Routine Investigation of CO2 Sorption Enhancement for Extruded

State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China...
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Routine Investigation of CO2 Sorption Enhancement for Extruded-spheronized CaO-based Pellets Hongqiang Chen, Wenqiang Liu, Jian Sun, Yingchao Hu, Wenyu Wang, Yuandong Yang, Shun Yao, and Minghou Xu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00921 • Publication Date (Web): 03 Aug 2017 Downloaded from http://pubs.acs.org on August 10, 2017

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Routine Investigation of CO2Sorption Enhancement for Extruded-spheronized CaO-based Pellets Hongqiang Chena, Wenqiang Liu*a, Jian Suna,b, Yingchao Hua, Wenyu Wanga, Yuandong Yanga, Shun Yaoa and Minghou Xu*a a

State Key Laboratory of Coal Combustion, School of Energy and Power Engineering,

Huazhong University of Science and Technology, Wuhan 430074, China b

Jiangsu Provincial Key Laboratory of Materials Cycling and Pollution Control, School

of Energy and Mechanical Engineering, Nanjing Normal University, Nanjing 210042, China *Corresponding author: Tel: +86 27 87542417-8301; Fax: +86 27 87545526; E-mail: [email protected]; [email protected]; Postal address: State Key Laboratory of Coal Combustion, Huazhong University of Science & Technology, 1037 Luoyu Road, Wuhan 430074, PR CHINA. ABSTRACT For practical application in calcium looping, the CaO-based sorbents need to be pelletized. However, the pelleting process, particularly the extrusion process will lead to the loss of specific surface area, hence weakening CO2 sorption performance of sorbent pellets. In previous studies, only limited modification methods were developed for CO2 sorption performance enhancements of sorbent pellet, and the effects of modification are not satisfactory. In this work, the enhancement of CO2sorption performance of extrudedspheronized pellets was achieved by incorporating the MgO support into CaO-based

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pellets. Three routines of pellets preparation were studied including soaking (Routine I), mechanical mixing (Routine II) and simultaneous hydration-incorporation (Routine III). It was found that the sorbent pellets produced via Routine III possess the best cyclic sorption performance due to the generation of additional pores within the pellets when the release of CO2derived from the decomposition of magnesium acetate. In addition, the content of incorporated MgO significantly affects the cyclic CO2 capture performance of sorbent pellets. The pellets introduced to 25 wt.% MgO display the highest conversion of 66.39%, which is approximately1.52times that of the pellets introduced to 5 wt.% MgO. Moreover, the MgO-incorporated pellets prepared via extrusion-spheronization possess the high capability to resist attrition. Keywords: CO2 capture, routine investigation, MgO support, extruded-spheronized pellets 1. INTRODUCTION Global warming caused by massive CO2 emissions, has already received public attention, and CO2 capture, storage and utilization (CCSU) has been considered one of the most effective solutions for solving this problem 1. In the procedure of CCSU, CO2 captured from coal-fired power plants, is transported, and stored or utilized instead of being emitted into the atmosphere 1. The wide application of CCUS technology has been delayed by the expensive CO2 capture progress, accounting for approximately 75% of the total cost 1. Therefore, the research of low-cost CO2 capture technologies is crucial. The calcium looping process (CLP), as one of the promising technologies, has attracted extensive attention worldwide in recent years 2. The CLP is based on the reversible reaction:O +  ⇋  , capturing CO2 from flue gas by the circulation of CaO2 ACS Paragon Plus Environment

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based sorbent between the carbonator and regenerator. In the previous studies, the sorbents are mostly in the form of powder. However, for practical application in calcium looping, sorbent powder is easily elutriated from the carbonator and regenerator and is thus not suitable in a calcium looping system. To solve this problem, it is necessary to pelletize CaO-based sorbent powder into pellet. Researchers have adopted several methods for producing CaO-based sorbent pellets. There are three common methods for producing sorbent pellets, i.e. extrusion granulation 3-7

, rotary drum granulation

8-11

, extrusion-spheronization12 and gel-casting technique

13

.

The extrusion-spheronization method is a novel pelleting method proposed in our recent study. This method that combines the advantages of the extrusion granulation method and the rotary drum granulation method can produce near-spherical CaO-based sorbent pellets with high compression strength. Nevertheless, the CO2 capture capacity of the sorbent pellet sharply declines during long-term multiple carbonation/calcination cycles. It is mainly caused by the sorbent sintering

5

and the loss of specific surface area of

sorbent pellet due to pelleting process, particularly the extrusion process 12. Many methods have been used to cope with the loss-in-capacity, such as hydration treatment14-21, thermal pretreatment precursor

31-34

22-26

, doping

27-30

, adding the anti-sintering calcium

, and incorporating an inert solid support

7, 35-40

. However, these modified

methods are mostly applied in sorbent powder and rarely used in sorbent pellet. Only limited modification methods are developed for the CO2 sorption performance enhancements of sorbent pellet, as shown in Table 1. These modified methods mainly include doping biomass-based pore-forming materials12, 42

and adding cement or bentonite

3, 7, 11

41

, mechanical modification

. However, the modified effect of these

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modification methods is not very satisfactory. For example, the conversion of sorbent pellets that were modified via mechanical grinding was only approximately 37.4% after 25 carbonation/calcination cycles

41

, that of sorbent pellets that were modified with

cellulose was approximately 44.5% after 25carbonation/calcination cycles 12, and that of sorbent pellets that were modified with cement was approximately 44.88% after 25carbonation/calcination cycles 7. The MgO insert solid support has been extensively studied and proved to be efficient for enhancing the cyclic performance of the CaO-based sorbent

23, 43

due to its high

Tammann temperature (1276 °C). However, the incorporation methods of MgO into the CaO-based sorbent lead to different distribution of MgO in the CaO-based sorbents, which results in different CO2 sorption performances among the sorbents44, 45. Therefore, it is necessary to investigate the effect of the incorporation methods of MgO into the CaO-based sorbent pellets on their cyclic CO2 capture performance. In this work, the routine investigation of sorption enhancement for extrudedspheronization CaO-based pellets was conducted. The enhancements were achieved via the incorporation of MgO inert solid support, and three synthesis routines (soaking, mechanical mixing and simultaneous hydration-incorporation) that adopted different incorporation methods of MgO into the CaO-based pellets were applied. Then, the sorption performance and anti-attrition of the MgO-incorporated CaO-based pellets were studied. In addition, the effect of MgO mass ratios (5, 15 and 25 wt.%) was also researched in this study. The micromorphology and porosity of the sorbent pellets were characterized using the field emission scanning electron microscopy (FSEM). The specific BET surface area and pore structure were tested in an accelerated surface area 4 ACS Paragon Plus Environment

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and a porosimetry system (ASAP2020, Micromeritics) apparatus. 2. EXPENRIMENTAL SECTION 2.1. Preparation of Sorbent Pellets In this study, MgO-incorporated CaO-based pellets were synthesized via three routines (Routine I, Routine II and Routine III), as illustrated in Figure 1. Routine I (soaking). First, calcined lime is obtained from the calcination of natural limestone (containing 98.5 wt.% CaCO3). Then, excess deionized water was added to the obtained calcined lime. Following this, the suspension was dried at 105 °C in a drying oven overnight, which was thereafter ground to obtain a hydrated lime powder. The hydrated lime powder was sieved to less than 200µm and then pelletized via the extrusion–spheronization method to obtain the hydrated lime pellets (Pellet 1). Then, Sample 1 was obtained after calcination of Pellet 1. Pellet 1 was soaked in 1 mol/L magnesium acetate solution for 1 hour to obtain Pellet 2, and after calcination, the MgOincorporated CaO-based sorbents were obtained (Sample 2). Routine II (mechanical mixing). MgO powders and the abovementioned hydrated lime powders (CH-MM-5%>CH>CC. Compared with the cyclic performance of the CC, the performances of other sorbent pellets were enhanced through the hydration and/or the inert solid support MgO. In addition, compared with the performance of CH and the MgO-incorporated CaO-based sorbent pellets, it is obvious that the addition of MgO inert solid support can enhance the cyclic performance of sorbent pellets, which is consistent with the results of Radfarnia et al. 47, although the MgO inert solid support was incorporated into the sorbent powder in that paper. To compare the sorption performance of sorbents more clearly, the conversion loss rate and the last cycle conversion are collectively shown in Figure 3. It is found that CHWM-5% has the lowest conversion loss rate and the highest last-cycle conversion (43.70%). Therefore, CH-WM-5% is the best sorbent among several pellets, and it is concluded that Routine III is the best for incorporating MgO into the CaO-based sorbent pellet. The superior performance of CH-WM-5% is explained below. Figure 4(a-d) shows the FSEM images of sorbents after 25 cycles. It is found that the sintering of CC is most serious and that there are almost no cracks in the morphology. However, there are apparent cracks in the morphology of other pellets, which allow more CO2 to diffuse into the pellets and, therefore, result in a better sorption capacity. There are two explanations 9 ACS Paragon Plus Environment

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for the cracks observed on the surface of the MgO-incorporated CaO-based pellets. First, the cracks were formed during the steam prehydration of CaO48-50. Second, the addition of MgO support, which functions as a metal skeleton, can maintain the shell porosity51, 52 and hence slow the sintering tendency of CaO/CaCO3 particles during the carbonation/calcination cycles. The EDX mapping images for Ca and Mg and FESM images of the cross-section of CH-WM-5% before calcination are shown in Figure 4(e). And Figure 4(f) shows the morphology and EDX mapping images for Ca and Mg of a small area in the cross-section of CH-WM-5%. The two figures illustrate that Ca and Mg elements are homogeneously dispersed over the cross-section of the CH-WM-5%. Furthermore, the homogeneously dispersed inert solid support MgO can assist well in resisting sintering. Among the three MgO-incorporated CaO-based pellets, CH-WM-5% has the best cyclic performance. The cyclic performance of CH-WM-5% is better than that of CHMM-5% probably because Routine III can distribute MgO more homogeneously around the sorbent pellets. In addition, magnesium acetate decomposes into MgO, CO2 and other substances during the calcination process. Additional pores were created when the generated CO2 came out from the sorbent pellets 43. The cyclic performance of CH-WM5% is better than that of CH-SK probably because Routine III distributes MgO more homogeneously around the sorbent pellets than Routine I does. 3.2. Effect of MgO Content on the CO2 Sorption Performance of Pellets Synthesized via Routine III The higher quantity of the inert solid support leads to a better sintering-resistant performance of the sorbent but also decreases the theoretical maximum CO2 capture 10 ACS Paragon Plus Environment

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capacity of the sorbent

53

. Thus, it is necessary to find the optimum ratios of the MgO.

Therefore, the CaO-based sorbent pellets with three different mass ratios (5, 15 and 25 wt.%) of MgO prepared via the synthesis Routine III, which is the best routine for incorporating MgO into the CaO-based sorbent pellet, were studied. Generally, the introduction of MgO inert solid support effectively enhances the cyclic performance of CaO-based sorbent pellets. The more MgO inert solid support is introduced, the better CaO conversion and durability of the sorbents is obtained. The lastcycle conversion of all the sorbents can be ranked as follows: CH-WM-25%>CH-WM15%>CH-WM-5%>CH>CC (Figure 5). The last-cycle CO2sorption capacity of all sorbents can be ranked as follows: CH-WM-25%>CH-WM-15%>CH-WM-5%>CH>CC (Figure 6). Moreover, the decreasing trend of Cn and Xn of CH-WM-25% is the slowest. It is found that the conversion losses of the CH-WM-15% and the CH-WM-25% are approximately only half the loss of the CH (Figure 7). In addition, as shown in Figure 8, during the 25th carbonation process, the reaction rate (corresponding to the peak value of the solid lines) during the chemical-reaction-controlled stage of the sorbents with incorporated MgO is much faster than that of the CC and CH. Particularly CH-WM-25%, the sorption amount during the initial five minutes accounts for approximately 72% of the total sorption amount. In practical application, the effective sorption time of the sorbent is only a few minutes, so CH-WM-25% is more suitable for practical application than other sorbents. Therefore, the CH-WM-25% were chosen for further analysis and comparison. Figure 9 shows the FSEM images of the sorbent pellets after 25 cycles of carbonation/calcination experiments. It is observed that the sorbent particle size exhibits the reduction tendency with the increase of the inert support. And the smaller particles are 11 ACS Paragon Plus Environment

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better for increasing the BET surface area of sorbent. The BET surface area, pore volume and pore size of sorbents after 25 carbonation/calcination cycles are shown in Table 2. CH-WM-25% has the biggest BET surface area (12.10 m2/g), which is almost four times larger than that of CH (3.26 m2/g), and the biggest BJH desorption cumulative pore volume (0.09 cm3/g). It is found that the porosity of nanosized pores in the range of 10– 100 nm of the sorbents with the addition of the MgO is greater than CH (Figure 10). CHWM-25% had the largest porosity compared with the other sorbents, which corresponds to its superior performance for capturing CO2. Figure 11 compares the last-cycle carbonation conversion of CH-WM-25% and that of pellets reported in the literatures (Table 1). It is found that the last-cycle conversion of CH-WM-25% modified by simultaneous hydration-incorporation method is superior to that of sorbents modified by other methods, such as doping biomass-based pore-forming materials, mechanical modification and adding cement. This result indicates that Routine III is the best method to modify the CO2 sorption performance of sorbent pellet among these methods. The attrition of sorbent pellets leads to the mass loss of sorbents, which results in higher system operating costs. Therefore, the anti-attrition of sorbent pellet is an important criterion for evaluating the sorbent pellets. The anti-attrition of the sorbent pellets was tested using a CS-2 friability tester mentioned above. The weight loss of the sorbent pellets before the cycles are shown in Table 3. The weight losses of all sorbent pellets after 3000 rotations are less than 1%, which indicates that the addition of the MgO insert solid support does not affect the anti-attrition of the sorbent pellets which are produced by extrusion-spheronization method. 12 ACS Paragon Plus Environment

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3.3. Effect of the Existence of CO2during Calcination Stage on the CO2 Sorption Performance of Pellets Decomposition of CaCO3 is an endothermic reaction that requires additional energy to decompose CaCO3. Shimizu et al.54 originally proposed that if CLP is used for capturing CO2 from flue gas in coal-fired power plants, the additional energy can be obtained from the oxy–fuel combustion of coal. In this case, CO2 is generated from the coal combustion with pure oxygen in the regenerator. Hence, it is necessary to test the CO2 sorption performance of the sorbent pellets under the CO2-containing calcination conditions. The CO2 sorption performance of the CH-WM-25% and CC calcined in 50 vol.% CO2 and 50 vol.% N2 was investigated in this study. The CC-WM-25% calcined in 50 vol.% CO2is denoted as CH-WM-25%-50%CO2, and that of CC is denoted as CC50%CO2. The carbonation conversions of those sorbent pellets are shown in Figure 12. With the introduction of 50 vol.% CO2 during calcination, the 25th conversion of CC decreased. Similarly, the 25thconversion of CH-WM-25%reduced from 66.39% to 54.15%. The result indicates that the existence of CO2 during the calcination stage has a negative impact on the CO2 sorption performance of the sorbent pellets, consistent with the results in these literature55-57. The existence of CO2 prolongs the process of CaCO3 complete decomposition55. The exposure time of CaCO3 under the high calcination temperature is lengthened due to the prolonged process of CaCO3complete decomposition. The longer time of exposure of CaCO3 under the high calcination temperature causes more severe sintering because the Tammann temperature of CaCO3 is much lower than that of CaO46.

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However, even under 50 vol.% CO2 during the calcination, the CH-WM-25%50%CO2 still exhibit an excellent cyclic performance. Figure 12 illustrates that CO2 conversion of CH-WM-25%-50% CO2 after 50 cycles is 47.36%. The results further proved that MgO particles with a high Tammann temperature can act as a framework for the CaO particles to prevent the sintering and aggregation of the CaCO3 nanoparticles. 3.4. Effect of SO2 during the Carbonation Stage on the Sorbent Performance SO2 exists in the flue gas of the coal-fired plants and therefore investigation about the sulfation of CaO-based pellets during the carbonation stage is important. CaSO4 could be formed through the reaction of SO2 with CaO and CaCO3 and cannot be decomposed under the operation temperature. Thus, the amount of active CaO would be quickly decreased 58, 59. The performance of CH-WM-25% under 1000 ppm SO2 during the carbonation stage was tested on the fixed bed reactors and the results are provided in Figure 13. It is clearly to see that, with the introduction of SO2, the cumulative sulfation conversion of the pellets fast increased, indicating the fast accumulation of the thermally stable CaSO4(meaning less active CaO available for CO2 capture).As a result, the CO2 capture performance of the pellets decayed quickly, which is consistent with the previous results58, 59. Therefore, for the efficient CO2 removal for coal-fired flue gas using the current pellets, prior SO2 elimination should be conducted for calcium looping system. 4. CONCLUSIONS In this study, the MgO-incorporated CaO-based pellets, produced by extrusionspheronization method were synthesized via the three different routines. Compared with 14 ACS Paragon Plus Environment

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the naturally occurring limestone sorbent, all MgO-incorporated CaO-based pellets exhibit enhanced cyclic performance. Moreover, the sorbent pellets which are produced by Routine III exhibit the lowest conversion loss and the highest last-cycle conversion. It was found that the introduction of MgO inert solid support effectively enhances the cyclic performance of CaO-based sorbent pellets and Routine III is the best routine. In addition, the sorbent pellets with three different MgO mass ratios (5, 15 and 25 wt.%) produced by Routine III were studied. The sorbent pellet with 25 wt.% of MgO exhibit the highest last-cycle conversion, the highest CO2sorption capacity and the best durability. This result indicates that the more MgO inert solid support is introduced, the better CaO sorption capacity and durability of the sorbents is obtained. Furthermore, even under 50 vol.% CO2 during calcination process, the MgO-incorporated CaO-based pellets still exhibit an excellent cyclic performance. ACKNOWLEDGMENTS The authors thanks very much for the financial supports from the National Natural Science Foundation of China (51306063) the Foundation of State Key Laboratory of Coal Combustion (FSKLCCB1602). The support from the Analytical and Testing Centre at Huazhong University of Science and Technology is also appreciated. REFERENCES 1. Metz, B.; Davidson, O.; de Coninck, H.; Loos, M.; Meyer, L.; Change, W. G. I. o. t. I. P. o. C., IPCC, 2005: IPCC special report on carbon dioxide capture and storage. 2005. 2. 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, (5), 2751-2767. 15 ACS Paragon Plus Environment

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capture in an FBC environment. Chemical Engineering Journal 2003, 96, (1), 187-195. 28. Laursen, K.; Grace, J. R.; Lim, C. J., Enhancement of the sulfur capture capacity of limestones by the addition of Na2CO3 and NaCl. Environmental Science & Technology 2001, 35, (21), 4384-4389. 29. Roesch, A.; Reddy, E. P.; Smirniotis, P. G., Parametric study of Cs/CaO sorbents with respect to simulated flue gas at high temperatures. Industrial & Engineering Chemistry Research 2005, 44, (16), 6485-6490. 30. Li, Y.; Zhao, C.; Chen, H.; Duan, L.; Chen, X., Cyclic CO2 capture behavior of KMnO4-doped CaO-based sorbent. Fuel 2010, 89, (3), 642-649. 31. Barker, R., The reactivity of calcium oxide towards carbon dioxide and its use for energy storage. Journal of Applied Chemistry and Biotechnology 1974, 24, (4 5), 221227. 32. Lu, H.; Reddy, E. P.; Smirniotis, P. G., Calcium oxide based sorbents for capture of carbon dioxide at high temperatures. Industrial & Engineering Chemistry Research 2006,45, (11), 3944-3949. 33. Hu, Y.; Liu, W.; Sun, J.; Li, M.; Yang, X.; Zhang, Y.; Liu, X.; Xu, M. Structurally improved CaO-based sorbent by organic acids for high temperature CO2 capture. Fuel 2016, 167, 17-24. 34. Gupta, H.; Fan, L.-S., Carbonation-calcination cycle using high reactivity calcium oxide for carbon dioxide separation from flue gas. Industrial & Engineering Chemistry Research 2002, 41, (16), 4035-4042. 35. Aihara, M.; Nagai, T.; Matsushita, J.; Negishi, Y.; Ohya, H., Development of porous solid

reactant

for

thermal-energy

storage

and

temperature

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upgrade

using

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carbonation/decarbonation reaction. Applied Energy 2001, 69, (3), 225-238. 36. Dobner, S.; Sterns, L.; Graff, R. A.; Squires, A. M., Cyclic calcination and recarbonation of calcined dolomite. Industrial & Engineering Chemistry Process Design and Development 1977, 16, (4), 479-486. 37. Hu, Y.; Liu, W.; Chen, H.; Zhou, Z.; Wang, W.; Sun, J.; Yang, X.; Li, X.; Xu, M. Screening of inert solid supports for CaO-based sorbents for high temperature CO2 capture. Fuel 2016, 181, 199-206. 38. Lu, H.; Smirniotis, P. G., Calcium oxide doped sorbents for CO2 uptake in the presence of SO2 at high temperatures. Industrial & Engineering Chemistry Research 2009, 48, (11), 5454-5459. 39. Hu, Y.; Liu, W.; Sun, J.; Yang, X.; Zhou, Z.; Zhang, Y.; Xu, M. High temperature CO2 capture on novel Yb2O3-supported CaO-based sorbents. Energ Fuel 2016, 30, (8), 66066613. 40. Wang, K.; Hu, X.; Zhao, P.; Yin, Z., Natural dolomite modified with carbon coating for cyclic high-temperature CO2 capture. Applied Energy 2016, 165, 14-21. 41. Sun, J.; Liu, W.; Chen, H.; Zhang, Y.; Hu, Y.; Wang, W.; Li, X.; Xu, M. Stabilized CO2 capture performance of extruded–spheronized CaO-based pellets by microalgae templating. Proc Combust Inst 2017, 36, (3), 3977-3984. 42. Sun, J.; Liu, W. Q.; Li, M. K.; Yang, X. W.; Wang, W. Y.; Hu, Y. C.; Chen, H. Q.; Li, X.; Xu, M. H., Mechanical modification of naturally occurring limestone for hightemperature CO2 capture. Energy & Fuels 2016, 30, (8), 6597-6605. 43. Liu, W.; Feng, B.; Wu, Y.; Wang, G.; Barry, J.; Diniz da Costa, J. o. C., Synthesis of sintering-resistant sorbents for CO2 capture. Environmental Science & Technology 2010,

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44, (8), 3093-3097. 44. Chen, H. C.; Zhao, C. S.; Yu, W. W., Calcium-based sorbent doped with attapulgite for CO2 capture. Applied Energy 2013, 112, 67-74. 45. Luo, C.; Zheng, Y.; Xu, Y.; Ding, N.; Shen, Q.; Zheng, C., Wet mixing combustion synthesis of CaO-based sorbents for high temperature cyclic CO2 capture. Chemical Engineering Journal 2015, 267, 111-116. 46. Hu, Y.; Liu, W.; Sun, J.; Li, M.; Yang, X.; Zhang, Y.; Xu, M., Incorporation of CaO into novel Nd2O3 inert solid support for high temperature CO2 capture. Chemical Engineering Journal 2015, 273, 333-343. 47. Radfarnia, H. R.; Iliuta, M. C., Metal oxide-stabilized calcium oxide CO2 sorbent for multicycle operation. Chemical Engineering Journal 2013, 232, 280-289. 48. Hughes, R. W.; Lu, D.; Anthony, E. J.; Wu, Y. H., Improved long-term conversion of limestone-derived sorbents for in situ capture of CO2 in a fluidized bed combustor. Industrial & Engineering Chemistry Research 2004, 43, (18), 5529-5539. 49. Manovic, V.; Anthony, E. J.; Lu, D. Y., Sulphation and carbonation properties of hydrated sorbents from a fluidized bed CO2 looping cycle reactor. Fuel 2008, 87, (13), 2923-2931. 50. Arias, B.; Grasa, G. S.; Abanades, J. C., Effect of sorbent hydration on the average activity of CaO in a Ca-looping system. Chemical Engineering Journal 2010, 163, (3), 324-330. 51. Grasa, G. S.; Abanades, J. C., CO2 capture capacity of CaO in long series of carbonation/calcination cycles. Industrial & Engineering Chemistry Research 2006, 45, (26), 8846-8851.

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52. Chen, Z.; Song, H. S.; Portillo, M.; Lim, C. J.; Grace, J. R.; Anthony, E., Long-term calcination/carbonation cycling and thermal pretreatment for CO2 capture by limestone and dolomite. Energy & Fuels 2009, 23, (3), 1437-1444. 53. Radfarnia, H. R.; Iliuta, M. C., Development of zirconium-stabilized calcium oxide absorbent for cyclic high-temperature CO2 capture. Industrial & Engineering Chemistry Research 2012, 51, (31), 10390-10398. 54. Shimizu, T.; Hirama, T.; Hosoda, H.; Kitano, K.; Inagaki, M.; Tejima, K., A twin fluid-bed reactor for removal of CO2 from combustion processes. Chemical Engineering Research and Design1999, 77, (A1), 62-68. 55. Chen, C.; Zhao, C.; Liang, C.; Pang, K., Calcination and sintering characteristics of limestone under O2/CO2 combustion atmosphere. Fuel Processing Technology 2007, 88, (2), 171-178. 56. Beruto, D.; Barco, L.; Searcy, A. W., CO2 Catalyzed surface area and porosity changes in high surface area CaO aggregates. Journal of the American Ceramic Society 1984, 67, (7), 512-516. 57. Borgwardt, R. H., Calcium oxide sintering in atmospheres containing water and carbon dioxide. Industrial & Engineering Chemistry Research 1989, 28, (4), 493-500. 58. Hu, Y.; Liu, W.; Wang, W.; Sun, J.; Yang, X.; Chen, H.; Xu, M. Investigation of novel naturally occurring manganocalcite for CO2 capture under oxy-fuel calcination. Chem Eng J 2016, 296, 412-419. 59. Luo, C.; Zheng, Y.; Yin, J.; Qin, C.; Ding, N.; Zheng, C.; Feng, B. Effect of sulfation during oxy-fuel calcination stage in calcium looping on CO2 capture performance of CaO-based sorbents. Energ Fuel 2013, 27, (2), 1008-1014. 22 ACS Paragon Plus Environment

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List of Table Captions Table 1. Summary of the typical modification methods for the CO2 sorption performance enhancements of sorbent pellets and the testing conditions (only representative results). Table 2. Characteristics of the sorbent pellets with different MgO mass ratios after 25 cycles. Table 3. Weight loss of the sorbent pellets.

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Table 1

Ref.

Modified method

Pelletization method

Carbonation condition

Calcination condition

Cycle No.

Last cycle conversion

Sun et al. 12

Doping cellulose

Extrusionspheronization

650 °C, 15 vol.% CO2, 30 min

850 °C, 100 vol.% N2, 5 min

25

~52.66%

Sun et al. 42

Mechanical modification

Extrusionspheronization

650 °C, 15 vol.% CO2, 30 min

850 °C, 100 vol.% N2, 2 min

25

~37.34%

Doping microalgae

Extrusionspheronization

650 °C, 15 vol.% CO2, 30 min

850 °C, 100 vol.% N2, 2 min

25

~44.53%

Qin et al. 7

Adding cement

Extrusion

650 °C, 15 vol.% CO2, 30 min

900 °C, 100 vol.% N2, 2 min

18

~44.88%

Manovic et al. 3

Adding bentonite

Extrusion

850 °C, 100 vol.% CO2, 10 min

850 °C, 100 vol.% N2, 10 min

35

~40.61%

Adding cement and flour

Rotary drum granulation

650 °C, 15 vol.% CO2, 20 min

950 °C, 100 vol.% CO2, 10 min

20

~41.07%

Sun et al.

41

María et al.

11

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Table 2

Sorbent

BET surface area (m2/g)

Pore volume (cm3/g)

Pore size (nm)

CH

3.26

0.02

27.87

CH-WM-5%

5.09

0.03

37.18

CH-WM-15%

9.68

0.07

35.24

CH-WM-25%

12.10

0.09

34.54

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Table 3

Sample

Weight loss (%)

Weight loss (%)

After 1000 rotations

After 3000 rotations

CC

0.50

0.69

CH

0.53

0.72

CH-MM-5%

0.40

0.81

CH-WM-5%

0.41

0.72

CH-WM-15%

0.20

0.41

CH-WM-25%

0.52

0.72

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List of Figure Captions Figure 1. Synthesis routines of CaO-based sorbent pellets via extrusionspheronization. Figure 2. Carbonation conversion of the sorbent pellets produced via different routines. Figure 3. Carbonation conversion loss rate (black line) and last-cycle conversion (blue line) of sorbent pellets produced via different routines. Figure 4. FSEM images of (a) CC, (b) CH-SK, (c) CH-MM-5%, (d) CH-WM-5% and (e) themorphology and EDX mapping for Ca and Mg of the CH-WM-5% crosssection and (f) themorphology and EDX mapping for Ca and Mg of a small area in the cross-section of CH-WM-5%. Figure 5. Carbonation conversion of the sorbent pellets with different MgO ratios. Figure 6.CO2 capture capacity of the sorbent pellets with different MgO ratios. Figure 7. Carbonation conversion loss rate of sorbent pellets with different MgO ratios. Figure 8. Reaction rate (solid lines) and conversion (dotted lines) of sorbent pellets with different MgO mass ratios in the 25th cycle. Figure 9. FSEM images of the sorbent pellets with different MgO mass ratios after 25 cycles, (a) CH, (b) CH-WM-5%, (c) CH-WM-15%, (d) CH-WM-25%. Figure 10. Pore size distribution of the sorbent pellets with different MgO mass ratios after 25 cycles. Figure 11. Comparison of the last-cycle conversion of the sorbent pellets modified by 27

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different methods. Figure 12. Carbonation conversion of sorbent pellets under more realistic calcination condition. Figure 13. Carbonation and sulfation conversions of the sorbents under SO2 concentrations (calcination: 850 °C, N2, 10 min; carbonation: 650 °C, 15% CO2, 5% O2, N2 balance, 1000 ppm SO2, 30 min).

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Figure1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 7

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Figure 9

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Figure 13

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