CO2 Carrying Behavior of Calcium Aluminate Pellets under High

Jan 26, 2010 - loss of sorbent activity. The influence of severe calcination conditions on the CO2 carrying behavior of calcium aluminate pellets is i...
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Ind. Eng. Chem. Res. 2010, 49, 6916–6922

CO2 Carrying Behavior of Calcium Aluminate Pellets under High-Temperature/ High-CO2 Concentration Calcination Conditions Vasilije Manovic and Edward J. Anthony* CanmetENERGY, Natural Resources Canada, 1 Haanel DriVe, Ottawa, Ontario, Canada K1A 1M1

Sintering and a resulting loss of activity during calcination/carbonation can introduce substantial economic penalties for a CO2 looping cycle using CaO-based sorbents. In a real system, sorbent regeneration must be done at a high temperature to produce an almost pure CO2 stream, and this will increase both sintering and loss of sorbent activity. The influence of severe calcination conditions on the CO2 carrying behavior of calcium aluminate pellets is investigated here. Up to 30 calcination/carbonation cycles were performed using a thermogravimetric analyzer apparatus. The maximum temperature during the calcination stage in pure CO2 was 950 °C, using different heating/cooling rates between two carbonation stages (700 °C, 20% CO2). For comparison, cycles were also done using N2 during the calcination stages. In addition, the original Cadomin limestone, used for pelletization, was also examined in its original form and the results obtained were compared with those for the aluminate pellets. As expected, high temperature during calcination strongly reduced CO2 carrying capacities of both sorbents. However, aluminate pellets showed better resistance to these severe conditions. The conversion profiles obtained are significantly different to those obtained under milder conditions, with significant increased activity during the slower, diffusion-controlled, carbonation stage. Moreover, scanning electron microscopy analysis of samples after prolonged carbonation showed that pore filling occurred at the sorbent particle surfaces preventing diffusion of CO2 toward the particle interior. 1. Introduction It is widely accepted that climate change is being driven by increasing concentrations of greenhouse gases due to human activity, and this key problem requires urgent solutions. Fossil fuel-fired power plants represent a major source of anthropogenic CO2. The negative environmental effects of such combustion represent a growing problem as the utilization of fossil fuels such as coal is increasing and this can be expected to continue for the near- to medium-term future.1,2 One possible approach is the capture of CO2 from flue gas followed by its sequestration in geological formations or perhaps ocean storage.3-5 The purpose of CO2 capture is to produce a concentrated stream of CO2 suitable for compression and piping to a storage site. Although, in principle, the entire gas stream containing low concentrations of CO2 could be transported and sequestered underground, the energy and other associated costs generally make this approach impractical. It is, therefore, necessary to produce a nearly pure CO2 stream suitable for sequestration. The capture/separation step for CO2 from large point sources is a critical one with respect to the technical feasibility and cost of the overall carbon sequestration scenario. CO2 separation is the first and most technically challenging and energy intensive step of the carbon capture and storage process; thus, much research has been targeted at improving current technologies or developing new approaches for CO2 separation and capture. For power plants, CO2 separation and capture processes can be divided into several scenarios: postcombustion processes for a traditional coal-fired power plant, precombustion processes for gasification or reforming, and oxy-fuel processes, sometimes referred to as oxy-firing or oxy-combustion.3-7 Important new classes of technologies for CO2 separation are based on solid looping cycles.7 These cycles employ a solid carrier to bring oxygen to the fuel, or remove CO2 from combustion or gasification gases to be released as a pure CO2 * To whom correspondence should be addressed. E-mail: [email protected]. Tel.: (613) 996-2868. Fax: (613) 992-9335.

stream, with subsequent regeneration of the O2 (O2 cycles) or CO2 carrier (CO2 cycles). Chemical-looping combustion involving O2 cycles is a combustion technology with inherent CO2 separation.6-8 In this process an oxygen carrier, typically a metal oxide, transferring the oxygen from the air to the fuel, circulates between two reactors. This means that the combustion air and the fuel are never mixed, and the flue gas from the fuel reactor consists of a concentrated CO2 stream, ready for sequestration. CO2 looping cycles, which employ a solid CaO-based carrier, may inexpensively and effectively remove CO2 from combustion (or gasification gases), allowing it to be regenerated as a pure CO2 stream suitable for sequestration.7,9 Preliminary economic analyses10-12 suggest that such processes are economically attractive, and an important advantage of using CaO is that limestone (CaCO3) is an abundant and relatively inexpensive material when used at the industrial scale. CO2 capture by CaO-based sorbents is based on the reversible chemical reaction CaO(s) + CO2(g) ) CaCO3(s)

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CO2 separation from flue gas is possible in a multicycle process in a dual reactor. This involves reaction of CaO with CO2 from flue gas in a carbonator, and regeneration of sorbent in a calciner.13 However, this relatively simple reaction process, which is in the ideal case limited only by the chemical equilibrium and kinetics determined by the temperature and CO2 partial pressure, in practice is subject to some difficulties. Namely, flue gas from fossil fuel combustion typically contains SO2, which under CO2 looping cycle conditions, irreversibly reacts with CaO forming CaSO4. A portion of the CaO sorbent is, therefore, lost as CaSO4, but more importantly, the CaO reaction surface is also covered by this product, preventing contact of CaO and CO2 with a resulting rapid decrease of capture capacity.14-16 Attrition of sorbent is also a potentially significant problem for fluidized-bed combustion (FBC) systems,17 leading to significant sorbent elutriation from the reactor,

10.1021/ie901795e  2010 American Chemical Society Published on Web 01/26/2010

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and this has been confirmed in our first pilot-scale demonstration of CO2 looping cycles using a dual FBC reactor.18 However, the most thoroughly investigated problem is the loss of activity of sorbent with increasing numbers of reaction cycles,19-21 which is typically explained by sorbent sintering. Sorbent activity decay strongly influences the efficiency and economics of the overall process10-12 and it is imperative that this problem be resolved or alleviated.22 Promising methods published in the literature include steam hydration,23,24 thermal pretreatment of the sorbent,25,26 sorbent doping by Al2O3,27,28 and hydration/doping/pelletization.29,30 The studies on sorbent activity (decay) presented in the literature are more often carried out using a thermogravimetric analyzer (TGA) apparatus14,16,19,21,23,25-30 and laboratory- or pilot-scale FBC reactors.15,18,20,24,28 The conditions employed during experiments are often isothermal,25,26,28-30 with N2, air, Ar, or diluted CO2 used during the calcination stage to enable sorbentregenerationtooccuratlowertemperatures.14-16,18-21,23-31 Thus, it can be concluded that most studies on CO2 looping cycles to date have been done under milder calcination conditions, that is, lower temperature and CO2 concentration than those expected in real FBC capture plants (>900 °C and >90% CO2),13 in order to produce concentrated CO2 streams. In the early phases of research this is a fairly reasonable approach, since it enables both faster and more simplified experimental procedures, which produce more experimental data necessary for sorbent screening and exploration of methods for sorbent enhancement, as well as allowing a quick comparison of results obtained in different laboratories. However, further development of the technology requires experiments performed under more realistic conditions, because it has already been established that activity for natural sorbents under carbonation/calcination cycles is significantly reduced under high-temperature and high-CO2 concentration conditions,32,33 and this has also been shown in detailed parametric studies.34-36 The main reason for faster loss of activity under more severe conditions appears to be enhanced sintering at elevated temperature37-39 and CO2 partial pressure.40,41 New research27-31 on CaO-based sorbents showed that the presence of Al2O3 in the form of mayenite (Ca12Al14O33), has a strong beneficial effect on sorbent performance during CO2 capture cycles. An additional major improvement in performance has been made by preparing pellets using calcium aluminate cement as a binder, which imparts mechanical hardness to these pellets. Aluminate cements are also a source of Al2O3 suitable for developing the nanoporous structure desirable for enhanced absorption activity.29 In a long series of 1000 cycles, such pellets showed an extremely high long-term residual activity of 28%,30 which compares very favorably to the activity of natural sorbents (7.5%34 and 8-16%42 so far reported). Moreover, in a series of 30 cycles, aluminate pellets showed similar or even better performance than that reported for similar synthetic sorbents prepared using more expensive materials and procedures.29 However, this testing was carried out at isothermal conditions, 850 °C, with calcination under an atmosphere of N2, similar to the bulk of experiments so far reported in the literature. Aluminate cements are also relatively inexpensive and commercially available at industrial scale, which with relatively simple procedures for their preparation, could enable easy scaleup, and allow their commercial utilization for CO2 capture. Thus, the next step is their laboratory testing under more realistic conditions, as expected in real CO2 capture processes, to confirm their superior performance. Hence, the CO2 carrying capacities of aluminate pellets exposed to high temperature/CO2 concen-

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tration during the regeneration stage were examined in this study and results are presented here. 2. Experimental Section Materials. Cadomin limestone (CD) from Canada, 0.25-1.4 mm, was used here as a typical natural material (50.67% CaO) examined in numerous studies as a CaO-based sorbent for both SO2 and CO2 capture. This sorbent was tested here as is and also used in pelletized form. Commercial calcium aluminate cement, CA-14, produced by Almatis Inc., was used at the same time as binder for pelletization and as a source of Al2O3 (71% Al2O3 and 28% CaO in cement) for sorbent microstructure enhancement. It was a very fine powder; >80% of the particles are 900 °C. However, we do not believe this makes a significant difference when natural limestone is used because this material does not convert before temperatures approach 900 °C under a CO2 atmosphere, and the time for the first calcination step is relatively short. However, the pellets contain portlandite, Ca(OH)2, which reacts with CO2 at lower temperatures as can be seen in Figure 1. The mass change and sample temperature during calcination and the first carbonation/calcination cycle are presented for the natural sorbent and the aluminate pellets. The dashed vertical lines labeled by numbers show characteristic temperature, reactive gas, and mass changes. Figure 1 shows that at the beginning of the run, the pellet mass did not change significantly. With the temperature increasing above ∼320 °C (1), pellet mass increased as a result of direct carbonation of Ca(OH)2. After the temperature reached ∼570 °C (2), sample mass was almost constant because all available CaO in the pellet was in the form CaCO3, and further changes were not noticed before a temperature of ∼880 °C (3), when its decomposition started. This performance is similar to that previously noticed for hydrated lime.32 A slight limestone mass loss before that temperature was probably due to decomposition of some hydrates. After this point, mass change profiles for both samples are very similar. Their calcination finished at 950 °C and the 100% CO2 was switched with a gas mixture of 20% CO2/80% N2 and cooling started (4). At a temperature of ∼820 °C (5) chemical equilibrium allowed carbonation, which at the beginning was fast, but interestingly, the slow carbonation stage started when the temperature reached 700 °C (6). It is notable that the conversion rate during the slow, diffusioncontrolled calcination stage was not negligible, as is the case with sorbents calcined under an atmosphere of N2 and lower temperatures.29 Thus, it appeared that carbonation duration in this case is a very important parameter, which led us to explore carbonation conversions for longer carbonation times than 10 min, and these results are presented below. When the gas was switched to 100% CO2 and the temperature was raised (7) in order to calcine the sorbent, the carbonation rate accelerated due to the increase in both temperature and CO2 concentration. This has a negative effect in that the sorbent loses activity due to an “ineffective and unwanted additional carbonation” process occurring in the calciner. However, the duration

Figure 2. Conversions of aluminate pellets (a) and limestone (b) during carbonation/calcination cycles with calcination in 100% CO2 (carbonation in 20% CO2, N2 balance), under regime presented in Figure 1. Bold solid lines represent conversions under isothermal (800 °C) conditions, calcination in 100% N2, carbonation in 50% CO2 (N2 balance), both for 10 min.

of this undesired stage, and the resulting degree of additional carbonation, before the temperature that ensures calcination is achieved (8), depends on the heating rate in TGA experiments. The heating rate in Figure 1 was 50 °C/min which allowed additional carbonation for more than 3 min. Under these conditions, this represented ∼15% of the total carbonation conversion reached in a cycle. The first cycle was completed when the temperature reached 950 °C (9), that is, at this point the sorbent is fully calcined. The mass change profiles were qualitatively similar in further cycles and sorbent conversions are presented in Figure 2. The conversions in this study are calculated taking into account only CaO from limestone, but not CaO from cement. This approach gives somewhat higher results for the beginning cycles because some of the CaO from cement can react with CO2 under the cycling conditions used here. During cycles, Al2O3 from the cement reacts with CaO forming mayenite (Ca12Al14O33), even using some of the CaO from the limestone,29 which means that conversions in later cycles are underestimated. Furthermore, this confirms that CaO from cement should not be taken into account in order to obtain more realistic values for conversions. From the practical point of view, uptake, that is, the amount of CO2 captured relative to the total mass of sorbent, is meaningful. These values (uptake) can be easily calculated from conversions presented in this paper, using CaO contents in limestone (50.67%) and cement (28%) and the limestone/cement ratio in the pellets (9:1). The conversions presented in this paper can be converted to uptake (g CO2 captured/100 g sorbent) by multiplying them by 0.52 (for pellet) or by 0.60 (limestone). It can be seen that significant loss of activity for pellets (Figure 2a) occurs, with only ∼22% conversion after 30 cycles. This means that calcination at high temperature in CO2 during cycling reduced sorbent carrying activity for the given carbon-

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Figure 4. Conversions of aluminate pellets during carbonation/calcination cycles with calcination in 100% CO2, and 15th carbonation prolonged for 12 h.

Figure 3. SEM images of aluminate pellets after 30 carbonation/calcination cycles with calcination in 100% CO2: (a) particle outer surface, and (b and c) particle interior.

ation duration (10 min) by more than 1000 cycles under milder conditions.30 This was also similar to the case of limestone where only ∼18% conversion was achieved after the 30th carbonation (Figure 2b) and is in agreement with previous studies.32,33 The conversions of pellets were slightly higher, but the major unfavorable result here is the much higher loss of activity of pellets (in comparison with conversions under milder conditions (bold solid line)) than that seen for limestone. Thus, additional experimentation was needed to explore this effect and find more favorable conditions suitable for utilization of pellets under realistic conditions. The SEM images of pellets after 30 cycles are presented in Figure 3. It can be seen that very porous structure both at the particle surface and in the interior may provide enough room

for more product, that is, for additional conversion. This structure also enables gas diffusion toward the interior of the pellet particle. It is also interesting that nanograins can be seen at higher magnification (Figure 3c), which is important for pellet reactivity.29 This SEM observation, compared with those for the uncycled material,29 showed that morphological properties of pellets were not responsible for their poorer conversion (Figure 2a). Moreover, at the end of the carbonation stage, pellets were still reactive, which means that their longer exposure to calcination conditions can result in significantly higher conversions. Figure 4 presents carbonation/calcination cycles under the same conditions as those in Figure 2, but with prolonged (12 h) 15th carbonation. It can be seen that conversion increased during the prolonged carbonation, reaching values >100%, which means that some of the CaO from the cement, which was unreactive under milder conditions,43 also reacted here with CO2. An interesting result was that conversions were higher in cycles after the prolonged carbonation step, which was also noticed for natural sorbents.42 These results are important because they show that longer carbonation durations can lead to higher conversions during CO2 capture cycles. This is additional confirmation that porosity is not the limiting factor, because it enables complete conversion. Thus, a possible explanation for slow, but long-term conversion increase could be additional stabilization of the sorbent crystal structure at higher calcination temperatures in an atmosphere of CO2. It is expected that such a stabilized solid structure would be less reactive, resulting in slower conversion,44 but this morphology enables complete conversions after longer times. The differences in performance of the pellets and limestone in 5-h carbonation/10-min calcination cycles are shown in Figure 5. It can be seen that complete conversion of pellets occurs at the end of cycles 1-3. It should be noted here that this experiment was not performed with the aim of actually using the sorbents in 5-h carbonation cycles, but rather to further explore what factors hinder conversion rate and limit final conversion. It can be seen that after the third cycle, the maximum conversion of the pellets decreases. However, these conversions were superior in comparison with those for the limestone. It was very interesting that in later cycles, when a maximum conversion was reached, there was no noticeable further increase during 3-4 h. The SEM images in Figure 6 can fully explain limits during long-carbonation cycles. The left column of the images presents calcined samples, while the right column presents samples at the end of the carbonation step. It can be seen that the outer surface of the pellets is of low porosity and images obtained at high magnification (Figure 6b) show very small pores at the

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Figure 5. Conversions of aluminate pellets and limestone during carbonation/ calcination cycles with calcination in 100% CO2 and carbonation steps for 5 h.

surface of calcined particles (left), which are totally filled after carbonation (right). The dimensions of CaCO3 grains/crystals are of an order of magnitude of hundreds of nanometers. They form a totally nonporous surface and prevent diffusion of CO2 toward the particle interior, where unreacted CaO is found in a very similar manner to the situation for sulfation. Despite the very porous structure of the pellet particle (Figure 6c,d), carbonation is blocked by the outer carbonate shell and, by analogy with sulfation, might suggest that the carbonation reaction is relatively fast to allow the formation of a shell despite the presence of ample porosity in the interior of the particle. However, clear proof that formation of the carbonate layer at the beginning of carbonation/calcination cycles is not the rate limiting step, is the observation that complete carbonation is achieved during the first three cycles (Figure 5). Instead, we suggest that during cycles the sorbent particles shrink, as can

Figure 6. SEM images of pellets after 12 cycles (Figure 4): (a and b) particle outer surface, and (c and d) particle interior. Left column, calcined sample; right column, carbonated sample.

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Figure 7. Conversions of aluminate pellets and limestone during carbonation/ calcination cycles with calcination in 100% CO2 and carbonation steps for 1 h.

be demonstrated by measuring pellet dimensions before and after cycling,30 where loss of particle volume was about 15-20%. A similar phenomenon has also been found by mercury porosimetry44 and powder and helium displacement pycnometry results33 for natural limestones. Furthermore, it is supposed that loss of sorbent volume occurs more strongly at the particle surface than from its interior; that is, this is an effect similar to surface tension, as seen in liquids.45 Namely, the grains inside sorbent particles are totally surrounded by other grains, but at the particle surface this is not the case. Thus, the interior structure is more resistant and less liable to see a loss of volume, and this enables it to retain a network of pores and solid skeleton (Figure 6c,d). On the other hand, the grains at the surface are more mobile. Hence, during carbonation/ calcination cycles they migrate closer, and finally, due to their swelling resulting from the formation of the more voluminous CaCO3 product (VM(CaCO3):VM(CaO) ) 36.9 cm3/mol:16.9 cm3/mol), form a nonporous shell (Figure 6a,b, right). This finding is applicable also to natural sorbents, which display a similar limit for full carbonation even after a very long time (Figure 5), and we have noticed the same phenomenon during our SEM work for our other studies. The more pronounced effect of volume shrinkage is another possible explanation for better resistance of smaller pellet particles to cracking/breaking.30 The durations employed for carbonation cycles, presented in Figure 2 (10 min) and Figure 5 (5 h), showed that a short carbonation time is not sufficient for high conversion, but longer time causes the formation of a carbonate shell at the sorbent surface, which limits reaction in longer series of cycles, and moreover, a long reaction time is not practical for real capture processes. Thus, a balance needs to be found and one example with a 1-h carbonation time is presented in Figure 7. These results again show the superior performance of aluminate pellets in comparison with that for natural limestone. The pellet conversion after 18 cycles was 70%, which is more than double that for limestone (32.5%). These results also confirmed the high efficiency of pellets under realistic CO2 capture conditions, that is, under high temperature and high CO2 concentration during the calcination step. However, an optimal time needs to be found for each particular sorbent to obtain high average conversion in a reasonable/practical number of cycles, taking into account the high amount of sorbent that must circulate in a FBC reactor to ensure a high CO2 capture level. One difference between conditions employed in this study and conditions expected in a FBC capture reactor is the heating rate of sorbent particles. The heating rate in the FBC carbonator will be much higher than that employed here (50 °C/min). Thus,

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Figure 8. Influence of heating rate during calcination in CO2 on sorbent conversion during carbonation/calcination cycles (10 min carbonation time).

Figure 9. Separate effects of high temperature and high CO2 concentration during calcination steps on sorbent activity in carbonation/calcination cycles (heating/cooling rate is 50 °C/min; carbonation for 10 min).

as a first step, the influence of heating rate on sorbent carrying capacity was tested in the TGA employing 100 °C/min and results are presented in Figure 8. It can be seen that the conversions obtained are not significantly higher, regardless of the fact that the carbonation peak during the calcination stage (Figure 1) is almost eliminated due to the higher heating rate. This means that the carbonation that occurred during the beginning stage of calcination, due to high temperature and CO2 concentration, probably has two opposite effects on sorbent carrying activity. The negative effect is that sorbent loses activity due to an “ineffective and unwanted additional carbonation” process occurring in the calciner, while the positive process appears to be an effective sorbent reactivation step due to higher conversions,42 which results in a slightly higher net effective conversion with faster heating rate. This conclusion should also be noted with respect to overall carbonation duration, because the ineffective carbonation process is negligible when carbonation time was longer (Figures 5 and 7). Finally, the separate effects of high temperature and high CO2 concentration during the calcination steps were explored in this study and the results are presented in Figure 9. The main conclusion from this figure is that both effects are important, but their relative influence depends on the sorbent. Thus, for limestone the change in temperature does not significantly affect the sorbent by the 10th cycle, whereas for pellets, the change is significant over all the cycles examined. However, for both limestone and pellets the largest effect on activity is the change in calcination environment (50% CO2/50% N2 T 100% CO2). Conclusions The CO2 carrying capacity of calcium aluminate pellets was investigated in this study under high-temperature and high-CO2-

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concentration calcination conditions using the TGA. The real CO2 capture conditions (temperature and CO2 concentration) expected in the FBC capture processes were simulated. The results obtained were compared with that for natural limestone tested under the same conditions. The main conclusion is that aluminate pellets are more efficient sorbents under more realistic CO2 capture conditions. However, their superior performance is more pronounced with longer carbonation time and there appears to be an optimal carbonation duration. However, it should also include consideration of practical reactor sizing. This can enable the best exploitation of these sorbents because an overly long carbonation period causes pore closure and formation of a nonporous carbonate shell at the surface of the pellet, which prevents diffusion of CO2 into the particle interior for reaction with unreacted CaO. This same phenomenon is also noticed for natural limestone. A new explanation is offered for this phenomenon, which can explain more pronounced shrinkage and sintering at the particle surface. Literature Cited (1) Mohr, S. H.; Evans, G. M. Forecasting coal production until 2100. Fuel 2009, 88, 2059–2067. (2) Hook, M.; Aleklett, K. Historical trends in American coal production and a possible future outlook. Int. J. Coal Geol. 2009, 78, 201–216. (3) Metz, B.; Davidson, O.; de Coninck, H.; Loos M.; Meyer, L., Eds. Special Report on Carbon Dioxide Capture and Storage, IntergoVernmental Panel on Climate Change; Cambridge University Press: Cambridge, U.K., 2005. (4) Herzog, H. What future for carbon capture and sequestration. EnViron. Sci. Technol. 2001, 35, 148–153. (5) Bachu, S. CO2 storage in geological media: Role, means, status, and barriers to deployment. Prog. Energy Combust. Sci. 2008, 34, 254–273. (6) Hossain, M. M.; de Lasa, H. I. Chemical-looping combustion (CLC) for inherent CO2 separationssA review. Chem. Eng. Sci. 2008, 63, 4433–4451. (7) Anthony, E. J. Solid looping cycles: A new technology for coal conversion. Ind. Eng. Chem. Res. 2008, 47, 1747–1754. (8) Sedor, K. E.; Hossain, M. M.; de Lasa, H. I. Reactivity and stability of Ni/Al2O3 oxygen carrier for chemical-looping combustion (CLC). Chem. Eng. Sci. 2008, 63, 2994–3007. (9) Stanmore, B. R.; Gilot, P. ReviewsCalcination and carbonation of limestone during thermal cycling for CO2 sequestration. Fuel Process. Technol. 2005, 86, 1707–1743. (10) MacKenzie, A.; Granatstein, D. L.; Anthony, E. J.; Abanades, J. C. Economics of CO2 capture using the calcium cycle with a pressurized fluidized bed combustor. Energy Fuels 2007, 21, 920–926. (11) Romeo, L. M.; Lara, Y.; Lisbona, P.; Escosa, J. M. Optimizing make-up flow in a CO2 capture system using CaO. Chem. Eng. J. 2009, 147, 252–258. (12) Romeo, L. M.; Lara, Y.; Lisbona, P.; Martı´nez, A. Economical assessment of competitive enhanced limestones for CO2 capture cycles in power plants. Fuel Process. Technol. 2009, 90, 803–811. (13) Abanades, J. C.; Anthony, E. J.; Wang, J.; Oakey, A. Fluidized bed combustion systems integrating CO2 capture with CaO. EnViron. Sci. Technol. 2005, 39, 2861–2866. (14) Li, Y.; Buchi, S.; Grace, J. R.; Lim, C. J. SO2 Removal and CO2 capture by limestone resulting from calcination/sulfation/carbonation cycles. Energy Fuels 2005, 19, 1927–1934. (15) Ryu, H.; Grace, J. R.; Lim, C. J. Simultaneous CO2/SO2 capture characteristics of three limestones in a fluidized-bed reactor. Energy Fuels 2006, 20, 1621–1628. (16) Grasa, G. S.; Alonso, M.; Abanades, J. C. Sulfation of CaO particles in a carbonation/calcination loop to capture CO2. Ind. Eng. Chem. Res. 2008, 47, 1630–1635. (17) Jia, L.; Hughes, R.; Lu, D.; Anthony, E. J.; Lau, I. Attrition of calcining limestones in circulating fluidized bed systems. Ind. Eng. Chem. Res. 2007, 46, 5199–209. (18) Lu, D. Y.; Hughes, R. W.; Anthony, E. J. Ca-based sorbent looping combustion for CO2 capture in pilot-scale dual fluidized beds. Fuel Process. Technol. 2008, 89, 1386–1395. (19) Abanades, J. C.; Alvarez, D. Conversion limits in the reaction of CO2 with lime. Energy Fuels 2003, 17, 308–315.

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ReceiVed for reView November 12, 2009 ReVised manuscript receiVed December 15, 2009 Accepted December 21, 2009 IE901795E