Ind. Eng. Chem. Res. 2010, 49, 12429–12434
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Ca(OH)2 Superheating as a Low-Attrition Steam Reactivation Method for CaO in Calcium Looping Applications Vlatko Materic´,* Susan Edwards, Stuart I. Smedley, and Robert Holt Industrial Research Ltd., 69 Gracefield Road, SeaView, Lower Hutt 5010, New Zealand
Steam hydration of lime is an effective method for restoring CO2 capture activity but gives rise to high particle attrition rates in a fluid bed reactor. This paper describes the phenomenon of Ca(OH)2 superheating, also referred to as superheated dehydration (SD). The potential of an attrition-free lime reactivation process using this phenomenon is also investigated. Attrition rates of the sorbent are measured when a reactivation step using steam hydration is implemented every three carbonation/calcination cycles. It has been shown that the presence of CO2 during the dehydration step reduces attrition during subsequent cycles. Experiments performed in a small fluid bed reactor show that the presence of 40-100% CO2 during the dehydration step increases the initiation temperature of the decomposition of Ca(OH)2 from 445 to 618 °C. The thermodynamic equilibrium water vapor pressure for the dehydration reaction at 618 °C is 516 kPa, whereas no water vapor was detected in the reactor during the dehydration step before the temperature reached 618 °C. Under these circumstances it is proposed that the Ca(OH)2 is in a nonequilibrium “superheated state”. A CO2 capture cycling experiment, with a reactivation step every three carbonation/calcination cycles, maintained an average activity of 60%, creating only 3.25% of fines < 150 µm after 28 carbonations. The reactivation step consisted of hydrating the sorbent at a temperature of 270 °C and dehydrating it in 100% CO2 with a 23 min hold at 520 °C. It is proposed that the SD phenomenon may be a key step in the development of an industrially feasible method of lime reactivation for use in CO2 capture and in thermal energy storage applications. Introduction The lime-limestone (CaO-CaCO3) thermochemical cycle, also referred to as calcium looping, has shown the potential to form the basis of economical solutions for both CO2 capture1 and heat storage and upgrade applications.2 The chemistry of this cycle is very simple because it is based on the following carbonation/calcination reaction: CaO + CO2 T CaCO3 ∆H < 0
(1)
However, one of the principal practical challenges to calcium looping is the irreversibility of the carbonation/calcination reaction which leads to the progressive reduction in the conversion of the carbonation reaction. Steam hydration of lime was shown to be an effective method for restoring its CO2 captureactivity.Thermogravimetricanalysis(TGA)investigations3-7 have shown that steam hydration can be used repeatedly to reactivate the sorbent after a number of CO2 capture (carbonation/ calcination) cycles. After reactivation, the sorbent’s CO2 capture activity typically returns to 60-80% of the theoretical capacity and then decreases with further CO2 capture cycling. A desired activity of the sorbent can be realized by selecting the appropriate frequency of reactivation, i.e., the number of CO2 capture cycles performed between reactivations. However, during steam hydration the lime particles undergo considerable expansion,6 and on the basis of experiences with steam hydration of lime for improved SO2,8 it is expected that the steam reactivation process is likely to cause substantial attrition when used in fluid bed reactors. This in turn greatly limits the practical applicability of the steam reactivation method as most calcium looping applications require the use of fluidized bed reactors in order to achieve the required heat- and masstransfer rates. * To whom correspondence should be addressed. E-mail:
[email protected].
This paper describes the discovery of a phenomenon leading to reduced attrition during fluid bed CO2 capture cycling experiments with periodic reactivation by steam hydration. In these experiments, the presence of CO2 during the dehydration step caused Ca(OH)2 to superheat, i.e., remain stable at temperatures higher than those at which it usually decomposes, up to 618 °C. This phenomenon has been named superheated dehydration (SD). Chaix-Pluchery et al.9 propose that the presence of proton donor species adsorbed on the surface of the Ca(OH)2 crystals can cause an increase in the temperature of dehydration of Ca(OH)2 Further, it is thought that the defects formed during hydration can be annealed by thermal treatment of the hydroxide.10 The annealing of crystal defects that occurs during the superheating of Ca(OH)2 could be the cause of the observed reductions in attrition rates induced by this step. With this hypothesis in mind a series of experiments, described below, were performed to further explore the phenomenon of SD. The potential for SD to reduce sorbent attrition rates in long CO2 capture cycling experiments is investigated. The effects of CO2 concentration on the decomposition temperature of the hydroxide and the effect of the time Ca(OH)2 spends in the superheated state on the attrition rates of the particles are also investigated. Method Limestone was supplied by Taylors Lime, Makareao, Otago, New Zealand. The limestone provided was a highly pure calcite with about 2% silica and traces of iron oxide. The limestone was sieved, washed, and then sieved between 300 and 600 µm. Gases were supplied by BOC. All experiments were performed in a fluidized bed reactor with a diameter of 32 mm, with a bed volume of about 0.13 L and a height of 1.3 m. The reactor was externally heated with a vertical electric 2.4 kW furnace, and the bed temperature was controlled with a Cal3300 thermocontroller. Mass flow control-
10.1021/ie100265x 2010 American Chemical Society Published on Web 11/08/2010
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Figure 1. Schematic of experimental fluid bed reactor system. Preheater (PH), mass flow controllers (MFC), mass flow meter (MFM), vacuum pump (VP), differential pressure transmitter (dP), temperature of the bed and the outlet gas (T), and relative humidity sensor (RH).
Figure 2. Water vapor pressure and temperature during dehydration of Ca(OH)2 in nitrogen.
lers (MFC) supplied a N2/CO2 gas mixture which was preheated to >300 °C before being injected into the reactor plenum chamber. In hydration experiments, controlled amounts of liquid water were injected into flowing N2 preheated at >200 °C, and the resulting mixture was further heated to >300 °C. Gases leaving the reactor were analyzed for relative humidity with a sensor (RH) maintained at 105 °C and passed through a mass flow meter (MFM). To lower calcination temperatures and thereby maintain reactor integrity, the calcination reactions were performed under reduced pressure. Figure 1 shows the experimental setup. In all experiments, the differential pressure across the bed was monitored. The temperatures of the bed and the outlet gas were also monitored. The mass flow controllers, water pump, vacuum pump, reactor furnace, and solenoid valves were controlled through a computer with custom-written Labview control software. Output from the analysis devices was recorded through the control software and logged every 3 s. The control software allowed programming of automated experimental sequences, thus ensuring the repeatability of the experimental conditions in multicycling experiments. In all experiments, raw limestone was first calcined and then subjected to the CO2 capture cycle (repeated carbonation/ calcination steps). Every three capture cycles, a reactivation step was performed (hydration/dehydration steps). This high frequency of reactivation was chosen to allow faster investigation of the effect of repeated reactivation steps on the sorbent. It is expected that in practical applications the reactivation frequency would be lower, probably once every five to 10 capture cycles. Calcination reactions were performed under a flow of pure N2 at reduced temperature, at 805 °C, and under reduced pressure (about 22 kPa) in order to preserve the integrity of the stainless steel reactor. Carbonation reactions were performed in a flow of CO2 (37.5%) in N2 at 620 °C. The CO2-laden gas was contacted with the solid at temperatures around 620 °C. Because the reactor was not actively cooled, the heat of carbonation increased the bed temperature to about 680 °C. Lime was hydrated in a flowing mixture of water vapor (20% absolute humidity) and N2 (3 slpm). The wet gas was contacted with the solid at temperatures around 360 °C for 40 min. Because the reactor was not actively cooled, the heat of hydration increased the bed temperature to 400 °C. Typically,
hydration proceeded to a breakthrough point in about 10 min, when the humidity in the output flow rose markedly. Dehydration reactions were performed by heating the freshly hydrated material in pure N2 (4 slpm). The heating sequence was composed of four steps; after hydration, the material was heated to a temperature of 520 °C and held there for 23 min. After that, the material was heated to 620 °C and held at that temperature for 22 min. Some dehydration experiments were performed in different mixtures of N2 and CO2 (12.5, 25, 37.5, 70, and 100%) at 4 slpm, but using the same heating sequence. Further dehydration experiments were performed in CO2 (37.5%)/N2 at 5 slpm. The same heating sequence was used, but the hold time at 520 °C was changed to 0 (no hold), 23, and 40 min. Longer hold times of 60 and 100 min were used in 100% CO2 dehydrations. In all experiments, the minimum fluidization gas velocity (Umf) was determined and the bed was usually operating at 1.5-2.5 Umf. Experiments performed in a glass vessel of identical dimensions have shown that the bed is in the slugging regime at these gas velocities. The particle size distribution of the sorbent was periodically measured by sieving the sorbent extracted from the bed after calcination. Particles that passed through the 150 µm sieve were considered fines in this work. After sieving, the different fractions were remixed, returned to the fluid bed reactor, and subjected to further cycling. CO2 capture activity was determined by two methods. First, during calcination, the mass flow meter at the outlet of the system detected gas (assumed to be CO2) in excess of the fluidizing N2 gas. An integration was performed, and the quantity of CO2 released was calculated in standard liters. This method is used to calculate the amount of CO2 captured during the experiments depicted in Figures 2-4. Second, during carbonation the pressure sensor (P) detected a sudden increase in pressure just above the bed, which was attributed to CO2 that did not react with the sorbent. This point is also called the breakthrough point of CO2, because after this moment the sorbent in the reactor was not able to capture the entirety of the CO2 introduced in the bed. The quantity (standard liters) of CO2 captured by the lime before the breakthrough point is the product of the CO2 flow rate and the time until
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Figure 5. Effect of CO2 concentration on the decomposition of Ca(OH)2.
Figure 3. Water vapor pressure and temperature during dehydration of Ca(OH)2 in nitrogen and carbon dioxide (37.5%).
Figure 4. Water vapor pressure and temperature during dehydration of Ca(OH)2 in nitrogen and carbon dioxide (37.5%), with removal of CO2 at minute 65.
breakthrough is reached. The quantity of CO2 captured after the breakthrough point is calculated by integration. The amounts of CO2 captured before and after breakthrough are added, and a total quantity of CO2 captured by lime during carbonation is obtained. In all cycling experiments, CO2 capture activity of the sorbent is calculated using the second method and is expressed as a percentage of the maximum capacity of lime (0.785 g of CO2/g of CaO). The average CO2 capture activity of the sorbent is expressed on that same basis and does not include the CO2 captured during the annealing step or the carbonation that immediately follows this step. The water content of the gas leaving the reactor was measured with a relative humidity sensor maintained at constant temperature, thereby enabling calculation of the absolute humidity of the outlet gas. Results Demonstration of the Superheated Dehydration Phenomenon. For the experiment depicted in Figure 2, dehydration was performed in an atmosphere of pure N2. Figure 2 shows the
variation with time of the temperature of the bed and the water vapor pressure of the gas leaving the bed. At time equal to 35 min, the bed material is completely hydrated, and having reached saturation, all the water that is introduced leaves the reactor and is detected by the sensor. At 35 min, the water flow was stopped and the heating of the bed started. As the temperature of the bed increased, the vapor pressure of the water vapor leaving the bed showed a sudden increase at 445 °C. This increase was attributed to the dehydration of the Ca(OH)2, which took about 6 min to complete. Note that this method of determining the dehydration temperature was used for all subsequent experiments. The CaO formed from the dehydration of the Ca(OH)2 was subsequently carbonated in a stream of N2 and CO2 (20%) at 620 °C. The resulting material was then calcined, and the CO2 capture capacity of the CaO was measured at 71% of its theoretical maximum capacity. In a second experiment dehydration was performed in an atmosphere containing 37.5/62.5 CO2/N2. Figure 3 shows the variation with time of the temperature of the bed and the vapor pressure of water vapor leaving the bed. In this experiment, dehydration occurred only as the bed reached 618 °C, as testified by a sudden increase in the water vapor pressure of the gas leaving the reactor. Dehydration was completed in 6 min. This result implies that the Ca(OH)2 was stable at a temperature higher than its usual decomposition temperature. For this reason the Ca(OH)2 is said to have been superheated. After dehydration, the material was left in contact with CO2 for 22 min and subsequently calcined; the material had captured 58% of its theoretical maximum capacity. In a third experiment, dehydration was performed in an atmosphere containing 37.5/62.5 CO2/N2 until the end of the 520 °C hold. At that moment, the gas was switched to 100% N2. Figure 4 shows the variation with time of the temperature of the bed and the vapor pressure of water vapor leaving the bed. Dehydration starts 7 s after the CO2 is removed from the inlet gas. The dehydrated material was then calcined, and 15% of this material was shown to be calcium carbonate. This carbonate must have formed in the presence of CO2 while the material largely comprised Ca(OH)2. This experiment illustrates that Ca(OH)2 reacts very slowly with CO2 in these conditions. Effect of CO2 Concentration on Dehydration Temperature. To examine the effect of CO2 concentration on the dehydration temperature of Ca(OH)2, five dehydration experiments were performed. These entailed a 23 min hold at 520 °C under a flow with different mixtures of N2 and CO2 (12.5, 25, 37.5, 70, and 100%). Figure 5 illustrates that the dehydration temperature increased with increasing CO2 content up to about 40%, whereafter it remains constant.
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Table 1. Results of Extended Multicycling Experiments % fines
av % CO2 capture
calcination
10
19
49
19
49
% CO2 during dehydration 37.50 0
3.1 5.0
3.6 6.1
6.7 9.0
37 38
33 33
Effect of Superheated Dehydration on Attrition Rates. Two cycling experiments were performed to investigate the effect of repeated reactivation steps on the sorbent. Dehydration was either performed in pure nitrogen or in a mixture of nitrogen and carbon dioxide (37.5%). A total of 48 capture (carbonation/ calcination) cycles was performed in each of these experiments with a reactivation step every three capture cycles, totaling 15 reactivation steps. Table 1 sums up the results obtained for these experiments. The data show greatly reduced attrition rates in the experiment where CO2 was present during dehydration, but very similar CO2 capture activity for both experiments. Ca(OH)2 always superheated when CO2 was present in the dehydration gas, but a major transition occurred after the first four reactivation steps. The temperature of the onset of dehydration shifted from 610 to 560 °C, and dehydration was completed earlier during the experiment, as shown in Figure 6. As a result, the hydroxide spent less time in the superheated state after the fifth dehydration. Concurrent with this reduction in superheating time, the attrition rate increased after the 19th calcination as shown in Table 1. Nevertheless, the amount of fines at the end of the experiment was lower when CO2 was present during dehydration. Particles that were superheated during SD experiments (37.5% CO2) also exhibit a net increase in size as compared to particles from the normal dehydration experiment (100% N2). Table 2 shows the evolution of particle sizes of the entirety of the sorbent during the experiments. Effect of Hold Time on the Attrition Rate. To test the hypothesis that annealing in the superheated state is responsible for attrition rate reductions, five separate cycling experiments were performed, each with a different hold time in the superheated state, as shown in Table 3. This table shows that the longer the material spends in the superheated state, the greater the reduction in attrition rate with no loss of CO2 capture activity. The experiment in which the sorbent spent the most time in the superheated state (strongest annealing conditions) had attrition levels lower than attrition occurring in simple CO2 capture cycling (around 2% fines after 19 calcinations). Optimization Experiment. A single optimization experiment was performed to evaluate the potential of the SD method for the reactivation for lime. Reactivation parameters from previous
Figure 6. Evolution of dehydration temperature and time to completion in multicycling SD experiment.
experiments that gave the best results for lime reactivation were assembled in a 29 capture cycles experiment with reactivation step every three capture cycles, totaling nine reactivations. The chosen conditions were 100% CO2 and a 23 min hold at 520 °C for dehydration. Hydration starting temperature was set to 270 °C. During this experiment the sorbent has maintained an average activity of 60%, creating only 3.25% of fines < 150 µm. Discussion These experiments show that the presence of 40-100% CO2 during the dehydration step increases the decomposition temperature of Ca(OH)2 from 445 to 618 °C. The thermodynamic equilibrium water vapor pressure for the dehydration reaction at 618 °C is 516 kPa, whereas no water vapor was detected in the reactor until the material had reached 618 °C. Under these circumstances we propose that the Ca(OH)2 was in a nonequilibrium superheated state. For this reason, the phenomenon was named superheated dehydration or SD. Furthermore, during the superheated phase of the experiments, CO2 was present at partial pressures far above the equilibrium partial pressure for CaCO3, The apparent stability of Ca(OH)2 implies that the reaction of Ca(OH)2 and CO2 proceeds only slowly under these conditions. Carbonation of Ca(OH)2 is usually reported to be fast at these temperatures.11,12 A possible explanation for the observed phenomena can be found in the literature.9,10,13 Evidence suggests that the mechanism of decomposition of Ca(OH)2 proceeds via several steps. Initially, as Ca(OH)2 is heated below the normal decomposition temperature, the hydroxyl groups dissociate into mobile H+ ions (protons) and O2- sites (proton defects), leading to a measurable proton conductivity As the temperature increases further, these mobile H+ ions attach to hydroxide ions and form water molecules, leaving an excess of proton defects in the lattice.13 As the temperature approaches the dehydration temperature, a critical concentration of proton defects is reached and the water molecules aggregate into water-rich zones and eventually escape from the crystal via channels or pathways in the crystal lattice.9 Proton conductivity in Ca(OH)2 has been shown to be enhanced by CH3COOH vapor (proton donor) and reduced by NH3 gas (proton acceptor).13 Furthermore, in the presence of proton donor molecules, such as chemisorbed water, the normal decomposition temperature of Ca(OH)2 can be elevated by many tens of degrees, 90 °C for 236 Pa of D2O.9 It was suggested that the H+ ions injected by chemisorbed water lead to a reduction in the concentration of proton defects at a given temperature through recombination. This in turn leads to an increase in the temperature where the critical concentration of proton defects is reached. As a result, the onset temperature for dehydration is increased. A similar mechanism is proposed to explain the results obtained here. It is proposed that water, chemisorbed on the surface or present beneath the surface of the crystals, can react with CO2 to release H+ ions. The H+ ions released by this reaction are injected into the solid Ca(OH)2 and result in the significant elevation of the decomposition temperature following the mechanism described above.9 Higher CO2 partial pressures would increase the concentration of injected H+ ions and therefore increase the decomposition temperature further, as shown in Figure 5. Correspondingly, a sudden reduction in the CO2 partial pressure when Ca(OH)2 is in the SD state would lead to a rapid decrease in the concentration of adsorbed CO2 and injected H+ ions and therefore to the rapid decomposition
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Table 2. Evolution of Particle Sizes during Long-Term Cycling pure N2 dehydrations (% mass)
37.5% CO2 dehydrations (% mass)
calcination
initial
10
19
49
10
19
49
size (µm) >600 300-600 0-150
0.0 100 0.0
0.0 90.9 5.0
0.0 89.0 6.1
1.2 85.0 9.0
0.6 90.6 3.1
2.4 91.1 3.6
11.1 79.5 6.7
Table 3. Effect of Hold Time on Fragmentation and CO2 Capture superheating % CO2 during % fines after nth calcination % CO2 capture time superheating activity (min) (%) 10 19 (av, %) 100 60 40 23 0
100 100 37.5 37.5 37.5
2.2 3.4 4.7
0.6 1.7 2.6 4.1 5.8
50 52 42 31 45
of the Ca(OH)2. This phenomenon has been observed by experiment and depicted in Figure 3. Following the line of reasoning described above, it would be expected that other acid compounds, e.g., acid gases such as SOx and NOx, if in contact with Ca(OH)2 from temperatures well below the normal dehydration temperature of 450 °C, would result in an increase in the dehydration temperature and improved attrition resistance in a fluidized bed reactor. The slow rate of carbonation of Ca(OH)2 at high temperature can be explained by the increased stability of Ca(OH)2. Carbonation of Ca(OH)2 at ambient temperature is reported to be slow, and it was shown in earlier work6,8 that the carbonation reaction of Ca(OH)2 becomes rapid only at about 350-400 °C, close to the temperature of the onset of dehydration. It is likely that the rate of carbonation of Ca(OH)2 below the temperature of decomposition of Ca(OH)2 is limited by the diffusion of CO2 into the particles and H2O out of the particles. In SD experiments, carbonation of Ca(OH)2 in the 400-600 °C region is slow because the Ca(OH)2 is still stable and therefore the reaction is limited by diffusion at these temperatures, which is not the case when Ca(OH)2 is not superheated. Not all calcium hydroxide materials superheat in the presence of CO2.11,12 Carbonation, and therefore dehydration, is usually completed before calcium hydroxide reaches 550 °C. The difference in the behavior of Ca(OH)2 in this work and those reported in the literature11,12 can only be explained by a difference in the physical properties of Ca(OH)2 obtained by the hydration method described here. Since the exact mechanism by which the presence of CO2 in the gas phase leads to an injection of protons into Ca(OH)2 is unknown, it is difficult to postulate what particular property of Ca(OH)2 enables it to superheat in the presence of CO2. The ability of the SD process to reduce attrition has been observed repeatedly and consistently in over 30 cycling experiments. It is proposed that during the superheated phase the stability of the Ca(OH)2 with rising temperature allows the annealing of stacking faults and mechanical strain formed during hydration and allows crystal growth. These effects should strengthen the particle to resist the forces of attrition. The occurrence of particles larger in size than the initial particles, see Table 2, is probably due to the expansion by hydration and to improved resistance to attrition provided by the annealing occurring in the superheated state. The data in Table 3 show that the particle fragmentation rate, resulting from repeated cycling, decreases the longer the Ca(OH)2 remains at 520 °C under CO2/N2 mixtures. This result
is as predicted by the annealing hypothesis. Note also that the longer annealing times do not reduce CO2 capture activity of the lime and may even improve it. The annealing hypothesis also explains the increase in the attrition rate observed with prolonged cycling, as shown in Table 1. As the dehydration temperature decreases with prolonged cycling, the freshly formed Ca(OH)2 crystal spends less time annealing in the superheated state. Fewer crystal defects are annealed, and the resulting material is less able to resist the forces of attrition in the fluidized bed environment. This progressive reduction in SD temperature is the phenomenon limiting the full potential of the SD process for long-term cycling of lime and is the focus of further work. During an optimized cycling experiment, natural limestone maintained an average activity of 60%, creating only 3.25% of fines < 150 µm over 28 carbonations. Such a result is an encouraging indicator of the potential of the SD method as a reactivation process for lime. Conclusions When lime, steam hydrated at 350 °C, is heated to decomposition in the presence of CO2, the resulting particles have a significantly reduced attrition rate and improved CO2 capture activity. Under such conditions, it is proposed that Ca(OH)2 enters into a superheated state with respect to its equilibrium temperature at the water vapor pressures observed in the reactor. It is postulated that the superheated state may result from the absorption of H+ ions generated by water and CO2. It is further postulated that thermal annealing during the superheated state relieved the crystal of strain created by hydration. Experimental results obtained concur with these hypotheses. Using the superheated dehydration phenomenon, an industrially feasible method of lime reactivation using steam hydration could be developed for use in CO2 capture and thermal energy storage applications. Acknowledgment We acknowledge Mr. Del Puerto for ensuring successful operation of the fluidized bed. Literature Cited (1) Abanades, J. C.; Grasa, G.; Alonso, M.; Rodriguez, N.; Anthony, E. J.; Romeo, L. M. Cost structure of a postcombustion CO2 capture system using CaO. EnViron. Sci. Technol. 2007, 41, 5523–5527. (2) Kyaw, K.; Matsuda, H.; Hasatani, M. Applicability of carbonation/ decarbonation reactions to high-temperature thermal energy storage and temperature upgrading. J. Chem. Eng. Jpn. 1996, 29 (1), 119–125. (3) Abanades, J. C.; Alvarez, D. Conversion limits in the reaction of CO2 with lime. Energy Fuels 2003, 17, 308–315. (4) Fennell, P. S.; Davidson, J. F.; Dennis, J. S.; Hayhurst, A. N. Regeneration of sintered limestone sorbents for the sequestration of CO2 from combustion and other systems. J. Energy Inst. 2007, 80 (2), 116– 119. (5) Hughes, R. W.; Lu, D.; Anthony, E. J.; Wu, Y. Improved long-term conversion of limestone-derived sorbents for in situ capture of CO2 in a fluidized bed combustor. Ind. Eng. Chem. Res. 2004, 43, 5529–5539.
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(6) Manovic, V.; Anthony, E. J. Sequential SO2/CO2 capture enhanced by steam reactivation of a CaO-based sorbent. Fuel 2008, 87 (8-9), 1564–1573. (7) Sun, P.; Grace, J. R.; Lim, C. J.; Anthony, E. J. Investigation of attempts to improve cyclic CO2 capture by sorbent hydration and modification. Ind. Eng. Chem. Res. 2008, 47, 2024–2032. (8) Scala, F.; Montagnaro, F.; Salatino, P. Enhancement of sulfur uptake by hydration of spent limestone for fluidized-bed combustion application. Ind. Eng. Chem. Res. 2001, 40, 2495–2501. (9) Chaix-Pluchery, O.; Bouillot, J.; Ciosmak, D.; Niepce, J. C.; Freund, F. Calcium hydroxide dehydration early precursor states. J. Solid State Chem. 1983, 50 (2), 247–255. (10) Beruto, D.; Spinolo, G.; Barco, L.; Tamburini, U. A.; Belleri, G. On the nature of the crystallographic disorder in submicrometer particles of Ca(OH)2 produced by vapour phase hydration. Ceram. Int. 1983, 9 (1), 22–25.
(11) Fan, L. S.; Gupta, H. Sorbent from separation of carbon dioxide (CO2) mixtures. U.S. Pat. 7,067,456, B2, 2006. (12) Manovic, V.; Lu, D.; Anthony, E. J. Steam hydration of sorbents from a dual fluidized bed CO2 looping cycle reactor. Fuel 2008, 87 (1516), 3344–3352. (13) Freund, F.; Wengeler, H. Proton conductivity of simple ionic hydroxides, part 1. Ber. Bunsen-Ges. Phys. Chem. 1980, 84, 866–873.
ReceiVed for reView February 2, 2010 ReVised manuscript receiVed August 1, 2010 Accepted October 21, 2010 IE100265X
