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CO2 Capture Capacity of CaO in Long Series of Carbonation/Calcination Cycles Gemma S. Grasa*,† and J. Carlos Abanades‡ Instituto de Carboquı´mica, CSIC, C/Miguel Luesma Casta´ n No. 4, 50015 Zaragoza Spain, and Instituto Nacional del Carbo´ n, CSIC, C/Francisco Pintado Fe, No. 26, 33011 OViedo, Spain
Calcium oxide can be an effective sorbent to separate CO2 at high temperatures. When coupled with a calcination step to produce pure CO2, the carbonation reaction is the basis for several high-temperature CO2 capture systems. The evolution with cycling of the capture capacity of CaO derived from natural limestones is experimentally investigated in this work. Long series of carbonation/calcination cycles (up to 500) varying different variables affecting sorbent capacity have been tested in a thermogravimetric apparatus. Calcination temperatures above T > 950 °C and very long calcination times accelerate the decay in sorption capacity, while other variables have a comparatively modest effect on the overall sorbent performance. A residual conversion of about 7-8% that remains constant after many hundreds of cycles and that seems insensitive to process conditions has been found. This residual conversion makes very attractive the carbonation/calcination cycle, by reducing (or even eliminating) sorbent purge rates in the system. A semiempirical equation has been proposed to describe sorbent conversion with the number of cycles based on these new long data series. Introduction The separation of a pure CO2 stream, combined with a wellmanaged geological storage site, is being considered as a mitigation option for climate change.1 It could be applied using existing technologies, because many of the components in these systems are commercially available. However, it is widely accepted that there is a large scope for cost reduction and energy efficiency improvements in CO2 capture systems. From the different approaches to separate gases from a flue gas stream, we focus in this paper on the use of regenerable solid sorbents based on the carbonation-calcination loop of CaO/CaCO3. The basic separation principle in these systems is depicted in Figure 1. Combustion-based systems of this type are described in more detail elsewhere,2 and several other options are being considered for the precombustion route.3 One of the advantages of this type of looping cycle is that it uses a very cheap and widely available regenerable sorbent that allows high makeup flows of fresh sorbent at a reasonable cost.4 A synergy could exist for a cement plant and a power plant in these conditions. However, it is obvious that lower makeup flows will be more attractive for most applications. This requires higher circulation rates of solids between carbonator and calciner and the sorbent to operate a larger number of cycles. The subject of this paper refers to the second point, investigating how the sorbent capacity evolves when particles have experienced in the loop up to several hundreds of cycles. All previous studies investigating the reversibility of the carbonation/calcination reaction showed that carbonation is far from reversible in practice.5-10 After a fast, chemically controlled, initial reaction stage, a second slower reaction stage controlled by the diffusion in the CaCO3 layer takes place. It was also observed that the transition between the fast and slow regimes takes place quite suddenly at a given level of conversion and this level of conversion decreases when the number of carbonation/calcination cycles is increased. The manufacture of synthetic Ca-based sorbents11-13 or a reactivation step14 can * To whom correspondence should be addressed. Tel.: +34 976733977. Fax: +34 976733318. E-mail:
[email protected]. † Instituto de Carboquı´mica. ‡ Instituto Nacional del Carbo´n.
Figure 1. Diagram of the proposed calcination/carbonation loop.
substantially reduce this decay in carbonation capacity, but the benefit of the very low cost of natural limestones may be lost.4 The effective removal of CO2 from any capture system using the carbonation-calcination cycle of CaO/CaCO3 requires availability in the carbonation reactor of a sufficient amount of active CaO, i.e., sorbent particles reacting in the fast reaction regime. Only a fraction of the CaO present in every particle of sorbent cycling in the system is able to react with CO2 to produce CaCO3. When the fast carbonation period is finished (after only 1-3 min at temperatures around 650 °C and typical combustion flue gas atmospheres), a product layer of CaCO3, about 50 nm thick, is formed15 on all the free surfaces of CaO in the original calcine. This product layer makes inaccessible a large fraction of the CaO in the interior of the particles (typically more than 70% after 20 cycles). For fluidized bed systems, this limiting conversion determines the overall performance of the reactor system, because the gas-solid contact times can be sufficiently long and of sufficient quality to ensure that most particles complete their fast reaction stage and achieve a conversion close to the maximum. Therefore, the characteristic conversion marking the end of the carbonation period as a function of the number of cycles is a critical design parameter for the design of capture systems using a CaO/CaCO3 chemical loop to separate CO2. The purpose of this work was to enlarge the experimental database of information on this parameter, investigating the effect of operating variables (sorbent type, particle size, calcination temperature, calcination time, CO2 concentration) on the rate of this decay.
10.1021/ie0606946 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/09/2006
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data series depending on the extent of the experimental work (10 cycle series vs 500 cycle series) for each set of conditions. Results and Discussion
Figure 2. Description of the TGA used during the experimental work.
Experimental Section The cyclic carbonation and calcination reactions were experimentally studied in a new thermogravimetric analyzer (TGA) especially designed for long multicycle carbonation-calcination tests. The TGA consisted of a quartz tube (1.8 × 10-2 m i.d.) placed inside a two-zone furnace capable of working at temperatures up to 1000 °C. The sample holder was a platinum basket of 8 × 10-3 m diameter and 2 × 10-3 m height. Temperature and sample weight were continuously recorded in a computer. The reacting gas mixture (CO2, O2/air) was set by mass flow controllers and fed to the bottom of the quartz tube. A special characteristic in the design of this TGA was the existence of two zones in the furnace capable of working at different temperatures. This furnace could be displaced (by means of a pneumatic piston) up and down. The position with respect to the platinum basket alternated between calcination conditions (>850 °C) and carbonation conditions (around 650 °C). Figure 2 shows a schematic diagram of the TGA. For each run in the TGA around 15 mg of sorbent was introduced in the sample holder. Calcination and carbonation reactions were carried out in the same atmosphere (CO2/air mixtures). Initial experiments were carried out to determine the total gas flow needed to eliminate external diffusion effects around the sample basket. This flow was set at a value higher than 4 × 10-6 m3/s (STP), which corresponds to a spatial gas velocity of 0.06 m/s at 650 °C. Also, some initial experiments were done with an empty sample holder and an inert material in order to determine disturbances on the weight readings when the furnace was displaced from the calcination to the carbonation position. Figure 3 shows the typical data recorded in the computer in the form of weight changes vs time. After correction of the data with the previous blank tests, plots of conversion vs time for each cycle were obtained from the measured weight losses (see Figure 4), assuming that the CaO was converted to CaCO3 during carbonation. At the end of each run, the samples were weighed in a different balance to check the accuracy of the TGA experiment. Good agreement was found in all the cases between the overall conversion calculated from this final weight difference and the conversion from the TGA. Figure 4 represents the same data series with two different scales in the number of cycles, to highlight the relative importance of deviations between
Five narrow particle size fractions (0.1-0.25 mm, 0.25-0.4 mm, 0.4-0.6 mm, 0.6-0.8 mm, 0.8-1 mm) have been investigated with a limestone named La Blanca. As seen in Figure 5, the particle size did not influence the sorption capacity of the sorbent that remains determined only by the number of calcination/carbonation cycles. The particle size affected only the reaction rate at the initial fast carbonation stage that lasted for 1-3 min. Because the carbonation part of cycle on these experiments was longer than 5 min, diffusion resistance inside the particles did not influence the sorbent maximum conversion capacity. However, diffusion resistance may play a role in the particle reaction rate. Furthermore, in view of these results one would expect a homogeneous carbonation pattern inside the particles. The issue of how much the type of limestone (or dolomite) affects the carbonation capacity has been a subject of debate. There is a body of literature showing that limestone type can strongly affect the performance of CaO as a SO2 from combustion gases to form CaSO4.16 Different limestones generate on calcination very different textures, and this can lead to very different sulfation patterns and maximum levels of sulfation.17 However, the reaction mechanism is different in the sulfation and carbonation reactions. In the first case, there is an inherent pore blockage mechanism due to the higher molar volume of CaSO4 with respect to CaCO3 or CaO precursor. This mechanism is obviously not so strong in the formation of CaCO3 during carbonation, although this has also been detected in some special conditions.18 In fact, results reviewed in previous works5,7,9-11 showed that there is a similarity in the decay trends of several limestones for a variety of carbonates (although all of them were of high purity and the data were restricted to a small number of cycles). Therefore, we have tested here six different types of limestones from very different locations and a dolomite (Sierra de Arcos, approximately 50% MgCO3). Results are plotted in Figure 6. As can be seen in Figure 6, some small differences exist among most limestones except for the case of dolomites and one limestone showing a remarkably poor performance from the first few cycles. For the case of dolomite, calcium conversion is higher, but around 50% of the sorbent is unconverted MgO. If the weight fractions (grams of CO2 captured per gram of parent sorbent) are plotted instead, the dolomite shows a trend very similar to the majority of the limestones, as can be seen in Figure 7. From a practical point of view, it is clear that, although differences in limestone performance do exist, they all decay in a similar way. On the other hand, the differences detected among some limestones will have important implications in the design of the system. The difference between the best limestone and the worst limestone tested here will not be relevant for conditions with a high purge rate of solids in the carbonationcalcination loop. In this case, performance will be dominated by the conversion of sorbent particles being cycled a small number of times, and all the limestones can achieve high average conversions under these conditions. However, if the capture system is designed with low purge rates (higher circulation rate of solids between carbonator and calciner to compensate for low sorbent average conversions), the differences between limestones from different origins become more prominent and long duration tests are needed to investigate sorbent performance. However, as a general rule, it can be said that most
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Figure 3. Typical weight changes vs time for a repeated number of calcination/carbonation cycles. Limestone: Piaseck, particle size 0.4-0.6 mm. Calcination temperature 850 °C, 5 min; carbonation temperature 650 °C, 5 min; both at pCO2 of 0.01 MPa in air.
Figure 4. Sorbent conversion vs number of cycles for the experiment in Figure 3. Solid line corresponds to data from eq 5; k ) 0.52, Xr ) 0.075 (see Table 1).
Figure 5. Conversion vs cycle number for experiments carried out with different particle size intervals. Limestone: La Blanca. Calcination temperature 850 °C, 20 min; carbonation temperature 650 °C, 20 min; pCO2 of 0.01 MPa. Solid line corresponds to the data obtained from eq 5; k ) 0.52 and Xr ) 0.075 (see Table 1).
limestones behave in a very similar way (except in some cases that are currently not well understood), and the selection of the sorbent should be based on factors other than their capture capacity along cycling (availability, cost, mechanical stability, ...). Figure 8 shows the effect of calcination temperature on the sorbent performance up to the maximum temperature (1000 °C) achievable in the present thermogravimetric analyzer. As can be seen, there is a range in temperatures (up to 950 °C) where the calcination temperature does not affect the sorbent performance much. At the lower calcination temperature there is only a modest improvement in the results. This is consistent with the existing results in the literature.6,11 In these works a similar decay in activity was observed with calcination temperatures
Figure 6. Conversion vs number of cycles for experiments carried out with different types of limestones. Particle size 0.4-0.6 mm. Calcination temperature 850 °C, 10 min; carbonation temperature 650 °C, 10 min; pCO2 of 0.01 MPa. The lines represent the data obtained with eq 5. From top to bottom: k ) 0.28 and Xr ) 0.22; k ) 0.52 and Xr ) 0.075; k ) 1.96 and Xr ) 0.075 (see Table 1).
Figure 7. Comparison of sorbent capture capacity (for Piaseck limestone and dolomite) along the cycles in terms of grams of CO2 captured per gram of parent sorbent. Calcination temperature 900 °C, 10 min.; carbonation temperature 650 °C, 10 min; pCO2 of 0.01 MPa.
as low as 750 °C in N2. On the other hand, increases in calcination temperature up to 1000 °C clearly cause the behavior of the sorbent to deteriorate. The sintering mechanism responsible for the decay in activity is drastically enhanced with temperature over 950-1000 °C. By contrast, results from Curran et al.5 showed high conversion at a calcination temperature of 1060 °C operating in continuous mode (ensuring a very fast calcination rate of sorbent particles when entering the hightemperature calciner). Further investigation is needed at this point to explain this discrepancy, and to understand the effect of very short calcination times. Good performance of the sorbent at these high temperatures (as good as that obtained at milder
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Figure 8. Conversion vs number of cycles for experiments carried out at different calcination temperatures. Limestone: La Blanca, 0.4-0.6 mm. Carbonation temperature 650 °C, 5 min; pCO2 of 0.01 MPa. The lines correspond to the data obtained from eq 5. From top to bottom: k ) 0.52 and Xr ) 0.075; k ) 0.73 and Xr ) 0.075; k ) 1.73 and Xr ) 0.075; k ) 2.11 and Xr ) 0.06 (see Table 1).
Figure 10. Sorbent conversion along the number of cycles for different calcination times. Limestone: La Blanca, 0.4-0.6 mm. Calcination temperature 950 °C; carbonation temperature 650 °C, 5 min; pCO2 of 0.01 MPa. The lines correspond to the data obtained from eq 5. From top to bottom: k ) 0.60 and Xr ) 0.075; k ) 0.76 and Xr ) 0.075; k ) 1 and Xr ) 0.075 (see Table 1).
Figure 9. Conversion vs number of cycles for two experiments carried out with dolomite calcined at two different temperatures. Carbonation temperature 650 °C, 10 min; pCO2 of 0.01 MPa. The lines correspond to the data obtained from eq 5. From top to bottom: k ) 0.28 and Xr ) 0.22; k ) 0.39 and Xr ) 0.11 (see Table 1).
Figure 11. Conversion vs cycle number for different carbonation/calcination atmospheres. Limestone La Blanca 0.4-0.6 mm. Calcination temperature 950 °C; carbonation temperature 650 °C, 5 min. The line corresponds to the data obtained from eq 5; k ) 0.52 and Xr ) 0.075 (see Table 1).
conditions) is essential to make the carbonation/calcination cycle19-21 technically viable operating at high pressures. For atmospheric pressure systems (postcombustion applications) calcination temperatures around 950 °C seem to be feasible for calcination in pure CO2, without deteriorating the sorbent performance even with extended calcination times (cumulative as the number of cycles increases). Finally, for illustrative purposes, the data from Deutch and Heller-Kallai22 have been included in the graph as an example of high sintering conditions (calcination temperature over 1200 °C for several minutes, under pCO2 of 0.1 MPa). These conditions already destroy the sorbent activity after the first calcination, bringing the value of conversion at the end of the fast reaction period to below 10%. Dolomite follows the same trend as limestone, with faster deactivation for calcination temperatures over 1000 °C as shown in Figure 9. At a given temperature, calcination time is known to affect the texture of the calcines derived from limestone. This in turns determines the maximum level of conversion in highly cycled samples through the value of the product layer critical thickness.15 Figure 10 shows that, for the first cycle, the maximum conversion can be very sensitive to the calcination time (temperatures above 900 °C). This difference disappears when the number of cycles is increased. The sintering mechanism associated with each carbonation-calcination cycle imposes a decay process much stronger than the one given by extended calcination times. Therefore, as with other variables investigated
here, the effect of calcination time on maximum calcination conversions is only visible for the first few cycles and it becomes modest when looking at the long cycle performance curves that will be characteristic of continuous systems operating with modest purge rates of solids. Finally, some tests have been conducted varying CO2 concentration during carbonation and calcination (0.01-0.1 MPa). Figure 11 presents the results and include the data from Curran et al.5 equivalent to a CO2 partial pressure of 0.4 MPa. The carbonation reaction rate of CaO particles has been described in the literature as a first-order reaction with respect to the CO2 partial pressure.8,9 But the maximum carbonation conversion is again highly insensitive to the partial pressure of CO2. It can be seen that sorbent performance is slightly worse for the first 10 cycles under a reaction atmosphere of pCO2 of 0.1 MPa, probably due to some pore blockage.18 This difference disappears when the cycle number is increased and is relatively modest compared with the general trend of deactivation. A few runs were made varying carbonation temperatures, obtaining results very similar to those presented in Figure 5. Semiempirical Equation Describing Decay in Activity. With all the previous data, a fitting exercise has been carried out to reformulate semiempirical equations describing the decay in sorbent capture capacity with the number of cycles. In earlier works10,15 that included detailed observations by mercury porosimetry and scanning electron microscopy of the textural changes of limestones along cycling, it was concluded that the main mechanism of sorbent deactivation is the progressive
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sintering, or grain growth, of the originally rich texture of the first calcine. According to this model, at a particular number of cycles, CaCO3 formed during carbonation will fill up all the available porosity made up of small pores plus a small fraction of the large voids, limited by the thickness of the product layer that marks the onset of the slow carbonation period. Secondorder effects (pore mouth blockage, isolated voids in the calcine, particle shrinking) can also be detected in some special sorbents and conditions.18 However, it is the grain sintering mechanism as the number of cycles increases, and the modest product layer thickness allowed on the resulting large voids, what that marks the decay of capture capacity. Based on these observations a correlation was proposed:10
XN ) fmN(1 - fw) + fw
1 1 + kN
(2)
This was derived by similarity with second-order deactivation rate equations for catalyst deactivation through sintering. Studies of sintering kinetics of catalyst indeed produce equations similar to the above.24 It has also been proved that many intrinsic mechanisms of catalyst deactivation lead to identical deactivation curves24 and, if the dominating mechanism is sintering, a residual activity of the catalyst is a characteristic feature of these decay curves.24,25 On the other hand, as a result of the experimental work presented in this paper, and especially attending to the long data series up to 500 cycles, it can be confirmed that there is a residual conversion that remains almost constant from N > 50 up to 500 cycles. This residual conversion, Xr, is around 0.0750.08 and is consistent with measured low values of surface area of highly cycled samples and a product layer thickness of around 50 nm.15 This proportionality between conversion and surface area through the product layer thickness makes more prominent the similarity with typical catalyst deactivation curves, where a change in normalized surface area takes place with time. As in Wang and Anthony,23 time can be replaced by number of cycles and the result is, for sufficiently long series of cycles24
-
(
)
Sr d(S/S0) S )k dN S0 S0
solids
T(calcination) (°C)
t(calcination) (min)
950 °C and calcination time is shown in Table 1. These two variables seem to have the strongest effect on sorbent performance along the cycles. As can be seen, k (the “deactivation constant” in eqs 3-5) increases with more extreme calcination conditions (longer times, higher temperatures). Work is in progress to elucidate more fundamental, microscopic mechanisms behind the observed sintering process. However, the correlation proposed here, and the trends observed in its critical parameters, should be valuable tools for the design of pilot installations intended to prove the concept of a range of CO2 capture systems based on CaO as a regenerable sorbent. In particular, the confirmation of a residual conversion capacity of the sorbent after hundreds of cycles is a relevant result for the design of CO2 capture cycles using a very low purge rate of sorbent.26
(3)
As described above, the work on textural analysis15 determined a product layer thickness with a very constant value along the cycles. Then, the CaCO3 formed (on each cycle), and therefore particle conversion, would be proportional to this product layer thickness and the available surface to react with CO2. Spatial requirements should be taken into account when the sorbent particle pore network is formed by pores of small diameter (due to different molar volumes of CaO and CaCO3), but this is not the case for highly cycled particles. Integrating eq 3 and considering the proportionality between available
Conclusions From all the experiments presented here, it can be concluded that the decay in maximum carbonation capacity along the cycles is a common feature in all the series of data irrespective of limestone type and process conditions, especially when comparing sorbent performance at a high number of cycles (calcination temperatures below 950 °C). Capture capacity decreases dramatically in the first 20 cycles and tends to stabilize when increasing the cycle number around a residual conversion of 0.075-0.08 that remains nearly constant up to 500 cycles. Calcination temperatures over 950 °C and/or extended calcina-
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tion times accelerate sorbent degradation, reaching the residual capture capacity at a lower number of cycles. The deactivation constant increases in these conditions, but the value of this residual activity seems to be insensitive. A semiempirical equation has been proposed to describe the experimental results by analogy with catalyst deactivation due to sintering. There is a good agreement between experimental results and those obtained with the proposed equation, with a couple of values for the deactivation constant (k ) 0.52) and the residual activity (Xr ) 0.075) that seem to be valid for a wide range of sorbents and conditions. The existence of a residual conversion could be important in the design of CO2 capture systems based on limestone as a regenerable sorbent. Acknowledgment This work is partially funded by the European Commission (ISCC and C3Capture projects) and the Spanish Ministry of Education (“Juan de la Cierva” contract to G.S.G.). We also thank Dr. E. J. Anthony from CANMET and Dr. H. Kruczec from Wroclaw University for supplying samples of Havelock and Katowice limestones. Literature Cited (1) Special Report on Carbon Dioxide Capture and Storage, IntergoVernmental Panel on Climate Change; Metz, B., Davidson, O., de Coninck, H., Loos, M., Meyer, L., Eds.; Cambridge University Press: Cambridge, U.K., 2005. (2) 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 (8), 2861. (3) Collot, A. G. Prospects for hydrogen from coal; CCC/78; IEA Clean Coal Centre: London, 2003. (4) 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. (5) Curran, G. P.; Fink, C. E.; Gorin, E. Carbon dioxide-acceptor gasification process: studies of acceptor properties. AdV. Chem. Ser. 1967, 69, 141. (6) Silaban, A.; Harrison, D. P. High-temperature capture of carbon dioxide: characteristics of the reversible reaction between CaO(s) and CO2(g). Chem. Eng. Commun. 1995, 137, 177. (7) Barker, R.. The reversibility of the reaction CaCO3 ) CaO + CO2. J. Appl. Chem. Biotechnol. 1973, 23, 733. (8) Bathia, S. K.; Perlmutter, D. D. Effect of the product layer on the kinetics of the CO2-lime reaction. AIChE J. 1983, 39, 79. (9) Shimizu, T.; Hirama, T.; Hosoda, H.; Kitani, K.; Inagaki, M.; Tejima, K.; A twin fluid-bed reactor for removal of CO2 from combustion processes. Trans. Inst. Chem.Eng. 1999, 77 (part A), 62. (10) Abanades, J. C.; Alvarez, D. Conversion Limits in the Reaction of CO2 with lime. Energy Fuels 2003, 17, 308.
(11) Ahiara, M.; Nagai, T.; Matsushita, J.; Negishi, Y.; Ohya, H. Development of porous solid reactant for thermal-energy storage and temperature upgrade using carbonation/decarbonation reaction. Appl. Energy 2001, 69, 225. (12) Gupta, H.; Fan, L. S. Carbonation-calcination cycle using high reactivity calcium oxide for carbon dioxide separation from flue gas. Ind. Eng. Chem. Res. 2002, 41, 4035. (13) Lu, H.; Reddy, E. P.; Smirniotis, P. G. Calcium Oxide Based Sorbents for Capture Carbon Dioxide at High Temperatures. Ind. Eng. Chem. Res. 2006, 45, 3944. (14) Hughes, R. W.; Lu, D.; Anthony, E. J.; Wu, Y. Improved longterm conversion of limestone derived sorbents for in situ capture of CO2 in a fluidised bed combustor. Ind. Eng. Chem. Res. 2004, 43, 5529. (15) 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. (16) Anthony, E. J.; Granatstein, D. L. Sulfation Phenomena in Fluidised Bed Combustion Systems. Prog. Energy Combust. Sci. 2001, 27, 215. (17) Laursen, K.; Duo, W.; Grace, J. R.; Lim, J. Sulfation and reactivation characteristics of nine limestones. Fuel 2000, 79 (2), 153. (18) 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. (19) Bandi, A. The effect of CO2 pressure and alkali salt on the mechanism of decomposition of dolomite. Thermochim. Acta 1976, 14 (12), 221. (20) Lin, S. Y.; Suzuki, Y.; Hatano, H.; Harada, M. Developing an innovative method, HyPr-Ring, to produce hydrogen from hydrocarbons. Energy ConVers. Manage. 2002, 43 (9-12), 1283. (21) Wang, J.; Anthony, E. J.; Abanades, J. C. Clean and efficient use of petroleum coke for combustion and power generation. Fuel 2004, 83, 1341. (22) Deutch, Y.; Heller-Kallai, L. Decarbonation and recarbonation of calcites heated in CO2. Part 1. Effect of the thermal regime. Thermochim. Acta 1991, 182, 77. (23) Wang, J.; Anthony, E. J. On the Decay Behavior of the CO2 Absorption Capacity of CO2-Based sorbents. Ind. Eng. Chem. Res.. 2005, 44, 627. (24) Bartholomew, C. H. Sintering kinetics of supported metals: new perspectives from a unifying GPLE treatment. Appl. Catal., A: Gen. 1993, 107, 1. (25) Corella, J.; Adanez, J.; Monzon, A. Some Intrinsic kinetic equations and deactivation mechanisms leading to deactivation curves with a residual activity. Ind. Eng. Chem. Res. 1988, 27, 375. (26) Abanades, J. C.; Alvarez, D.; Grasa, G.; Soley, E.; Pajares, J. A new fluidized bed combustion system to capture CO2 with CaO. Presented at the International Conference on Coal Science and Technology, Okinawa, Oct 2005.
ReceiVed for reView May 31, 2006 ReVised manuscript receiVed September 20, 2006 Accepted September 20, 2006 IE0606946
