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Enhanced Cyclic Stability and CO2 Capture Performance of MgO-Al2O3 Sorbent Decorated with Eutectic Mixture
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 114 (2017) 2421 – 2428
13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, 14-18 November 2016, Lausanne, Switzerland
Enhanced cyclic stability and CO2 capture performance of MgOAl2O3 sorbent decorated with eutectic mixture Vishwanath Hiremath, Soonha Hwang and Jeong Gil Seo* Department of Energy Science and Technology, Myongji University, Myongji-ro 116, Nam-dong, Cheoin-gu, Yongin-si, Gyeonggi-do 17058, South Korea
Abstract Recently, alkali nitrate activated MgO has been proposed as an alternative solid sorbent for high temperature CO 2 capture due to the facile solvation of MgO in the activation step. However, it tends to be unstable under multi-cyclic sorption-regeneration process due to continuous deformation. In this work, it is demonstrated that MgO-Al2O3 composite sorbent decorated with eutectic mixture (KNO3-LiNO3) can be a feasible candidate for high temperature CO2 capture without losing cyclic performance. It is noteworthy that the eutectic mixture could activate both MgO and MgO-Al2O3 composite, while NaNO3 can only activate MgO. Also, the inactive MgO-Al2O3 is activated at high temperature sorption with the help of the eutectic mixture. Further, it is demonstrated that the EM-MgO-Al2O3 could overcome aggregation and shows stable CO2 sorption performance for 10 cycles corresponding to ~8.7 wt.%. ©2017 2017The TheAuthors. Authors. Published Elsevier © Published by by Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the organizing committee of GHGT-13. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. Keywords: High temperature CO2 capture, MgO-Al2O3 sorbents, Eutectic mixture, Cyclic stability
1. Introduction Global warming associated with the increasing amount of greenhouse gases, mainly carbon dioxide (CO 2), is regarded as one of the key environmental issue of the 21st century. To halt the increase in the CO2 concentration in the atmosphere, various advanced technologies have been proposed including capture by solid sorbents. However, the lack of suitable sorbent which matches the energy criteria comprised of temperature window, heat of sorption
*
Corresponding author. Tel.: +82-31-324-1338, fax: +82-31-336-6336. E-mail address: [email protected]
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. doi:10.1016/j.egypro.2017.03.1389
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and regeneration, thermal stability of the sorbent, and the operational cost still need to be evaluated to establish an effective approach. It is well known that alkaline-earth metal oxides, especially MgO and CaO, provide the basic sites for acid gas sorption either by temperature- or pressure-swing operation. However, these solid sorbents suffer from poor resistance towards aggregation or irreversible CO 2 sorption due to continuous changes in their morphology and phase during CO2 capture. Moreover, the slow kinetics coupled with the surface saturation inhibit further CO2 uptake. To overcome these limitations, alkaline-earth metal oxides especially, MgO-based sorbents with promoters such as alkali/alkaline carbonates/nitrates have been proposed [1]. Massive efforts are made to understand the mechanism of CO2 sorption and it was derived that these promoters act as CO 2 carriers thereby providing an alternative pathway for bulk MgO to adsorb CO2. However, limited CO2 uptake and large collapse in the regeneration has been cited consistently. This is mainly due to the structural deformations during the high temperature regeneration of sorbent. To resolve the issue of structural deformations in the pure MgO, composites of MgO such as, MgO-Al2O3 [2], hydrotalcites [3] and MgO dispersed in various supporting materials [4] have been proposed. However, limited CO2 uptake due to the inactive support cannot be avoided. Also, the modified basic sites will restrict the high temperature application. Recently, eutectic mixture promoted MgO evolved as the potential candidate with higher sorption capacity and fast kinetics for CO2 sorption [5]. However, inhibition of morphological and crystallographic changes originating from sorption and regeneration have remained challenging. Hence, in this work we demonstrate the MgO-Al2O3 composite decorated with eutectic mixture (KNO3-LiNO3) with an aim to enhance sorption performance in terms of sorption capacity and cyclic durability. The synergistic effect between the stability of MgO-Al2O3 composite and the activation ability of eutectic mixture is combined in order to evaluate the CO2 sorption characteristics. 2. Experimental The MgO-Al2O3 composite sorbent (Mg/Al ratio=5) was synthesized by using magnesium acetate and aluminium tetra butoxide as precursors in ethanolic solution. HNO 3 was used as the catalyst for the hydrolysis step. The solid obtained was calcined at 550 oC for 5h. The sample obtained was designated as MgO-Al2O3. The binary eutectic mixture was prepared by thoroughly mixing and grinding KNO 3 (59 mol%) with LiNO3 (41 mol%) [6]. Furthermore, the MgO-Al2O3 sorbent decorated with KNO3-LiNO3 (EM) was prepared by a physical mixing technique. The fused nitrate mixture was comminuted and added to the as-synthesized MgO-Al2O3 in a quantified proportion. The solid mixture (30 wt.% EM and 70 wt.% MgO-Al2O3) was fused at 100 °C above the eutectic temperature (~ 120 oC) for 12 hours to homogenize the melt [7]. The obtained sample was designated as EM-MgOAl2O3. Similarly, NaNO3 decorated MgO-Al2O3 sample was prepared by wet impregnation method using water as solvent. After drying the wet cake, it was calcined at 450 oC in air. The NaNO3 loading was maintained at 20 wt.% in both the cases. For the purpose of comparison, EM and NaNO3 decorated MgO was synthesized physical mixing and wet impregnation and designated as EM-MgO and NaNO3-MgO respectively. 3. Characterization The material characterization was carried out by using XRD (D-Max2500-PC, Rigaku), and FE-SEM (Helios 650). The CO2 sorption-regeneration performance of the developed adsorbents were examined by using thermogravimetric analyzer (SCINCO, TGA N-1000). 4. Results and Discussion Fig. 1 represents the conceptual illustration of a solid sorbent composed of the active material (MgO), promoter (eutectic mixture), and support (Al2O3) with respect to their relative amount and dispersion. It shows a clear disparity between stability and performance. It is expected that the increased fraction of MgO could enhance the CO2 sorption performance while stability decreases due to the irreversible changes in phase and morphology of MgO. It is also probable that the composites (i.e. MgO-Al2O3) can effectively suppress a significant structural transformation during CO2 sorption-regeneration and thereby promoting the higher stability and better CO 2 sorption performance.
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Therefore, in this study the effect of promoting agent such as EM and NaNO3 on MgO-Al2O3 composite to achieve a synergy for stable temperature-swing CO2 sorption was evaluated. Also, it is believed that, such promoting agents can alter the path of CO2 sorption and lower the adsorption energy. This is probably due to the new basic sites originated from Mg-O-Al structure that has a strong interaction with CO2 at high temperature.
MgO Support
Cyclic stability
Promoting agent
Composite MgO-Al2O3
MgO Mg M gO
Physical Mixtures (Al2O3 rich) Physical Mixtures (MgO-rich)
CO2 sorption activity Fig. 1. Correlation between stability and activity of CO2 solid sorbent containing MgO, support, and promoting agent
Fig. 2 represents the XRD patterns for the MgO-Al2O3 and EM-MgO-Al2O3 composites. From Fig. 2(a) it is observed that the diffraction peaks corresponding to both MgO and γ-Al2O3 were present. Furthermore, the presence of LiNO3 and KNO3 was confirmed from the respective diffraction peaks observed in EM-MgO-Al2O3 (Fig. 2(b)). On the other hand, the intensity of the MgO-Al2O3 peaks decreased in presence of EM. This suggests that a better distribution of the all the active species throughout the sample was attained.
a)
b) ̷
MgO-Al2O3 KNO3 z LiNO3
̷ MgO ϴ Al2O3
Intensity (A.U.)
Intensity (A.U.)
̷
̷ ϴ
20
z
40
50
60
2 theta (degree)
̷
ϴ
70
̷
ϴ ϴ
30
z
zz
80
20
30
40
50
60
70
80
2 theta (degree)
Fig. 2. XRD patterns for a) MgO-Al2O3 and b) EM-MgO-Al2O3 adsorbents
The FE-SEM images were collected for EM-MgO-Al2O3 as shown in the Fig. 3. It shows a rough surface morphology. This may be a result of the surface modification of the MgO-Al2O3 during decoration with EM and subsequent activation at high temperature melt. The superior distribution of all the active species (Mg, O, Al and K) suggests that physical mixing could be a feasible method to produce highly dispersed active species in a composite. After confirming the structure, composition, and morphology of EM-MgO-Al2O3, the CO2 adsorption studies were carried out by using the thermogravimetric method. The CO 2 sorption performance was evaluated by both nonisothermal and isothermal tests. Initially, a number of samples were prepared in order to examine the application
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towards high temperature CO2 sorption namely, EM-MgO, NaNO3-MgO, MgO-Al2O3, EM-MgO-Al2O3, and NaNO3-MgO-Al2O3. The screening tests were performed in order to evaluate the effect of metal nitrates such as NaNO3 and EM in activating the MgO-Al2O3 composite towards CO2 capture. Numerous studies have shown that, alkali metal nitrates and eutectic mixture activate bulk MgO due to solvation effect, and thus, provides an alternative pathway for the formation of MgCO3. a)
EM-MgO-Al2O3 Mg (wt.% )
Al (wt. %)
K (wt.%)
O (wt.%)
32.06
6.91
16.10
44.93
500nm
b)
Mg
c)
Al
K
d)
e)
O
Fig. 3. FE-SEM images for EM-MgO-Al2O3 150
(a) (b)
Weight gain (%)
135
120
(c) 105
(d)
90
(e) (f)
75
60 100
200
300
400
500
600
Temperature (oC) Fig. 4. Non-isothermal CO2 sorption behavior for a) EM-MgO, b) NaNO3-MgO, c) EM-MgO-Al2O3, d) MgO-Al2O3, e) MgO, and f) NaNO3MgO-Al2O3
Conversely, the biggest drawback associated with the MgO based sorbents is that they undergo continuous structural changes between cubic (MgO) to trigonal (MgCO 3) forms, resulting in the collapse of the CO2 sorption performance. One way to restrict the rapid structural changes is to form composites of MgO wherein, MgO is highly dispersed in atomic order. However, a number of reports evaluating the effect of MgO-based composites for CO2 capture have shown limited CO2 sorption capacity [8-16]. This may be attributed to the formation of bulk MgO which is inactive at high temperature. Consequently, efforts to activate MgO using promoters such as metal nitrates or EMs have been considered. Recently, Kim et al. reported the effect of Mg/Al molar ratio in hydrotalcites activated
Vishwanath Hiremath et al. / Energy Procedia 114 (2017) 2421 – 2428
with NaNO3 for CO2 capture [17]. The highest sorption performance was obtained from hydrotalcite having Mg/Al=20 at 240 oC corresponding to 9.27 mmol/g. However, the CO2 uptake dropped to 25% of its initial capacity after 16 cycles. We attribute this loss to the resulting structural deformations with high Mg/Al molar ratio. Motivated by this work, EM was chosen as the promoter in this study while systematically comparing the effect of activation towards high temperature CO2 sorption. Fig. 4 represents the non-isothermal screening of all the developed sorbents. Interestingly, the eutectic mixture could activate both MgO and MgO-Al2O3 composite, while NaNO3 only activates MgO. As shown in table 1, the CO2 sorption capacity increases in the order of NaNO3-MgO-Al2O3 < MgO < MgOAl2O3 < EM-MgO-Al2O3 < NaNO3-MgO < EM-MgO. Although EM-MgO-Al2O3 shows moderate CO2 sorption capacity among the sorbents tested, it could be a practical candidate as a solid sorbent for fluidized-bed CO2 capture, which requires strong resistance toward aggregation (by molten salt) and abrasion (by collision). However, the fast kinetics and higher sorption performance of EM-MgO suggests that it could be a potential candidate for further studies with compositional modifications. Table 1. The detailed CO2 sorption properties. Sorbent
CO2 uptake (wt.%)
EM-MgO
45
NaNO3-MgO
37
EM-MgO-Al2O3
5.1
MgO-Al2O3
1.4
MgO
0.2
NaNO3-MgO-Al2O3
0.0 o
o
Note: The results are obtained by temperature swing from 25 to 600 C at the rate of 5 C/min by using TGA.
Fig. 5 represents the isothermal CO2 sorption tests for EM-MgO-Al2O3 at different temperatures for 60 min. Since the high temperature CO2 sorption involves equilibrium, the CO2 uptake will be different at different temperatures. To find the optimum point, CO2 sorption tests were carried out at temperatures ranging from 250-375 oC above the eutectic point of EM. Initially, CO2 uptake increased with increase in temperature until 300 oC. The highest CO2 sorption performance was observed at 300 oC corresponding to 6.6 wt.%. Further increase in temperature resulted to a decrease in sorption performance with an abrupt decreases to 2.2 wt.% at 375 oC. Also, it was observed that at the optimum temperature, the CO2 uptake has better kinetics, while the sorption kinetics slows down at other points. Interestingly, at temperatures above 375 oC, CO2 uptake possess a different behavior. For initial 15 min, there is a consistent CO2 uptake, but after 15 min, a small curve was obtained. This suggests that there is a resistance for further uptake coupled with slow kinetics. This behavior may also be a result of the equilibrium that exists between the adsorbed-desorbed CO2. Fig. 6 shows the cyclic CO2 adsorption/desorption profile for EM-MgO and EM-MgO-Al2O3 conducted by using TSA method. As expected the pristine EM-MgO shows excellent adsorption in the first cycle corresponding to 34 wt.% at 350 oC for 1h. This uptake is due to the activation of bulk MgO by means of dissolution in the molten EM. However, a significant decrease in the cyclic performance was observed up to 17 successive cycles. This phenomenon clearly supports the fact that bulk MgO undergoes pronounced structural transformations which result in the destabilization and subsequent loss in CO 2 sorption performance. Notably, the initial adsorption capacity corresponding to 34 wt.% dropped to 8.4 wt.% after 16 cycles. The observed desorption temperature was also found to be high, corresponding to 500 oC. It is believed that this high temperature may further have a negative effect on regenerating the MgCO3 which undergoes abrupt structural changes during CO2 sorption.
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300 oC, 6.6 wt.% 103
275 oC, 5.2 wt.%
Weight gain (%)
102
250
oC,
4.8 wt.%
101
350 oC, 3.4 wt.% 100
375 oC, 2.2 wt.% 99
98
97 0
10
20
30
40
50
60
Time (min) Fig. 5. Isothermal CO2 sorption behavior for EM-MgO-Al2O3 at different temperatures.
a)
160
Temperature (oC)
Weight gain (%)
500 400
140 34 wt. %
300
120
200 8.4 wt. %
100
100 0 0
5
10
15
20
25
30
35
Time (h)
b) 110
400
Temperature (oC)
Weight gain (%)
105 300
100 95
7.6
8.9
8.0
9.1
8.9
8.8
8.5
9.2
8.9
8.7
8.7
200
90
100 85 80 0
5
10
15
20
0 25
Time (h) Fig. 6. Cyclic CO2 sorption and regeneration tests for a) EM-MgO and b) EM-MgO-Al2O3
Also, in order to evaluate the effect of MgO-Al2O3 composite in reproducible CO2 sorption, the cyclic test was performed under 100% CO2 for 1h and the regeneration was done under 100% N2 at 400 oC for 1h. It can be seen from Fig. 6(b) that the EM-MgO-Al2O3 has a stable CO2 uptake for 11 successive cycles. In fact, an improvement in
Vishwanath Hiremath et al. / Energy Procedia 114 (2017) 2421 – 2428
the sorption performance was observed, which may be attributed to the activation of MgO. Also, the optimum temperature for EM-MgO-Al2O3 decreased to 300 oC with concomitant decrease in the desorption temperature (400 o C). This signifies that, the resultant composite MgO-Al2O3 can control the nature of binding between MgO and CO2. It is interesting to note that the key mechanism involves the formation of MgCO 3 in both cases, the presence of MgO-Al2O3 resulted to a clear difference in the sorption performance, temperature window (adsorption/desorption temperature) and durability. The systematic approach carried out in this work clearly demonstrates the relationship between the presence of MgO-Al2O3 as a modifier together with the EM as a promoter for stabilizing MgO and enhancing the CO2 cyclic performance. 5. Conclusions The decoration of the MgO-Al2O3 with a certain amount of eutectic mixture (KNO3-LiNO3) has increased the CO2 sorption performance. Although a significant loss in CO2 sorption capacity occurred, MgO-Al2O3 decorated with eutectic mixture gave promising results for further development as a stable solid sorbent for high temperature CO2 capture. The structural deformations and the decay of CO2 sorption performance could be constrained with such approaches where it forms the different composites of MgO. In this work, it is concluded that EM-MgO-Al2O3 composite has some advantages over the EM-MgO. Acknowledgement This work was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant number: NRF-2013R1A1A2060638). References [1] Zhang K, Li XS, Duan Y, King DL, Singh P, Li L. Roles of double salt formation and NaNO3 in Na2CO3-promoted MgO absorbent for intermediate temperature CO2 removal. Int. J. Greenh. Gas Control 2013;12:351–358. [2] Han SJ, Bang Y, Lee H, Lee K, Song IK, Seo JG. Synthesis of a dual-templated MgO-Al2O3 adsorbent using block copolymer and ionic liquid for CO2 capture. Chem. Eng. J 2015;270:411–417. [3] Gao Y, Zhang Z, Wu J,. Yi X, Zheng A, Umar A, O’Hare D, Wang Q. Comprehensive investigation of CO2 adsorption on Mg–Al–CO3 LDHderived mixed metal oxides. J. Mater. Chem. A 2013;1:12782–12790. [4] Liu WJ, Jiang H, Tian K, Ding YW, Yu HQ. Mesoporous carbon stabilized MgO nanoparticles synthesized by pyrolysis of MgCl2 preloaded waste biomass for highly efficient CO2 capture. Environ. Sci. Technol. 2013;47:9397-9403 [5] Harada T, Simeon F, Hamad EZ, Hatton TA. Alkali metal nitrate-promoted high-capacity MgO adsorbents for regenerable CO2 capture at moderate temperatures. Chem. Mater 2015;27:1943−1949 [6] Janz GJ, Allen CB, Downey JR, Tomkins RPT. Physical properties data compilations relevant to energy storage. I. molten salts: Eutectic data. 1978 [7] Yeboah YD, Xu Y, Sheth A, Godavarty A, Agrawal PK. Catalytic gasification of coal using eutectic salts: Identification of eutectics. Carbon 2003;41:203–214 [8] Han SJ, Bang Y, Kwon HJ, Lee HC, Hiremath V, Song IK, Seo JG. Elevated temperature CO2 capture on nano-structured MgO–Al2O3 aerogel: Effect of Mg/Al molar ratio. Chem. Eng. J 2014;242:357–363. [9] Bhagiyalakshmi M, Lee JY, Jang HT. Synthesis of mesoporous magnesium oxide: Its application to CO2 chemisorption. Int. J. Greenhouse Gas Control 2010;4:51−56. [10] Han KK, Zhou Y, Chun Y, Zhu JH. Efficient MgO-based mesoporous CO2 trapper and its performance at high temperature. J. Hazard. Mater 2012;341−347. [11] Bian SW, Baltrusaitis J, Galhotra P, Grassian VH. A template-free, thermal decomposition method to synthesize mesoporous MgO with a nanocrystalline framework and its application in carbon dioxide adsorption. J. Mater. Chem 2010;20:8705−8710. [12] Ram Reddy MK, Xu ZP, Lu GQ. (Max), Diniz da Costa JC. Layered double hydroxides for CO2 capture: Structure evolution and regeneration. Ind. Eng. Chem. Res 2006;45:7504−7509. [13] Zukal A, Pastva J, Č ejka J. MgO-modified mesoporous silicas impregnated by potassium carbonate for carbon dioxide adsorption. Microporous Mesoporous Mater 2013;167:44−50. [14] Fu X, Zhao N, Li JP, Xiao FK, Wei W. Sun YH. Carbon dioxide capture by MgO-modified MCM-41 materials. Adsorpt. Sci. Technol 2009; 27:593−601. [15] Bhagiyalakshmi M, Hemalatha P, Ganesh M, Mei PM, Jang HT. A direct synthesis of mesoporous carbon supported MgO sorbent for CO 2 capture. Fuel 2011; 90: 1662−1667.
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[16] Jiao X, Li L, Zhao N, Xiao F, Wei W. Synthesis and low-temperature CO2 capture properties of a novel Mg−Zr solid sorbent. Energy Fuels 2013; 27: 5407−5415. [17] Kim S, Jeon SG, Lee K.B. High-Temperature CO2 Sorption on Hydrotalcite Having a High Mg/Al Molar Ratio. ACS Appl. Mater. Interfaces 2016;8:5763–5767.
ScienceDirect Energy Procedia 114 (2017) 2421 – 2428
13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, 14-18 November 2016, Lausanne, Switzerland
Enhanced cyclic stability and CO2 capture performance of MgOAl2O3 sorbent decorated with eutectic mixture Vishwanath Hiremath, Soonha Hwang and Jeong Gil Seo* Department of Energy Science and Technology, Myongji University, Myongji-ro 116, Nam-dong, Cheoin-gu, Yongin-si, Gyeonggi-do 17058, South Korea
Abstract Recently, alkali nitrate activated MgO has been proposed as an alternative solid sorbent for high temperature CO 2 capture due to the facile solvation of MgO in the activation step. However, it tends to be unstable under multi-cyclic sorption-regeneration process due to continuous deformation. In this work, it is demonstrated that MgO-Al2O3 composite sorbent decorated with eutectic mixture (KNO3-LiNO3) can be a feasible candidate for high temperature CO2 capture without losing cyclic performance. It is noteworthy that the eutectic mixture could activate both MgO and MgO-Al2O3 composite, while NaNO3 can only activate MgO. Also, the inactive MgO-Al2O3 is activated at high temperature sorption with the help of the eutectic mixture. Further, it is demonstrated that the EM-MgO-Al2O3 could overcome aggregation and shows stable CO2 sorption performance for 10 cycles corresponding to ~8.7 wt.%. ©2017 2017The TheAuthors. Authors. Published Elsevier © Published by by Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the organizing committee of GHGT-13. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. Keywords: High temperature CO2 capture, MgO-Al2O3 sorbents, Eutectic mixture, Cyclic stability
1. Introduction Global warming associated with the increasing amount of greenhouse gases, mainly carbon dioxide (CO 2), is regarded as one of the key environmental issue of the 21st century. To halt the increase in the CO2 concentration in the atmosphere, various advanced technologies have been proposed including capture by solid sorbents. However, the lack of suitable sorbent which matches the energy criteria comprised of temperature window, heat of sorption
*
Corresponding author. Tel.: +82-31-324-1338, fax: +82-31-336-6336. E-mail address: [email protected]
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. doi:10.1016/j.egypro.2017.03.1389
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and regeneration, thermal stability of the sorbent, and the operational cost still need to be evaluated to establish an effective approach. It is well known that alkaline-earth metal oxides, especially MgO and CaO, provide the basic sites for acid gas sorption either by temperature- or pressure-swing operation. However, these solid sorbents suffer from poor resistance towards aggregation or irreversible CO 2 sorption due to continuous changes in their morphology and phase during CO2 capture. Moreover, the slow kinetics coupled with the surface saturation inhibit further CO2 uptake. To overcome these limitations, alkaline-earth metal oxides especially, MgO-based sorbents with promoters such as alkali/alkaline carbonates/nitrates have been proposed [1]. Massive efforts are made to understand the mechanism of CO2 sorption and it was derived that these promoters act as CO 2 carriers thereby providing an alternative pathway for bulk MgO to adsorb CO2. However, limited CO2 uptake and large collapse in the regeneration has been cited consistently. This is mainly due to the structural deformations during the high temperature regeneration of sorbent. To resolve the issue of structural deformations in the pure MgO, composites of MgO such as, MgO-Al2O3 [2], hydrotalcites [3] and MgO dispersed in various supporting materials [4] have been proposed. However, limited CO2 uptake due to the inactive support cannot be avoided. Also, the modified basic sites will restrict the high temperature application. Recently, eutectic mixture promoted MgO evolved as the potential candidate with higher sorption capacity and fast kinetics for CO2 sorption [5]. However, inhibition of morphological and crystallographic changes originating from sorption and regeneration have remained challenging. Hence, in this work we demonstrate the MgO-Al2O3 composite decorated with eutectic mixture (KNO3-LiNO3) with an aim to enhance sorption performance in terms of sorption capacity and cyclic durability. The synergistic effect between the stability of MgO-Al2O3 composite and the activation ability of eutectic mixture is combined in order to evaluate the CO2 sorption characteristics. 2. Experimental The MgO-Al2O3 composite sorbent (Mg/Al ratio=5) was synthesized by using magnesium acetate and aluminium tetra butoxide as precursors in ethanolic solution. HNO 3 was used as the catalyst for the hydrolysis step. The solid obtained was calcined at 550 oC for 5h. The sample obtained was designated as MgO-Al2O3. The binary eutectic mixture was prepared by thoroughly mixing and grinding KNO 3 (59 mol%) with LiNO3 (41 mol%) [6]. Furthermore, the MgO-Al2O3 sorbent decorated with KNO3-LiNO3 (EM) was prepared by a physical mixing technique. The fused nitrate mixture was comminuted and added to the as-synthesized MgO-Al2O3 in a quantified proportion. The solid mixture (30 wt.% EM and 70 wt.% MgO-Al2O3) was fused at 100 °C above the eutectic temperature (~ 120 oC) for 12 hours to homogenize the melt [7]. The obtained sample was designated as EM-MgOAl2O3. Similarly, NaNO3 decorated MgO-Al2O3 sample was prepared by wet impregnation method using water as solvent. After drying the wet cake, it was calcined at 450 oC in air. The NaNO3 loading was maintained at 20 wt.% in both the cases. For the purpose of comparison, EM and NaNO3 decorated MgO was synthesized physical mixing and wet impregnation and designated as EM-MgO and NaNO3-MgO respectively. 3. Characterization The material characterization was carried out by using XRD (D-Max2500-PC, Rigaku), and FE-SEM (Helios 650). The CO2 sorption-regeneration performance of the developed adsorbents were examined by using thermogravimetric analyzer (SCINCO, TGA N-1000). 4. Results and Discussion Fig. 1 represents the conceptual illustration of a solid sorbent composed of the active material (MgO), promoter (eutectic mixture), and support (Al2O3) with respect to their relative amount and dispersion. It shows a clear disparity between stability and performance. It is expected that the increased fraction of MgO could enhance the CO2 sorption performance while stability decreases due to the irreversible changes in phase and morphology of MgO. It is also probable that the composites (i.e. MgO-Al2O3) can effectively suppress a significant structural transformation during CO2 sorption-regeneration and thereby promoting the higher stability and better CO 2 sorption performance.
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Therefore, in this study the effect of promoting agent such as EM and NaNO3 on MgO-Al2O3 composite to achieve a synergy for stable temperature-swing CO2 sorption was evaluated. Also, it is believed that, such promoting agents can alter the path of CO2 sorption and lower the adsorption energy. This is probably due to the new basic sites originated from Mg-O-Al structure that has a strong interaction with CO2 at high temperature.
MgO Support
Cyclic stability
Promoting agent
Composite MgO-Al2O3
MgO Mg M gO
Physical Mixtures (Al2O3 rich) Physical Mixtures (MgO-rich)
CO2 sorption activity Fig. 1. Correlation between stability and activity of CO2 solid sorbent containing MgO, support, and promoting agent
Fig. 2 represents the XRD patterns for the MgO-Al2O3 and EM-MgO-Al2O3 composites. From Fig. 2(a) it is observed that the diffraction peaks corresponding to both MgO and γ-Al2O3 were present. Furthermore, the presence of LiNO3 and KNO3 was confirmed from the respective diffraction peaks observed in EM-MgO-Al2O3 (Fig. 2(b)). On the other hand, the intensity of the MgO-Al2O3 peaks decreased in presence of EM. This suggests that a better distribution of the all the active species throughout the sample was attained.
a)
b) ̷
MgO-Al2O3 KNO3 z LiNO3
̷ MgO ϴ Al2O3
Intensity (A.U.)
Intensity (A.U.)
̷
̷ ϴ
20
z
40
50
60
2 theta (degree)
̷
ϴ
70
̷
ϴ ϴ
30
z
zz
80
20
30
40
50
60
70
80
2 theta (degree)
Fig. 2. XRD patterns for a) MgO-Al2O3 and b) EM-MgO-Al2O3 adsorbents
The FE-SEM images were collected for EM-MgO-Al2O3 as shown in the Fig. 3. It shows a rough surface morphology. This may be a result of the surface modification of the MgO-Al2O3 during decoration with EM and subsequent activation at high temperature melt. The superior distribution of all the active species (Mg, O, Al and K) suggests that physical mixing could be a feasible method to produce highly dispersed active species in a composite. After confirming the structure, composition, and morphology of EM-MgO-Al2O3, the CO2 adsorption studies were carried out by using the thermogravimetric method. The CO 2 sorption performance was evaluated by both nonisothermal and isothermal tests. Initially, a number of samples were prepared in order to examine the application
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towards high temperature CO2 sorption namely, EM-MgO, NaNO3-MgO, MgO-Al2O3, EM-MgO-Al2O3, and NaNO3-MgO-Al2O3. The screening tests were performed in order to evaluate the effect of metal nitrates such as NaNO3 and EM in activating the MgO-Al2O3 composite towards CO2 capture. Numerous studies have shown that, alkali metal nitrates and eutectic mixture activate bulk MgO due to solvation effect, and thus, provides an alternative pathway for the formation of MgCO3. a)
EM-MgO-Al2O3 Mg (wt.% )
Al (wt. %)
K (wt.%)
O (wt.%)
32.06
6.91
16.10
44.93
500nm
b)
Mg
c)
Al
K
d)
e)
O
Fig. 3. FE-SEM images for EM-MgO-Al2O3 150
(a) (b)
Weight gain (%)
135
120
(c) 105
(d)
90
(e) (f)
75
60 100
200
300
400
500
600
Temperature (oC) Fig. 4. Non-isothermal CO2 sorption behavior for a) EM-MgO, b) NaNO3-MgO, c) EM-MgO-Al2O3, d) MgO-Al2O3, e) MgO, and f) NaNO3MgO-Al2O3
Conversely, the biggest drawback associated with the MgO based sorbents is that they undergo continuous structural changes between cubic (MgO) to trigonal (MgCO 3) forms, resulting in the collapse of the CO2 sorption performance. One way to restrict the rapid structural changes is to form composites of MgO wherein, MgO is highly dispersed in atomic order. However, a number of reports evaluating the effect of MgO-based composites for CO2 capture have shown limited CO2 sorption capacity [8-16]. This may be attributed to the formation of bulk MgO which is inactive at high temperature. Consequently, efforts to activate MgO using promoters such as metal nitrates or EMs have been considered. Recently, Kim et al. reported the effect of Mg/Al molar ratio in hydrotalcites activated
Vishwanath Hiremath et al. / Energy Procedia 114 (2017) 2421 – 2428
with NaNO3 for CO2 capture [17]. The highest sorption performance was obtained from hydrotalcite having Mg/Al=20 at 240 oC corresponding to 9.27 mmol/g. However, the CO2 uptake dropped to 25% of its initial capacity after 16 cycles. We attribute this loss to the resulting structural deformations with high Mg/Al molar ratio. Motivated by this work, EM was chosen as the promoter in this study while systematically comparing the effect of activation towards high temperature CO2 sorption. Fig. 4 represents the non-isothermal screening of all the developed sorbents. Interestingly, the eutectic mixture could activate both MgO and MgO-Al2O3 composite, while NaNO3 only activates MgO. As shown in table 1, the CO2 sorption capacity increases in the order of NaNO3-MgO-Al2O3 < MgO < MgOAl2O3 < EM-MgO-Al2O3 < NaNO3-MgO < EM-MgO. Although EM-MgO-Al2O3 shows moderate CO2 sorption capacity among the sorbents tested, it could be a practical candidate as a solid sorbent for fluidized-bed CO2 capture, which requires strong resistance toward aggregation (by molten salt) and abrasion (by collision). However, the fast kinetics and higher sorption performance of EM-MgO suggests that it could be a potential candidate for further studies with compositional modifications. Table 1. The detailed CO2 sorption properties. Sorbent
CO2 uptake (wt.%)
EM-MgO
45
NaNO3-MgO
37
EM-MgO-Al2O3
5.1
MgO-Al2O3
1.4
MgO
0.2
NaNO3-MgO-Al2O3
0.0 o
o
Note: The results are obtained by temperature swing from 25 to 600 C at the rate of 5 C/min by using TGA.
Fig. 5 represents the isothermal CO2 sorption tests for EM-MgO-Al2O3 at different temperatures for 60 min. Since the high temperature CO2 sorption involves equilibrium, the CO2 uptake will be different at different temperatures. To find the optimum point, CO2 sorption tests were carried out at temperatures ranging from 250-375 oC above the eutectic point of EM. Initially, CO2 uptake increased with increase in temperature until 300 oC. The highest CO2 sorption performance was observed at 300 oC corresponding to 6.6 wt.%. Further increase in temperature resulted to a decrease in sorption performance with an abrupt decreases to 2.2 wt.% at 375 oC. Also, it was observed that at the optimum temperature, the CO2 uptake has better kinetics, while the sorption kinetics slows down at other points. Interestingly, at temperatures above 375 oC, CO2 uptake possess a different behavior. For initial 15 min, there is a consistent CO2 uptake, but after 15 min, a small curve was obtained. This suggests that there is a resistance for further uptake coupled with slow kinetics. This behavior may also be a result of the equilibrium that exists between the adsorbed-desorbed CO2. Fig. 6 shows the cyclic CO2 adsorption/desorption profile for EM-MgO and EM-MgO-Al2O3 conducted by using TSA method. As expected the pristine EM-MgO shows excellent adsorption in the first cycle corresponding to 34 wt.% at 350 oC for 1h. This uptake is due to the activation of bulk MgO by means of dissolution in the molten EM. However, a significant decrease in the cyclic performance was observed up to 17 successive cycles. This phenomenon clearly supports the fact that bulk MgO undergoes pronounced structural transformations which result in the destabilization and subsequent loss in CO 2 sorption performance. Notably, the initial adsorption capacity corresponding to 34 wt.% dropped to 8.4 wt.% after 16 cycles. The observed desorption temperature was also found to be high, corresponding to 500 oC. It is believed that this high temperature may further have a negative effect on regenerating the MgCO3 which undergoes abrupt structural changes during CO2 sorption.
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300 oC, 6.6 wt.% 103
275 oC, 5.2 wt.%
Weight gain (%)
102
250
oC,
4.8 wt.%
101
350 oC, 3.4 wt.% 100
375 oC, 2.2 wt.% 99
98
97 0
10
20
30
40
50
60
Time (min) Fig. 5. Isothermal CO2 sorption behavior for EM-MgO-Al2O3 at different temperatures.
a)
160
Temperature (oC)
Weight gain (%)
500 400
140 34 wt. %
300
120
200 8.4 wt. %
100
100 0 0
5
10
15
20
25
30
35
Time (h)
b) 110
400
Temperature (oC)
Weight gain (%)
105 300
100 95
7.6
8.9
8.0
9.1
8.9
8.8
8.5
9.2
8.9
8.7
8.7
200
90
100 85 80 0
5
10
15
20
0 25
Time (h) Fig. 6. Cyclic CO2 sorption and regeneration tests for a) EM-MgO and b) EM-MgO-Al2O3
Also, in order to evaluate the effect of MgO-Al2O3 composite in reproducible CO2 sorption, the cyclic test was performed under 100% CO2 for 1h and the regeneration was done under 100% N2 at 400 oC for 1h. It can be seen from Fig. 6(b) that the EM-MgO-Al2O3 has a stable CO2 uptake for 11 successive cycles. In fact, an improvement in
Vishwanath Hiremath et al. / Energy Procedia 114 (2017) 2421 – 2428
the sorption performance was observed, which may be attributed to the activation of MgO. Also, the optimum temperature for EM-MgO-Al2O3 decreased to 300 oC with concomitant decrease in the desorption temperature (400 o C). This signifies that, the resultant composite MgO-Al2O3 can control the nature of binding between MgO and CO2. It is interesting to note that the key mechanism involves the formation of MgCO 3 in both cases, the presence of MgO-Al2O3 resulted to a clear difference in the sorption performance, temperature window (adsorption/desorption temperature) and durability. The systematic approach carried out in this work clearly demonstrates the relationship between the presence of MgO-Al2O3 as a modifier together with the EM as a promoter for stabilizing MgO and enhancing the CO2 cyclic performance. 5. Conclusions The decoration of the MgO-Al2O3 with a certain amount of eutectic mixture (KNO3-LiNO3) has increased the CO2 sorption performance. Although a significant loss in CO2 sorption capacity occurred, MgO-Al2O3 decorated with eutectic mixture gave promising results for further development as a stable solid sorbent for high temperature CO2 capture. The structural deformations and the decay of CO2 sorption performance could be constrained with such approaches where it forms the different composites of MgO. In this work, it is concluded that EM-MgO-Al2O3 composite has some advantages over the EM-MgO. Acknowledgement This work was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant number: NRF-2013R1A1A2060638). References [1] Zhang K, Li XS, Duan Y, King DL, Singh P, Li L. Roles of double salt formation and NaNO3 in Na2CO3-promoted MgO absorbent for intermediate temperature CO2 removal. Int. J. Greenh. Gas Control 2013;12:351–358. [2] Han SJ, Bang Y, Lee H, Lee K, Song IK, Seo JG. Synthesis of a dual-templated MgO-Al2O3 adsorbent using block copolymer and ionic liquid for CO2 capture. Chem. Eng. J 2015;270:411–417. [3] Gao Y, Zhang Z, Wu J,. Yi X, Zheng A, Umar A, O’Hare D, Wang Q. Comprehensive investigation of CO2 adsorption on Mg–Al–CO3 LDHderived mixed metal oxides. J. Mater. Chem. A 2013;1:12782–12790. [4] Liu WJ, Jiang H, Tian K, Ding YW, Yu HQ. Mesoporous carbon stabilized MgO nanoparticles synthesized by pyrolysis of MgCl2 preloaded waste biomass for highly efficient CO2 capture. Environ. Sci. Technol. 2013;47:9397-9403 [5] Harada T, Simeon F, Hamad EZ, Hatton TA. Alkali metal nitrate-promoted high-capacity MgO adsorbents for regenerable CO2 capture at moderate temperatures. Chem. Mater 2015;27:1943−1949 [6] Janz GJ, Allen CB, Downey JR, Tomkins RPT. Physical properties data compilations relevant to energy storage. I. molten salts: Eutectic data. 1978 [7] Yeboah YD, Xu Y, Sheth A, Godavarty A, Agrawal PK. Catalytic gasification of coal using eutectic salts: Identification of eutectics. Carbon 2003;41:203–214 [8] Han SJ, Bang Y, Kwon HJ, Lee HC, Hiremath V, Song IK, Seo JG. Elevated temperature CO2 capture on nano-structured MgO–Al2O3 aerogel: Effect of Mg/Al molar ratio. Chem. Eng. J 2014;242:357–363. [9] Bhagiyalakshmi M, Lee JY, Jang HT. Synthesis of mesoporous magnesium oxide: Its application to CO2 chemisorption. Int. J. Greenhouse Gas Control 2010;4:51−56. [10] Han KK, Zhou Y, Chun Y, Zhu JH. Efficient MgO-based mesoporous CO2 trapper and its performance at high temperature. J. Hazard. Mater 2012;341−347. [11] Bian SW, Baltrusaitis J, Galhotra P, Grassian VH. A template-free, thermal decomposition method to synthesize mesoporous MgO with a nanocrystalline framework and its application in carbon dioxide adsorption. J. Mater. Chem 2010;20:8705−8710. [12] Ram Reddy MK, Xu ZP, Lu GQ. (Max), Diniz da Costa JC. Layered double hydroxides for CO2 capture: Structure evolution and regeneration. Ind. Eng. Chem. Res 2006;45:7504−7509. [13] Zukal A, Pastva J, Č ejka J. MgO-modified mesoporous silicas impregnated by potassium carbonate for carbon dioxide adsorption. Microporous Mesoporous Mater 2013;167:44−50. [14] Fu X, Zhao N, Li JP, Xiao FK, Wei W. Sun YH. Carbon dioxide capture by MgO-modified MCM-41 materials. Adsorpt. Sci. Technol 2009; 27:593−601. [15] Bhagiyalakshmi M, Hemalatha P, Ganesh M, Mei PM, Jang HT. A direct synthesis of mesoporous carbon supported MgO sorbent for CO 2 capture. Fuel 2011; 90: 1662−1667.
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[16] Jiao X, Li L, Zhao N, Xiao F, Wei W. Synthesis and low-temperature CO2 capture properties of a novel Mg−Zr solid sorbent. Energy Fuels 2013; 27: 5407−5415. [17] Kim S, Jeon SG, Lee K.B. High-Temperature CO2 Sorption on Hydrotalcite Having a High Mg/Al Molar Ratio. ACS Appl. Mater. Interfaces 2016;8:5763–5767.