Al2O6 synthesis process, and therefore, the soft mechanochemical synthesis mechanism of Ca3Al2O626,27 is cited to understand the mechanism of preparation and reaction process of high reactivity Cabased CO2 sorbent. The reaction between CaO and Al2O3 starts at about 800 °C. Al2O3 reacts with CaO through an exothermic reaction, forming a transient phase, Ca12Al14O33 (see Figure 4). The theoretical volume increment due to Ca12Al14O33 formation is 36.20%. Thermodynamic data indicate that reaction 1 has the lowest Gibbs free energy in the CaO-Al2O3 system and is therefore favored at lower temperatures.27 Another transient phase, CaAl2O4, can be produced at 1100 °C by two reactions. Thermodynamic calculations indicate that reaction 3 has lower Gibbs free energy than reaction 2 and is therefore favored at lower temperatures.27 When Al2O3 is limited, CaAl2O4 cannot be produced, for all the Al2O3 has already reacted with CaO to form Ca12Al14O33. The formation of Ca3Al2O6 takes place at 1100 °C. The following reactions are given to interpret the observation:
7Al2O3+ 12CaO f Ca12Al14O33
(1)
5Al2O3 + Ca12Al14O33 f 12CaAl2O4
(2)
Al2O3 + CaO f CaAl2O4
(3)
7CaAl2O4 + 5CaO f Ca12Al14O33
(4)
Ca12Al14O33 + 9CaO f 7Ca3Al2O6
(5)
Figures 4 and 5 show schemes of the reaction mechanism. When sample is calcined at higher temperature (1100 °C) Ca2+ from the aggregates would diffuse through mayenite layers to react and produce a homogeneous and porous phase of Ca3Al2O6 as the final product,27 which decreases the ratio of CaO to binder in sorbent particle. The sintering process would take place at temperatures higher than 1200 °C among grains of pure Ca3Al2O6. Concerning the microstructure and specific surface area, the material for 1.5-h calcination at 1300 °C exhibits a severe sintering, and a part of CaO grain is sintered into the interior of the Ca3Al2O6 particle, as shown in Figure 5. Because
Figure 5. Microstructure change mechanism sketch of the new Ca-based CO2 sorbent (Figure 5 is modified on the basis of Mercury et al.27).
of the formation of Ca3Al2O6 and the severe sintering of sorbents at higher calcination temperatures in the preparation processes, the CO2 capacities of sorbents begin to decline. The mechanism for the porosity development would be related to the release of H2O and NO2 during the dehydration of Ca(OH)2 and the decomposition of Al(NO3)3. The evolved gases would yield a
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Figure 6. Extended calcination-carbonation cycles with CaO/Ca12Al14O33 (CaO-CA-1) at 700 °C in TGA in a 20% CO2 stream at the mild calcination temperature of 850 °C, 100% N2. (A, B, C, D denote the 11th, 17th, 35th, 44th-50th cycles, in which 20% CO2 was introduced into calcination gas atmosphere).
homogeneous open pore structure with a narrow pore size distribution (Figure 3c). Finally, it can be concluded that the experimental evidence indicates that the optimal calcination temperature in the preparation process of mechanochemically treated CaO-Al(NO3)3 mixtures should not be higher than 1000 °C in order to obtain high reactivity sorbent. 3.3. Multiple Carbonation-Calcination Cycles under Mild Calcination Conditions. Regeneration temperature for CaCO3 decomposition in SERP system is critical and governed by the thermodynamics of the regeneration system. A temperature range from 800 °C without CO2 stream in calcination atmosphere at 101 325 Pa to 900 °C with 100% CO2 stream in calcination atmosphere at 101 325 Pa is required. When the heat for sorbent regeneration is provided by the combustion of the fuel-air mixture inside the regenerator, such as unmixed reforming for small-scale generation of hydrogen,3 the regeneration process of Ca-based sorbents can operate under mild calcination conditions. We carried out extended cycle testing of CaO/Ca12Al14O33 sorbent at 850 °C under mild calcination conditions. Figure 6 depicts the CO2 capture of the CaO/Ca12Al14O33, quantified as wt % CO2 captured by the calcined sorbent. An earlier study has shown that CaO/Ca12Al14O33 achieves high CO2 capture and cyclic stability toward carbonation as compared with other naturally occurring calcium sources.22 Multicycle testing on CaO/Ca12Al14O33, carried out in 14% CO2 for 30 min, did not show a significant drop in reactivity for 13 cycles. However, prior literature indicates a loss in reactivity over a higher number of carbonation-calcination reaction cycles. Extended cycle testing of this new sorbent at 700 °C was carried out. Figure 6 shows the results obtained over 50 cycles on CaO/ Ca12Al14O33 sorbent. The carbonation was carried out in a 20% CO2 stream at 700 °C, while calcination was carried out at 850 °C and in pure N2 except for the 11th, 17th, 35th, and 44th50th cycles, in which 20% CO2 was introduced into the calcination gas atmosphere. Each of the carbonation reaction and calcination stage was performed for 30 and 5 min, respectively. From Figure 6, the capture of CaO/Ca12Al14O33 is about 50 wt % in the first cycle, which drops to about 41 wt % by the 50th cycle, and it is evident that the adsorption capture of CaO/Ca12Al14O33 sorbent over 50 cycles is high. The cyclic CO2 capture attained by CaO/Ca12Al14O33 (CaO-
CA-1) and some other high temperature Ca-based sorbents reported in the literature for multiple cycles is depicted graphically in Figure 7. The experimental conditions in calcinations-carbonation cycles, such as carbonation and calcination temperatures, reaction and calcination times, and the CO2 concentration in the gas mixture during the reaction and calcination stages, have important effects on the cyclic CO2 capture. All the other sorbents except for dolomite, richer in CaO with respect to their materials, behave better in the first cycle; however, dolomite is an exception because of its low CaO content. The long-term performance of the sorbent is more important. CaO/Ca12Al14O33 (CaO-CA-1) attains a 50 wt % CO2 capture in 30 min at the end of the second cycle. Because of its high cyclic stability, CaO/Ca12Al14O33 (CaO-CA-1) attains 41 wt % CO2 capture for the 50th cycle; in contrast, PCC-CaO21 also attained a 41 wt % increase at the end of the 50th cycle, which is almost the same as that of CaO/Ca12Al14O33 (CaO-CA-1). However, the 850 °C calcination temperature and 20% CO2 in some calcination cycles for CaO/Ca12Al14O33 are more severe than that of PCC-CaO (700 °C and 100% N2 in calcination process). Although 10-nm CaO particles obtained repeated 93% conversion over 30 cycles with a carbonation time of 24 h under 100% CO2 at 577 °C, the bulk density of this high surface area CaO was very low (∼0.1 g/mL) and it would need a large reactor, which is difficult to operate continuously with these 10-nm CaO particles. The particles had been compressed in the work of Barker13 to increase the density, and the surface area of the pressed sample fell from 38.4 to 0.5 m2/g while the capture decreased from 77.5 to 37.6 wt % after the carbonate through 18 decomposition-back-reaction cycles. Because of product layer formation, pore plug, pore mouth closure, and sintering, some natural sorbents suffer from a decline of CO2 capture, as shown in Figure 7. The high reactivity of PCC-CaO was attributed to the predominant mesoporous structure, which allows the reactant gases to access the entire surface of the particle through the larger pores. While the mechanism of high reactivity CaO/Ca12Al14O33 was different from that of PCC-CaO, it may be due to the binder formed among the CaO ultrafine particles retarding the sintering of CaO particles. However, when 20% CO2 was introduced into the calcination gas atmosphere, the adsorption capture of CaO/Ca12Al14O33 declines a little. This fact indicates that CO2 concentra-
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Figure 7. CO2 capture of various high-temperature Ca-based sorbents over multiple carbonation-calcination cycles under mild calcination conditions (A, B, C, D denote the 11th, 17th, 35th, 44th-50th cycles, in which 20% CO2 was introduced into calcination gas atmosphere. PCC and LC denote precipitated calcium carbonate and Linwood carbonate, respectively).
Figure 8. CO2 capture of Ca-based sorbents over multiple carbonationcalcination cycles under severe calcination conditions.
tion in the calcination stage plays a significant role in determining the capture for the same sorbent. 3.4. Multiple Carbonation-Calcination Cycles under Severe Calcination Conditions. If the processes of methane steam re-forming with Ca-based sorbent addition are operated in pursuit of as large CO2 sequestration as possible and large production of hydrogen, the regeneration of Ca-based CO2 sorbent happens in pure CO2 stream and at higher calcination temperature. Hence, the performance of the Ca-based sorbent needs to be evaluated at higher concentrations of CO2 and higher calcination temperatures in every cycle during regeneration. Extended cycle testing of CaO-CA-2 sorbent took place at 650 °C/100% CO2 carbonation condition and 980 °C/100% CO2 calcination condition, and every cycle of the carbonation reaction and calcination stage for CaO-CA-2 was performed for 30 and 5 min, respectively. The cyclic CO2 capacities attained by CaOCA-2, dolomite, and limestone under severe calcination conditions are depicted graphically in Figure 8. The limestone was allowed to react for 10 min for calcination at 950 °C, 100% CO2 and for 30 min for carbonation at 650 °C, 100% CO2.16
Dolomite was exposed to pure CO2 at 101 325 Pa, while the temperature was continually cycled between 800 °C for carbonation and 950 °C for calcination, and the temperature was held constant for 5 min at both the maximum and minimum temperatures and was changed at a rate of 10 °C/min between the temperature limits.14 The capture of limestone rapidly declined from 46 wt % capture in cycle 1 to 6 wt % capture in cycle 14, as shown in Figure 8. From Figure 8, the dolomite sorbent yielded a 35 wt % capture in the first cycle, which fell to 16 wt % by the 56th cycle, and the capture of CaO-CA-2 is 52 wt % in the first cycle, which drops to about 22 wt % by the 56th cycle. The degradation rate shown in Figure 8 was similar for both CaO-CA-2 and dolomite. Although dolomite also shows a similar degradation trend, it has low capture ability that is, to a large extent, due to the large MgO content. The maximum CO2 capture of dolomite on a weight basis was lower than that of the new CaO-CA-2; therefore, the capture of CaOCA-2 sorbent over 56 cycles is higher than that of dolomite and limestone, even under more severe calcination conditions. The degrading performance of CaO in cycles was mainly due to sintering. By mixing high melting point compounds, such as A12O3 or MgO to CaO, sintering may be inhibited to some extent; therefore, the performances of CaO-CA-2 and dolomite were better than that of limestone, as shown in Figure 8. Calcination temperature, which determines the mobility of CaO, and CO2 gas concentration, which can accelerate the sintering, have important adverse effect on the performance of sorbent. 4. Conclusions When calcination temperature in the preparation stage is lower than 1100 °C, the transient phase in the new kind of Ca-based regenerable CO2 sorbent is Ca12Al14O33. This novel sorbent is able to give high CaO conversion and cyclic CO2 capture and has excellent regenerability in cyclic use for carbonation reaction. When calcination temperature in the preparation stage is higher than 1100 °C, Ca12Al14O33 and Ca3Al2O6 are the main transient phases in the new sorbent, and the cyclic carbonation reactivity begins to decline. The lower CO2 capture may mainly be attributed to the formation of Ca3Al2O6, which decreases
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the ratio of CaO to binder in sorbents, and the severe sintering of sorbents when higher calcination temperatures were used in the preparation processes. The experimental results also indicate that the optimal calcination temperature in the preparation process of mechanochemically treated CaO-Al(NO3)3 mixtures is no higher than 1000 °C in order to obtain high-reactivity sorbent. The new kind of sorbent, CaO/Ca12Al14O33, attained 41 wt % CO2 adsorption capture after 50 carbonation-calcination cycles when the temperature of carbonation and calcination is 700 and 850 °C, respectively. The results obtained here indicate that the new sorbent has significantly improved CO2 capture and cyclic reaction stability over multiple carbonation-calcination cycles compared with other Ca-based CO2 sorbents under mild calcination conditions. In the case of more severe calcination conditions (980 °C, 100% CO2), the capture of CaO-CA-2 dropped from 52 wt % in the first cycle to about 22 wt % by the 56th cycle; however, the CO2 capture of CaO-CA-2 sorbent over 56 cycles is still higher than that of dolomite and limestone under the same severe calcination conditions. When a fixed bed is used as re-forming reactor in SERP, synthetic sorbents with improved capture ability and long-term performance would be highly beneficial for the SERP system in which continuous operation of reactor is more important for practical application. When a fluidized bed is used as the reactor, the decay of traditional sorbents’ activity can be compensated by a makeup flow of sorbents. The traditional sorbents cannot maintain long-term capture ability, and large quantities of sorbent makeup are required for CO2 separation. However, because of the rather low price of traditional sorbents, they perhaps can compete with synthetic or reactivated sorbents of high price, if the costs and benefits of sorbents are considered.30 Acknowledgment This work was supported by the National High Technology Development Program of China (No. 2003AA501330). Literature Cited (1) Williams, R. Hydrogen production. U.S. Patent 1,938,202, 1933. (2) Gorin, E.; Retallick, W. B. Method for the production of hydrogen. U.S. Patent 3,108,857, 1963. (3) Lyon, R. K.; Cole, J. A. Unmixed combustion: An alternative to fire. Combust. Flame 2000, 121, 249. (4) Balasubramanian, B.; Lopez, A.; Kaytakoglu, S.; Harrison, D. P. Hydrogen from methane in a single-step process. Chem. Eng. Sci. 1999, 54, 3543. (5) Kato, Y.; Ando, K.; Yoshizawa, Y. J. Study on a regenerative fuel reformer for a zero-emission vehicle system. Chem. Eng. Jpn. 2003, 36, 860-866. (6) Brun-Tsekhovoi, A. R.; Zadorin, A. N.; Katsobashvili, Y. R.; Kourdyumov, S. S. The process of catalytic steam-reforming of hydrocarbons in the presence of a carbon dioxide acceptor. In Hydrogen Energy Progress VII, Proceedings of the 7th World Hydrogen Energy Conference, Moscow, Russia, Sept 25-29, 1988; Veziroglu, T. N., Protsenko, A. N., Eds.; Pergamon Press: New York, 1988; Vol. 2, p 885.
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ReceiVed for reView November 1, 2005 ReVised manuscript receiVed January 18, 2006 Accepted January 20, 2006 IE051211L
