Salt-Composition-Controlled Precipitation of Triple-Salt-Promoted

Subscriber access provided by UNIVERSITY OF ADELAIDE LIBRARIES

Article

Salt Composition-Controlled Precipitation of Triple Salt-Promoted MgO with Enhanced CO Sorption Rate and Working Capacity 2

Seongmin Jin, Keon Ho, Anh-Tuan Vu, and Chang-Ha Lee Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01428 • Publication Date (Web): 08 Aug 2017 Downloaded from http://pubs.acs.org on August 8, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 38

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

Energy & Fuels

Schematic illustration of the liquid phase sintering during cyclic tests 121x92mm (150 x 150 DPI)

ACS Paragon Plus Environment

Energy & Fuels

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

Salt Composition-Controlled Precipitation of Triple Salt-Promoted MgO with Enhanced CO2 Sorption Rate and Working Capacity Seongmin Jina, Keon Hoa, Anh-Tuan Vuab, and Chang-Ha Lee*a a

Department of Chemical and Biomolecular Engineering, Yonsei University, Korea

b

School of Chemical Engineering, Hanoi University of Science and Technology, Vietnam

*Corresponding Author: Tel.: + 82-02-2123-2762; Fax: + 82-02-312-6401; E-mail address: [email protected];

Abstract Triple salt-promoted MgO composites (NaNO3, Na2CO3, and LiNO3) for pre-combustion CO2 capture were developed via a precipitation method with a controllable salt composition. MgO precursors were mixed and aged with salts to control the composition and the morphology. The MgO composites exhibited a CO2 sorption capacity of 73 wt% in pure CO2 at 240 min and 300 °C and achieved a sorption capacity of 25 wt% within 10 min due to a high sorption rate. When a cyclic test was conducted with pure CO2 sorption for 60 min at 325 °C and N2 regeneration for 15 min at 425 °C (60/15 min cycle) as a reference, the cyclic capacity was 45 1


Page 2 of 38

Page 3 of 38

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

Energy & Fuels

wt% after 30 cycles. On the other hand, considering the applicable capture processes, the sorption capacity during a fast cycle (10/5 min cycle) was 18 wt% under the same gas and temperature conditions. Finally, the working capacity of the MgO composite was evaluated under a simulated emission gas (29% CO2, 3% H2O, and balance N2) at 300 °C for sorption and CO2 at 450 °C for regeneration due to the importance of water vapor and CO2 regeneration in the evaluation. The rearrangement of salts and MgO grains during initial cycles led to enhanced working capacity. However, the working capacity declined along the subsequent cycles due to sintering and it was severe under CO2 regeneration. However, the working capacity at the wet mixture sorption and CO2 regeneration stabilized after 20 cycles at 23 wt% and 4.6 wt% for 60/15 min and 10/5 min cycles, respectively. The results indicated that the as-synthesized MgO composites are feasible for the practical application of the precombustion CO2 capture. Keywords: MgO composite, Precipitation, CO2 capture, Sorption rate, Intermediate temperature, Sintering.

2


Energy & Fuels

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

Page 4 of 38

1. Introduction

With increasing fossil fuel consumption, vast emissions of CO2 have consequently resulted in an increased atmospheric CO2 concentration. One feasible solution proposed for reducing CO2 emissions is its capture and sequestration. Among various capture technologies, a precombustion capture is known as one of the representative carbon mitigation ways because it has advantages of producing various products from electricity to hydrogen.1,

2

Pre-

combustion capture can be applied to power generation processes, such as the Integrated Gasification Combined Cycle (IGCC)1, 3, 4, which require intermediate to high temperature conditions for CO2 capture. If CO2 can be efficiently captured from the effluent gas of a water-gas-shift reactor (WGSR), it will guarantee better heat integration without any gas cooling step than post-combustion capture.5 For the CO2 capture process, a number of sorbents have been proposed.6-9 Each sorbent has advantages, but pragmatic problems of capacity and economical applicability in real operating conditions still need to be overcome. Recently, magnesium oxide (MgO) has received appreciable interest as a catalyst and an excellent sorbent for toxic chemicals and CO2.10-13 MgO composites have been reported to capture CO2 over a wide range of temperatures. In pre-combustion capture, intermediate temperature conditions between 250 and 450 °C can be utilized for the sorption and regeneration cyclic process using MgO-based sorbents. In an intermediate temperature range, various alkali salts have been applied for MgO sorbents to improve the CO2 capture capacity; similarly, other sorbents have been modified with alkali salts for operation at lower or higher temperatures.14-20 Although this process considerably enhances the sorption capacity, further improvement of sorption rate is needed to be comparable with the performance of CaO or alkali ceramic-based sorbents.14, 21-23 The 3


Page 5 of 38

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

Energy & Fuels

slow sorption of salt-promoted MgO has to be improved for practical capture processes because a large volume of a sorption bed to compensate the low kinetics make it difficult to control the temperature and high volumes of gases.5, 24 If a high sorption rate and working capacity for short sorption and regeneration time can be achieved, a large amount of energy and capital cost can be saved.25, 26 The issues of economic preparation and stability have remained as serious problems for practical use of CO2 sorbents. For example, mesoporous MgO sorbents with various salts and their composites were developed by the aerogel method and evaluated under dry and wet gas conditions.5,

27

Although the MgO-based sorbents had excellent performances, a high

synthetic cost was estimated for practical application because the method involves various steps including a supercritical drying step.28 Therefore, the development of a cost-effective synthetic method is essential for high-performance MgO-based sorbents. The conventional precipitation method for salt-promoted MgO is known as one of the most cost-effective synthetic methods. In this method, residual salts are used as a promoter after incomplete filtration and washing of a MgO precursor.17,

18, 29

However, in the synthetic

process for salt promoted-MgO, the filtration step makes it difficult to control and reproduce the salt composition owing to the unknown residual salt amount in the MgO precursor.17 Additionally, in the precipitation method, the morphological and textural characteristics can be controlled, and those are important factors which determine the performances of sorbents.30-33 MgO and its precursors, such as magnesium carbonate hydrates,32, 34, 35 have been studied to obtain improved properties and controlled morphologies by developing specific synthetic conditions, such as pH, reaction temperature, concentration, and surfactants. Therefore, an efficient synthetic pathway via the precipitation method needs to be developed to control the salt compositions and to achieve the sorbents with excellent textural properties 4


Energy & Fuels

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

and capture performances. When evaluating the stability and working capacity for a cyclic test, test conditions can result in different performances, i.e. a partial pressure of CO2 and presence of H2O for sorption and a type of gas for regeneration.36,

37

For example, saturated sorbents were

regenerated with captured CO2, which typically requires higher temperature than with N2.38, 39 As a result, a high regeneration temperature may cause the decay of working capacity along the cycles due to the sintering of metal oxide.21 In this regard, the stability evaluation and its improvement of as-prepared materials under wet CO2 mixture feeds and CO2 regeneration gas is essential in determining feasibility and adaptability.36, 40 In the study, triple salt-promoted MgO sorbents with NaNO3, Na2CO3, and LiNO3 were prepared by a precipitation method to enhance the sorption capacity, rate, and stability for CO2 capture at intermediate temperatures. The synthetic procedure in the precipitation method was developed to control not only the composition of the salts, but also the physical properties. During the synthetic process, the morphology of the sorbent was controlled by changing the aging time and drying method. MgO composites with the desired triple-salt compositions were successfully prepared because the salts were added to a slurry solution after completely washing a mixture of Mg(NO3)2 and K2CO3. Firstly, a CO2 sorption capacity was evaluated at CO2 sorption and thermal N2 regeneration. The working capacity in a cyclic test was further evaluated using a simulated pre-combustion emission gas (29% CO2, 3% H2O, and balance N2) for sorption and CO2 for regeneration and compared with the results at a dry feed condition. In addition, the performance obtained at the fast cycle (10 min for sorption and 5 min for regeneration: 10/5 min), which is applicable for a practical capture process, was compared with that at 60/15 min cycle.

5


Page 6 of 38

Page 7 of 38

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

Energy & Fuels

2. Experimental 2.1. Materials Magnesium nitrate hexahydrate (Aldrich, 99%), K2CO3 (Dunksan, Korea; 99.0%), LiNO3 (Sigma-Aldrich), Na2CO3 (Sigma-Aldrich, 99.5%) and NaNO3 (Daejung, Korea; 99.0%) were used without any further purification. N2 (Daedeok, Korea; 99.999%) was used as a purge gas during activation and regeneration in the cyclic test. CO2 (Deokyang, Korea; 99.99%) was applied for sorption and regeneration test. Air (O2 21% / balance N2) was used in the calcination procedure. De-ionized water was used to prepare sorbents.

2.2. Sorbent Preparation Triple-salt-promoted MgO composites were synthesized via a precipitation method. Details about the preparation steps are specified in Figure 1. In various studies, MgO was prepared by using K2CO3 and Mg(NO3)2 and morphology controls and reaction mechanisms were also reported.32,

34

In

the

study,

to

prepare

the

MgO

precursor

(hydromagnesite,

Mg5(CO3)4(OH)2·4H2O)34, aqueous K2CO3 solution (50 mL, 1 M) was added dropwise to aqueous Mg(NO3)2 solution (100 mL, 0.5 M). Then, the solution was stirred for 30 min and maintained for 1 h at room temperature without stirring. Generally, salts are simultaneously included in the solution. However, it is difficult to control the salt composition in MgO due to incomplete filtration and washing steps. To overcome the problems, the white precipitate was first collected from the solution, filtered with a vacuum pump, and then thoroughly washed with distilled water. The obtained slurry was mixed with 50 mL of distilled water. Then, the desired amount of salts was added to the solution. Since the purpose of this study was to develop triple salt-promoted MgO sorbents with 6


Energy & Fuels

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

enhanced CO2 sorption rates and working capacities by using the salt-composition controllable precipitation method, the molar ratio of the total triple-salt to Mg(NO3)2 was fixed at 0.1 or 0.2 and the molar fraction among salts was LiNO3:NaNO3:Na2CO3 = 0.2:0.76:0.04. The molar fraction between NaNO3 and Na2CO3 (0.04 and 0.76) and the molar ratio between total salt amount and MgO (0.1 or 0.2) were determined from the previous study, which prevented pore blockage of MgO from excessive salt amounts and showed high performance in double-salt MgO5. LiNO3 was adopted to decrease the melting temperature of the salt mixture because it has the lowest melting temperature among alkali nitrates and carbonates. When LiNO3 is mixed with NaNO3 at the ratio of 0.2 used in this study, the melting temperature of NaNO3 is reduced from approximately 310 to 275 °C.41 Alternatively, the melting temperature reduction for Na2CO3 was estimated from the phase diagram for the NaNO3 and Na2CO3 system. When the ratio of NaNO3 to (NaNO3 + Na2CO3) is 0.83, which is the ratio of LiNO3 to Na2CO3 used in the study, the melting temperature is about 550 °C, which is about 300 °C lower than the melting temperature of Na2CO3 (858 °C).42 Therefore, based on the phase diagrams of binary salt systems, the melting temperature of Na2CO3 is expected to be considerably decreased by adding LiNO342

7


Page 8 of 38

Page 9 of 38

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

Energy & Fuels

Figure 1. Synthesis procedure of triple salt-promoted MgO.

Synthesis conditions, such as mixing and aging time with salts and solution drying with or without stirring, were varied during the preparation procedure to control the morphology and textural property. The aqueous solution with the MgO precursor and salts was vigorously stirred and then maintained without stirring for a fixed time (mixing and aging time). The solution was evaporated and dried overnight in an oven at 110 °C without stirring. As another method (two-step drying) to minimize the aging effect during drying, the aqueous solution was dried with stirring at 110 °C until most of the water had evaporated, and then the solution slurry was dried in an oven at 110 °C. The samples prepared at each condition were denoted as MgO molar ratio of salts to Mg8


Energy & Fuels

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

mixing time(h)-aging time(h)-Y or N. Y indicated two-step drying with stirring and N indicated oven drying only without stirring. For example, MgO0.1-10-10-Y indicates a MgO sorbent with a 0.1 molar ratio of salts to Mg, 10 h mixing time, and 10 h aging time that was dried with stirring. As a final step, the samples were calcined by ramping the temperature to 450 °C at a heating rate of 1 °C/min and maintaining the sample for 9 h under an air flow.

2.3. Characterization X-ray diffraction (XRD) of the as-prepared samples was carried out using an X-ray diffractometer (Ultima IV) with Cu Kα radiation (λ = 1.5418 Ǻ) operated at 40 kV and 100 mA. The XRD patterns were recorded from 10° to 100° (2θ) with a scanning step of 0.02°. The sample morphology and size were observed via field emission scanning electron microscopy (FE-SEM, JEOL-7800F). The textural properties were evaluated via N2 adsorption/desorption isotherms using a gas sorption analyzer (Quantachrome Instruments, Autosorb IQ, version 3.0). To confirm that control of the salt ratios was achieved in the developed precipitation method, the metal compositions of the samples were quantified by ICP-MS (Perkin Elmer, NexlON300). To analyze the salt melting temperatures, differential scanning calorimetry (Perkin Elmer, DSC 8000) was performed under the following conditions: the samples were heated to 450 °C under argon flow at a heating rate of 20 °C/min, maintained for 1 h, cooled to 50 °C, and heated again to 450 °C.

2.4. CO2 Sorption Measurements To evaluate the CO2 sorption features, CO2 uptake experiments were conducted over a range of intermediate temperatures using a thermogravimetric analyzer (Perkin Elmer, TGA 4000). To accurately measure the temperature of samples, a thermocouple is located as close 9


Page 10 of 38

Page 11 of 38

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

Energy & Fuels

as possible to the sample pan (within 5 mm). The difference in the temperature measurements among samples was negligible because of their similar compositions and heat capacities, as shown in Table S1 and discussed later. In each experiments, 8–10 mg of samples was loaded. Before all experiments, the assynthesized samples were activated at 500 °C for 1 h with a heating rate of 10 °C/min in a 60 mL/min flow of N2. For dynamic thermogravimetric analyses, temperature increased from ambient temperature to 550 °C with a heating rate of 10 °C/min in a 40 mL/min flow of CO2. According to these results, the temperature range for the CO2 uptake experiments was determined, and the uptake experiments were conducted under fixed temperature conditions for 4 h in a 40 mL/min flow of CO2. Cyclic tests were carried out to evaluate the working capacity and stability of the samples under dry and wet gas conditions. In pre-combustion CO2 capture, the effluent gas generally contains 20–30 vol% CO2 with other gases such as H2, CO, CH4, N2, and H2O. In this study, the gas mixture for sorption was simplified by using CO2, N2, and H2O as follows: 29 vol% CO2 and balance N2 or 29 vol% CO2, 3 vol% H2O, and balance N2. For regeneration of the sorbent, pure N2 or CO2 was used. The gas pressure in all CO2 sorption measurements was fixed at 1 bar.

3. Results and discussion 3.1. Morphology and characterization The SEM images of as-prepared MgO and Mg5(CO3)4(OH)2·4H2O with triple salts are presented in Figure 2. The sorbents prepared in solution with only 1 h mixing, MgO0.2-1-0-N and MgO0.1-1-0-N in Figure 2(a, b), show a spherical flower shape.31 According to the SEM 10


Energy & Fuels

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

images, the concentration of the salts does not significantly affect to the morphology of MgO. On the other hand, when 10 h mixing and 10 h aging of the solution were applied with a twostep drying process (MgO0.1-10-10-Y), the sorbent became rod-shaped, as shown in Figure 2(c), with a “house of cards” structure.35 The morphology of each sample was not greatly changed after calcination, as shown in Figures 2(d-f). In addition, as shown in the elemental map images of MgO0.2-1-0-N (no aging of the solution and no stirring in the drying step) in Figures 2(g–i), sodium is dispersed throughout the MgO particles, but partially aggregated salts seem to have a different melting temperature because of a different distribution of lithium and sodium salts.

Figure 2. SEM images of ((a) and (d)) MgO0.2-1-0-N, ((b) and (e)) MgO0.1-1-0-N and ((c) and (f)) MgO0.1-10-10-Y; (a), (b), and (c) are the images before calcination and (d), (e), and

11


Page 12 of 38

Page 13 of 38

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

Energy & Fuels

(f) are the ones after calcination, respectively; (g) Mg, (h) O, and (i) Na elemental map images of (d) calcined MgO0.2-1-0-N.

The textural properties of triple salt-promoted MgO before and after calcination were measured using N2 adsorption/desorption isotherms (Table 1). The surface area and pore volume of the samples were significantly reduced from Mg5(CO3)4(OH)2·4H2O with triple salts after calcination because the calcination would be attributed to the collapse of a structure under high temperature and coating of salts. The same phenomenon was also observed in the previous studies10, 27 Compared with MgO0.1-1-0-N, the sample prepared with an extended aging time and stirring during drying (Mg0.1-10-10-Y) had a higher mesopore volume (0.342 cc/g), but a similar surface area and micropore volume. When the salt amount was increased from MgO0.1-1-0-N to MgO0.2-1-0-N, a reduction of the surface area and meso- and micropore volumes was observed, as shown in Table 1. Based on the SEM images and textural properties, the morphology was influenced by the mixing and aging conditions during the salt-coating step, whereas the textural properties were mainly affected by the salt amount.

Table 1. Textural properties of triple salt-promoted MgOs Samples

BET surface area (m2/g)

BJH mesopore volume (cc/g)

MgO0.2-1-0-N 28 0.092 MgO0.1-1-0-N 48 0.295 MgO0.1-10-10-Y 49 0.342 a MgO0.2-1-0-N-HY 95 0.371 MgO0.1-1-0-N-HY 116 0.476 MgO0.1-10-10-Y-HY 139 0.535 a HY implied Mg5(CO3)4(OH)2·4H2O with triple salts before calcination 12


SF micropore volume (cc/g) 0.008 0.014 0.013 0.030 0.033 0.039

Energy & Fuels

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

Wide-angle XRD patterns were used to investigate the crystal phases of the as-synthesized samples. Figures 3(a–c) show that all the samples have the same MgO diffraction planes, and peaks corresponding to each salts were also confirmed. Because the compositions of LiNO3 were very small in all the samples, the peaks of LiNO3 were not clearly observed. It is notable that sharper peaks were observed in the sample prepared with an extended aging time and two-step drying (MgO0.1-10-10-Y) than in the other samples (MgO0.1-1-0-N and MgO0.2-1-0-N). Based on the differences in peak intensity, it appears that the salts may be dispersed in the MgO grains with different crystal sizes according to the synthetic conditions.27, 43 On the other hand, the XRD patterns of the samples became more similar after two cyclic tests, as shown in Figures 3(d–f). The results will be discussed in detail with the CO2 sorption results.

13


Page 14 of 38

Page 15 of 38

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

Energy & Fuels

Figure 3. XRD patterns of (a) MgO0.1-10-10-Y, (b) MgO0.1-1-0-N, (c) MgO0.2-1-0-N, and ((d), (e), and (f)) corresponding samples after two cyclic tests.

To confirm the salt composition, the metal ions were quantified using ICP-MS (Table 2). All of the samples retained the desired amounts of metal ions and only small amounts of potassium were detected. The Li+ ion which was difficult to be observed in XRD patterns were detected with the desired amount by ICP-MS analysis. The results indicated that the salt amount was successfully controlled by the developed preparation step in the precipitation method and that reproducible MgO composites could be achieved.

14


Energy & Fuels

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

Page 16 of 38

Table 2. Calculated atomic ratios of metal ions to Mg in as-synthesized samples.( Numbers in brackets indicate a desired ratio.) Samples MgO0.2-1-0-N MgO0.1-1-0-N MgO0.1-10-10-Y

Li/Mg 0.038(0.04) 0.018(0.02) 0.022(0.02)

Na/Mg 0.164(0.168) 0.076(0.084) 0.087(0.084)

K/Mg 0.007(0) 0.003(0) 0.003(0)

3.2. CO2 Sorption and desorption behavior Dynamic thermogravimetric analyses were conducted for the as-prepared samples. Figure 4 shows that the sorption and desorption characteristics with increasing temperature are different among the sorbents. As shown in Figure 4, the peaks of the derivative weights for MgO0.2-1-0-N and MgO0.1-1-0-N are located at approximately 255 and 285 °C, respectively. As presented in Table S1, during the temperature difference of 30 °C, the different heat capacities of the samples can result in a temperature difference of about 0.3 °C between the two samples. Therefore, in this study, the temperature difference among samples in the thermogravimetric analysis experiments was neglected. In Figure 4, the sorption amount of all sorbents increased slightly and linearly with temperature until around 250 °C. The MgO composite with less salt (MgO0.1-1-0-N) showed accelerated sorption of CO2 at a relatively low temperature than MgO0.2-1-0-N. A “premelting” or “surface melting” might be responsible for the difference in the onset temperature observed for MgO0.1-1-0-N and MgO0.1-10-10-Y, despite the same salt composition.20 The different synthetic conditions may give rise to distinct salt dispersion over MgO and coating of the salts over MgO sorbents, resulting in the dissimilar premelting. Furthermore, the sorption behavior could be affected by the salt amount when comparing the MgO0.1-1-0-N and MgO0.2-1-0-N. 15


Page 17 of 38

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

Energy & Fuels

Figure 4. Dynamic thermogravimetric analyses and derivative weight curves of MgO0. 2-1-0-N, MgO0.1-1-0-N, and MgO0.1-10-10-Y in pure CO2 flow.

The features of the dynamic thermogravimetric results agreed well with the experimental results of CO2 uptake sorption capacity, as shown in Table 3 and Figure 5. The sorption rates and capacities of the triple salt-promoted MgO samples were much higher than those of the double sodium salt-promoted MgO sample prepared by an aerogel method with a NaNO3/Na2CO3 molar ratio of 0.05 at 300 °C in pure CO2 flow.5 The results indicated that the developed precipitation method is superior to the aerogel method, as well as being simple and cost-effective. The sorption capacity of MgO0.1-10-10-Y at 300 °C reached 73 wt% at 240 min, which is high among the reported values for MgO-related sorbents. Furthermore, the 16


Energy & Fuels

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

rod-shaped MgO composite showed a higher sorption capacity than the flower-shaped MgO composite with sorption capacities of 66 wt% and 69 wt%, respectively. (Table 3).

Table 3. Sorption capacity of MgO0.2-1-0-N, MgO0.1-1-0-N, and MgO0.1-10-10-Y for 10, 60, and 240 min in pure CO2 flow Time

10 min.

60 min.

240 min.

Samples MgO0.2-1-0-N MgO0.1-1-0-N MgO0.1-10-10-Y MgO0.2-1-0-N MgO0.1-1-0-N MgO0.1-10-10-Y MgO0.2-1-0-N MgO0.1-1-0-N MgO0.1-10-10-Y

Temperature (°C) / Sorption capacity (wt%) 250 275 300 325 350 15 21 20 4 1 15 24 16 5 3 7 22 25 5 1 26 47 49 30 19 31 45 49 39 27 27 43 62 54 38 38 60 66 55 46 36 59 69 66 64 40 55 73 73 70

The sorption capacity of all the sorbents increased significantly at 275 °C and improved further at 300 °C. However, further temperature increases led to decrease of the sorption capacity and the initial sorption rate became lower, as shown in Figure 5. As sorption processes of 240 min or longer are rather impractical for real CO2 capture, the evaluation of sorption capacity and rate at a sorption time of 10 min or 30 min is more reasonable for CO2 capture reactors.24, 44 The sorption capacity behavior of the rod-shaped MgO composite at 10 min was different from that of the flower-shaped MgO composite as the temperature changed from 250 to 300 °C, but the sorption behavior of all the sorbents became similar at higher temperatures.

17


Page 18 of 38

Page 19 of 38

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

Energy & Fuels

18


Energy & Fuels

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

Figure 5. CO2 uptakes of (a) MgO0.2-1-0-N, (b) MgO0.1-1-0-N, and (c) MgO0.1-10-10-Y at different temperature conditions in pure CO2 flow (MgONaNa indicates the uptake of aerogel MgO with a NaNO3 to Na2CO3 molar ratio of 0.05 at 300 °C in pure CO2 flow 5)

As reported in previous studies, the melting characteristics of salts at the operating temperature determine the sorption behavior owing to the role of liquid channels.17, 27 As all the salt phases became molten at around 300 °C, as revealed by the DSC results in Figure S1 and Table S2 (Supporting information), most of the MgO grains would be surrounded by the liquid phase of the salts, resulting in considerable enhancement of the sorption capacity and rate. However, the initial sorption rate at 325 and 350 °C was significantly decreased (Figure 5 and Table 3). These results indicated that a high temperature is not desirable for CO2 sorption over short times. For practical purposes, the regeneration characteristics of the sorbents should be examined.

19


Page 20 of 38

Page 21 of 38

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

Energy & Fuels

The sorbents saturated by CO2 flow at 325 °C for 240 min were regenerated by increasing the temperature from 60 °C to 550 °C under a flow of N2 or CO2. As MgO0.2-1-0-N (flower shape) and MgO0.1-10-10-Y (rod shape) showed higher sorption capacities at 300 °C within 10 and 60 min, their results are presented in Figure 6. Under N2 flow, desorption was accelerated from 350 °C and 360 °C for MgO0.2-1-0-N and MgO0.1-10-10-Y, respectively. Practically, desorption has to be operated under CO2 flow to obtain high purity CO2. Under CO2 flow, the regeneration temperature increased and the difference in the temperature for complete regeneration under N2 and CO2 flows was approximately 100 °C. From the result in Figure 6, the regeneration temperature under CO2 flow was determined as 450 °C, which is as low as possible for complete regeneration; a higher regeneration temperature could result in more severe sintering and decay of the working capacity, which will be discussed later. This result implied that the thermal conditions for regeneration should be evaluated with respect to the type of regeneration gas because this can significantly affect the efficiency of CO2 capture and the energy requirements.

20


Energy & Fuels

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

Figure 6. Desorption curve of MgO0.2-1-0-N and MgO0.1-10-10-Y under N2 flow or CO2 flow after the saturation under CO2 at 325 °C

Interestingly, the temperatures at which the weight started to decrease under CO2 flow from the dynamic analysis (Figure 4) and the desorption curve (Figure 6) were different. This difference is likely due to differences in the CO2 sorption amounts because the desorption curve test was carried out under saturated conditions. This result implied that the saturated sorbent needs a higher regeneration temperature and that its desorption rate is lower than that of the unsaturated sample. As the sorption amount increases, some bulk MgCO3 can be directly exposed to the CO2 atmosphere because the wetting degree by the salts on MgCO3 depends on the sorption amount. According to the Born–Haber cycle, MgCO3 not covered by liquid-phase salts requires a higher activation energy for decomposition to MgO than liquidsalt-covered MgCO3.19 Therefore, a higher temperature seems to be necessary to regenerate 21


Page 22 of 38

Page 23 of 38

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

Energy & Fuels

the sorbent with a larger amount of MgCO3.

3.3. Cyclic stability evaluation Cyclic stability evaluation of MgO sorbents in various studies have been performed for relatively long sorption time of 30 min or 60 min.5, 17 However, the time conditions for cyclic test can vary depending on the application. A fluidized bed reactor for carbon capture was proposed to overcome some limitations of the fixed bed reactor, such as temperature control and reactor volume, and required a short sorption and regeneration time from a few seconds to 10 min.24, 44 From this viewpoint, the stability evaluation in this study was carried out for sorption/regeneration times of 60/15 min and 10/5 min. It was reported that the decay of stability resulting from physical aggregation of metal oxide crystals and sintering strongly depends on the regeneration temperature and time.21,

45

Consequently, in this study, the

stability was evaluated under the conditions where the above results were obtained and at practically available temperatures in a pre-combustion process. Figure 7(a) represents the results of 30 cyclic tests at the short sorption/regeneration times (10/5 min cycle). As a preliminary test, the cyclic tests were carried out under pure CO2 sorption and N2 regeneration conditions. In the short cycle (Figure 7(a)), the sorption capacities of MgO0.2-1-0-N and MgO0.1-10-10-Y were stabilized at approximately 18 and 10 wt%, respectively, at the sorption temperature of 325 °C. However, MgO0.2-1-0-N was not stabilized at the sorption temperatures of 300 °C even after 30 cycles, whereas MgO0.110-10-Y was relatively stable with a slightly higher capacity than at 325 °C. The improvement of the sorption capacity at 325 °C during the initial cycles was much higher than at 300 °C for both MgO0.2-1-0-N and MgO0.1-10-10-Y. This result could be attributed to the dispersion and rearrangement of the molten salt phase over MgO during the initial 22


Energy & Fuels

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

cyclic period. Working capacities improved along the initial cycles, and it made the developed sorbent more efficient under the short sorption time condition. The improvement in the sorption rate and working capacity indicates the applicability of the developed sorbent to the fluidized bed reactor which can capture CO2 from massive emission gases.

23


Page 24 of 38

Page 25 of 38

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

Energy & Fuels

Figure 7. Cyclic stability tests for (a) sorption for 10 min at 300 or 325 °C and regeneration for 5 min at 425 °C, and (b) sorption for 60 min at 325 °C and regeneration for 15 min at 425 °C in pure CO2 sorption flow and pure N2 regeneration flow.

Sintering of MgO0.2-1-0-N was clearly confirmed in SEM images (See Figure 8). The MgO0.2-1-0-N was originally consisted of small and separated nano-grains, but those were aggregated to form compact grains because of sintering after the cyclic test. It was reported that significant sintering resulted in decreased surface area and working capacity.5 The working capacity of the flower-shaped MgO composite (MgO0.2-1-0-N) at both sorption temperatures was higher than that of the rod-shaped MgO composite (MgO0.1-10-10-Y). However, the rod-shaped MgO composite appeared to have better stability.

Figure 8. SEM images of MgO0.2-1-0-N (a) before and (b, c) after 30th cyclic test of sorption (10min) and regeneration (5min) at 325 and 425 oC, respectively.

In the cyclic test, two noticeable features were found: the sorption capacity improved during the initial cyclic period and the two sorbents had different stabilities. The former phenomenon can be explained by the mechanism of liquid-phase sintering as shown in Figure 9. The sintering of a liquid phase in inorganic materials consists of three steps: rearrangement, 24


Energy & Fuels

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

solution-reprecipitation, and densification.46, 47 The results of the cyclic test demonstrated that the rearrangement started during the initial several cycles by diffusion of Mg2+ and the liquidphase salt, followed by solution-reprecipitation and densification during the subsequent cycles (Figure 9). This mechanism was confirmed by the XRD results shown in Figure 3. The intensity of the XRD peaks of NaNO3 decreased after two consecutive cycles compared with those of the as-prepared sorbents. Furthermore, the crystallite size of NaNO3 decreased considerably, whereas sintering resulted in an increase in MgO grain sizes (Table 4). Melting and rearrangement of salts occurs during the initial calcination, sorption, and regeneration steps because the temperatures used are all above the melting temperatures of LiNO3 and NaNO3. In particular, the formation and decomposition of MgCO3 during the sorption and regeneration steps may accelerate rearrangement because of the difference in the densities of MgO (3.58 g/cm3) and MgCO3 (2.96 g/cm3) and their wetting properties in molten salts.48 These results indicated that the rearrangement process contributes to improving salt dispersion over the MgO grains and reducing the crystallite size of the salts.27, 43

25


Page 26 of 38

Page 27 of 38

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

Energy & Fuels

Figure 9. Schematic illustration of the liquid phase sintering during cyclic tests.

Table 4. Crystallite size of triple salt-promoted MgOs before CO2 sorption and after two cyclesb. Crystallite size (nm) a Before CO2 sorption After two cycles MgO NaNO3 MgO NaNO3 21.2 43.1 26.1 27.6 MgO0.2-1-0-N 26.1 57.6 29.2 52.0 MgO0.1-1-0-N 18.7 50.3 27.1 38.8 MgO0.1-10-10-Y a Estimated by Scherrer equation from XRD pattern. b The cycle was operated at CO2 sorption of 10 min at 325 °C and N2 desorption of 5 min at 425 °C Samples

The second phenomenon of stability variation could be explained by complete wetting and different sorption amounts. The clear stability decay of MgO0.2-1-0-N may be attributed to the rapid sintering and rearrangement owing to complete wetting of MgO by the liquid salt phase at the higher salt ratio. To accomplish the rapid liquid-phase sintering, a complete wetting by the liquid phase is required.46 On the other hand, a lower sorption amount and low salt ratio led to weaker sintering in MgO0.1-10-10-Y. MgCO3 can preferentially induce diffusion of Mg2+ because Mg2+ is more easily dissolved from MgCO3 than from MgO due to a lower energy difference between ionic and covalent bonds.19 Accordingly, the lower working capacity of MgO-0.1-10-10-Y leads to the formation of less MgCO3, and the diffusion of Mg2+ is less. Therefore, stability decay was not observed for MgO0.1-10-10-Y in 10/5 min cycle, unlike MgO-0.2-1-0-N. In the 60/15 min cycle (Figure 7(b)), an increasing tendency during the initial several cycles was also observed, similar to that in the 10/5 min cycle, although the sorption capacities were much higher than those in the 10/5 min cycle. Furthermore, the working capacities in the 26


Energy & Fuels

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

cyclic tests improved compared to the previous results. Following the enhancement of the sorption capacity, the sorbents seemed to be stabilized. However, the sorption capacities gradually decreased again from the 15th cycle, finally approaching 45 wt% after 30 cycles. Nevertheless, considering that the working capacity of MgO promoted by double sodium salts was 32 wt% under the same cycling conditions,5 the working capacity of as-prepared MgO was significantly improved. As a sorption time of 60 min was sufficient for rearrangement of the liquid salts over the MgO grains, including the small pores, it seemed that the entire MgO surface can be covered by the liquid phase during the initial several cycles. Then, solution-reprecipitation gradually progressed during the subsequent cycles, and sintering had a dominant effect on the sorption capacity from the 15th cycle onwards. As a result, from the viewpoint of stability, the sorption time should be controlled to protect the sorbent from sintering. The stability and working capacity tests for CO2 capture materials using CO2 gas mixtures are practically meaningful for evaluating the feasibility of these materials because the real discharge gas contains CO2 with other gases and water vapor. In addition, the regeneration should be carried out with captured CO2 rather than N2 or mixture gases to prevent further purification of the captured CO2.

In this study, the gas conditions for the cyclic test were amended as follows: simulated dry syngas (30 vol% CO2 and balance N2) or wet syngas (29 vol% CO2, 3 vol% H2O, and balance N2) for sorption and pure CO2 for regeneration. The sorption gas was supplied for the cooling period after regeneration and the regeneration gas was used for the heating period after sorption (see Figure S2 in a Supporting Information). In addition, a higher temperature for complete regeneration was applied for CO2 regeneration (450 °C) than for N2 regeneration 27


Page 28 of 38

Page 29 of 38

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

Energy & Fuels

(425 °C) because desorption occurred at a higher temperature in the desorption curve (Figure 6). It was reported that a negligible amount of water vapor at intermediate temperatures is sorbed on double salt-promoted MgO using NaNO3 and Na2CO3 in a flow of 2.5% vol. H2O and balance N2.5 Therefore, it is expected that the weight change could be attributed to CO2 sorption. MgO0.1-10-10-Y, which showed better stability relative to the other sorbents (Figure 5), was used for the cyclic test. Because the pretreatment was carried out by using CO2 flow at 500 °C, the highest sorption capacity in Figure 10 was observed at the first cycle and the increase of the sorption capacity during the initial cyclic period was eliminated. As shown in Figure S2, the weight decreased after the pretreatment due to the desorption of preadsorbed components. The weight right after the pretreatment was used to calculate the working capacity. In addition, the working capacity did not include the weight increase after switching the gas to CO2 for regeneration. The results with the dry gas mixture showed the obvious effect of CO2 partial pressure and CO2 regeneration on the working capacity. A significant decrease of working capacity was observed, and the capacity stabilized at 1.8 wt% and 11.2 wt% for the 10/5 min and 60/15 min cycles, respectively. However, the sorption capacity during the first cycle using the wet gas mixture was comparable to that using pure CO2, as seen in Figure 7, and the working capacity was higher than that using the dry gas mixture. This result implied that the water vapor changes the sorption mechanism of MgO and contributes to improving the sorption capacity and rate.5, 36

28


Energy & Fuels

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

Figure 10. Cyclic tests of MgO0.1-10-10-Y for 10 min and 60 min of sorption at 300 °C in the dry (30 vol.% CO2 and balance N2) or wet (29 vol.% CO2, 3 vol.% H2O, and balance N2) syngas flow, and 5 and 15 min of corresponding regeneration at 450 oC in pure CO2 flow.

Compared with the cyclic test using pure CO2 for sorption and N2 for regeneration (Figure 7), the performance deteriorated. The main reason for this was the use of CO2 as a regeneration gas at a high temperature because sintering and agglomeration of the sorbents can be accelerated at a higher temperature46. It is difficult to use large aggregated sorbents in fluidized bed reactors because the capture performance deteriorates with cycling.49 Therefore, a greater amount of sorbent should be supplied to the reactor to replace the aggregated sorbents with fresh sorbents during capture process operations. In this regard, the sintering of MgO grains and the agglomeration and attrition of MgO particles should be overcome for industrial applications.50 29


Page 30 of 38

Page 31 of 38

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

Energy & Fuels

In the wet gas mixture, the working capacity of triple-salt MgO prepared by the developed precipitation method was eventually stabilized at 4 wt% and 23 wt% for the 10/5 min and 60/15 min cycles, respectively. Because the sintering of MgO composites is sensitive to temperature, the sorption and regeneration temperatures need to be optimized for CO2 mixtures containing water vapor. As the developed MgO composite showed high performance with the wet gas mixture, it could be applied to CO2 capture processes with relatively long cycles.

4. Conclusion Triple salt-promoted MgO composites (NaNO3, Na2CO3, and LiNO3) were prepared by using a precipitation method to enhance the sorption capacity, rate, and stability for CO2 capture at an intermediate temperature. The salt composition-controlled precipitation method was successfully developed and it will contribute to designing an effective saltpromoted MgO sorbent. By controlling the salt amount and synthetic conditions, the textural properties, CO2 sorption rates, and working capacities were enhanced. By adopting Li salts as the third salt, as-prepared MgO exhibited a high sorption capacity of 73.0 wt% at 325 °C for pure CO2 uptake. Furthermore, the sorption rate was considerably improved compared with that of MgO promoted by NaNO3 and Na2CO3.5 The capture capacity achieved 60 wt% at the 14th cycle in the cyclic test (sorption for 60 min in pure CO2 flow and regeneration for 15 min in pure N2 flow), which is also much higher than the result for sodium double-salt MgO under the same conditions (32 wt%). Furthermore, as the contact time between sorbent and gas is limited in practical capture operations, the relatively fast sorption rates of the developed sorbents could contribute to high working capacities, even at short sorption times (10 min for 30


Energy & Fuels

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

sorption/ 5 min for regeneration). In future, the cost of lithium salts should be evaluated relative to the cost reduction achieved for improved CO2 capture because utilization of lithium salts in renewable energy and energy storage applications is expected to increase. In CO2 capture processes, sorbents have to be regenerated by using CO2. Under a wet CO2 gas mixture for sorption and CO2 regeneration, the working capacities were 23 wt% and 4.6 wt% for 60/15 min and 10/5 min cycles, respectively. Although the as-prepared sorbent achieved high working capacity under the wet CO2 mixture/CO2 regeneration and the fast cycle condition, CO2 regeneration led to an increase of the optimum regeneration temperature compared with that using N2 regeneration. In addition, higher regeneration temperature resulted in greater sintering of sorbents and energy consumption. Therefore, further advanced sorbents are necessary to improve capture capacity and stability for practical adaptability.

Associated content Supporting Information Details about calculated data of heat capacities for as-synthesized MgO samples (Table S1), data calculated from the DSC curves of as-synthesized samples (Table S2), DSC curves for as-synthesized samples (Figure S1), and explanation about conditions and curves in the cyclic test with simulated dry and wet syngas (Figure S2).

Acknowledgements This work was financially supported by the Korea Institute of Energy Technology Evaluation and Planning (20158510011280) and the Ministry of Trade, Industry & Energy.

References 31


Page 32 of 38

Page 33 of 38

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

Energy & Fuels

1. Moon, D.-K.; Lee, D.-G.; Lee, C.-H., H2 pressure swing adsorption for high pressure syngas from an integrated gasification combined cycle with a carbon capture process. Appl. Energy 2016, 183, 760-774. 2. Haszeldine, R. S., Carbon capture and storage: how green can black be? Science 2009, 325, (5948), 1647-1652. 3. Lee, J. C.; Lee, H. H.; Joo, Y. J.; Lee, C. H.; Oh, M., Process simulation and thermodynamic analysis of an IGCC (integrated gasification combined cycle) plant with an entrained coal gasifier. Energy 2014, 64, 58-68. 4. Luberti, M.; Friedrich, D.; Brandani, S.; Ahn, H., Design of a H2 PSA for cogeneration of ultrapure hydrogen and power at an advanced integrated gasification combined cycle with pre-combustion capture. Adsorption 2014, 20, (2), 511-524. 5. Vu, A.-T.; Ho, K.; Jin, S.; Lee, C.-H., Double sodium salt-promoted mesoporous MgO sorbent with high CO2 sorption capacity at intermediate temperatures under dry and wet conditions. Chem. Eng. J. 2016, 291, 161-173. 6. Nandi, S.; De Luna, P.; Daff, T. D.; Rother, J.; Liu, M.; Buchanan, W.; Hawari, A. I.; Woo, T. K.; Vaidhyanathan, R., A single-ligand ultra-microporous MOF for precombustion CO2 capture and hydrogen purification. Sci. Adv. 2015, 1, (11), e1500421. 7. Kim, S.; Jeon, S. G.; Lee, K. B., High-Temperature CO2 Sorption on Hydrotalcite Having a High Mg/Al Molar Ratio. ACS Appl. Mater. Interfaces 2016, 8, (9), 5763-5767. 8. Xu, F.; Yu, Y.; Yan, J.; Xia, Q.; Wang, H.; Li, J.; Li, Z., Ultrafast room temperature synthesis of GrO@ HKUST-1 composites with high CO2 adsorption capacity and CO2/N2 adsorption selectivity. Chem. Eng. J. 2016, 303, 231-237. 9. Hefti, M.; Joss, L.; Bjelobrk, Z.; Mazzotti, M., On the potential of phase-change adsorbents for CO2 capture by temperature swing adsorption. Faraday Discuss. 2016, 192, (0), 153-179. 32


Energy & Fuels

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

10. Vu, A.-T.; Jiang, S.; Kim, Y.-H.; Lee, C.-H., Controlling the physical properties of magnesium oxide using a calcination method in aerogel synthesis: its application to enhanced sorption of a sulfur compound. Ind. Eng. Chem. Res. 2014, 53, (34), 13228-13235. 11. Vu, A.-T.; Ho, K.; Lee, C.-H., Removal of gaseous sulfur and phosphorus compounds by carbon-coated porous magnesium oxide composites. Chem. Eng. J. 2016, 283, 1234-1243. 12. Li, Y. Y.; Han, K. K.; Lin, W. G.; Wan, M. M.; Wang, Y.; Zhu, J. H., Fabrication of a new MgO/C sorbent for CO2 capture at elevated temperature. J. Mater. Chem. A 2013, 1, (41), 12919-12925. 13. Gunathilake, C.; Jaroniec, M., Mesoporous calcium oxide–silica and magnesium oxide– silica composites for CO2 capture at ambient and elevated temperatures. J. Mater. Chem. A 2016, 4, (28), 10914-10924. 14. Seggiani, M.; Puccini, M.; Vitolo, S., Alkali promoted lithium orthosilicate for CO2 capture at high temperature and low concentration. Int. J. Greenhouse Gas Control 2013, 17, 25-31. 15. Xiao, G.; Singh, R.; Chaffee, A.; Webley, P., Advanced adsorbents based on MgO and K2CO3 for capture of CO2 at elevated temperatures. Int. J. Greenhouse Gas Control 2011, 5, (4), 634-639. 16. Harada, T.; Simeon, F.; Hamad, E. Z.; Hatton, T. A., Alkali Metal Nitrate-Promoted HighCapacity MgO Adsorbents for Regenerable CO2 Capture at Moderate Temperatures. Chem. Mater. 2015, 27, (6), 1943-1949. 17. Zhang, K.; Li, X. S.; Duan, Y.; King, D. L.; Singh, P.; Li, L., Roles of double salt formation and NaNO3 in Na2CO3-promoted MgO absorbent for intermediate temperature CO2 removal. Int. J. Greenhouse Gas Control 2013, 12, 351-358. 18. Lee, C. H.; Mun, S.; Lee, K. B., Characteristics of Na–Mg double salt for high33


Page 34 of 38

Page 35 of 38

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

Energy & Fuels

temperature CO2 sorption. Chem. Eng. J. 2014, 258, 367-373. 19. Zhang, K.; Li, X. S.; Li, W. Z.; Rohatgi, A.; Duan, Y.; Singh, P.; Li, L.; King, D. L., Phase Transfer‐Catalyzed Fast CO2 Absorption by MgO‐Based Absorbents with High Cycling Capacity. Adv. Mater. Interfaces 2014, 1, 1400030. 20. Zhang, K.; Li, X. S.; Chen, H.; Singh, P.; King, D. L., Molten Salt Promoting Effect in Double Salt CO2 Absorbents. J. Phys. Chem. C 2015, 120, 1089-1096. 21. Liu, W.; Feng, B.; Wu, Y.; Wang, G.; Barry, J.; Diniz da Costa, J. O. C., Synthesis of sintering-resistant sorbents for CO2 capture. Environ. Sci. Technol. 2010, 44, (8), 3093-3097. 22. Qin, C.; Liu, W.; An, H.; Yin, J.; Feng, B., Fabrication of CaO-based sorbents for CO2 capture by a mixing method. Environ. Sci. Technol. 2012, 46, (3), 1932-1939. 23. Broda, M.; Müller, C. R., Synthesis of Highly Efficient, Ca‐Based, Al2O3‐Stabilized, Carbon Gel‐Templated CO2 Sorbents. Adv. Mater. 2012, 24, (22), 3059-3064. 24. Li, L.; Li, Y.; Wen, X.; Wang, F.; Zhao, N.; Xiao, F.; Wei, W.; Sun, Y., CO2 capture over K2CO3/MgO/Al2O3 dry sorbent in a fluidized bed. Energy Fuels 2011, 25, (8), 3835. 25. Prashar, A. K.; Seo, H.; Choi, W. C.; Kang, N. Y.; Park, S.; Kim, K.; Min, D. Y.; Kim, H. M.; Park, Y. K., Factors affecting the rate of CO2 absorption after partial desorption in NaNO3-promoted MgO. Energy Fuels 2016, 30, (4), 3298-3305. 26. Martín, C. F.; Sweatman, M. B.; Brandani, S.; Fan, X., Wet impregnation of a commercial low cost silica using DETA for a fast post-combustion CO2 capture process. Appl. Energy 2016, 183, 1705-1721. 27. Vu, A.-T.; Park, Y.; Jeon, P. R.; Lee, C.-H., Mesoporous MgO sorbent promoted with KNO3 for CO2 capture at intermediate temperatures. Chem. Eng. J. 2014, 258, 254-264. 28. Kim, Y.-H.; Tuan, V. A.; Park, M.-K.; Lee, C.-H., Sulfur removal from municipal gas using magnesium oxides and a magnesium oxide/silicon dioxide composite. Microporous 34


Energy & Fuels

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

Mesoporous Mater. 2014, 197, 299-307. 29. Lee, C. H.; Kwon, H. J.; Lee, H. C.; Kwon, S.; Jeon, S. G.; Lee, K. B., Effect of pHcontrolled synthesis on the physical properties and intermediate-temperature CO2 sorption behaviors of K–Mg double salt-based sorbents. Chem. Eng. J. 2016, 294, 439-446. 30. Sutradhar, N.; Sinhamahapatra, A.; Pahari, S. K.; Pal, P.; Bajaj, H. C.; Mukhopadhyay, I.; Panda, A. B., Controlled synthesis of different morphologies of MgO and their use as solid base catalysts. J. Phys. Chem. C 2011, 115, (25), 12308-12316. 31. Qu, Y.; Zhou, W.; Ren, Z.; Pan, K.; Tian, C.; Liu, Y.; Feng, S.; Dong, Y.; Fu, H., Fabrication of a 3D Hierarchical Flower‐Like MgO Microsphere and Its Application as Heterogeneous Catalyst. Eur. J. Inorg. Chem. 2012, 2012, (6), 954-960. 32. Zhang, Z.; Zheng, Y.; Ni, Y.; Liu, Z.; Chen, J.; Liang, X., Temperature-and pH-dependent morphology and FT-IR analysis of magnesium carbonate hydrates. J. Phys. Chem. B 2006, 110, (26), 12969-12973. 33. Purwajanti, S.; Zhou, L.; Ahmad Nor, Y.; Zhang, J.; Zhang, H.; Huang, X.; Yu, C., Synthesis of Magnesium Oxide Hierarchical Microspheres: A Dual-Functional Material for Water Remediation. ACS Appl. Mater. Interfaces 2015, 7, (38), 21278-21286. 34. Zhang, Z.; Zheng, Y.; Zhang, J.; Zhang, Q.; Chen, J.; Liu, Z.; Liang, X., Synthesis and shape evolution of monodisperse basic magnesium carbonate microspheres. Cryst. Growth Des. 2007, 7, (2), 337-342. 35. Mitsuhashi, K.; Tagami, N.; Tanabe, K.; Ohkubo, T.; Sakai, H.; Koishi, M.; Abe, M., Synthesis of microtubes with a surface of “house of cards” structure via needlelike particles and control of their pore size. Langmuir 2005, 21, (8), 3659-3663. 36. Zarghami, S.; Hassanzadeh, A.; Arastoopour, H.; Abbasian, J., Effect of Steam on the Reactivity of MgO-Based Sorbents in Precombustion CO2 Capture Processes. Ind. Eng. 35


Page 36 of 38

Page 37 of 38

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

Energy & Fuels

Chem. Res. 2015, 54, (36), 8860-8866. 37. Masala, A.; Vitillo, J. G.; Mondino, G.; Grande, C. A.; Blom, R.; Manzoli, M.; Marshall, M.; Bordiga, S., CO2 Capture in Dry and Wet Conditions in UTSA-16 Metal–Organic Framework. ACS Appl. Mater. Interfaces 2017, 9, (1), 455-463. 38. Kim, C.; Cho, H. S.; Chang, S.; Cho, S. J.; Choi, M., An ethylenediamine-grafted Y zeolite: a highly regenerable carbon dioxide adsorbent via temperature swing adsorption without urea formation. Energy & Environmental Science 2016, 9, (5), 1803-1811. 39. González, B.; Liu, W.; Sultan, D. S.; Dennis, J. S., The effect of steam on a synthetic Cabased sorbent for carbon capture. Chem. Eng. J. 2016, 285, 378-383. 40. Donat, F.; Florin, N. H.; Anthony, E. J.; Fennell, P. S., Influence of high-temperature steam on the reactivity of CaO sorbent for CO2 capture. Environ. Sci. Technol. 2012, 46, (2), 1262-1269. 41. Campbell, A.; Kartzmark, E.; Nagarajan, M., The binary (anhydrous) systems NaNO3– LiNO3, LiClO3–NaClO3, LiClO3–LiNO3, NaNO3–NaClO3 and the quanternary system NaNO3–LiNO3–LiClO3–NaClO3. Can. J. Chem. 1962, 40, (7), 1258-1265. 42. Bale, C.; Bélisle, E.; Chartrand, P.; Decterov, S.; Eriksson, G.; Gheribi, A.; Hack, K.; Jung, I.-H.; Kang, Y.-B.; Melançon, J., FactSage thermochemical software and databases, 2010–2016. Calphad 2016, 54, 35-53. 43. Inoue, M.; Hirasawa, I., The relationship between crystal morphology and XRD peak intensity on CaSO4·2H2O. J. Cryst. Growth 2013, 380, 169-175. 44. Park, Y. C.; Jo, S.-H.; Lee, S.-Y.; Moon, J.-H.; Ryu, C. K.; Lee, J. B.; Yi, C.-K., Performance analysis of K-based KEP-CO2P1 solid sorbents in a bench-scale continuous dry-sorbent CO2 capture process. Korean J. Chem. Eng. 2016, 33, (1), 73-79. 45. Grasa, G. S.; Abanades, J. C., CO2 Capture Capacity of CaO in Long Series of 36


Energy & Fuels

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

Carbonation/Calcination Cycles. Ind. Eng. Chem. Res. 2006, 45, (26), 8846-8851. 46. German, R. M.; Suri, P.; Park, S. J., Review: liquid phase sintering. J. Mater. Sci. 2009, 44, (1), 1-39. 47. Kingery, W.; Niki, E.; Narasimhan, M., Sintering of Oxide and Carbide‐Metal Compositions in Presence of a Liquid Phase. J. Am. Ceram. Soc. 1961, 44, (1), 29-35. 48. Jo, S.-I.; An, Y.-I.; Kim, K.-Y.; Choi, S.-Y.; Kwak, J.-S.; Oh, K.-R.; Kwon, Y.-U., Mechanisms of absorption and desorption of CO2 by molten NaNO3-promoted MgO. Phys. Chem. Chem. Phys. 2017, 19, (8), 6224-6232. 49. Ciborowski, J.; Wlodarski, A., On electrostatic effects in fluidized beds. Chem. Eng. Sci. 1962, 17, (1), 23-32. 50. Broda, M.; Manovic, V.; Anthony, E. J.; Müller, C. R., Effect of Pelletization and Addition of Steam on the Cyclic Performance of Carbon-Templated, CaO-Based CO2 Sorbents. Environ. Sci. Technol. 2014, 48, (9), 5322-5328.

37


Page 38 of 38

Salt-Composition-Controlled Precipitation of Triple-Salt-Promoted

Recommend Documents