Superheating on the CO2 Capture

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Influence of Hydration by Steam/Superheating on the CO2 Capture Performance and Physical Properties of CaO-Based Particles Junjun Yin,† Changlei Qin,† Hui An,‡ Ananthanarayanan Veeraragavan,† and Bo Feng*,† †

School of Mechanical & Mining Engineering, the University of Queensland, St Lucia QLD 4072, Australia School of Chemical Engineering, the University of Queensland, St Lucia QLD 4072, Australia

‡

S Supporting Information *

ABSTRACT: This paper studied the effect of the cyclic carbonation/calcination reactions and the hydration-based reactivation methods (i.e., hydration by steam and hydration by superheating) on the CO2 capture performance and physical properties of pelletized CaO-based sorbent particles in a fixed bed reactor. Hydration by superheating consisted of first a steam hydration process in 15 vol % H2O at 300 °C and a subsequent superheating process in 100% CO2 at 550 °C. It was found that during the successive carbonation/calcination cycles, the chemical activity of CaO sorbent particles fell rapidly while their physical properties (i.e., weight loss after rotations and compressive strength) were largely maintained. In addition, both hydration by steam and hydration by superheating were effective in reactivating the chemical activity of CaO-based pellets to nearly the same level. Hydration by steam, however, significantly weakened the physical strength of CaO-based pellets whereas hydration by superheating largely maintained their mechanical properties over 20 cycles. The positive effect of hydration by superheating was ascribed to the carbonation of Ca(OH)2 during the superheating step at 550 °C.

1. INTRODUCTION Calcium looping is a promising technology to concentrate CO2 from both pre-1,2 and postcombustion systems.3 Due to its economic advantages, a number of pilot-scale calcium looping plants have been setup worldwide to move the technology toward commercialization.4 The large-scale demonstrations that have capacity over 1 MWth include the Caoling project in Spain (1.7 MWth),5 the LISA process in Germany (1 MWth),6 and the carbonation−calcination−hydration process in Taiwan (2 MWth) integrated with a cement plant.7 However, both in pre- and postcombustion applications, calcium looping suffers from well-known problems of loss-in-capacity and attrition of CaO sorbent,8−11 which lead to a large makeup flow of fresh sorbent and/or an increase in the solid circulation rate to maintain the CO2 capture efficiency.12 Consequently, the huge consumption of CaO sorbent and the cost associated with the disposal of spent sorbent significantly increase the overall cost of the calcium looping process. Therefore, there is scope to reduce the cost of calcium looping by manufacturing CaObased sorbent with desirable chemical reversibility and mechanical stability. The reduction in the chemical activity of CaO sorbent is mainly caused by the sintering of CaO at high temperatures and CO2 concentrations.13 The methods to maintain the activity of CaO-based sorbents include thermal pretreatment,14 sorbent doping,4 synthesis with inert supports,15−17 recarbonation,18 and hydration.19−21 Among these methods, hydration is one of the most promising ways to be directly applied in calcium looping. And, the new calcium looping process integrated with an intermediate hydration process even has a lower energy penalty than the typical calcium looping process, due to the reduced solid feed rate as a result of enhanced chemical activity of CaO sorbents.22 However, the formation of Ca(OH)2 during hydration process significantly weakens the mechanical strength © 2013 American Chemical Society

of natural limestone, thereby further increasing the attrition rate of sorbent in fluidized-bed reactors.23,24 Recently an interesting hydration-based method, referred to as “Ca(OH)2 superheating”, was proposed to reduce the attrition rate of limestone in fluidized-bed reactors.25 The reduction in attrition rate was ascribed to the annealing of Ca(OH)2 in 100% CO2 at 520 °C that is higher than the corresponding equilibrium temperature, which, however, is not persuasive because the phenomenon of “superheating state of Ca(OH)2” was later observed only for particles larger than 30 μm by Blamey et al.26 Therefore, a better understanding of the mechanism of hydration by superheating is needed. In contrast to the problem of loss-in-capacity, the attrition problem of CaO sorbent in fluidized beds has received less attention. It is generally accepted that attrition is more intense during the initial calcination and becomes quite stable afterward.11,27 The mechanical behavior of the sorbent particles during cyclic reactions is governed by (1) the internal stresses due to thermal shocks and/or internal excess-pressures associated with carbonation/calcination reactions; (2) the severe surface abrasion between particles and particles and reactor wall; and (3) the rounding off of the particle roughness. The first and second effects are believed to be responsible for the high attrition rate during initial cycles, while the rounding off effect stabilizes the attrition rate after certain cycles.27,28 In fluidized beds, the influence of these effects on the physical properties of CaO sorbents is often studied as a whole. It would be of interest to understand the individual effect of cyclic carbonation/calcination reactions on the physical properties of Received: Revised: Accepted: Published: 18215

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CaO sorbents because the switching between carbonation and calcination would lead to cyclically repeated thermal shocks. This must be accomplished in a fixed-bed reactor where the effects of surface abrasion and rounding off can be excluded. The methods to enhance the physical strength of CO2 sorbent include pelletization with various binders28,29 and light sulphation.10 The latter is easily achieved in practice but will reduce the CO2 uptake because CaO has stronger affinity for SO2. On the other hand, pelletized sorbent particles generally show improved mechanical properties (e.g., reduced attrition rate) and better chemical reversibility, compared with natural limestone.2,30 However, most of the pellets reported in the literature still noticeably lose CO2 uptake.30,31 According to Manovic and Anthony’s work,29 the combined hydration and pelletization method could be a possible solution to simultaneously maintain the chemical reversibility and mechanical stability of CaO-based sorbents.11,29 However, so far no work has been carried out to investigate the influence of hydration-based methods on the physical properties of the synthetic CaO pellets. Therefore, the objectives of this work were the following: (1) to investigate the individual effect of successive carbonation/ calcination reactions on the physical properties of CaO sorbent particles in a fixed-bed reactor, (2) to investigate the influence of hydration-based methods on the mechanical stability of synthetic CaO pellets, which has not been studied yet, and (3) to better understand the possible mechanism of hydration by superheating. This work was carried out in a fixed-bed reactor by examining the physical and chemical properties of four synthetic sorbents before and after hydration-based treatments, or after cycles without hydration treatments. Four synthetic sorbents were selected from a group of extruded CaO pellets by assessing their chemical and physical performances.

Table 1. Physical Properties of CaO Particles after 3 h Calcination at 900 °C sample CC12 CC43 CCA43 CCA53 CA43 CA64 CHC20 CHC25 CHC40 CHC50 CHC75 a

raw material lime:cement lime:cement: Al2O3 lime:Al2O3 lime:high aluminate cementa

mass ratio

weight loss [%]

compressive strength [MPa]

3:17 1:1 2:1:1 3:1:1 1:1 7:3 1:4 1:3 4:6 1:1 7:3

18.97 40.71 25.04 23.84 0.83 8.5 9.66 10.94 10.96 11.76 3.77

5.80 0.85 2.51 0.78 5.99 4.50 22.38 20.46 13.87 3.17 4.37

Refers to the mass ratio of CaO and high aluminate cement.

and other components (such as Al and Si). The commercial Ca(OH)2 was also tested as a reference. 2.3. Determination of the Physical Properties of Sorbent Particles. The physical properties of CaO sorbent particles are represented by two parameters, namely the weight loss of particles after 2000 rotations and the compressive strength. According to Vaux and Keairns’s work,32 the former shows the particles’ resistance to abrasion while the latter indicates the particles’ resistance to breakage. The particles used in the physical property measurements were obtained before and after various carbonation/calcination cycles, with or without hydration-based treatments, as described in the fixedbed tests below. The procedures for determining those two parameters are described in detail as follows. 2.3.1. Weight Loss Measurement. A rotating drum friability tester (Vanderkamp) with a vankel-type USP drum (28.5 cm of inner diameter) was used to determine the weight loss of sample particles. In each test, about 1 g of sample particles (larger than 1 mm) was loaded in the rotator and subjected to 2000 rotations at a speed of 25 rpm. Then, the particles were sieved to exclude the particles smaller than 1 mm that was considered to be the elutriated fines as a result of abrasion. The weight loss was defined as the mass ratio of the weight of the elutriated fines over the initial weight. 2.3.2. Compressive Strength Test. The compressive strength of sample particles was determined using an Instron Universal Tester (Type of 4505). One sample particle with the uniform length of 5 mm was placed between two compressive plates, one of which was set to move toward the other still one at a speed of 0.1 mm/min. A uniaxial force was slowly applied until the sorbent crushed. The compressive strength, defined as the ratio of the maximum load applied over the cross-sectional area of the sorbent, was then obtained. The final value for the compressive strength was a mean value of 5 measurements. 2.4. Characterization of Samples. 2.4.1. X-ray Diffraction (XRD) Analysis. The X-ray diffraction pattern of sorbent was analyzed in a Bruker AXS D8 Advance X-ray diffractometer equipped with a scintillation counter, graphite monochromators, and copper target. The XRD analysis was conducted at room temperature with a 0.4 s step time, 10° start test angle, and 70° end angle. 2.4.2. Scanning Electron Microscope (SEM) Imaging. The sample morphologies were observed with a Jeol JSM-6610 Scanning Electron Microscope (SEM) with the accelerating voltage of 20 kV under high vacuum. Before the SEM analysis,

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The CaO-based sorbent particles were extruded by a PRISM Euro lab 16XL twin screw extruder from commercial calcium hydroxide with a binder of Portland cement, aluminum oxide, a mixture of them, or high aluminate cement. The produced particles were in cylindrical form with a diameter of 1.5 mm and a length between 3 and 7 mm. Then, the particles were calcined at 900 °C for 3 h in a furnace to obtain the final sample.2,28 The mass ratio of CaO in each sorbent particle varied in order to investigate the effect of CaO loading on the mechanical strength, with details shown in Table 1. Note that, these cylindrical particles can be directly used in fixed-bed reactors or easily crushed into micrometer-sized particles for use in fluidized-bed reactors without losing mechanical strength.2 2.2. Determination of the CO2 Capture Performance of Sorbent Particles. A thermogravimetric analyzer (TGA, Cahn TG-131) was used to determine the CO2 capture performance of samples during cyclic carbonation/calcination reactions. In each run, about 30−40 mg of sample was loaded into a quartz holder and subjected to carbonation at 650 °C for 30 min in 15 vol % CO2 balanced with N2 and calcination at 900 °C for 10 min in a N2 stream. The carbonation/calcination process was repeated 48 times. The mass variation of sample during the test was continuously recorded. The conversion and CO2 capture capacity of CaO sorbent particles can be accordingly calculated. It should be noted that the conversion of CaO was calculated on the basis of the predetermined CaO content without taking into account the reaction between CaO 18216

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°C in 100% CO2 for 5 min. The cycle was repeated 20 times for each sample. At the cycle numbers 11 and 16, the sorbent was reactivated by either steam hydration or hydration by superheating. Before and after the hydration treatment, about 1 g of sorbent was taken out from the reactor for mechanical property measurements under the conditions as aforementioned. After the 20th cycle, another 1 g of sorbent was subjected to the same mechanical property measurements. To investigate the individual effect of the successive carbonation/calcination cycles on the mechanical properties of CaO sorbent particles, each sorbent was also subjected to 20 cycles without hydration treatments. The weight loss and compressive strength were measured with sorbent particles after 11, 15, and 20 cycles, as seen in Table 2. The gas exiting the reactor passed through a condenser and a silica trap to remove steam before a Fourier Transform Infrared Spectrometer (FTIR Nicolet 5700, Thermo), which was used to continuously measure the CO2 concentration in the gas. The amount of CO2 absorbed by particles can be calculated by integrating the peak as a result of decomposition of CaCO3 during the heating stage from 650 to 950 °C, as shown in Figure 2. Therefore, the conversion of CaO in the sorbent

the sample was dispersed on a conductive adhesive tab placed on a SEM mount and coated with platinum using a Bal-Tec MED 020 coater for 200 s at the sputtering current of 15 mA. 2.5. Lab-Scale Fixed-Bed Reactor Tests. A lab-scale fixed-bed system was used to investigate the influence of cyclic carbonation/calcination reactions and hydration-based reactivation methods (i.e., hydration by steam and hydration by superheating) on the CO2 capture performance and physical properties of sorbent particles. A schematic diagram of the system is shown in Figure 1. The system consists of a gas

Figure 1. Schematic diagram of the fixed-bed reactor system: (1) N2 cylinder; (2) CO2 cylinder; (3) liquid metering pump; (4) vaporizer; (5) mass-flow controller; (6) gas mixer; (7) red area-heating tape; (8) quartz support; (9) CaO sorbent; (10) Inconel 600 reactor; (11) three-stage furnace; (12) thermocouple; (13) condenser; (14) silica trap; (15) FTIR; (16) controlling system; (17) temperature controller.

feeding system, a reactor system with temperature control, and a gas analyzer. The reactor system contains a three-stage furnace and an Inconel 600 reactor that has an internal diameter of 27 mm and a length of 550 mm. About 10 g of particles was placed in the center of the reactor and supported by a quartz tube, to form a bed with a height of 3.5 mm and a bed porosity of 0.47−0.49. Quartz rods were inserted in the quartz tube to enhance heat transfer. The volumetric mass flowrates of the CO2 stream and N2 stream were regulated to be 200 mL/min and 800 mL/min, respectively. Then, the carbonation was conducted at 650 °C in 20 vol % CO2 (N2 balance) for 10 min, while the calcination was conducted at 950

Figure 2. Variation of CO2 concentration in the exiting gas from the fixed-bed reactor at the sixth cycle with CA40 as a CO2 sorbent.

particle can be approximately calculated on the basis of the following equations: t

FCO2 ≈

∫t 2 CCO2(t ) dt ·Q N2 1

Vm

(1)

Table 2. Physical Properties of CHC75 after Various Treatmentsa without hydration

initial calcination 10 cycles first treatment second treatment 20 cycles

steam

superheating

weight loss [%]

compressive strength [MPa]

weight loss [%]

compressive strength [MPa]

weight loss [%]

compressive strength [MPa]

3.77 5.46 6.11 5.64 6.98

4.37 7.99 9.76 8.12 5.66

3.77 7.69 16.32 31.98 35.22

4.37 9.33 5.04 1.81 1.61

3.77 13.19 7.74 7.78 12.80

4.37 12.87 22.02 13.57 9.45

a

The 1st and 2nd treatments were conducted after cycle numbers 10 and 15, respectively. When hydration was not applied, the measurement for physical properties was conducted with particles after 11, 15, and 20 cyles. 18217

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Figure 3. Variations of conversion and CO2 capture capacity of CaO-based sorbent particles over 48 cycles. Carbonation in 15 vol % CO2 (N2 balance) at 650 °C for 30 min; calcination in N2 at 900 °C for 10 min.

Xconversion ≈

FCO2WCaO m0XCaO

× 100%

(2)

In eqn 1, CCO2(t) (%) is the CO2 fraction in the mixed gas stream, t (s) is time, QN2 (l/s) is the volumetric flow rate of N2, Vm (l/mol) is the molar volume of CO2, and FCO2 (mol) is the amount of CO2 absorbed by CaO-based sorbent particles, while in eq 2, WCaO (g/mol) is the molecular weight of CaO, m0 (mg) is the sample mass after complete decomposition at 950 °C, and XCaO (%) is the CaO content in the particle. Steam hydration was conducted at 300 °C for 1 h in 50 vol % steam (N2 balance). A predetermined amount of water was injected into a vaporizer by a liquid metering pump, as shown in Figure 2. The generated steam was then carried into the reactor by the N2 stream. The tubing after the vaporizer was wrapped with heating tapes to maintain the temperature at 170 °C to avoid water condensation. The process of hydration by superheating in fact consisted of hydrating the particle at 300 °C for 1 h in 50 vol % steam (N2 balance) and subsequently superheating it at 550 °C in 100% CO2 for 1 h for annealing.

Figure 4. XRD patterns of CaO sorbent particles after 3 h calcination at 900 °C: (A) CaO, (B) Ca12Al14O33, (C) Ca2SiO4, (D) ferrite.

cement presented very weak compressive strength, although Al2O3 was added purposely to enhance the chemical activity and mechanical strength. In contrast to the particles bound with Portland cement, the CHC and CA particles showed noticeable improvements both in chemical activity and physical properties. For instance, the conversion of CA43 (∼30%) was higher than that of Ca(OH)2 over 48 cycles, though the CO2 capture capacity of CA43 was lower because of the lower CaO loading. As the CaO loading increased to 75%, CHC75 showed a higher CO2 capture capacity than Ca(OH)2 after 48 cycles. The enhancement in the activities of CA particles and CHC particles could be ascribed to the formation of Ca12Al14O33 (as seen in Figure 4) that separates the CaO grain to prevent its growth.15,16,33 Also, the high CO2 uptake of CA43 indicates that CaO did not completely react with Al2O3 to form Ca12Al14O33 although the molar ratio of Ca/Al was stoichiometric. In addition, the CA particles and CHC particles generally exhibited much lower weight loss than the CC and CCA particles, as shown in Table 1. Particularly, CA43 had the lowest weight loss of 0.83% among all sorbent particles. With respect to the compressive strength, the CHC particles with less than 40% CaO were

3. EXPERIMENTAL RESULTS 3.1. Screening of CaO Sorbent Particles with Desirable Physical Strength. This part aims at identifying the CaO-based sorbents with better chemical and physical properties for subsequent fixed-bed tests. Figure 3 presents the variations of CaO conversion and CO2 capture capacity of CaO-based sorbent particles as a function of cycle numbers. It can be clearly seen that Portland cement is not a good binder for pelletization. Specifically, the CC particles (i.e., CC12 and CC43) showed much lower conversions (e.g., 13.5% for CC43) than that of the reference material (∼21.3%) over 48 cycles. The reason for the negative effect of Portland cement on the chemical activity of CaO sorbent particles could be the presence of calcium silicate as found in the XRD patterns of those pellets (as seen in Figure 4), which enhances the sintering of CaO.31 In addition, the CC particles showed greater weight loss and lower compressive strength than other types of particles made with different binders, indicating that those particles were mechanically weaker, as shown in Table 1. Similarly, the CCA particles with the addition of Portland 18218

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Figure 5. Variation of conversion of CaO sorbent particles in the fixed-bed reactor over 20 cycles. Carbonation in 20 vol % CO2 (N2 balance) at 650 °C for 10 min; calcination in 100% N2 at 950 °C for 5 min. Hydration at 300 °C in 50 vol %H2O (N2 blance) for 1 h at the 11th and 16th cycles; dehydration in hydration by superheating at 550 °C in 100% CO2 for 1 h at the 11th and 16th cycles. (solid line−without hydration, □−hydration by steam, ○−hydration by superheating).

observed in Figure 3, in accordance with the literature results.2,28,34 In this work the method of hydration by steam or superheating was used to reactivate four selected sorbent particles in a fixed-bed reactor and the effects of hydration and cyclic reactions on the chemical and physical properties were investigated. Figure 5 shows the variation of CaO conversion of CaObased sorbent particles over 20 cycles in the fixed-bed reactor, with and without hydration treatments at cycle numbers 11 and 16. It can be clearly seen that all the CaO particles without hydration treatments exhibited a decrease in the conversion of CaO over 20 cycles (see solid line in Figure 5), in accordance with the TGA results in Figure 3. The difference is that sorbents tested in the fixed-bed reactor lost activity much faster than that in TGA. This could be attributed to the higher reaction temperature (950 °C) and higher CO2 concentration (100%) during calcination in the fixed-bed tests, both of which would enhance the sintering of CaO.13,35 When hydration was applied after the 10th and 15th cycles, the activity of CaO particles was significantly recovered, but again quickly decreased in the following 5 cycles. It is worthy to note that the particles treated by steam hydration presented very similar conversion to those treated via hydration by superheating,

much higher than the particles made with other binders. However, as the CaO content exceeded 40% in the particles, their compressive strength decreased to values lower than that of the CA particles. Furthermore, the effect of binder content on the chemical and physical properties of sorbent particles was also analyzed. As seen from Figure 3, higher binder content in the sorbent led to a lower CO2 capture capacity, although some of them (e.g., CHC20 and CHC25) presented better reversibility over 48 cycles. This result indicates that the binder content in the synthetic sorbent should be as low as possible for effective CO2 capture. However, low binder contents resulted in fragile particles. It can be clearly seen in Table 1 that the compressive strength of particles generally decreased as the binder content decreased, although no regular trend for the variation of weight loss of sorbent particles was observed. Therefore, there exists a trade-off when selecting the mass ratio of CaO precursor and binder to produce CaO sorbent particles with acceptable CO2 capture capacity and mechanical strength. 3.2. Reactivation of CaO-based Sorbent Particles by Hydration in the Fixed-Bed Reactor. Although the chemical reversibility of those CaO-based sorbent particles was improved to some extent, the problem of loss-in-capacity was still 18219

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Figure 6. Mass variations of hydrated sorbents after (a) superheating at 300 °C and (b) superheating at 550 °C at elevated temperatures. Hydration was conducted at 300 °C for 1 h in 50 vol % steam (N2 balance) while subsequent particle superheating was performed at 300 or 550 °C in 100% CO2 for 1 h.

indicating that the subsequent superheating step at 550 °C in 100% CO2 in the process of hydration by superheating did not affect the activity of particles. Therefore, either hydration by steam or hydration by superheating was effective in reactivating the spent CaO-based particles. The influence of the successive carbonation/calcination reactions and different hydration treatments on the physical properties of the sorbent particles was studied by comparing their weight loss and compressive strength after various treatments in the fixed-bed reactor, as tabulated in Table 2 and Table S1 in the Supporting Information. It should be noted that the changes in the weight loss and compressive strength of the sorbent particles over 20 cycles without hydration treatments were very small. Specifically, CHC75 presented a very small increase in compressive strength from 4.37 to 5.66 MPa over 20 cycles. And, the weight loss of CHC75 also increased from 3.77% to 6.98%, which was insignificant compared with the changes caused by steam hydration. It should be noted that other CaO sorbent particles also presented similar variations, as shown in Table S1. The observation indicates that the cyclic carbonation/calcination reactions exerted insignificant influence on the physical properties of sorbent particles. That is to say, the effect of thermal shocks and internal stresses associated with cyclic carbonation/calcination reactions on the physical properties of CaO sorbent particles was very limited. Note that, the difference found in the values of weight loss and compressive strength of the CaO particles in the cases of hydration by steam and hydration by superheating could be caused by the selection of different particles and apparatus error. After the first steam hydration, however, the weight loss of sorbent particles during rotation (see section 2.3.1 for the definition of weight loss measurements) increased significantly, indicating that the presence of steam reduced the resistance of sorbent particles to abrasion. Specifically, the weight loss of CHC75 increased from 3.77% after the initial calcination to 16.32% after the first hydration by steam. The value almost doubled to 31.98% after the second hydration by steam,

indicating further weakening of CHC75 particles, as shown in Table 2. In contrast, hydration by superheating increases the weight loss of CHC75 insignificantly, from 3.77% after initial calcination to 7.78% after the second hydration by superheating. Therefore, the results show that hydration by superheating is able to maintain the particles’ resistance to abrasion, which is not the case for hydration by steam. Similarly, the variation of compressive strength of CHC75 pointed to the same conclusion. It is clearly seen that the compressive strength of CHC75 increased from 4.37 MPa after initial calcination to 9.33 MPa after 10 cycles, then quickly decreased to 5.04 MPa after the first hydration by steam. The second hydration by steam even decreased the value of compressive strength to 1.81 MPa. Meanwhile, the compressive strength of CHC75 was 22.02 and 13.57 MPa after the first and second hydration by superheating, respectively. In general, all CHC75 particles reactivated via hydration by superheating showed considerably lower weight loss and higher compressive strength than the steam hydrated particles. And the test results for other sorbent particles also imply the same conclusion, as seen in Supporting Information Table S1. In other words, hydration by superheating is capable of maintaining the physical properties of CaO-based sorbent particles (i.e., weight loss and compressive strength) while hydration by steam has adverse effects. This finding is consistent with the results reported by Materić et al.,25 although the possible mechanism responsible for this is different, as discussed later. Therefore, it can be concluded that the successive carbonation/calcination cycles did not influence the physical properties of CaO particles significantly in the fixed-bed reactor. Hydration by steam played a more important role than the cyclic reactions in weakening the CaO particles, whereas hydration by superheating did not reduce the mechanical stability. These tests indicate that the use of Al2O3 or high aluminate cement as binder material along with hydration by superheating as the recovery process, can 18220

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the treated particles were heated to 900 °C in TGA to confirm the existence of Ca(OH)2. As seen in Figure 6a, the decomposition of particles started at about 350 °C and finished at 450 °C. With further increase in temperature, a small proportion of sorbent decomposed at about 600 °C. Obviously, the first decomposition can be attributed to the decomposition of Ca(OH)2 while the second one was a result of CaCO3 decomposition. This indicates that the “superheated state” of Ca(OH)2 was observed when the sorbent particles were superheated at a lower temperature. However, the CHC75 particles with Ca(OH)2 remaining in them did not display increased mechanical strength (see the values of weight loss and compressive strength after heating at 300 °C in Table 3), which further implied that the previous improvement in the physical stability of CaO particles was not caused by the existing of Ca(OH)2. Then, the particles were subjected to another 5 cycles and the second steam hydration. Again, the particles were further weakened with a weight loss of 34.56% and a compressive strength of 2.35 MPa, as shown in Table 3. Finally, the second steam hydrated particles were further heated at 550 °C in 100% CO2 for 1 h to carbonate Ca(OH)2. Interestingly, the carbonated CaO particles became physically stronger compared to the previously steam hydrated particles. The weight loss of CHC75 particles was reduced to 12.31%, and the compressive strength increased to 6.85 MPa. It should be noted that other sorbent particles showed similar trend, as shown in Supporting Information Table S2. Therefore, it is evident that the enhancement in the mechanical properties of CaO particle was a result of the formation of CaCO3. Figure 7 presents the morphology change of CA64 during the substeps of hydration by superheating, which provides clues to the positive effect of hydration by superheating on both the CO2 capture performance and the physical structure of CaO particles. It can be clearly seen that the hydrated CA64 particles (Figure 7b) had a very spongy and fluffy structure compared to the freshly calcined CA64 (Figure 7a). This porous structure is believed to be responsible for weakening the sorbent’s resistance to abrasion. After the dehydration of Ca(OH)2 (reverse reaction of eq 3), the CA64 particles show recovered morphology, as shown in Figure 7c. In some areas, the grain size even became smaller compared to that of the fresh CaO particles as shown in Figure 7a. Consequently, the chemical activity of CA64 was recovered. After the second treatment of hydration by superheating at cycle 15, Ca(OH)2 adsorbed CO2 to form CaCO3, which occupied a greater volume than Ca(OH)2. Accordingly, the porosity of the carbonated particle reduced significantly, leading to a more compact surface (see Figure 7d and e). This compact surface indicates higher resistance of sorbent particles to abrasion.36 Also, the decomposed CA particles after superheating showed similar morphology as the fresh calcined CA64 particle, although the grain size partially became bigger. This could be the reason why the chemical activity of superheated CaO particles was recovered to nearly the same level as steam hydrated particles. The improvement in mechanical properties by the formation of CaCO3 has also been observed in strengthening concrete structures. It has been reported that the carbonation of Ca(OH)2 can slightly enhance the compressive strength and splitting strength of concrete (the maximum load that the concrete beam can bear before cracking occurs), although the formed CaCO3 can easily cause corrosion due to the contact between CO32− and water in air.37,38

simultaneously yield high CO2 capture performance and material reliability for calcium looping based CO2 capture.

4. DISCUSSIONS 4.1. Mechanism of Hydration by Superheating. In Materić et al.’s work,25 the best performing reactivation procedure of hydration by superheating involved the following: (1) hydrating calcined sorbent in 15 vol % H2O (N2 balanced) at 270 °C; (2) switching to a pure CO2 environment and heating the hydrated sorbent to 520 °C; (3) holding at 520 °C in 100% CO2 for 100 min for annealing; and (4) heating to 620 °C for carbonation. Step 1 follows the reaction described in eq 3. Steps 2 and 3 are so-called superheated dehydration of Ca(OH)2, which was responsible for the reduced attrition rate of limestone in the fluidized-bed reactor. Then, step 4 is the carbonation of Ca(OH)2, as shown in eq 4 CaO + CO2 ⇄ Ca(OH)2

(3)

Ca(OH)2 + CO2 ⇄ CaCO3 + H 2O

(4)

However, it was found that the Ca(OH)2 phase could not be preserved during steps 2 and 3 in this work. This was confirmed by heating the particles obtained after step 3 in TGA. As seen from Figure 6b, the particles began to decompose at temperatures higher than 600 °C, which is the typical decomposition temperature for CaCO3 in pure N2. It indicates that Ca(OH)2 in the particles formed in step 1 was fully converted into CaCO3 in steps 2 and 3, as described by eq 4. Even so, the physical properties of the carbonated CaO particles were still found to be maintained. It is possible that this positive effect of hydration by superheating was more likely to be caused by the formation of CaCO3 rather than the superheated phase of Ca(OH)2. To further verify the hypothesis above, the process of hydration by superheating was divided into two independent substeps (i.e., the hydration process and the subsequent superheating process). And, the experimental condition for the hydration process remained unchanged while the superheating process was conducted at two different temperatures (i.e., 300 and 550 °C) for the same duration under the same CO2 concentration (i.e., 100%). The weight loss and compressive strength of sorbent particles after each substep were determined using the same apparatus under the same conditions as mentioned before. As tabulated in Table 3, the results show that the CHC75 particles became fragile after the first steam hydration as expected, showing higher weight loss and lower compressive strength, which is in reasonable agreement with the values in Table 2. The sorbent particles were then heated at 300 °C in 100% CO2 for 1 h. Subsequently, Table 3. Physical Properties of CHC75 after Each Substep of Hydration by Superheatinga

1st steam hydration 1st steam hydration + superheating at 300 °C 2nd steam hydration 2nd steam hydration + superheating at 550 °C

weight loss [%]

compressive strength [MPa]

18.78 22.31

6.87 6.04

34.56 12.31

2.35 6.85

a

The 1st and 2nd treatments were conducted at cycle numbers 11 and 16, respectively. 18221

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Figure 7. SEM images of CA64 after (a) 3 h calcination, in 5 μm scale; (b) 10 cycles + steam hydration, in 5 μm scale; (c) 10 cycles + steam hydration/dehydration, in 5 μm scale; (d) 15 cycles + hydration by superheating, in 5 μm scale; (e) 15 cycles + hydration by superheating, in 1 μm scale; (f) 15 cycles + hydration by superheating/decomposition, in 1 μm scale.

4.2. Implications to the Practical Calcium Looping Process. The findings of this work can be extended to the practical calcium looping process that is usually configured with a dual fluidized-bed reactor system. First, the successive carbonation/calcination cycles reduce the chemical activity of sorbent but have negligible influence on the mechanical strength. Thus, more attention should be paid to reduce the surface abrasion of sorbent in the fluidized-bed reactor. Second, the positive effect of hydration by superheating indicates that the hydrated sorbent should be directly fed into the carbonator. Finally, the pelletization along with hydration by superheating as a reactivation method could be an effective way to simultaneously maintain the chemical activity and mechanical strength of CaO sorbent in calcium looping. When this combined technique is applied in calcium looping, the cost associated with pelletization should be the major concern. This is because the new calcium looping process integrated with an intermediate hydration step has been proven to deliver a lower energy penalty than the conventional calcium looping process.39 In our previous work,40 the cost for raw materials (commercial Ca(OH)2 and aluminate cement) would be in the range of $300−400 per ton. With the PRISM Eurolab 16XL extruder, the maximum yield of particle is 15 kg/h with power of 5.5 kW. Hence, the extrusion process would cost about $70 per ton synthetic CaO particles assuming the electricity price is $0.2 per kWh. Therefore, the whole cost for the extruded particles would be $370−470 per ton. Taking CHC75 for example, 2.5 tons of sorbents are necessary to capture 1 ton of CO2, provided the average CO2 capture capacity of CHC75 is 0.4 kg CO2/ kg sorbent by controlling the hydration level.24 In this scenario, the sorbent cost would be $925−1175 per ton CO2 captured. In contrast, the sorbent cost per ton of CO2 captured of natural limestone is about $1250 assuming the cost of raw limestone is $100 per ton and the average CO2 capture capacity is 0.08 kg CO2/ kg sorbent.41 That is to say, the cost of synthetic sorbent is comparable to that of natural limestone on the basis of the assumption above. It should be noted that above estimation is on the basis of a one-time use of CaO-based

sorbent. If the sorbent could be cyclically used 500 times, the sorbent cost would fall to $1.85−2.35 per ton CO2 captured, which is quite acceptable for the process to be commercialized.

5. CONCLUSIONS This paper investigated the influence of hydration by steam or superheating on the chemical and physical properties of CaObased pellets in a fixed-bed reactor. The use of Al2O3 and high aluminate cement as binder was critical in enhancing the chemical and mechanical properties of CaO sorbent particles. Also, both steam hydration and hydration by superheating were effective to reactivate the CaO sorbent particles after cycles in the fixed-bed reactor. Moreover, it was found that the successive carbonation/calcination cycles exerted negligible influence on the physical properties of CaO sorbent particles. However, steam hydration significantly weakened the CaO sorbent particles’ mechanical strength. On the other hand, hydration by superheating delivered similar enhanced recovery without the loss in mechanical strength. It is concluded that the use of Al2O3 or high aluminate cement as the binder material along with hydration by superheating as the recovery process, can simultaneously yield high performance and material reliability for calcium looping based CO2 capture.

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ASSOCIATED CONTENT

S Supporting Information *

Values of weight loss and compressive strength of CA43, CA64, and CHC40 before and after cyclic carbonation/calcination reactions and/or hydration treatments. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +61-7-33469193. Fax: +617-33654799. Notes

The authors declare no competing financial interest. 18222

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ACKNOWLEDGMENTS

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REFERENCES

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The authors are grateful for the financial support by the Department of Resources, Energy, and Tourism under the Australia−China Joint Coordination Group on Clean Coal Technology Grant Scheme. The authors also acknowledge the facilities, and the scientific and technical assistance, of the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy and Microanalysis, the University of Queensland.

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