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Steam Hydration of Calcium Oxide for Solid Sorbent Based CO Capture: Effects of Sintering and Fluidized Bed Reactor Behavior Alan Wang, Niranjani Deshpande, and Liang-Shih Fan Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 04 Dec 2014 Downloaded from http://pubs.acs.org on December 5, 2014

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Steam Hydration of Calcium Oxide for Solid Sorbent Based CO2 Capture: Effects of Sintering and Fluidized Bed Reactor Behavior

Alan Wang, Niranjani Deshpande, and L.-S. Fan*

William G. Lowrie Department of Chemical and Biomolecular Engineering The Ohio State University 140 W. 19th Avenue, Columbus, Ohio 43210 U.S.A.

*To whom correspondence may be addressed. Telephone: (614)-688-3262 Fax: (614)-292-3769 Email: [email protected]

ACS Paragon Plus Environment

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Abstract: This study reports on the hydration characteristics in the three-step CarbonationCalcination Reaction (CCR) process with intermediate steam hydration. Specifically, experimental results of sorbent reactivation in a fluidized bed steam hydration reactor are presented along with the effect of the operating gas velocity (Ug) and different calcination conditions on the hydration reaction. The enhancing effect of increasing Ug on the hydration rate is evident, but limited with a threshold at 0.3 m/s, above which the bulk phase mass transfer effects are less significant. The highest hydration conversion of 83% is achieved in 30 min at the highest Ug of 0.5 m/s. However, increasing Ug also yields lower steam utilization during hydration, thus an optimal operating condition respect to both the solid and steam conversions shall be determined via an overall process analysis. In addition, the parametric effect of different calcination conditions, specifically the calcination temperature, time, and reactor type, on the hydration reaction mechanism is investigated.

The

sorbent

reactivity

and

morphology is

characterized

using

thermogravimetric (TGA) and Brunauer–Emmett–Teller (BET) analysis as well as scanning electron microscopy (SEM) imaging. Sorbents produced at lower calcination temperatures and in a reactor with sufficient mixing exhibit larger surface area and pore volume, and superior reactivity. The effect of calcination time, within the scope of this study, is not as significant. Lastly, sorbent reactivity toward steam hydration depends strongly on its surface kinetics but not to the same extent as carbonation. The hydration and carbonation reactions are compared using a modified Jander’s equation,

[1−1−∝ ] = .


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1. Introduction The growing population coupled with the accelerated development in many areas around the world has led to an increasing energy demand on fossil fuels, along with a simultaneous increase in carbon dioxide (CO2) emissions, with neither showing signs of slowing down in the near future.1-2 However, there is evidence that governing entities and policy makers are becoming increasingly aware of the need to reduce the global carbon footprint to maintain the delicate balance of the earth’s ecosystems.3-4 In recent years, plethora of CO2 capture strategies and technologies are coming into effect at various levels internationally. 5-8 The calcium looping process is one such promising technology for post-combustion CO2 capture from large-scale point sources, such as coal-fired power plants.9-10 This technology uses a limestone-based solid sorbent, either calcium oxide (CaO) or calcium hydroxide (Ca(OH)2), to chemically absorb CO2 from the flue gas stream to form calcium carbonate (CaCO3). The CaCO3 is then decomposed while releasing a concentrated stream of CO2 that is ready for compression and sequestration.11 In this reversible manner, the sorbent can be regenerated and utilized repeatedly in a cycle. The different aspects of the CaO-based calcium looping technology are being researched, leading to the demonstration of a number of large pilot-scale projects worldwide. 12-14. Unfortunately, the high-temperature reaction cycle of calcium looping technology results in a rapid decrease of sorbent reactivity due to thermal sintering.15 Over the last decade, various methods have been proposed, researched, and developed to overcome this decay in sorbent reactivity, including: synthesizing structurally engineered sorbents to improve mechanical strength, doping the sorbent with chemical additives to increase resistance to sintering, using dolomitic sorbents that are more resistant towards sintering (but at the expense of lower CO2


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carrying capacity), and the inclusion of a recarbonation step at a higher CO2 partial pressure prior to calcination to increase the sorbent’s maximum CO2 capture capacity.16-25 The hydration of CaO sorbent to form Ca(OH)2 (Eqn. 1) is another promising approach because of the latter sorbent’s increased reactivity towards CO2.26 + ↔

(1)

The CaO hydration reaction has been previously reported by a number of researchers using a wet gas or water instead of high temperature steam.27-28 Using steam hydration to reactivate sintered sorbent for CO2 and SO2 capture has received increasing attention in recent years.18,29-32 Hydration reaction is believed to restore sorbent reactivity by improving surface morphology and cracking of the product shell in case of sulfation reaction.32-33 In addition, the reversible high temperature steam hydration/dehydration cycle has potential applications in chemical heat pump systems.34-35 The mechanism of high temperature hydration can be explained by two different governing equations for reactions close to and away from the thermodynamic equilibrium.35 Various options exist for applying hydration as a sorbent reactivation method such as sorbent pre-treatment with water/steam, hydration after a specific number of cycles, or in-line partial/complete hydration of the calcined solids after every cycle.33,36 The last option necessitates the inclusion of an additional hydration reactor. The effective integration of steam hydration as a third step in the CCR process requires operating the hydrator reactor near equilibrium conditions for efficient heat recovery. This three-step reaction concept, i.e., carbonation-calcination-hydration, forms the basis of the Ohio State University’s (OSU) Carbonation-Calcination-Reaction (CCR) process.26 Steam hydration can effectively reactivate spent CaO sorbent30, but the success of the three-step calcium looping technology as a commercially viable technology for CO2 capture depends


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largely on the successful development of a hydration reactor. Accordingly, the design of a benchscale, high-temperature fluidized bed steam hydrator was published with preliminary results on the effect of operating temperature and steam partial pressure.37 Operating the hydrator isothermally, however, proved to be challenging due to the high exothermicity of reaction. Control of the reaction temperature is critical because the relationship between equilibrium partial pressure of steam and temperature determines reactor’s operating limitations.10 At atmospheric steam pressure, the maximum temperature at which steam hydration can occur is ~512 °C. Without pressurizing the reactor, harnessing the heat of hydration reaction for electricity generation necessitates an extremely narrow operating window of 450-512 °C.38-39 This study is intended to contribute to three objectives, namely, the development of a scalable reactor model, the enhancement of solid conversion within the system design constraints, and the investigation of sintering effects on the hydration reaction. Updated hydration results, including the effect of operating gas velocity (Ug) on both the solid and steam conversions, are presented. In addition, the effects from other components of the process - specifically the calcination conditions, including the calcination temperature and time as well as the calciner design - on the steam hydration mechanism are discussed. The calcined CaO sorbent is subjected to Brunauer– Emmett–Teller (BET) and scanning electron microscopy (SEM) analysis to determine its microstructure and surface morphology. Lastly, the hydration and carbonation data are fitted by a modified Jander’s equation, and fundamental differences in carbonation and steam hydration reaction are discussed. 2. Experimental The following section outlines the procedure, analysis techniques used, and sorbent materials, and describes the reactor used for this experiment.


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2.1 Reactor: Figure 1 shows a schematic of the bench-scale, stainless steel hydrator setup. The principal design consisting of a mechanical mixer and internal baffles has been described in another publication.37 This reactor consists of three sections: one 5” ID cylindrical section (12” tall) with an internal wall baffle, a 5”-to-2.5” reducer section that connects the main reactor body to a 2.5” ID riser (6” tall), and lastly a 2.5” ID riser section (16” tall). The riser section allows for steam and entrained solids to exit to a bag house. Solid sampling was achieved by capturing the entrained solids at the bottom of the riser using a sampling tube. Four type-K thermocouples (TCs) are positioned along the vertical axis of the reactor. Two TCs, immersed in the solid bed, measure the reactor temperature while the other two measure the temperature of the inlet gas and outlet gas/solid streams. The reaction temperature was calculated based on the average of the two TCs in contact with the solids. The steam generator consists of stainless steel heating coils enclosed by two high-temperature ceramic heaters (OMGEA Engineering Inc.). The steam generator can superheat steam to 300400 °C, depending on the water flow rate that is controlled by an Optos 3HM liquid pump (Eldex Laboratories) with a precision of 0.01 ml/min. Lastly, the actual overall water flow rate was verified by measuring the weight change of the water reservoir before and after each experiment. 2.2 Sorbent Material: Two types of sorbents, namely pulverized limestone (CaCO3 > 90 %, d50= 20 um) and pulverized lime (CaO > 95%, d50= 20 um), were obtained from Graymont, Inc. The specifications of the pulverized limestone are listed in Table 1. Due to its fine particle size, this calcium sorbent can be classified as Geldart Group C, which is known to be cohesive and difficult to fluidize especially at high temperatures.40 The minimum fluidization velocity (Umf) for the calcined


6

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sorbent, CaO, was measured to be 0.04 m/s from cold model experiments. Initiation of entrainment occurs at Ug above 0.7 m/s due to the aggregative fluidization behavior. 2.3 Sorbent Preparation: In lab, the pulverized limestone samples were calcined, using either a rotary calciner (FEECO) or a high-temperature fixed bed (FB) furnace, before being subjected to steam hydration. The specifications of the rotary calciner (RC) were described in another publication.26 A average solid residence time of 1 hr was required to reach complete calcination at 900 °C in the RC. Alternatively, samples were also prepared in the high-temperature fixed bed furnace, with precise control of temperature and time ranging from 900-1000 °C and 1-3 hr, respectively. The lack of sweeping gas means the calcination condition, in either the rotary calciner or the fixed bed furnace, consists of high concentration CO2 similar to the gas composition in a commercial limestone kiln. The conditions of this parametric study are described in detail in Table 2. 2.4 Experiment Procedure: Hydration experiments were carried out in the bench-scale hydrator following a standard operating protocol. Before each experiment, the starting solid sample was weighed, and analyzed using a thermogravimetric analyzer (Perkin-Elmer Pyris1 TGA) for its composition (at 700 °C, 100% N2). The solid bed and the steam were preheated to 450 °C and 300 °C, respectively. Reaction temperature data was recorded using a data-acquisition system (Measurement Computing) and DAQFactory software (AzeoTech Inc.). Solid samples were extracted from the reactor’s effluent stream every 5, 10, 20, and 30 min. An additional sample was taken from the bulk solids remaining in the reactor after each experiment to verify that the sample composition is representative of the solid composition in the reactor.


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2.5 Sorbent Characterization: The sorbent’s carbonation capacity and the extent of hydration are determined in a thermogravimetric analyzer (TGA). The test program as well as the calculation methodology is described in a prior publication.37 Characterization of sorbent morphology was performed using a BET analyzer (Quantachrome NOVA 4200e series). The samples were degassed at 300 °C for at least 6 hrs and analyzed using N2 adsorption and desorption isothermally at -196 °C. The surface area was calculated based on the multipoint BET method. Particle surface morphology was observed on a Quanta 200 SEM analyzer (FEI Instruments). 3. Results and Discussion Following sections present experimental results on factors affecting the steam hydration of CaO sorbent. Further, comparative analysis between the carbonation and hydration reaction using a modified Jander’s equation is also discussed. 3.1 Effect of Steam Flow Rate The reaction conversion in such a bench-scaled fluidized bed reactor is a strong function of bulk phase mass transfer limitations; therefore the effect of operating gas flow rate, which correlates to the fluidization regime in the reactor, is investigated. Due to the thermal expansion of steam, the actual gas velocity depends on the reactor temperature. Figure 2, the average temperature profile of ten randomly selected hydration tests, shows the temperature curve being bounded by the initial temperature of 450 °C and the thermodynamic equilibrium temperature of 512 °C. At the same time, the consumption of fluidization gas (steam) by reaction, the rate of which varies with time, reduces the actual gas velocity. Due to the offsetting effects of thermal expansion and reaction consumption on the gas flow rate, the target superficial gas velocity (Ug) is calculated at


8

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the initial reaction temperature of 450 °C. Cold-flow model results indicate that Ug between 0.05 and 0.5 m/s are suitable for the steam hydration reactor. Each hydration experiment presented in paper was repeated at least three times to demonstrate data reproducibility. The standard error of each run is presented as error bars in Figures 3, 6, and 8. The maximum error of ± 3.3 % confirms the excellent repeatability of these experiments. The results from Figure 3 suggest that the hydration rate increases with respect to increasing Ug. After 30 min, the solid conversion increased from 58.9% to 76.3% when Ug was doubled from 0.05 m/s to 0.1 m/s. Further increasing the Ug to 0.5 m/s yielded the maximum conversion of 82.6% after 30 min. The correlation between Ug and solid conversion can be attributed to two factors - better fluidization and a higher steam:calcium (S:C) ratio. Increasing the operating Ug in the hydrator causes more vigorous mobilization of the solid bed, creating better gas-solid mixing and reducing the bulk phase mass transfer resistance. In addition, at higher S:C ratios, more steam is available to react with the same amount of solids while the excess steam exits as fluidization gas. Otherwise insufficient steam feed can potentially cause local defluidization and limit the reaction rate. However, this enhancing effect is diminished when the Ug exceeds the critical value of 0.3 m/s, above which the bulk phase mass transfer resistance is no longer rate limiting. For instance, the relative gain in solid conversion when Ug is increased from 0.05 to 0.1 m/s (17.4%) far exceeds that when Ug is quintupled from 0.1 to 0.5 m/s (6.3%). The analysis of the steam conversion as a function of steam flow rate reveals a different story, as shown in Figure 4. The steam conversion to Ca(OH)2 is calculated based on the steam flow rate, solid conversion at each time interval, and the solids charge. Higher steam velocities, and in effect more excess reacting gas, results in greater gas bypassing through the solids. This causes decreased steam utilization or ‘steam conversion’. Thus, an increase in gas velocity results in an


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increase in solid conversion at the expense of excess steam. Due to the semi-batch nature of operation of the reactor, the amount of CaO solids fed is relatively constant, which necessarily causes a proportional increase in the gas:solid ratios with an increase in gas velocity, implying that excess steam that passes through the bed is unreacted. This places an upper limit on the amount of steam that can be consumed. For example, when Ug=0.5 m/s, the total steam fed (over 30 minutes duration of the experiment) is approximately 10 times the stoichiometric requirement of the molar amount of sorbent present. Thus, at this Ug, the cumulative maximum theoretical steam conversion at 30 minutes would not exceed 10%; hence, a large portion of the steam would act as fluidizing gas instead of reactant gas. Hence, increasing steam velocity improves the solids conversion, but at the expense of wasting excess steam in the absence a steam recycle stream. Considering the energy required to generate high quality steam (>300 °C), excessive steam bypassing can be detrimental to the overall process efficiency. In conclusion, increasing the steam flow rate may not be a cost-effective method to enhance the reaction conversion without a means of recycling the unreacted steam. Through an overall process analysis, a suitable operating window that balances steam consumption and solid conversion can be determined. 3.2 Effect of Calcination Temperature The effect of calcination temperature on the surface morphology and the subsequent hydration reactivity of the calcined CaO sorbent was investigated using a high-temperature, fixed bed furnace with precise control of its internal temperature and heating duration. Atmospheric calcination is thermodynamically feasible at approximately 900 °C in a high CO2 concentration, the temperature is further increased to 1000 °C for comparison. One hour residence time was required to achieve full calcination, and the duration was subsequently increased to 3 hr to


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investigate the effect of prolonged exposure at such calcination temperatures. Particle morphology characterization was performed, and the results are presented in Figure 5. Increasing the calcination temperature from 900 to 1000 °C resulted in an approximate 50% reduction in specific surface area and total pore volume of the CaO sorbent. This result indicates that increasing calcination temperature beyond 900 °C can significantly worsen the sorbent surface structure, specifically causing the closure of surface pores. One potential explanation is that higher temperature leads to stronger atomic vibrational energy at the grain boundary, which is one of the main driving forces of particle coarsening.41 As a result of this coarsening mechanism, solid micro-grains coalesce, surface defects disappear, and a smoother grain surface with reduced surface area and pore volume is produced. The relationship between the surface morphology of CaO sorbent and its reactivity toward carbonation and sulfation has generally been recognized.42 However, a corresponding relationship with respect to the sorbent’s reactivity for steam hydration is not well understood. The change in steam hydration conversion at various calcination conditions is shown in Figure 6 & 7. A substantial difference of 27% exists when comparing the steam hydration conversion at 30 min of samples calcined at 900 and 1000 °C, respectively. The increase in calcination temperature likely accelerated the rate of sintering as evident from the BET data. As a gas-solid reaction, the rate of steam hydration depends on the number of active sites on the grain surface, involving simultaneous nucleation and growth of the Ca(OH)2 product. Therefore, the steam hydration rate should be affected by sintering similar to the known carbonation and sulfation mechanism of CaO sorbent. 3.3 Effect of Calcination Time


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The effect of calcination time is not as strong as that observed with calcination temperature. Increasing the calcination time by a factor of three only resulted in a minor deterioration of particle surface morphology, as seen in Figure 5. Holding the calcination temperature constant at 900 °C, the CaO sorbent surface area is reduced from 4.57 to 4.24 m2/g, a 7.2% loss, when the calcination time is increased from 1 to 3 hr. Similarly, a 7.7% reduction in surface area is observed for the 1000 °C sample. The reduction in CaO sorbent pore volume is consistent with the surface area loss. This observation implies that change in sorbent microstructure is more sensitive to calcination temperature than calcination time within the chosen experimental conditions. One explanation is that the change in sorbent microstructure requires a certain activation energy that is temperature dependent. Borgwardt has shown that the rate of sintering, which is measured through a reduction in sorbent surface area and porosity, accelerates drastically at temperatures slightly above 900 °C.42 In the temperature range of 900 – 1000 °C, others even claimed that this transformation in sorbent morphology would occur in the order of seconds.43 Therefore, the difference that can be observed within the time range of 1-3 hr as part of this study may be minimal. The steam hydration conversions at 30 min are 63.4%, 58.9%, 37.1%, and 31.0% for the four samples calcined at 900°C-1hr, 900°C-3hr, 1000°C-1hr, and 1000°C-3hr, respectively. In other words, the hydration conversion decreased with respect to increasing calcination temperature and calcination time. However, the change in steam hydration conversion due to increasing calcination time is minimal compared to that observed with increasing calcination temperature. On average, the hydration conversion after 30 min is reduced by 27% when the temperature is increased from 900 to 1000 °C, whereas the average time derived reduction in conversion is just 5.5%, as seen in Figure 7. Knowing how the hydration conversion corresponds to sorbent surface


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area and total pore volume, this outcome is expected because of the minimal change observed in sorbent surface morphology with variation in calcination time. In conclusion, from an operational perspective, precise control of the calcination temperature is important so that thermal sintering can be minimized, decay in sorbent reactivity reduced, and sorbent steam hydration rate maximized. 3.4 Effect of Calcination Reactor Design The effect of type of calcination reactor on steam hydration reactivity was investigated using CaO sorbents produced from a rotary kiln, a fixed bed furnace, and a commercial limestone kiln. From Figure 8, the highest steam hydration conversion is achieved using the rotary kiln calcined sample. From Figure 5, the rotary kiln calcined sample exhibits the largest surface area and total pore volume, which provides more favorable surface kinetics for the reaction to proceed. The advantage of the rotary kiln derives from the mixing motion that inherently produces a more uniform temperature distribution in the reactor. Therefore, local hotspots are less likely to develop in the rotary kiln than in a fixed bed furnace, where the solids closer to the wall are exposed to higher temperatures. In addition, the mixing motion reduces the inter-particle grain growth that tends to develop at high temperatures in comparison to stationary samples.41 Lastly, the presence of CO2 at the macroscopic level and within the microstructure of the particle itself is another sintering inducing factor. Better solid-gas mixing leads to rapid evolution of CO2 that is less likely to be trapped within the pore structure of the grain preventing the inner-core to be completely calcined. From Figures 5, 7, & 8, the Graymont pulverized lime sample, produced from a commercial limestone kiln, shows comparable hydration conversion, surface area, and pore volume to that of the fixed bed furnace sample calcined at 1000 °C for 3 hrs. Without knowing the specific


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operating conditions of the commercial limestone kiln, the following speculation may serve as the explanation for the apparent difference between samples produced in the in-house rotary kiln and that produced commercially. Some of these factors may have contributed to the Graymont lime sample’s low reactivity. First, the obvious difference in processing capacity between the pilot and commercial scale can introduce complications regarding the temperature profile and the mixing behavior in the reactor, both may affect the extent of sintering. Second, the rotary kiln in lab has three electrically heated zones, controlled separately by external heaters to ensure a uniform axial temperature distribution within the reactor; however, commercial kilns are usually directly-fired using coal or natural gas. Directly-fired reactors are more energetically efficient at the cost of being able to maintain a uniform temperature profile and to avoid the development of local hot spots close to the flame. Exposure to higher temperatures due to the aforementioned reasons can induce further sintering, thus making the CaO product less reactive. Lastly, the combustion products of coal and natural gas may also aid sintering. The reasons mentioned above all contribute to reducing the reactivity of CaO sorbent post-calcination, and should be considered when designing a calcination reactor aimed at producing reactive CaO product. 3.5 SEM Imaging of Sorbent Morphology Figure 9 (a-c) reports select SEM images of the calcined CaO sorbent samples prepared at different calcination conditions. The surface morphology of sample (a) obtained from the rotary calciner is distinctly different from that of the fixed bed furnace samples (b) & (c), which are more severely sintered. The SEM image of sample (a) reveals a network of rod-shaped defects, separated by pores in the range of 0.5-2 micron, on the grain surface. The presence of these pores allows for access to the inner core during reaction. Furthermore, the tip of each grain in sample (a) exhibits uniform step-like defects, creating additional surface area. This sample is considered


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relatively ‘unsintered’. In comparison, the ‘sintered’ samples (b) and (c) show signs of individual grains fusing together into a uniform layer. The surface pores consist of irregular and interconnected channels, which are visibly shallower than the pores observed on sample (a). This channel/crack formation can be a result of pore closure due to thermal sintering. The evolution of surface morphology between samples (b) & (c) in relation to their respective calcination temperatures is also evident. The transition from a rough surface (b) to a smooth surface (c) suggests a loss of finer surface structures. Overall, the SEM images, in agreement with the BET results, illustrate the changes in the shape, size, and organization of the surface structures under different calcination environments. Figure 9(d) shows the surface morphology of a Ca(OH)2 sample, which exhibits a similar surface texture to sample (a) with respect to their shape and arrangement; however, the hydrated sample has a visibly smoother surface with smaller surface pores. The formation of Ca(OH)2 product layer seemingly reduces the amount of surface defects and the size of surface pores. Despite the change in surface morphology, the hydrated sample still retains the individuality of the rod-shaped structure on its surface layer. Some surface pores are still present, albeit the pore size has been reduced. Complete densification of the grain surface due to product layer formation is not observed for the Ca(OH)2 sample. 3.6 Comparative Analysis of Carbonation and Hydration As heterogeneous gas-solid reactions, the carbonation and steam hydration reactions share some similarities; however, definite differences exist in the extent of effects caused by sintering. In sections 3.2, 3.3 and 3.4, the effect of calcination conditions on the extent of sintering as well as the hydrator performance has been explored. Carbonation conversion is known to be linearly related to the sorbent surface area and total pore volume, but the relationship between that and steam hydration conversion appears non-linear. This difference is evident when comparing the


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rotary calciner and fixed bed furnace samples produced at the identical calcination condition of 900 °C for 1 hr. The rotary calciner sample produced only 13.1% more hydrate even though its surface area is more than double that of the fixed bed furnace sample. Their respective carbonation conversions, however, differ by a factor of 2, pointing to the conclusion that the effect of surface kinetics is more relevant for carbonation than steam hydration. According to the core-shell model, the carbonation reaction proceeds via two reaction phases: a fast reaction controlled regime and a slow diffusion limited phase.44 Majority of the carbonation conversion is typically achieved during the reaction controlled phase, after which the reaction rate is too slow for practical applications as the diffusion resistance increases with respect to product layer thickness. This diffusional limitation can result in an unreacted core within particle grain, thus limiting the maximum carbonation conversion. Consequently, the traditional two-step carbonation-calcination reaction cycle shows a dramatic decay in sorbent carbonation capacity when the diffusional limitation is worsened by thermal sintering over multiple reaction cycles. The Jander’s equation can explain the effects of solid phase diffusion on reaction rate for this type of solid-gas reaction. Kondo et al.45 have classified reaction behaviors into three groups based on the order ‘N’ using a modified Jander’s equation (2):

[1−1−∝ ] =

(2)

where α is the extent of reaction; K is the overall rate constant; t is the reaction time; and N is the order. 1. If the reaction order N ≤ 1, the reaction is controlled by reaction kinetics occurring on the grain surface.


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2. If the reaction order 1 ≤ N ≤ 2, the reaction is controlled by the diffusion of reactants through a porous layer of reaction products. 3. If the reaction order N ≥ 2, the reaction is controlled by the diffusional resistance of reactants through a dense layer of product formation. In Figure 10, Eqn (2) is fitted to the data obtained from carbonation conversions for the calcined CaO samples. The carbonation reactivity of the calcined CaO sorbents was tested in a TGA isothermally at 650 °C in 10% CO2 with balance nitrogen. The corresponding slopes represent the inverse of the order (1/N). The distinction between the fast and slow reaction phases, labeled as N1 and N2 respectively, is evident due to the presence of an inflection point. Similarly, the Jander’s equation can be fitted to the hydration conversion data from the fluidized bed reactor to provide a comparative analysis between steam hydration and carbonation, as shown in Figure 11. Due to the difference of the scale, the reaction rate in this case is governed by kinetic factors as well as bulk phase mass transfer rates, effect of which has been explained in Section 3.1. The rate data obtained from a bench scale fluidized bed reactor cannot be directly used to construct empirical kinetic models. Thus, the following analysis is solely for the relative comparison. Furthermore, due to operational limitations of the bench-scale hydrator, the sampling frequency is restricted to four data points over the 30 min span. The lack of sampling points during the initial 5 min limits the source of information for the understanding of the reaction during the induction phase where the reaction conversion exceeded 10% in some cases. This missing information does not allow an empirical model to be developed for given operating conditions. However, more than 60% of the observable hydration conversion can be measured. The linear trend, observed in Figure 11, without an inflection point suggests that steam hydration is less hindered by the diffusional resistance due to the product layer formation. Once again, the


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parallel slopes suggest that a consistent order N can be observed for the steam hydration. By Kondo’s definition, steam hydration can be characterized by the diffusion rate through a porous product layer which is a mechanism fundamentally different from the fast and slow regimes observed for the carbonation. Physically, this difference can be attributed to the additional surface defects created during Ca(OH)2 product layer formation and hence making the Ca(OH)2 layer more porous than CaCO3.46-47 This change in surface morphology is caused by the differences in the molar densities of CaO and Ca(OH)2, such that the reaction leads to a volume expansion outward of the particle grain forming cracks and porous channel due to the low tensile strength of Ca(OH)2 product. Furthermore, the sustained reaction observed in Figures 3, 6, and 8 confirms that the steam reactant can diffuse through the porous Ca(OH)2 product layer and react with the inner core of the calcium sorbent. The steam hydration reaction thus behaves differently from the slow diffusion limited phase of the carbonation reaction, as dense CaCO3 product layer formation can reduce the carbonation rate to practically negligible. Such a disparity helps explain why the carbonation capacity of CaO sorbent decays drastically but the hydration capacity can be maintained, over multicyclic operation.35 4. Concluding Remarks A relationship between the operating velocity and the solid conversion is determined in this study. The maximized solid conversion is obtained at higher steam velocities but at the expense of excessive waste steam. Thus, establishing a balance between maximizing solid conversion and minimizing steam wastage is needed through a detailed process analysis of the effects of steam utilization and target solid conversions on the overall solid circulation rates, energy and steam requirements, and purge and makeup rates. Further, the effect of upstream calcination conditions on the performance of the hydration reactor requires to be understood. In this study, a correlation


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is found between the rate of hydration and the sorbent surface morphology, which depends on the extent of sintering during calcination. Surface characterization studies performed using BET and SEM analyses indicate the sorbent morphology is more sensitive to calcination temperature and reactor type than calcination time under our experimental conditions. According to Jander’s classification, unlike the carbonation reaction, which is limited by the formation of a dense product layer that severely reduces the reaction rate and hence limits the practical maximum conversion, the hydration reaction is proposed to be governed by the diffusional resistance through a porous product layer that does not affect the maximum conversion. From the perspective of the CCR process, the intermediate hydration reaction reactivates the calcined sorbent, restores the sorbent carbonation capacity, and reduces the solid circulation rate. Due to design constraints, the hydrator operates near the equilibrium condition of 500 °C and the steam partial pressure of 1 atm. Limited by the reaction thermodynamics, the rate of steam hydration is relatively slow under such conditions. There is great impetus to improve the steam hydration rate as it will reduce the required hydrator size, solid residence time, and steam consumption. This study reveals that the reactivity of sintered sorbent post-calcination affects the steam hydration rate, in addition to the previously known effect on the carbonation capacity. Future research efforts need to be focused on a calciner design that can produce a reactive CaO product with minimum sintering.


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References (1) (2) (3) (4) (5) (6) (7)

(8) (9)

(10) (11)

(12) (13)

(14)

(15)

(16)

U. S. Energy Information Administration Annual Energy Outlook 2014. DOE/EIA0383(2014) Washington DC, 2014. Intergovernmental Panel on Climate Change. Climate Change 2013: The Physical Science Basis; Fifth Assessment; 2007. Protocol, K. United Nations Framework Convention on Climate Change; Kyoto, 1997. Steffen, W.; Noble, I.; Canadell, J.; Apps, M.; Schulze, E.-D.; Jarvis, P. G. The Terrestrial Carbon Cycle: Implications for the Kyoto Protocol. Science 1998, 280, 1393–1394. Ellerman, A. D.; Convery, F. J.; Perthuis, C. de. Pricing Carbon: The European Union Emissions Trading Scheme; Cambridge University Press, 2010. Folger, P. Carbon Capture: A Technology Assessment. Congr. Res. Serv. Rep. 2010. Othman, M. R.; Martunus; Zakaria, R.; Fernando, W. J. N. Strategic Planning on Carbon Capture from Coal Fired Plants in Malaysia and Indonesia: A Review. Energy Policy 2009, 37, 1718–1735. US EPA. Clean Power Plan Proposed Rule http://www2.epa.gov/carbon-pollutionstandards/clean-power-plan-proposed-rule (accessed July 12, 2014). Sánchez-Biezma, A.; Ballesteros, J. C.; Diaz, L.; de Zárraga, E.; Álvarez, F. J.; López, J.; Arias, B.; Grasa, G.; Abanades, J. C. Postcombustion CO2 Capture with CaO. Status of the Technology and next Steps towards Large Scale Demonstration. Energy Procedia 2011, 4, 852–859. Fan, L.-S. Chemical Looping Systems for Fossil Energy Conversions; John Wiley & Sons, 2011. Shimizu, T.; Hirama, T.; Hosoda, H.; Kitano, K.; Inagaki, M.; Tejima, K. A Twin FluidBed Reactor for Removal of CO2 from Combustion Processes. Chem. Eng. Res. Des. 1999, 77, 62–68. Blamey, J.; Anthony, E. J.; Wang, J.; Fennell, P. S. The Calcium Looping Cycle for Large-Scale CO2 Capture. Prog. Energy Combust. Sci. 2010, 36, 260–279. Arias, B.; Diego, M. E.; Abanades, J. C.; Lorenzo, M.; Diaz, L.; Martínez, D.; Alvarez, J.; Sánchez-Biezma, A. Demonstration of Steady State CO2 Capture in a 1.7 MWth Calcium Looping Pilot. Int. J. Greenh. Gas Control 2013, 18, 237–245. Fan, L.-S.; Zeng, L.; Wang, W.; Luo, S. Chemical Looping Processes for CO2 Capture and Carbonaceous Fuel Conversion – Prospect and Opportunity. Energy Environ. Sci. 2012, 5, 7254–7280. Dean, C. C.; Blamey, J.; Florin, N. H.; Al-Jeboori, M. J.; Fennell, P. S. The Calcium Looping Cycle for CO2 Capture from Power Generation, Cement Manufacture and Hydrogen Production. Chem. Eng. Res. Des. 2011, 89, 836–855. Florin, N. H.; Blamey, J.; Fennell, P. S. Synthetic CaO-Based Sorbent for CO2 Capture from Large-Point Sources. Energy Fuels 2010, 24, 4598–4604.


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(17) Manovic, V.; Anthony, E. J. Lime-Based Sorbents for High-Temperature CO2 Capture— A Review of Sorbent Modification Methods. Int. J. Environ. Res. Public. Health 2010, 7, 3129–3140. (18) Yu, F.-C.; Phalak, N.; Sun, Z.; Fan, L.-S. Activation Strategies for Calcium-Based Sorbents for CO2 Capture: A Perspective. Ind. Eng. Chem. Res. 2012, 51, 2133–2142. (19) Liu, F.-Q.; Li, W.-H.; Liu, B.-C.; Li, R.-X. Synthesis, Characterization, and High Temperature CO2 Capture of New CaO Based Hollow Sphere Sorbents. J. Mater. Chem. A 2013, 1, 8037–8044. (20) Liu, W.; Low, N. W.; Feng, B.; Wang, G.; Diniz da Costa, J. C. Calcium Precursors for the Production of CaO Sorbents for Multicycle CO2 Capture. Environ. Sci. Technol. 2010, 44, 841–847. (21) Li, Y.; Sun, R.; Liu, H.; Lu, C. Cyclic CO2 Capture Behavior of Limestone Modified with Pyroligneous Acid (PA) during Calcium Looping Cycles. Ind. Eng. Chem. Res. 2011, 50, 10222–10228. (22) Albrecht, K. O.; Wagenbach, K. S.; Satrio, J. A.; Shanks, B. H.; Wheelock, T. D. Development of a CaO-Based CO2 Sorbent with Improved Cyclic Stability. Ind. Eng. Chem. Res. 2008, 47, 7841–7848. (23) Li, Z.; Cai, N.; Huang, Y. Effect of Preparation Temperature on Cyclic CO2 Capture and Multiple Carbonation−Calcination Cycles for a New Ca-Based CO2 Sorbent. Ind. Eng. Chem. Res. 2006, 45, 1911–1917. (24) Arias, B.; Grasa, G. S.; Alonso, M.; Abanades, J. C. Post-Combustion Calcium Looping Process with a Highly Stable Sorbent Activity by Recarbonation. Energy Environ. Sci. 2012, 5, 7353–7359. (25) Valverde, J. M.; Sanchez-Jimenez, P. E.; Perez-Maqueda, L. A. High and Stable Capture Capacity of Natural Limestone at Ca-Looping Conditions by Heat Pretreatment and Recarbonation Synergy. Fuel 2014, 123, 79–85. (26) Wang, W.; Ramkumar, S.; Li, S.; Wong, D.; Iyer, M.; Sakadjian, B. B.; Statnick, R. M.; Fan, L.-S. Subpilot Demonstration of the Carbonation−Calcination Reaction (CCR) Process: High-Temperature CO2 and Sulfur Capture from Coal-Fired Power Plants. Ind. Eng. Chem. Res. 2010, 49, 5094–5101. (27) Ramachandran, V. S.; Sereda, P. J.; Feldman, R. F. Mechanism of Hydration of Calcium Oxide. Nature 1964, 201, 288–289. (28) Hartman, M.; Trnka, O. Reactions between Calcium Oxide and Flue Gas Containing Sulfur Dioxide at Lower Temperatures. AIChE J. 1993, 39, 615–624. (29) Manovic, V.; Anthony, E. J. Steam Reactivation of Spent CaO-Based Sorbent for Multiple CO2 Capture Cycles. Environ. Sci. Technol. 2007, 41, 1420–1425. (30) Phalak, N.; Deshpande, N.; Fan, L.-S. Investigation of High-Temperature Steam Hydration of Naturally Derived Calcium Oxide for Improved Carbon Dioxide Capture Capacity over Multiple Cycles. Energy Fuels 2012, 26, 3903–3909.


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(31) Rong, N.; Wang, Q.; Fang, M.; Cheng, L.; Luo, Z.; Cen, K. Steam Hydration Reactivation of CaO-Based Sorbent in Cyclic Carbonation/Calcination for CO2 Capture. Energy Fuels 2013, 27, 5332–5340. (32) Anthony, E. J.; Bulewicz, E. M.; Jia, L. Reactivation of Limestone Sorbents in FBC for SO2 Capture. Prog. Energy Combust. Sci. 2007, 33, 171–210. (33) Sun, Z.; Chi, H.; Fan, L.-S. Physical and Chemical Mechanism for Increased Surface Area and Pore Volume of CaO in Water Hydration. Ind. Eng. Chem. Res. 2012, 51, 10793– 10799. (34) Schaube, F.; Wörner, A.; Tamme, R. High Temperature Thermochemical Heat Storage for Concentrated Solar Power Using Gas–Solid Reactions. J. Sol. Energy Eng. 2011, 133, 031006–031006. (35) Schaube, F.; Koch, L.; Wörner, A.; Müller-Steinhagen, H. A Thermodynamic and Kinetic Study of the de- and Rehydration of Ca(OH)2 at High H2O Partial Pressures for ThermoChemical Heat Storage. Thermochim. Acta 2012, 538, 9–20. (36) Liu, W.; An, H.; Qin, C.; Yin, J.; Wang, G.; Feng, B.; Xu, M. Performance Enhancement of Calcium Oxide Sorbents for Cyclic CO2 Capture—A Review. Energy Fuels 2012, 26, 2751–2767. (37) Wang, A.; Wang, D.; Deshpande, N.; Phalak, N.; Wang, W.; Fan, L.-S. Design and Operation of a Fluidized Bed Hydrator for Steam Reactivation of Calcium Sorbent. Ind. Eng. Chem. Res. 2013, 52, 2793–2802. (38) Wang, W.; Ramkumar, S.; Fan, L.-S. Energy Penalty of CO2 Capture for the Carbonation– Calcination Reaction (CCR) Process: Parametric Effects and Comparisons with Alternative Processes. Fuel 2013, 104, 561–574. (39) Steam, Its Generation and Use; Babcock & Wilcox., 1913. (40) Geldart, D.; Harnby, N.; Wong, A. C. Fluidization of Cohesive Powders. Powder Technol. 1984, 37, 25–37. (41) Chiang, Y. M.; Birnie, D.; Kingery, W. D. Physical Ceramics: Principle for Ceramic Science and Engineering; Wiley Publishings, 1997. (42) H. Borgwardt, R. Sintering of Nascent Calcium Oxide. Chem. Eng. Sci. 1989, 44, 53–60. (43) Stanmore, B. R.; Gilot, P. Review—calcination and Carbonation of Limestone during Thermal Cycling for CO2 Sequestration. Fuel Process. Technol. 2005, 86, 1707–1743. (44) Bhatia, S. K.; Perlmutter, D. D. Effect of the Product Layer on the Kinetics of the CO2Lime Reaction. AIChE J. 1983, 29, 79–86. (45) Kondo, R.; Lee, K.; Daimon, M. Kinetics and Mechanism of Hydrothermal Reaction in Lime-Quartz-Water System. J. Ceram. Soc. Jpn. 1976, 84, 573–578. (46) Lin, S.; Harada, M.; Suzuki, Y.; Hatano, H. CaO Hydration Rate at High Temperature (∼1023 K). Energy Fuels 2006, 20, 903–908. (47) Song, H. S.; Kim, C. H. The Effect of Surface Carbonation on the Hydration of CaO. Cem. Concr. Res. 1990, 20, 815–823.


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Figure 1. Schematics of hydrator setup.


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500

T_eq = 512 C T_initial = 450 C

400

Temperature (°C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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300

200

100

0 0

5

10

15

20

25

30

35

Time (min) Figure 2. Hydration experiment average temperature profile.


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1.0

0.8

Solid Conversion

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6

0.4

Ug=0.05m/s Ug=0.1m/s Ug=0.3m/s Ug=0.5m/s

0.2

0.0 0

10

20

30

Time (min) Figure 3. Sorbent hydration conversion with respect to Ug.


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1.0 Ug=0.05 m/s Ug=0.1 m/s Ug=0.3 m/s Ug=0.5 m/s

0.8

Steam Conversion

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6

0.4

0.2

0.0 0

10

20

30

Time (min) Figure 4. Steam hydration conversion with respect to Ug.


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12 Surface Area

Pore Volume

0.06

10

8 0.04 6

0.03

4

0.02

2

Pore Volume (cc/g)

0.05

Surface Area (m2/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.01

0

0 RC-900C-1hr FB-900C-1hr FB-900C-3hr FB-1000C-1hr FB-1000C-3hr Graymont Lime

Figure 5. Sorbent surface area and pore volume after calcination.


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1.0 FB-900C-1hr FB-900C-3hr FB-1000C-1hr FB-1000C-3hr

0.8

Solid Conversion

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6

0.4

0.2

0.0 0

10

20

30

Time (min) Figure 6. Sorbent hydration conversion of sorbent calcined at different conditions.


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90% Carbonation (30min) 80%

63.2% 58.6%

60% 50%

Hydration (30min)

76.3%

70%

Solid Conversion

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44.9% 37.7%

40%

31.2%

30% 19.6%

27.9%

18.3%

20%

11.4%

9.7%

12.8%

10% 0% RC-900C-1hr

FB-900C-1hr

FB-900C-3hr

FB-1000C-1hr

FB-1000C-3hr Graymont Lime

Figure 7. Comparison of sorbent hydration and carbonation conversions.


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1.0 RC-900C-1hr FB-900C-1hr FB-1000C-3hr Graymont Pulverized Lime

0.8

Solid Conversion

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6

0.4

0.2

0.0 0

10

20

30

Time (min) Figure 8. Hydration Conversion of sorbent calcined in different reactors.


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a

c

b

d

Figure 9. Sorbent SEM images: (a) upper left, RC-900C-1hr sorbent, (b) upper right, FB-900C1hr sorbent, (c) bottom left, FB-1000C-3hr sorbent, (d) bottom right, RC-900C-1hr hydrated for 30 min.


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-0.5 N2=2.70 -1.0

-1.5

1/3

Log [1-(1-α) ]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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N1=0.60 -2.0

-2.5 RC-900C-1hr FB-900C-1hr FB-1000C-3hr Graymont Pulverized Lime

-3.0

-3.5 -1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Log (t) (min) Figure 10. Carbonation kinetics of sorbent produced at different calcination conditions.


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-0.2 -0.4 N=1.25

-0.6 1/3

Log [1-(1-α) ]

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-0.8 -1.0 -1.2 -1.4

RC-900C-1hr FB-900C-1hr FB-1000C-1hr Graymont Pulverized Lime

-1.6 -1.8 0.6

0.8

1.0

1.2

1.4

1.6

Log (t) (min) Figure 11. The Jander’s equation fitted to hydration conversion of sorbent produced at different calcination conditions


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Table 1. Sorbent Physical Properties. Compound CaCO3 MgO SiO2 Fe2O3+Al2O3 S LOI

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Wt % >90.0%

Steam Hydration of Calcium Oxide for Solid ... - ACS Publications

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