Article pubs.acs.org/EF
Structurally Improved, Core-in-Shell, CaO-Based Sorbent Pellets for CO2 Capture Jian Sun, Wenqiang Liu,* Yingchao Hu, Mingkui Li, Xinwei Yang, Yang Zhang, and Minghou Xu* State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, People’s Republic of China S Supporting Information *
ABSTRACT: The pelletization of CaO-based sorbents is necessary for its practical application in the calcium looping process. In this work, three groups of composite pellets with different structures (non-shell pellets, core-in-shell pellets with inert shells, and structurally improved core-in-shell pellets with semi-reactive shells) were prepared from limestone powder and calcium aluminate cement. For the core-in-shell pellets, 2 wt % rice husks were added to the shells to enable the formation of relatively porous and strong shells. Both the CO2 uptake and mechanical strength of the cement-bound pellets were investigated to find the promising structure for the pelletization of the CaO-based sorbent. Moreover, wet curing was used for the first time, and prolonging the curing time could be effective to enhance the mechanical strength of the pellets. It was found that the core-in-shell pellets with semi-reactive shells via adding a moderate amount of limestone to the outer shell was able to largely improve the overall CO2 uptake capacity and, meanwhile, maintain the relatively good mechanical property. Particularly, when the limestone content of the core was fixed at 80 wt %, the pellets containing 60 wt % limestone in the shell exhibited a high total CO2 uptake capacity of 2.97 g/g during 17 cycles, a value more than twice that of the pellets that did not have limestone in the shells. As a result of limestone addition, the average crushing force of the cured pellets decreased by only 11.8%. Comprehensively, considering the CO2 uptake and mechanical strength, the core-in-shell pellets consisting of highly reactive cores and semi-reactive shells were the most promising to be used in the calcium looping process. drum granulation.18−20 Extrusion granulation is an effective way to obtain cylindrical pellets with high compression strengths, but the extrudates need to be cut off and made into pellets to be used in calcium looping systems. Spherical sorbent pellets, which can be directly obtained through rotary drum granulation, are easy to fluidize between the carbonation and calcination reactors. In addition, the cost of rotary drum granulation is lower and, hence, more amenable to scale-up. Therefore, rotary granulation has attracted the interest of many researchers. Lu et al.18 verified the feasibility of using a disc pelletizer to prepare sorbent pellets from a lime-based powder and bentonite. Manovic et al.19 also produced sorbent pellets from pulverized lime-based materials and commercial calcium aluminate cement using a mechanical pelletizer. Using a bubbling fluidized bed, they found that the cement-bound pellets resisted attrition. This method also allows for the reactivation and pelletization of the spent sorbents eluted from the calcium looping systems.20 To further improve the mechanical properties of the sorbent pellets, the concept of the core-in-shell sorbent pellet was proposed. Akiti et al.21,22 produced structure-enhanced, core-inshell pellets composed of highly reactive cores and highmechanical strength shells for flue gas desulfurization. They found that the surfaces of sorbent pellets that possessed cores consisting entirely of limestone or shells consisting entirely of cement easily cracked; with the addition of limestone to the
1. INTRODUCTION As the problem of global warming becomes increasingly serious, CO2 emissions must be significantly reduced if fossil fuels are to going to continue to be used.1 Many CO2 capture technologies exist; calcium looping systems are promising for this application because the adopted CaO-based sorbents have high CO2 capture capacities and are mainly produced from raw limestone, which is abundant and inexpensive.2−4 Therefore, calcium looping systems have garnered intense interest among researchers in recent years. These systems consist of two interconnected reactors: a carbonation reactor and a calcination reactor. CO2 contained in the flue gas is stripped in the carbonation reactor via the carbonation reaction of CaO, and formed CaCO3 decomposes in the calcination reactor to produce concentrated CO2 gas stream for sequestration. Then, regenerated CaO is passed back to the carbonation reactor for recycling.5−7 To maintain stable CO2 uptake, the sorbents in the calcium looping system should have good mechanical strengths during cyclic carbonation/calcination reaction processes. However, collisions between CaO-based pellets and pipe walls can cause severe attrition; in addition, the pellets can crumble during the circulation process, leading to a rapid degradation in their mechanical strength. Moreover, the thermal stress caused by the temperature difference between the carbonation and calcination reactors can also promote degradation.8−11 Researchers have tried many different granulation methods to produce CaO-based sorbent pellets with enhanced mechanical strengths for CO2 capture. Two of the most common methods are extrusion granulation8,11−17 and rotary © XXXX American Chemical Society
Received: June 24, 2015 Revised: September 25, 2015
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DOI: 10.1021/acs.energyfuels.5b01419 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
shell sorbent pellets (CC100, CC95, CC80, and CC60), core-in-shell sorbent pellets with inert cement shells (CC95−100S, CC80−100S, and CC60−100S), and core-in-shell sorbent pellets with semi-reactive shells containing added limestone (CC80−90S, CC80−70S, CC80− 60S, and CC80−40S). The sorbent pellets were named as follows: CC@ (non-shell sorbent pellets) or CC@−&S (core-in-shell sorbent pellets), where @ and & refer to the weight percent of limestone in the core and the weight percent of cement in the shell, respectively. For example, a sorbent that contains 95 wt % limestone in the core and 98 wt % cement in the shell (plus 2 wt % rice husk) is denoted as CC95− 100S (the outer shells of all of the core-in-shell sorbent pellets contained 2 wt % rice husk). More details of sorbent pellet preparation can be found in the Supporting Information. 2.2. Testing the CaO-Based Pellets. A mini precision compression tester (LYYS-1000N) was used to measure the crushing forces of the cured and calcined pellets (calcined at 900 °C for 0.5 h in a muffle furnace). The peak force required to crush a single pellet was recorded, and 20 pellets from each sample were tested. The mean of the crushing forces for each sample was used to evaluate the mechanical property of the pellets. Student’s t distribution was applied to calculate the error bars of the crushing force, and a 95% confidence interval was chosen. The detailed calculation method could refer to the way used by Kawatra et al.26 The CO2 sorption capacity of the pellets was tested using a TGA (Pyris 1, PerkinElmer). In each test, two pellets were placed in a platinum sample pan hung in a quartz tube. Before the cyclic carbonation/calcination tests, the pellets were heated to 900 °C at a rate of 20 °C/min under a N2 flow rate of 100 mL/min. The pellets were held under these conditions for 10 min to ensure that contained CaCO3 was decomposed completely. Then, the pellets were subjected to the cyclic tests and cooled to 650 °C at a rate of −25 °C/min. The gas was switched from pure N2 to 15 vol % CO2 (the total gas flow was kept at 100 mL/min) as soon as the temperature reached 650 °C. These conditions were maintained for 30 min to achieve sorbent carbonation. Then, the temperature was brought to 900 °C at a rate of 20 °C/min again under a N2 flow rate of 100 mL/min. These conditions were maintained for 10 min to achieve sorbent calcination. In this way, one carbonation/calcination cycle was completed, and each sample was subjected to this process 17 times. It is worth noting that, although 30 min of carbonation is unrealistic for a practical system, especially for the fluidized-bed-based reactors, the selection of carbonation time aims to comprehensively study the two-stage carbonation reaction of sorbent pellets. Actually, it is accepted that the longer time is needed to achieve the same carbonation conversion in the TGA as that in a practical fluidized-bed reactor.27 The CO2 sorption capacity (Cn, grams of CO2 per gram of initial calcined sorbent, g/g) and the maximum CaO carbonation conversion (Xn, %) of the sorbents were calculated on the basis of the weight change of the pellets that were tested, assuming that it was caused by only the carbonation and regeneration of CaO. Moreover, the carbonation conversion loss rate (Ln, %) was used to evaluate the stability of the CO2 uptake of the sorbent during multiple carbonation/calcination cycles. Detailed calculations of Cn, Xn, and Ln can be found in our recent paper.28 The cross-sectional microstructure of the sorbent pellets was investigated using field-emission scanning electron microscopy (FSEM, Nova NanoSEM 450, FEI, Inc.). Elemental mapping of the cross-sections of the pellets were obtained using energy-dispersive analysis X-ray spectroscopy (EDX).
shell or cement to the core, crack formation was less likely. The sulfur uptake ability of the pellets was enhanced when the limestone was added to the outer shell, and this property was attributed to the increase in the overall CaO content of the pellet.23 Liu et al.24 produced core-in-shell sorbent pellets consisting of calcium carbonate cores and clay shells using a modified planetary ball mill. The pellets exhibited stable CO2 sorption capacities during the first three carbonation/ calcination cycles. The average crushing force of the core-inshell pellets was found to be superior to that of the calcium carbonate pellets without clay shells, and the coated clay shells also enhanced the ability of the pellets to withstand attrition. Manovic et al.25 used a mechanical pelletizer to prepare core-inshell, CaO/CuO-based pellets with different ratios of CuO, CaO, and cement in their shells. The thermogravimetric analyzer (TGA) and bubbling fluidized bed testing results showed that the pellets with cement-bound shells resisted attrition better than both of those with shells that did not contain cement and homogeneous pellets without shells. Moreover, the addition of cement to the outer shell inhibited the deterioration of the surface of the core-in-shell pellets during sintering. Although the cement-bound outer shells improved the mechanical strength of the pellets, it also hindered the penetration of CO2 into the inner core, prohibiting its reaction with CaO and causing the CO2 sorption capacity and durability of the sorbent pellets to decrease. This phenomenon has not been thoroughly studied. Therefore, this work mainly investigated the effect of the addition of limestone to the outer shell of these structures on their CO2 sorption properties and mechanical strength. The cement content of the core was also studied to find the optimum ratio that balanced CO2 sorption and mechanical strength. Moreover, rice husks, which decompose at high temperatures, were added to the outer shell to improve its porosity. The curing times of the cement-bound pellets are closely related to the hydration level of the cement, which affected the mechanical strength of the pellet. Therefore, the dependence of the mechanical strength on the curing time of the pellets was investigated.
2. EXPERIMENTAL SECTION 2.1. Preparation of CaO-Based Pellets. Raw limestone from a Fude chemical plant that passed through the 150 μm sieve was collected. Calcium aluminate cement containing between 50 and 60 wt % Al2O3 (CA-50, from a Hongyu refractory material plant) was used as the binder. The results of the compositional analysis of the limestone and cement are shown in Table 1. Raw rice husks with particle sizes less than 150 μm were used as the pore-forming material. A general scheme for the work is displayed in Figure 1. Three groups of sorbent pellets with different structures were prepared: non-
Table 1. X-ray Fluorescence (XRF) Analysis of Raw Limestone and CA-50 component
limestone (wt %)
CA-50 (wt %)
CaO SiO2 SO3 Fe2O3 Al2O3 TiO2 K2O loss on ignition (LOI)
55.27 0.58 0.13 0.1
34.89 8.39 1.06 1.87 50.94 1.97 0.14
3. RESULTS AND DISCUSSION CaO contained in the calcium aluminate cement (CA-50) in the form of aluminates is regarded as an inactive component in CO2 capture. This assumption can be verified through cyclic CO2 capture tests of pure CA-50 cement. As shown in Figure S2, an increase in the CO2 capture capacity did not occur at the carbonation stage during 7 cycles; thus, the contribution of CA50 to the CO2 uptake of the sorbent pellets could be ignored. To investigate the effect of the rice husk used as the pore-
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DOI: 10.1021/acs.energyfuels.5b01419 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 1. Schematic for the investigation of CO2 uptake for core-in-shell pellets.
Additionally, SiO2 contained in the residual rice husk ash reacts with CaO to form anti-sintering Ca2SiO4, which might also improve the cyclic CO2 capture performance of the pellets.30 However, the further addition of rice husk will cause the mechanical strength of the sorbent pellets to decrease. Therefore, in this work, 2 wt % rice husk was added to the shells of the core-in-shell pellets to produce both relatively porous and strong shells. Three types of sorbent pellets (CC80, CC80−100S, and CC80−40S), representing the three groups of pellets with different structures, were analyzed by FSEM/EDX. The images in Figure 3a reveal that the elements of calcium and aluminum were homogeneously dispersed over the cross-section of the CC80 pellet, representing the homogeneous distributions of limestone and calcium aluminate cement. In panels b and c of Figure 3, the core-in-shell structures could be easily distinguished from the calcium and aluminum maps. It is obvious that the aluminum resides mainly in the shells rather than in the cores, because the shells contain more calcium aluminate cement. 3.1. Non-shell, CaO-Based Pellets. The changes in the CO2 sorption capacities of the non-shell pellets over 17 cycles are displayed in Figure 4. The addition of cement improved the CO2 carbonation conversions of the sorbent pellets. In particular, when 5 wt % cement was added, the CO2 capture capacity of CC95 was as high as 0.241 g/g in the 17th carbonation cycle, corresponding to a carbonation conversion of 34.1%. However, the 17th carbonation conversion was 21% for CC100, which contained only limestone. The stability of the CO2 uptake could be reflected in the carbonation conversion loss (Ln). CC100 showed the lowest Ln of −72.0% compared to −53.3% for CC95 (a decrease of 35.08%). The Ln values of CC80 and CC60 were 15.8 and 10.0% higher than that of CC100, respectively. Therefore, the cement addition effectively enhanced the CO2 uptake stability of the sorbent pellets. It is likely that the cement caused the formation of mayenite (Ca12Al14O33), which could enhance the ability of the CaO-
forming material on the CO2 capture performance of the sorbent pellets, three types of limestone-based pellets containing different amounts of rice husk were produced. In Figure 2, it was found that the pellets containing 10 and 2 wt %
Figure 2. Carbonation conversion of limestone-based pellets with different amounts of added rice husk over 17 cycles (magnified profiles are over the 30 min carbonation steps in the 1st and 17th cycles): calcination, 10 min at 900 °C in 100 vol % N2; carbonation, 30 min at 650 °C in 15 vol % CO2.
rice husk had higher carbonation conversions (26.0 and 23.3%, respectively) in the 17th carbonation cycle than the pellets that did not contain rice husk (21%). This enhancement in CO2 uptake was mainly attributed to the improved morphology of the pellets that resulted from the addition of the rice husk.29 The improved pore structure allowed CO2 to reach the inside of the pellet, where it could react with internal CaO. From the magnified profiles of the 1st and 17th carbonation cycles, it is clearly observed that the addition of rice husk promotes the carbonation reaction rate of pellets at chemical-reactioncontrolled stages, especially when 10 wt % rice husk is used. C
DOI: 10.1021/acs.energyfuels.5b01419 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 3. EDX mapping images for elements Ca (red dots) and Al (green dots) of cross-sections of samples (a) CC80, (b) CC80−100S, and (c) CC80−40S with their corresponding FSEM images of cross-sections of samples (first column).
based sorbent pellets to resist sintering.11,31,32 However, the cement could cause more active CaO to be bound as mayenite during the carbonation/calcination process.12 The carbonation conversion was calculated on the basis of the initial active CaO content; the fact that the calculated value was lower than the real value for the amount of consumed CaO was not taken into account. Thus, the larger the amount of CA-50 added into the pellets, the greater the discrepancy between the real and calculated carbonation conversions. In addition to the CO2 capture performance, it is important to consider the mechanical properties of the pellets because these properties are also important for their use in practical applications. The addition of cement enhanced the mechanical strength of the sorbent pellets. In Table 2, the average crushing
force of CC95 was only 1.54 N, whereas that of CC80 and CC60 was 24.44 and 70.92 N, respectively. However, the average crushing force of CC80 and CC60 declined sharply after calcination (2.69 and 12.43 N, respectively), and cracks even appeared on the surface of calcined CC95. The sharp decline in the crushing forces of the calcined pellets can be attributed to the decomposition of the limestone and cement hydration products, which causes the volume expansion of and internal cracking within the pellets.21 3.2. Core-in-Shell, CaO-Based Pellets with Inert Shells. To further improve the mechanical strength of the sorbent pellets, the concept of producing core-in-shell pellets with reactive cores encased in inert cement shells was proposed. As shown in Table 2, the average crushing force of CC95−100S D
DOI: 10.1021/acs.energyfuels.5b01419 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 4. (a) CO2 capture capacity and (b) carbonation conversion of non-shell pellets during 17 cycles. The conditions are the same as for the samples shown in Figure 2.
Figure 5. (a) CO2 capture capacity and (b) carbonation conversion of core-in-shell pellets with inert cement shells during 17 cycles. The conditions are the same as for the samples shown in Figure 2.
was 34.18 N, which was over 21 times that of CC95 (without a cement shell). The increase in the average crushing force for CC80−100S compared to CC80 was moderate, approximately 56.46%. However, in comparison to CC60, the average crushing force of CC60−100S increased by only 6.64%; because more cement was mixed in the core, there was less room for improvement. After calcination, the average crushing forces of the pellets coated with cement shells sharply declined and the surface of the calcined CC95−100S pellets severely cracked. The cracks were partly caused by the large amount of CO2 escaping from the low-porosity cement shell during the calcination process. They could also be attributed, in part, to the compositional differences between the core and shell and the resulting heterogeneous thermal deformation between them.33 The cracking could be eliminated by decreasing the limestone content of the core; cracking did not occur with the calcined CC60−100S and CC80−100S samples. For the calcined samples, the cement shells greatly increased the crushing force of the CC80 sample (by 1.5 times) but not that of the CC60 samples (Table 2). Although the coated cement shells enhanced the mechanical strength of the sorbent pellets, their CO2 capture capacities dropped dramatically, as shown in Figure 5a. For instance, CC60 had a CO2 sorption capacity of 0.236 g/g in the first
cycle, but this value for CC60−100S was only 0.092 g/g. This decrease in the overall CO2 sorption capacity was foreseeable because the cement shell does not contribute to the CO2 uptake capacity of the pellet. Instead, it hinders the diffusion of CO2 into the pellet, where it can react with the active CaO in the core. As shown in Figure 5b, the carbonation conversions of the pellets coated with cement shells were always approximately 5−10% lower than those of the corresponding pellets without shells over 17 cycles. Therefore, although the cement shells allowed for increases in mechanical strength, they decreased the CO2 uptake of the sorbent pellets. As a compromise, the concept of adding limestone into the outer shell was proposed. 3.3. Core-in-Shell, CaO-Based Pellets with Semireactive Shells. The amount of limestone added to the shell was closely related to the enhancement in the CO2 uptake of the pellet. To avoid massive losses in mechanical strength caused by the addition of limestone to the shell, the limestone content of the core was fixed at 80 wt %. The CO2 sorption capacities of the pellets containing between 10 and 60 wt % limestone in their outer shells were better than that the CC80− 100S pellets without limestone in their shells, as shown in Figure 6a. It was found that, as more limestone was added to the shell, higher CO2 uptake capacities were obtained. In the first cycle, CC80−40S (60 wt % limestone added to the shell)
Table 2. Average Crushing Forces of the Cured and Calcined CaO-Based Pellets crushing force (N) sorbents CC95 CC80 CC60 CC95−100S CC60−100S a
crushing force (N)
cured sample
calcined sample
sorbents
± ± ± ± ±
na 2.69 ± 0.30 12.43 ± 1.30 na 13.58 ± 1.19
CC80−100S CC80−90S CC80−70S CC80−60S CC80−40S
1.54 24.44 70.92 34.18 75.63
0.09 2.13 4.79 2.95 6.06
cured sample 38.24 35.97 35.44 34.76 33.73
± ± ± ± ±
3.99 4.29 3.45 3.57 2.55
calcined sample 6.86 6.94 6.43 5.47 4.82
± ± ± ± ±
0.94 0.92 0.83 0.46 0.54
Cracks were found in the calcined CC95 and CC95−100S samples, and further tests were not carried out. E
DOI: 10.1021/acs.energyfuels.5b01419 Energy Fuels XXXX, XXX, XXX−XXX
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sorbent pellets. The dependence of the addition of limestone to the shells of the pellets upon their total CO2 sorption capacity and average crushing force are displayed in Figure 8. With an
Figure 8. Average crushing forces of cured core-in-shell pellets. The right y axis shows the total CO2 uptake capacity of the pellets during 17 cycles. The limestone content of the shell is 0, 10, 30, 40, and 60 wt % (corresponding to CC80−100S, CC80−90S, CC80−70S, CC80− 60S, and CC80−40S, respectively).
Figure 6. (a) CO2 capture capacity and (b) carbonation conversion of core-in-shell pellets with semi-reactive shells with added limestone during 17 cycles. The conditions are the same as for the samples shown in Figure 2.
increasing limestone content, the total CO2 uptake capacity increased largely and the average crushing force decreased slightly. In particular, the total CO2 sorption capacity of CC80−40S (60 wt % limestone in the shell) was approximately 2.97 g/g during 17 cycles, more than twice that of the CC80− 100S sample (pellet without limestone in the shell). However, the average crushing force of cured CC80−40S decreased by only 11.8% compared to that of cured CC80−100S. Hence, considering both the CO2 sorption capacity and the mechanical strength, CC80−40S was regarded as the most promising sample to be applied in the calcium looping system. Moreover, it is worth recognizing that it would be more helpful for practical application if the sorbent pellets could be tested under nearly realistic conditions. The CO2 uptake performance of the CC80−40S sample calcined at 950 °C in 35 vol % CO2 (the maximal CO2 concentration limited by the TGA) was investigated. The schematic representation of the experiment is displayed in Figure S3. The lower calcination temperatures (850 and 900 °C) were also studied for comparison purposes. As shown in Figure 9, the existence of CO2 at the calcination stage had a negative impact on the CO2
showed the highest CO2 capture capacity (0.293 g/g); the CO2 capture capacities were 0.165 and 0.132 g/g for CC80−90S (10 wt % added limestone in the shell) and CC80−100S (limestone not added to the shell), respectively. The carbonation conversion was also improved when the limestone content of the shell was increased (especially for CC80−60S and CC80− 40S), as shown in Figure 6b. This increase was mainly attributed to the improvement in shell porosity, which allows for more CO2 gas to diffuse into the pellet, where it can react with internal CaO. However, the CO2 sorption capacity gradually decreased with increasing the cycle number; the phenomenon can be attributed primarily to the loss of the nanosized pores useful for carbonation (
