Calcination Loop to

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Sulfation of CaO Particles in a Carbonation/Calcination Loop to Capture CO2 Gemma S. Grasa,*,† Mo´ nica Alonso,‡ and J. Carlos Abanades‡ Instituto de Carboquı´mica, (CSIC), Miguel Luesma Casta´ n 4, 50015 Zaragoza, Spain, and Instituto Nacional del Carbo´ n, (CSIC), Francisco Pintado Fe 26, 33011 OViedo, Spain

CaO is being proposed as a regenerable sorbent of CO2 via a carbonation/calcination loop. It is well known that natural sorbents lose their capacity to capture CO2 with the number of cycles due to textural degradation. In coal combustion systems, reaction with the SO2 present in flue gases also causes sorbent deactivation. This work investigates the effect of partial sorbent sulfation on the amount of CaO used in systems where both carbonation and sulfation reactions are competing. We have found that SO2 reacts with the deactivated CaO resulting from repetitive calcination/carbonation reactions. Therefore, the deactivation of CaO as a result of the presence of SO2 is lower than one would expect if one assumes that SO2 reacts only with active CaO. This work shows that changes in the texture of the sorbent due to repetitive carbonation/calcination cycles tend to increase the sulfation capacity of the sorbents tested. This suggests that the purge of deactivated CaO obtained from a CO2 capture loop could be a more effective sorbent of SO2 than fresh CaO. Introduction The use of CaO as a regenerable CO2 sorbent via calcination/ carbonation reactions is at the core of several processes involving CO2 capture in post-combustion applications for coal power plants1-3 where SO2 is present within the flue-gas stream. Figure 1 shows a possible scheme for the proposed CO2 capture process.3 The system consists of two interconnected circulating fluidized bed reactors: a carbonator and a regenerator or calciner. In the carbonator, the CO2 present in the flue-gas stream coming from the boiler meets a flux of CaO and reacts to form CaCO3. In the calciner, a second fuel (coal with sulfur) is fired with pure oxygen to supply the necessary heat to calcine the CaCO3 formed in the carbonator and decompose it into CaO (which is returned to the carbonator) and CO2, suitable for final purification, compression, and geological storage. The economics of the process4,5 compared to other CO2 capture systems for power plants has been proven to be very sensitive to the reduction in activity that natural limestone has shown6,7 in this chemical loop. The carbonation reaction and the subsequent calcination of the CaCO3 formed has been extensively studied but so far mainly in clean, air/CO2 environments.6,7 From these studies, it is known that the initially rich structure of the small pore diameters developed after the first calcination evolves toward macropores in the interior of particles as the number of carbonation/calcination cycles increases. This change in pore structure together with the limiting thickness of the product layer, which marks the beginning of the slow carbonation period, leads to the decay in the capture capacity of CaO.8,9 Despite this decay in capture capacity, CO2 capture systems for postcombustion application based on the carbonation/calcination loop could be designed to operate with a sufficiently high makeup flow of fresh sorbent, taking advantage of the low cost of limestone.10-12 It might also be possible to operate the system with low make-up flows by increasing the solid circulation rates between reactors, exploiting the residual activity detected in natural sorbents even after hundreds of cycles.7,10 In addition * To whom correspondence should be addressed. Tel.: +34 976733977. Fax: +34 976733318. E-mail: [email protected]. † Instituto de Carboquı´mica, (CSIC). ‡ Instituto Nacional del Carbo ´ n, (CSIC).

Figure 1. Schematics of the carbonation-calcination loop and relevant reaction and residence times.

to the deactivation mechanism derived from multiple calcinations, CaO will also deactivate as a result of CaSO4 formation in combustion environments with an excess of oxygen, for example in the flue gases coming from the boiler and in the calciner. In earlier works, it was assumed that deactivated CaO particles produced by the large number of carbonationcalcination cycles would be also unable to react with SO2.13 It has been experimentally confirmed that SO2 does decrease CaO capture capacity,14-16 even when the SO2 concentrations are 2 orders of magnitude lower than those of CO2. Cyclic co-capture (of SO2 and CO2) studies have been carried out in TGA15 (at both atmospheric and pressurized conditions) and in a fluidized bed reactor16 for up to 10-15 carbonation-sulfation/calcination cycles. These studies confirmed that the capacity of the sorbent to capture CO2 decreased more quickly when SO2 was present and when the concentration of SO2 was increased, for different types of limestone tested. Total calcium utilization (CO2 and SO2) was found to decrease with the number of cycles, although the effect of SO2 concentration varied in the types of limestone tested.16 The mechanism of deactivation in these tests must be similar to the known mechanism in the CaO-SO2 reaction (blockage of the pores by the CaSO4 product as the molar volume of CaSO4 is higher than CaCO3).17,18 However, it should be mentioned that, in the co-capture studies carried out so far,14-16 the Ca/S ratio during the experiments is much higher

10.1021/ie070937+ CCC: $40.75 © 2008 American Chemical Society Published on Web 01/30/2008

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than the ratio that would be present in a real system based in Figure 1. A Ca/C molar fraction of 3-10 is necessary to sustain the CO2 capture loop. Therefore, with reasonable values of make-up flow of limestone in the system, very high Ca/S ratios are to be expected (as the sulfur molar fraction in fuels is much lower than the carbon molar fraction). In these conditions, it is not possible to achieve the high calcium conversions to CaSO4 necessary for extensive pore plugging. The key flow rates and residence times of the solid materials relevant for this work are also shown in Figure 1. The stream of solids, FR, (unreacted CaO plus the CaCO3 formed, if no ash and sulfur are present) is constantly circulating between the two reactors and determines the average residence time of the circulating solids in each reactor. Because of the low activity of natural sorbents,6,7 a large amount of solids needs to be circulating between the reactors (FR) for high CO2 capture efficiencies to be achieved (with Ca/C molar ratio >5 in the carbonator).10,12 The particle residence time in each reactor can be calculated from the inventory of solids in each reactor divided by the recirculation stream of solids (tr_carb ) W/FR). This residence time will be on the order of a few minutes because of the high value of FR and the need to moderate the solids inventory in the risers. As a result of the deactivation that the sorbent undergoes with the number of cycles, a fresh sorbent make-up flow (F0) and a purge to extract the spent sorbent (including ashes and CaSO4 if they are present) are needed to maintain a desirable level of sorbent activity.10 F0 determines the average residence time of particles in the system through, tr_sys ) (Wcarb + Wcalc)/F0. The difference between the residence time in the individual reactors and the average residence time of the particles in the system is typically 1 order of magnitude (tr_sys >> tr_carb or tr_calc) as the make-up flow of fresh limestone (expressed as F0/FR) is typically lower than 0.1 (ref 10 for details). In these conditions, the average residence time of the sorbent particles in the system, tr_sys, can be on the order of hours. This is the relevant residence time for the sulfation reaction of CaO. The progressive conversion of CaO to CaSO4 would take place in these time scales and conditions (both similar to the existing circulating fluidized bed combustors), but the CaO particles will only stay a few minutes in each reactor in every carbonation-calcination cycle. In this situation, CaO conversion to CaSO4 every time that the solids pass through the regenerator or the carbonator (residence time tr_calc on the order of minutes) would be less than 1% (molar basis). In other words, a typical particle of sorbent circulating between reactors will experience many carbonation/calcination cycles before it reaches its maximum conversion with respect to CaSO4. Therefore, one key question that arises at this point is how this low level of sulfation conversion that takes place on each particle recirculating from the carbonator to the regenerator affects the capacity of the sorbent to capture CO2 during the next cycle. Another key question is whether the SO2 will react only with the active part of the CaO or whether it will react also with the nonactive CaO (nonactive with respect to the carbonation reaction). It would also be of interest to study the effect of the type of limestone on these coupled reactions. Experimental Section Two limestones (Imeco and Piaseck, Polish limestone) with particle sizes in the range of 300-600 µm were studied in this work. The experimental installation has been described elsewhere.7 It consists of a thermogravimetric analyzer that records the sample weight changes while the reaction is taking place

Figure 2. XCaCO3 vs cycle number. Carbonation, 5 min at 650 °C; calcination, 5 min at 900 °C both in 10 vol % CO2/air.

inside a quartz tube under a desired reaction atmosphere. Commercial pressurized gas bottles were used to simulate the reaction atmosphere: CO2 in air/N2 and SO2 in air. This TGA is built inside a double furnace system that works at different temperatures (650 °C for carbonation and >850 °C for the calcination reaction). This makes it possible to perform fast changes in the sample temperature and to alternate the calcination and carbonation conditions. Different series of tests were carried out to determine: the carbonation capacity of the limestone tested with the number of cycles, their sulfation capacity in long tests (180 min), and their evolution after they have been subjected to different numbers of calcination/ carbonation cycles (1, 15, 100). Finally, the effect of partial sulfation on limestone carbonation capacity with the number of cycles was also studied. This last set of experiments included a sulfation step between the calcination and carbonation of the sorbent, in order to achieve a CaO molar conversion (to CaSO4) of 0.5 and 1% in two series of experiments. The experimental conditions, in terms of reaction atmosphere and temperature, are specified in the figure captions. Scanning electron microscopy (SEM) characterization has been also carried out to observe the features of the sulfated particles. After the tests, some samples were mildly crushed, dispersed on a graphite tab, and gold-coated with a ∼20 nm thick film for observation under SEM. Results The two limestones tested presented the same behavior when subjected to repeated carbonation/calcination cycles (under a CO2/air atmosphere) as shown in Figure 2. Molar conversion to carbonate, XCaCO3, after 5 min reached 0.65 for the first cycle, and this capture capacity decreased with the number of cycles toward a residual conversion typical of highly cycled CaO particles (around 0.075-0.08 after hundreds of cycles).7 These results are in agreement with the representative features of the CaO carbonation reaction discussed in previous work.6,7 This decrease in capture capacity with the number of cycles was attributed to a change in pore structure (from an initially rich microporous structure to larger pores in the interior of the particles), together with a product-layer thickness of 50 nm that marked the beginning of the slow carbonation reaction period.8,9 Both facts seem to be the limiting factors for the carbonation reaction. The reactivity of the limestones with respect to the formation of CaSO4 has been also tested in the TGA apparatus. Figure 3 shows the results in terms of CaO conversion to CaSO4 versus time. Sulfation took place after the total calcination of the limestones. The reaction atmosphere was 2200 ppm of SO2 in air at 900 °C.

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Figure 3. XCaSO4 vs time. Reaction atmosphere was 2200 ppmv SO2 in air at 900 °C. Note that the y axis scale goes only up to 0.6.

As can be seen in Figure 3, both limestones presented different levels of CaO conversion. This low CaO utilization on limestones, from 0.12 to 0.27 in this case, is typical during sulfation reactions.17,18 The higher molar volume of CaSO4 produces pore closure and prevents the reaction from continuing toward the interior of the particle and also leads to different sulfation patterns. It is, however, remarkable that limestones exhibiting such different sulfation patterns present identical behaviors with respect to the carbonation reaction (as shown in Figure 2). The limestones have been divided into three groups, depending on their sulfation pattern:17 unreacted core, network, and uniform. The unreacted core type is typical of limestones containing medium-sized grains and small micropores as main contributors to the overall porosity. These sulfated particles are characterized by a highly sulfated rim and a slightly sulfated (or nonsulfated) core. Network sulfation is typical of particles with large interconnected fractures (or a cluster of small micrograins). They present high sulfation around the periphery and in the proximity of these fractures, whereas they are slightly sulfated or nonsulfated in the core of the micrograins separated by the fractures. Finally, particles with large microfractures, large grains, large micropores, or macropores sulfate in an uniform pattern. To obtain a qualitative insight into the sulfation

characteristics of the limestones tested, some sulfated samples were examined by means of SEM. Figure 4 shows images of these sulfated samples. Figure 4 (left) corresponds to Imeco limestone. It shows a magnification of a hole in the CaSO4 external layer that shows the interior of the particle formed by unreacted CaO. This unreacted CaO shows a typical micrograin structure and has a grain diameter of around 0.1-0.2 µm. Figure 4 (center) shows very fine grains highly sulfated externally that could well represent the small grains formed when the initial limestone particles have been crushed (particle size 300 µm). Long and wide fractures that cross the grains can be observed together with finer fractures (some of which might be due to the sample preparation method for SEM). Figure 4 (right) shows the interior of a sulfated Piaseck particle where unreacted CaO is visible. After this initial analysis, several sulfation tests were carried out to some of the samples resulting from the different series of carbonation/calcination cycles. Figure 5 shows the results in terms of CaO conversion versus time for the sulfation reaction (c-1, c-15, and c-100 indicate that sulfation takes place in the 1st; 15th cycle, after 14 carbonation/calcination cycles; and 100th cycle, after 99 carbonation/calcination cycles, respectively). As can be seen in Figure 5, CaO conversion to CaSO4 increases when the number of carbonation/calcination cycles that particles have previously experienced increases. This phenomenon occurs for both limestones (increase in utilization from 0.27 up to 0.47 for Imeco limestone, from 0.12 up to 0.40 for Piaseck). This result agrees with the work of Li et al.14 where the sulfation capacity of limestones was increased by subjecting the samples to recarbonation followed by calcination. The initial slopes of the conversion curves for a given limestone are similar along the cycles, and changes in pore structure with the number of cycles seem to be responsible for this higher CaO utilization. The pore-sealing mechanism that occurs during the sulfation reaction is reduced for sorbent particles with open structures (typical of solids cycled a certain number of times in the

Figure 4. Images of limestone particles after a long sulfation test at 900 °C in 2200 ppmv of SO2. Left and center correspond to Imeco, right corresponds to Piaseck.

Figure 5. CaO sulfation conversion vs time. The reaction atmosphere was 2200 ppm SO2 in air at 900 °C. Sulfation took place after 1 calcination (c-1), 15 calcinations (c-15), and 100 calcinations (c-100). Left, Imeco; right, Piaseck.

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Figure 6. Images of limestone particles after a long sulfation test. Left, sulfate layer and big fracture after sulfation in cycle 100, Imeco; right, image of Piaseck particle after sulfation in cycle 15. Figure 8. Carbonation capacity vs cycle number for nonsulfated particles and particles sulfated up to 0.5 and 1%, molar conversion to CaSO4, in every cycle. Left, Imeco; right, Piaseck.

Figure 7. Sample mass change vs time during the calcination, sulfation, and carbonation stages; Piaseck limestone.

carbonation/calcination loop). SEM analysis was also carried out on these sulfated samples. Figure 6 shows the results for Imeco and Piaseck limestone. Figure 6 (left) shows the CaSO4 outer layer and a big fracture typical of Imeco limestone. The CaSO4 layer seems to be in the range of 1 to 3 µm. Figure 6 (right) shows a CaSO4 layer formed in a Piaseck particle sulfated after 15 carbonation/ calcination cycles. In contrast, inside the particle unreacted CaO can be seen in the same photograph. With these and other images alone it is difficult to determine the sulfation pattern that these limestones follow, but the low utilization of Piaseck and the strong particle size effects on the sulfation reaction may indicate an unreacted-core sulfation pattern. Figure 6 (right) shows a clear boundary between the external CaSO4 layer and the CaO inside the particle. From the images, it can also be seen that there are no differences between the CaSO4 layer formed on CaO particles calcined for the first time and the sulfate layer formed on particles that have experienced many calcination/ carbonation cycles. The results in Figure 5 show an increase in CaO utilization (to form CaSO4) when the number of calcinations that the particle experiences increases. This behavior is opposite to CaO utilization with respect to carbonation. It is therefore possible that SO2 is able to react with nonactive CaO with respect to carbonation. To prove whether this was the case, a series of tests were carried out including a sulfation step between the calcination and carbonation reactions. The routine for the experiments was complete calcination (in 10 vol % CO2/air), followed by sulfation (2200 ppm SO2 in air at 850 °C) in order to achieve a low partial conversion to sulfate (0.5% or 1% molar respectively) and finally, carbonation (in 10 vol % CO2/air) for 5 min. This routine was repeated up to 50 times for each experiment. Figure 7 shows an example of the results obtained in TGA for a typical experiment including a sulfation step. The results are plotted in terms of mass change during the calcination, sulfation, and carbonation stages versus time. Three cycles are visible in Figure 7. This example corresponds to the Piaseck limestone. Figure 8 shows the results from the experiments that include the sulfation step (left) and (right) for Imeco and Piaseck, respectively. Three experiments have been represented in each figure: a experiment with no sulfation step, a second experiment

Figure 9. Carbonation capacity vs cycle number for nonsulfated particles (XCaCO3(nonsulfated), curve 1) and sulfated particles (XCaCO3(sulfated) 0.5% molar conversion to CaSO4, in every cycle, curve 2). Accumulated sulfation with the number of cycles (XCaSO4, curve 3), CaO deactivation due to SO2 (Xdeactivated due SO2, curve 4), and total CaO utilization (Xtotal CaO, curve 5) are also plotted. Left, Imeco; right, Piaseck.

including a sulfation step to achieve 0.5% molar conversion to CaSO4, and a third experiment including partial sulfation to achieve up to 1% molar conversion to CaSO4 between each cycle. The curves represent CaO capture capacity (with respect to carbonation only) versus cycle number. As can be seen, the inclusion of a sulfation stage accelerates deactivation of the sorbent for both limestones when compared with sorbent behavior in only CO2/air reaction atmospheres (black dotted series). It is also clear that a higher partial conversion to sulfate before every carbonation stage also increases deterioration of the sorbent. Imeco limestone shows a much higher sensitivity to deactivation with SO2 than Piaseck limestone. Conversion to carbonate for Imeco is as low as 2% in cycle 30, for the series that includes a 0.5% sulfation step, and it is even lower for the series that includes a 1% sulfation step. Piaseck shows a different behavior. Although it also deactivates with the presence of SO2, it still maintains a CO2 capture capacity of around 5% molar conversion up to 50 cycles. This capture capacity is still maintained when the intermediate sulfation conversion increases up to 1%. The cause of this different behavior for these limestones must also be responsible for their behavior with respect to sulfation in the first cycle (with a lower CaO conversion to CaSO4 for Piaseck, Figure 3). Several curves extracted from the experimental work are plotted in Figures 9 and 10 to help with the interpretation of the results. Curve 1 represents the carbonation capacity, XCaCO3, versus cycle number for the nonsulfated samples; curve 2 represents the same magnitude for the sulfated samples; curve 3 represents the cumulated CaO conversion, in every cycle, to CaSO4 (XCaSO4) calculated as

XCaSO4 )

∑XCaSO4,i

for i ) 1 to N, (N ) number of cycles) (1)

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maintain at a residual capacity for capturing CO2 (for both levels of sulfation tested). These results show that SO2 is able to react with CaO, which is already useless for CO2 capture. This is very important for the design of real units. In the absence of other constraints, the overall objective for a practical application should be to maximize total sorbent utilization. This may vary strongly according to the type of limestone and the amount of SO2 present in the flue gas. Previous results also show that simulation exercises published in the past3,4,13 were too conservative as it was thought that SO2 would react only with active CaO for the carbonation reaction. Figure 10. Carbonation capacity vs cycle number for nonsulfated particles (XCaCO3(nonsulfated), curve 1) and sulfated particles (XCaCO3(sulfated) 1% molar conversion to CaSO4, in every cycle, curve 2). Accumulated sulfation with the number of cycles (XCaSO4, curve 3), CaO deactivation due to SO2 (Xdeactivated due SO2, curve 4), and total CaO utilization (Xtotal CaO, curve 5) are also plotted. Left, Imeco; Right, Piaseck.

Also plotted is the fraction of CaO deactivated due to SO2 in every cycle (curve 4), calculated as,

(Xdeactivated due SO2)i ) [XCaCO3(nonsulfated) - XCaCO3(sulfated)]i for i ) 1 to N (2) and finally the total amount of CaO utilized (curve 5) in every cycle calculated as

XtotalCaO,i ) [XCaCO3(sulfated) + XCaSO4]i for i ) 1 to N

(3)

As can be seen, the curve for the total amount of the CaO utilized (to form CaSO4 plus CaCO3) presents a minimum. The utilization of CaO decreases during the first 10-15 cycles (due to the strong textural deactivation of CaO plus deactivation due to the presence of SO2). For higher numbers of cycles, however, CaO utilization increases as a result of the residual carbonation that still takes place and the increasing sulfation of CaO in every cycle. For the case of Imeco limestone, the total amount of CaO utilized (curve 5) is very close to the cumulative sulfation (curve 3) when the cycle number is around 30. Its residual carbonation capacity is very low, and it tends to disappear at a higher number of cycles. In contrast, total CaO utilization (curve 5) for Piaseck limestone seems to flow parallel to cumulative sulfation (curve 3) for a cycle number higher than 20. This indicates that the residual carbonation conversion tends to maintain its value, at least up to 50 cycles. The same graphs have been plotted for the experiments sulfated up to 1% molar conversion in every cycle (Figure 10). The results show the same type of curve as for total CaO utilization. The residual activity of Imeco limestone tends to a very low value at an earlier stage due to the higher amount of CaO deactivated by SO2 during the initial cycles. Imeco limestone presents a good sulfation rate in Figure 3 and this results in a higher sorbent deactivation with respect to carbonation. In contrast, Piaseck limestone, which is a worse sorbent for SO2 capture, still maintains a residual activity with respect to carbonation (on the order of 5%, Figures 9 and 10, right). Regarding sulfation capacity, after 40-50 cycles, the limestones are still able to react with SO2. From left in Figure 9 and left in Figure 10 it can also be seen that for Imeco limestone there is a cumulative deactivation due to SO2, which increases with the number of cycles and reaches a level that almost makes the carbonation of the particles impossible during the next cycle. However, the deactivation that Piaseck limestone undergoes due to the presence of SO2 is almost constant from the first cycle (Figures 9 and 10, right). This feature allows the particles to

Conclusions Limestones may differ strongly in their performance during sulfation and still show a similar behavior with respect to the carbonation reaction (in clean CO2/air atmospheres). The deactivation process that limestone experiences during repetitive calcination/carbonation cycles is beneficial for its reaction with SO2. This is because large pores (typical in CaO particles after many carbonation/calcination cycles) are less subject to pore blockage during sulfation, and SO2 may react not only with active CaO for the carbonation reaction but also with deactivated CaO for carbonation. However, the presence of SO2 always accelerates deactivation of the sorbent with respect to CO2 capture. The additional sorbent deactivation due to SO2 is very different for the two limestones tested in this work despite their similarity during carbonation when there is no sulfur. One of the limestones undergoes a progressive deactivation due to the sulfur during the first 10-15 cycles, and it reaches a level of deactivation that makes carbonation in the next cycles almost impossible. However, the other limestone presents almost constant deactivation due to sulfur from the first cycle, and it maintains a residual carbonation capacity on the order of 5%. Total CaO utilization for both limestones presents a minimum with the number of cycles (due to textural degradation plus reaction with SO2) around cycles 10-15 and then tends to increase due to residual sulfation and carbonation capacities over long series of cycles. Acknowledgment This work is partially funded by the European Commission (C3-Capture) and the Spanish Ministry of Education (“Juan de la Cierva” program). Help from D. Alvarez in obtaining the SEM photographs is also acknowledged. Note Added after ASAP Publication. Since the publication of this paper on the Web 1/30/2008, some minor text changes have been made throughout the paper. The amended version of this paper was reposted to the Web 2/05/2008. Literature Cited (1) Shimizu, T.; Hirama, T.; Hosoda, H.; Kitano, K.; Inagaki, M.; Tejima, K. A Twin Fluid-Bed Reactor for Removal of CO2 from Combustion Processes. Trans. Inst. Chem. Eng. 1999, 77A, 62. (2) Abanades, J. C.; Anthony, E. J.; Lu, D. Y.; Salvador, C.; Alvarez, D. Capture of CO2 from Combustion Gases in a Fluidized Bed of CaO. AIChE J. 2004, 50, 1614-1622. (3) Abanades, J. C.; Anthony, E. J.; Wang, J.; Oakey, J. E. Fluidized Bed Combustion Systems Integrating CO2 Capture with CaO. EnViron. Sci. Technol. 2005, 39, 2861. (4) MacKenzie, A.; Granastein, 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.

Ind. Eng. Chem. Res., Vol. 47, No. 5, 2008 1635 (5) Abanades, J.C.; Grasa, G.; Alonso, M.; Rodrı´guez, N.; Anthony, E.J.; Romeo L.M. Cost Structure of a Postcombustion CO2 Capture System Using CaO. EnViron. Sci. Technol. 2007, DOI 10.1021/es070099a. (6) Abanades, J. C.; A Ä lvarez, D. Conversion Limits in the Reaction of CO2 with Lime. Energy Fuels 2003, 17 (2), 308. (7) Grasa, G.; Abanades, J. C. CO2 Capture Capacity of CaO in Long Series of Carbonation/Calcination Cycles. Ind. Eng. Chem. Res. 2006, 45(26), 8846. (8) Alvarez, D.; Abanades, J. C. Pore-Size and Shape Effects on the Recarbonation Performance of Calcium Oxide Submitted to Repeated Calcination/Recarbonation Cycles. Energy Fuels 2005, 19, 270. (9) Alvarez, D.; Abanades, J. C. Determination of the Critical Product Layer Thickness in the Reaction of CaO with CO2. Ind. Eng. Chem. Res. 2005, 44, 5608. (10) Rodriguez, N.; Alonso, M.; Grasa, G.; Abanades, J.C. Heat Requirements in a Calciner of CaCO3 Integrated in a CO2 Capture System Using CaO. Chem. Eng. J. 2007, DOI 10.1016/j.cej.2007.06.005. (11) Abanades, J. C.; Rubin, E. S.; Anthony, E. J. Sorbent Cost and Performance in CO2 capture systems. Ind. Eng. Chem. Res. 2004, 43 (13), 3462. (12) Grasa, G.S.; Abanades J.C.; Alonso, M.; Gonza´lez, B. Reactivity of Highly Cycled Particles of CaO in a Carbontion/Calcination Loop. Chem. Eng. J. 2007, DOI 10.1016/j.cej.2007.05.017.

(13) Wang, J.; Anthony, J, E.; Abanades, J. C. Clean and Efficient Use of Petroleum Coke for Combustion and Power Generation. Fuel 2004, 83, 1341. (14) Li, Y.; Buchi, S.; Grace, J. R.; Lim, J. C. SO2 Removal and CO2 Capture by Limestone Resulting from Carbonation/Sulfation/Carbonation Cycles. Energy Fuels 2005, 19, 1927. (15) Sun, P.; Grace, J. R.; Lim, C. J.; Anthony, E. J. Removal of CO2 by Calcium-Based Sorbents in the Presence of CO2. Energy Fuels 2007, 21, 163. (16) Ryu, H. J.; Grace, J. R.; Lim, J. Simultaneous CO2/SO2 Capture Characteristics of Three Limestones in a Fluidized-Bed Reactor. Energy Fuels 2006, 20, 1621. (17) Laursen, K.; Duo, W.; Grace, J. R.; Lim, J. Sulfation and Reactivation Characteristics of Nine Limestones. Fuel 2000, 79, 153. (18) Anthony, E. J.; Granatstein, D. L. Sulfation Phenomena in Fluidized Bed Combustion Systems. Prog. Energy Combust. Sci. 2001, 27, 215.

ReceiVed for reView July 10, 2007 ReVised manuscript receiVed November 20, 2007 Accepted December 18, 2007 IE070937+