Carbonation Performance of NaNO3 Modified MgO Sorbents

Dec 15, 2016 - Research Center for Clean Energy and Power, Chinese Academy of Sciences, Lianyungang, Jiangsu 222069, China. § University of Chinese A...
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Carbonation Performance of NaNO3 Modified MgO Sorbents Xinfang Yang,*,†,‡ Lifeng Zhao,†,‡ Yang Liu,†,§ Zhenli Sun,∥ and Yunhan Xiao†,‡ †

Key Laboratory of Advanced Energy and Power, Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China ‡ Research Center for Clean Energy and Power, Chinese Academy of Sciences, Lianyungang, Jiangsu 222069, China § University of Chinese Academy of Sciences, Beijing 100049, China ∥ State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: NaNO3 modified MgO sorbents have a large theoretical CO2 capture capacity, but their practical capture performance at medium temperatures (350−400 °C) is poor for the high thermodynamic equilibrium partial pressure of CO2 at these temperatures. The thermodynamic limitations for NaNO3 modified MgO sorbents can be relaxed by adding alkali or alkaline earth carbonates, and the effective carbonation temperature can be shifted to the higher temperatures (350−400 °C) to satisfy the requirement of fast kinetic reactions. In this paper, the CO2 absorption characteristics of NaNO3 modified MgO with or without CaCO3 are compared experimentally to verify the promoting effects of CaCO3. Based on the CaCO3 promoting theory, the methods of stage heating during carbonation and partial decomposition during regeneration were adopted to improve the CO2 absorption ability of NaNO3 modified MgO sorbents at 350 °C. Combined with the composition of fuel gas, effects of increasing CO2 partial pressure and H2O were tested, and they both accelerated the carbonation rates of the sorbents, which forecasted the application potential of NaNO3 modified MgO sorbents.

1. INTRODUCTION As a large amount of greenhouse gas CO2 is emitted from fossil fuel combustion, cost-effective CO2 capture and separation technology becomes imperative in fossil fuel utilization systems. In the system of integrated gasification combined cycle (IGCC), sorption enhanced water gas shift (SEWGS) process (eq 1) is one of the promising technologies for precombustion CO2 capture, which combines CO2 capture and H2-rich syngas production together. Recently, several concepts for the SEWGS process have been proposed,1,2 among which MgO-based sorbents are considered to be the ideal sorbents for removing CO2 from fuel gas at warm temperatures (200−400 °C).3−5 At this condition, the H2-rich syngas can be fed directly into the gas turbines above 200 °C to minimize the sensible heat loss during coal gas purification process, which improves the efficiency of the IGCC system.6,7 MgO-based sorbents with high and stable CO2 absorption ability are crucial for the SEWGS technique. © XXXX American Chemical Society

CO(g) + H 2O(g) ↔ CO2 (g) + H 2(g)

(1)

Because the CO2 absorption ability of pure MgO at medium temperature is as low as 1.1 wt %,8 the sorbent is not applicable for industrial use. Duan et al.9 calculated the CO2 capture properties of alkali metal carbonates promoted MgO sorbents, and found that mixing K2CO3, Na2CO3 or CaCO3 into MgO can increase the effective CO2 absorption temperature to satisfy practical industrial use. Many studies have been conducted on the CO2 absorption performance of alkali metal carbonates promoted MgO sorbents. For K2CO3 promoted sorbents, Hassanzadeh et al.4 indicated that the CO2 absorption ability of MgO−K2CO3 was 10.2−11.7 wt % at 300−450 °C with the CO2 partial pressure of 10 atm, whereas Xiao et al.10 found that Received: Revised: Accepted: Published: A

October 9, 2016 December 7, 2016 December 15, 2016 December 15, 2016 DOI: 10.1021/acs.iecr.6b03909 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research the CO2 absorption ability of MgO−K2CO3 at 375 °C was about 8.6 wt % in pure CO2 under atmospheric pressure. For Na2CO3 promoted MgO sorbents, researchers reported that the CO2 absorption ability of MgO−Na2CO3 sorbents with proper amount of NaNO3 could reach 15.3 wt % at 375 °C in pure CO2 under atmospheric pressure.11−13 For CaCO3 promoted MgO sorbents, Yang et al.14,15 reported that the CO2 absorption ability of NaNO3 modified dolomite at 400 °C with 100% CO2 at atmospheric pressure can reach 11.5 wt %. These studies verified the important effect of NaNO3 on facilitating the CO2 absorption ability of alkali or alkali earth metal carbonates promoted MgO sorbents. For the NaNO3 affecting mechanism, Zhang et al.16,17 reported the phase catalytic mechanism of the molten NaNO3 through the partial dissolving of MgO into Mg2+ and O2−, which removes the energy requirements of breaking the lattice bonds in MgO solid and makes Mg more reactive toward CO2 form MgCO3. Because the theoretical CO2 absorption ability of the pure MgO is much greater than that of alkali or alkali earth metal carbonates promoted MgO, researchers have also investigated NaNO3 or KNO3 modified MgO sorbents directly, without the introduction of alkali or alkali earth metal carbonates.16−19 The MgO conversion of NaNO3 modified MgO sorbents with 20 wt % NaNO3 addition is more than 70% at 330 °C, but the conversion gradually decreases by about 50% after the first several cycles. In addition, the carbonation rate is not fast for MgO sorbents with 20 wt % NaNO3, and the MgO conversion after 25 min can only reach about 10%.16 Lee et al.18 reported that the CO2 sorption capacity of KNO3 modified MgO sorbent was about 10.24 wt % at 325 °C after 20 min carbonation in the first cycle, but decreased to 76% of the first cycle (7.83 wt %) at 325 °C in the 12th cycle, whereas 90% (9.22 wt %) at 375 °C in the 12th cycle. The carbonation performance at 375 °C was obviously more stable than that at 325 °C. Prashar et al.19 also reported the cyclic carbonation ability at 330 °C for 5 h decreased in the first several cycles, and analyzed how the regeneration extent affected the carbonation rates. Because the carbonation rate is a key factor that affects the application of NaNO3 modified MgO sorbents, it is necessary to conduct experiments to analyze further its absorption ability combined with its carbonation rate. In addition, previous research did not compare the difference between the NaNO3 modified MgO sorbents with those promoted by alkali or alkali earth metal carbonate. Our previous investigation also just clarified the affecting mechanism of NaNO3 in the NaNO3 modified dolomite, without considering the CO2 absorption performance of the NaNO3 modified MgO sorbents without CaCO3 promoting. Therefore, the CO2 absorption performance of NaNO3 modified MgO sorbents with or without CaCO3 promoting needs to be further studied. The thermodynamic carbonation temperature for MgO sorbent is relatively low according to Zhang et al.,16,17 and then the carbonation performance of NaNO3 modified MgO sorbents at high temperature will be closely related to CO2 partial pressure. H2O also plays a role in the carbonation of NaNO3 modified MgO sorbents by inducing the switchover of surface oxide to hydroxide over MgO particle surface and coincidently producing Mg−OH.20 Siriwardane et al.3 have reported that the kinetics of CO2 absorption over Mg(OH)2 are much faster than those over MgO, and the Mg(OH)2 sorbent system is more thermodynamically favorable for CO2 capture than MgO at high temperature. Choi et al.,21 Li et

al.22,23 and Han et al.24 have also indicated that the presence of Mg(OH)2 formed by hydroxylation of MgO can increase the surface area of sorbents and facilitate the CO2 absorption of the sorbents. Therefore, it is necessary to evaluate the effects of CO2 partial pressure and H2O in the syngas on carbonation performance of NaNO3 modified MgO sorbents before the industrial use in SEWGS process. In this study, NaNO3 modified MgO sorbents with/without CaCO3 promoting were compared, and methods of the stage heating during carbonation and partial decomposition during regeneration were investigated to further analyze the carbonation performance of the NaNO3 modified MgO sorbent, with the consideration of the CO2 partial pressure and H2O.

2. EXPERIMENTAL SECTION 2.1. Sorbent Preparation. According to the optimized NaNO3 content of around 25 wt % in MgO sorbents reported by Zhang et al.,16 the NaNO3 modified MgO sorbents (0.3NaMgCO3 and 0.5Na-MgCO3) were prepared by wet mixing of MgCO3 in NaNO3 solution. During the sorbent preparation, NaNO3 was first dissolved in the deionized water at 80 °C, then a certain quantity of MgCO3 was put into NaNO3 solutions and stirred for 15 min using a magnetic stirrer, and then the slurry mixture was dried at 120 °C for 2 h in an oven to obtain the NaNO3 modified MgO sorbents. For 0.3Na-MgCO3, the mass ratio of NaNO3 to MgCO3 was 0.3:1, and the NaNO3 content in the mixture was 23 wt %, whereas the mass ratio was 0.5:1 and the NaNO3 content was 33 wt % for 0.5Na-MgCO3. Pure MgCO3 was prepared by the coprecipitation of Mg(NO3)2· 6H2O and Na2CO3·10H2O with a molar ratio of 1:1. The powder was dissolved in deionized water at 80 °C, and then the solutions were mixed and stirred at 80 °C for 15 min using a magnetic stirrer. The white slurry mixture was filtered with full rinsing by deionized water over the filter before dried at 120 °C for 2 h in an oven to obtain pure MgCO3. According to the CO2 absorption tests of the NaNO3 modified dolomites,15 the sorbents of NaNO3 modified MgO with CaCO 3 promoting (0.15Na-MgCO 3-CaCO3 ) were prepared by wet mixing of the obtained pure MgCO3 and limestone in NaNO3 solutions. The limestone was bought from Jinan Quandong Standard Substance Institute in Shandong, China, with main components of 74.4 wt % CaCO3 and 22.6 wt % MgCO3. The detail chemical compositions of the limestone used in this research are the same with the 3# limestone that was reported by Yang et al.25 in the previous investigation. The mass ratio of the NaNO3, MgCO3 and limestone was 0.2:0.387:1, and then, in the obtained 0.15Na-MgCO3-CaCO3 sorbent, the molar ratio of Mg to Ca was 1:1, and the mass ratio of NaNO3 to the mixture of MgCO3 and limestone was 0.15:1. After sorbents preparation, the obtained samples of NaNO3 modified MgO sorbents with CaCO3 promoting or not were all ground to powders with the diameter less than 100 μm before the activation tests. 2.2. Sorbent Characterization. Primary 0.5Na-MgCO3, activated 0.5Na-MgCO3 and carbonation products of activated 0.5Na-MgCO3 were all characterized by the Raman spectrometer (Enwave Optronics, Inc. USA) with 4 cm−1 resolution at the excitation energy of 785 nm. The specific surface area of the 0.5Na-MgCO3 sorbents regenerated with H2O present or not was measured by nitrogen adsorption using an ASAP 2000. 2.3. Carbonation/Calcination Tests. The carbonation/ calcination tests of MgO-based sorbents were carried out by B

DOI: 10.1021/acs.iecr.6b03909 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research thermogravimetric analysis (TGA) using a Thermax 500 analyzer that was manufactured by Thermo Cahn company. The diameter of the reactor tube was 38 mm, and the diameter of the sample pan was 24 mm. Before tests, 10 mg sample (pure MgCO3, 0.3Na-MgCO3, 0.5Na-MgCO3 or 0.15Na-MgCO3CaCO3) was uniformly distributed in the sample pan, and then activated under N2 at 450 °C for 2 h to obtain the MgO-based sorbents. During the activation process, the CaCO3 did not decompose because the temperature was not high enough. In addition, the weight loss from the decomposition of NaNO3 was less than 2 wt % as reported by Yang et al.15 Therefore, the weight loss during activation in Figure 1 varied mainly for the different contents of MgCO3 in the samples.

Figure 2. CO2 absorption curves of MgO-based sorbents.

showed good CO2 absorption performance during the temperature rising stage, but poor CO2 absorption ability at 400 °C. The CO2 absorption ability of the MgO produced from 0.15Na-MgCO3-CaCO3 at 400 °C in the second cycle was close to its absorption ability in the first cycle, and the little higher absorption ability in the first cycle was attributed to its carbonation starting from room temperature. The results agreed with the previous investigation of the CaCO3 promoted MgO sorbents.14 During the temperature rising stage, the CO2 absorption ability of the activated 0.3Na-MgCO3 and 0.5Na-MgCO3 were better than that of the activated 0.15Na-MgCO3-CaCO3, which was attributed to their much higher theoretic CO2 absorption capacity. The activated 0.15Na-MgCO3-CaCO3 had the best CO2 absorption performance when the carbonation happened at 400 °C under 100% CO2. The best carbonation performance of the activated 0.15Na-MgCO3-CaCO3 at 400 °C indicates that the introduction of CaCO3 shifts the thermodynamic equilibrium temperature of NaNO3 modified MgO sorbents to a higher temperature ranges or reduces the required equilibrium partial pressure of CO2 (Ceq) at a certain temperature, which agrees with the calculated results by Duan et al.9 The carbonation rate (r) reported by Abanades et al.26 as eq 2 indicated that the carbonation rate increased with the increase of CCO2 (the CO2 bulk concentration) and the decrease of Ceq. Therefore, the carbonation rate of NaNO3 modified MgO sorbents with CaCO3 promoting at 400 °C is much higher than the sorbents without CaCO3 promoting for the lower Ceq. Inspired by the promoting effect of CaCO3, it is speculated that the carbonation products at low temperature during the temperature rising stage might be crucial for the subsequent carbonation at higher temperature.

Figure 1. Weight loss curves during sample activation at 450 °C under N2.

During the carbonation/calcination tests, the flow rate of reaction gas was 600 mL/min at atmospheric pressure. When the test was conducted under 0.3 MPa, the flow rate increased to 1200 mL/min. In all experiments, the diffusion effects were ruled out in view of sample accumulation and flow gas diffusion. The sorbent mass was recorded once per second by the data acquisition system. The increase of the sample weight corresponded to the carbonation, while the decrease corresponded to the calcination. The CO2 capture capacity was calculated by the weight gain during carbonation divided by the weight of sorbents after activation.

3. RESULTS 3.1. Comparison of the NaNO 3 Modified MgO Sorbents with CaCO3 Promoting or Not. To identify the respective effects of NaNO3 and CaCO3, the CO2 absorption performances of MgCO3, 0.3Na-MgCO3, 0.5Na-MgCO3 and 0.15Na-MgCO3-CaCO3 are compared, and the results of the two cyclic carbonation/calcination cycles for every sorbent are shown in Figure 2. For the first cycle, the carbonation happened under atmospheric pressure with 100% CO2 during the temperature rising stage from room temperature to 400 °C with a heating rate of 6 °C/min, and then the calcination happened under atmospheric pressure with 100% N2 at 400 °C for 30 min. For the second cycle, the carbonation happened at 400 °C under 100% CO2 for 60 min, whereas the calcination happened at 400 °C under N2 for 30 min. Without NaNO3, the MgO produced from pure MgCO3 showed almost no CO2 absorption ability even during the temperature rising stage. The MgO produced from 0.3Na-MgCO 3 and 0.5Na-MgCO3

r = ksS(1 − X )2/3 (CCO2 − Ceq)

(2) 4

In eq 2, ks represent the kinetic constant (m /mol s), X is the sorbent conversion and S represents the surface area of the sorbent. 3.2. Effects of Carbonation Procedure on Carbonation Performance of NaNO3 Modified MgO Sorbents. The influence of carbonation procedure on NaNO3 modified MgO sorbents using a stage heating method was investigated to verify the promoting effect of the obtained carbonation products at relatively low temperature. Three tests have been conducted with fresh activated 0.3NaMgCO3 sorbents, and the three carbonation curves for each test C

DOI: 10.1021/acs.iecr.6b03909 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research have been drawn as (1), (2) and (3) in Figure 3. Figure 3 illustrates the CO2 absorption curves of activated 0.3Na-

Figure 4. Raman spectra for primary 0.5Na-MgCO3, activated 0.5NaMgCO3 and carbonation products of activated sorbent [(#1) activated 0.5Na-MgCO3, (#2) carbonation products of activated 0.5Na-MgCO3 after stage A, (#3) carbonation products of activated 0.5Na-MgCO3 after stage C, (#4) primary 0.5Na-MgCO3].

Figure 3. CO2 absorption curves for activated 0.3Na-MgCO3 under 100% CO2 at atmospheric pressure with the carbonation happening at different stages (stage A, temperature rising from room temperature to 300 °C; stage B, temperature at 300 °C and the following rising to 350 °C; stage C, temperature at 350 °C.).

were carbonated after stage A (#2) and stage C (#3) had an extra vibration peak around 1079 cm −1 , which was corresponded to the vibration peak (around 1079 cm−1) of CO32− in the newly produced carbonation products MgCO3. However, there was slight shift from the vibration peak (around 1079 cm−1) of CO32− in the carbonation products (#2 and #3) to the vibration peak (around 1121 cm−1) of CO32− in the fresh 0.5Na-MgCO3 before activation (#4). The shift indicated that the sorbents after activation were more active and had more lattice defects. The Raman results verified that there was only reactive MgCO3 formed during different carbonation stages. Therefore, the promoting effect should originate from the formed MgCO3 at the lower temperature. 3.3. Effects of Regeneration Extent on Carbonation Performance of NaNO3 Modified MgO Sorbents. Because the formed MgCO3 at low temperature can facilitate the subsequent carbonation at a higher temperature, it is speculated that the incomplete decomposition of MgCO3 during regeneration should also facilitate the subsequent carbonation. Therefore, the effects of the regeneration extent on the subsequent carbonation cycle have also been investigated. After activation, the activated 0.5Na-MgCO3 sorbent was carbonated from room temperature to 400 °C at the heating rate of 6 °C/ min (Figure 2), with the CO2 absorption ability of ∼45 wt %. After full carbonation, the carbonation products went on different extent of regeneration followed by a subsequent carbonation. The one regeneration and followed carbonation process are called a cycle. The carbonation was conducted at 350 °C under 100% CO2 for 10 min, and the regeneration was conducted at 375 °C under N2. At 375 °C, the decomposition was not very drastic, and the regeneration extent can be easily controlled. Therefore, the regeneration was conducted at 375 °C during this test. The heating and cooling rates between 350 and 375 °C were ±5 °C/min during the cyclic tests. From the first to the fifth cycle, the regeneration time at 375 °C gradually increased from 5, 10, 15, 25 to 50 min, and the weight loss percent (the weight loss during regeneration to the weight of the sorbent after activation) increased from about 20, 29, 35 and 40 wt % to 43 wt %, respectively. Because the CO2 absorption ability of activated 0.5Na-MgCO3 was about 45 wt %, the regeneration extent was calculated by the weight loss during regeneration to the CO2 absorption ability of 45 wt %

MgCO3 sorbents with the carbonation happening at different heating stages. The carbonation procedure was divided into three stages. Temperature rises from room temperature to 300 °C at stage A, keeps at 300 °C for 30 min and rises to 350 °C at stage B and then remains at 350 °C for 1 h at stage C. During the tests, the carbonation procedure was controlled by the switch of the reaction atmosphere (100% N2 or 100% CO2), and the heating rate was 10 °C/min. For curve (1), the carbonation happened at all A, B and C stages, and had the highest CO2 absorption ability. For curve (2), the introduction of CO2 began at 300 °C, whereas for curve (3), the introduction of CO2 began at 350 °C. Comparing (2) and (3), though the carbonation rate was slow and the CO2 absorption ability was lower than 10 wt % at stage B, the carbonation at stage B favored the following carbonation at stage C greatly. For (1), the carbonation began at room temperature, the carbonation rate was the fastest, and the CO2 absorption ability was the best. CO2 absorption tests of activated 0.5Na-MgCO3 were also conducted, and the results were consistent with that of activated 0.3Na-MgCO3 sorbents. The carbonation procedure investigation further indicated that the produced carbonation products at the lower temperature (from room temperature to ∼350 °C) favored the subsequent carbonation at the higher temperatures (350−400 °C). Therefore, the carbonation products after different carbonation stages were analyzed and compared by the Raman spectrometer. Raman spectra of activated 0.5Na-MgCO3 sorbent (#1), carbonation products after stage A (#2) and stage C (#3), and primary 0.5Na-MgCO3 (#4) were detected as shown in Figure 4. The peaks around 724 and 1068 cm−1 correspond to the vibration of NO3−.27,28 The Raman curve of 0.5Na-MgCO3 (#4) shows the vibration bands of CO32− at 713, 1121 and 1387 cm−1. After activation, MgCO3 mainly decomposed, but partial MgCO3 still existed, and the obvious vibration peaks of activated 0.5Na-MgCO3 (#1) existed also at 1387 cm−1 (CO32−), except for the vibration peaks of NO3− at 724 and 1068 cm−1. Comparing to activated 0.5Na-MgCO3 sorbent (#1), the carbonation products of activated 0.5Na-MgCO3 that D

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Industrial & Engineering Chemistry Research after the first full carbonation. The regeneration extent for the 5 cycles were 44%, 64%, 78%, 89% and 96%, as shown in Table 1.

equilibrium partial pressure of CO2 for MgO sorbent without the promotion of MgCO3 at high temperatures. Therefore, the influences of the high CO2 partial pressure and H2O were investigated in view of thermodynamics point. The coal gas contains about 10% CO2 and the IGCC product stream contains approximately 40% CO2 at the pressure of 20−30 atm.2 CO2 partial pressure is about 0.2−0.3 MPa at the beginning, and reaches 0.8−1.2 MPa after water gas shift reaction, which indicates the CO2 partial pressure might be higher than 0.3 MPa during water gas shift reaction. Therefore, the reaction conditions of 0.3 MPa, 100% CO2, or 80% CO2 with 20% H2O were adopted. Figure 6 shows the carbonation curves of NaNO3 modified MgO sorbents at 0.3 MPa and 350 °C. In comparison to 0.3Na-

Table 1. Regeneration Times and Regeneration Extents for the Five Regeneration Cycles cycle

first

second

third

fourth

fifth

regeneration time (min) weight loss percent (wt %) regeneration extent (%)

5 20 44

10 29 64

15 35 78

25 40 89

50 43 96

The carbonation curves of the five different regenerated sorbents after different regeneration times (Table1) are shown in Figure 5, which clearly indicates that the carbonation rate

Figure 5. Carbonation curves of activated 0.5Na-MgCO3 at 350 °C under atmospheric pressure after different regeneration extent (regeneration at 375 °C for different time (first, 5 min; second, 10 min; third, 15 min; fourth, 25 min; fifth, 50 min); carbonation at 350 °C for 10 min; the heating and cooling rate was ±5 °C/min).

Figure 6. Carbonation curves of NaNO3 modified MgO sorbents at 0.3 MPa and 350 °C.

MgCO3 with 100% CO2 introduced at stage C under atmospheric pressure (curve (3) in Figure 3), the carbonation rate of activated 0.3Na-MgCO3 under 100% CO2 at 0.3 MPa was higher. Under 100% CO2 at atmospheric pressure, the CO2 absorption ability of activated 0.3Na-MgCO3 sorbent was no more than 5 wt % after 60 min carbonation, whereas the CO2 absorption ability of activated 0.3Na-MgCO3 sorbent could reach 5 wt % after 11 min under 100% CO2 at 0.3 MPa. The increase of CO2 partial pressure effectively improved the CO2 absorption performance of NaNO3 modified MgO sorbents, which verified the high thermodynamic equilibrium partial pressure of CO2 for NaNO3 modified MgO sorbents indirectly. The carbonation curves of the activated 0.3Na-MgCO3 and 0.5Na-MgCO3 under 80% CO2 at 0.3 MPa with/without H2O in Figure 6 show that H2O improves the carbonation rates significantly. With H2O present, the CO2 absorption ability of the activated 0.3Na-MgCO3 sorbent reached 30 wt % within 3.5 min, whereas that of the activated 0.5Na-MgCO3 sorbent reached 32.5 wt % within 3.5 min. Without H2O, the CO2 absorption ability of the activated 0.3Na-MgCO3 sorbent was only 0.9 wt %, and that of the activated 0.5Na-MgCO3 sorbent was only 4 wt % within 3.5 min. The introduction of H2O greatly enhanced the carbonation rates. The comparison between the carbonation curves of 0.5Na-MgCO3 and 0.3NaMgCO3 indicated that the carbonation rate of 0.5Na-MgCO3 was higher than that of 0.3Na-MgCO3 under the same carbonation conditions for the more catalyzing effect of molten NaNO3.16,17 In addition, the carbonation tests of pure MgO

decreases as the regeneration extent increases. The CO2 absorption ability increased from 15.5, 20.9 and 26.1 wt % to 26.3 wt % when the regeneration extent increased from 44%, 64% and 78% to 89%. Though the CO2 absorption ability in the fourth cycle still increased, the carbonation rate decreased significantly in comparison to the previous 3 cycles. When the regeneration time increased to 50 min in the fifth cycle, the regeneration extent increased to about 96%, and the CO2 absorption ability was only about 7 wt % with low carbonation rate. The above results indicate that the regeneration extent of carbonation products has important effects on the CO2 absorption ability and the absorption rate of regenerated sorbents at the subsequent carbonation process. If the regeneration extent is too low, the absorption rate is fast due to the promoting effects of residual MgCO3, but the absorption ability is low for limited MgO in the regenerated sorbents. If the regeneration extent is too high, the subsequent carbonation at high temperature (350 °C) does not have enough promoting effects of MgCO3, and the absorption ability and absorption rates are poor. 3.4. Influence of CO2 Partial Pressure and H2O on Carbonation Performance of NaNO3 Modified MgO sorbents. Activated 0.3Na-MgCO 3 and 0.5Na-MgCO 3 sorbents have poor absorption ability under atmospheric pressure with 100% CO2 at 350 °C without the promotion of MgCO3, which is mainly attributed to the high required E

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Industrial & Engineering Chemistry Research without NaNO3 at 350 °C and 0.3 MPa under 100% CO2 or 80% CO2 and 20% H2O was also conducted. The results indicated that the carbonation of pure MgO with NaNO3 was not obvious under both carbonation conditions, which also verified the catalyzing effect of NaNO3 indirectly. Figure 6 also indicated that the CO2 absorption ability of 0.5Na-MgCO3 was smaller than that of 0.3Na-MgCO3 at 0.3 MPa under 100% CO2 or 80% CO2 and 20% H2O, for the less amount of MgO sorbents in 0.5Na-MgCO3, which was consistent with the results in Figure 2. The ability of 0.5Na-MgCO3 was bigger than that of 0.3Na-MgCO3 at 0.3 MPa under 80% CO2 and 20% N2, for the much slower carbonation rate of 0.3NaMgCO3. In addition, the H2O affecting tests of activated 0.5NaMgCO3 have also been conducted under 95% CO2 and 5% H2O or 90% CO2 and 10% H2O (shown as Figure S1), and the results verified that H2O facilitated the carbonation rate obviously even only 5% H2O was introduced. The H2O affecting results shown in Figure 6 were obtained with H2O introduced first in the carbonation process. To know the cyclic CO2 absorption ability of the sorbent with H2O, CO2 absorption abilities before or after H2O action were compared, and the results were shown in Figure 7. The first calcination

20% H2O or 100% CO2 at 0.3 MPa are drawn in Figure 8. It indicated that the CO2 absorption ability and the carbonation

Figure 8. Cyclic carbonation curves of activated 0.5Na-MgCO3 at 0.3 MPa under 80% CO2 + 20% H2O or under 100% CO2.

rate of 0.5Na-MgCO3 in the first cycle were obvious better than that of the second and third cycle under 80% CO2 with 20% H2O, whereas the CO2 absorption ability and the carbonation rate were stable during the cyclic tests under 100% CO2 at 0.3 MPa. The absorption and desorption of H2O over activated 0.5NaMgCO3 sorbents were then tested using the TGA equipment to explain how H2O resulted in the different weight increases among the first, second and third carbonations, and the results are shown in Figure 9. First, activated 0.5Na-MgCO3 sorbent

Figure 7. Calcination (cal) and carbonation (car) cycles of activated 0.5Na-MgCO3 sorbent under 0.3 MPa (first cal: 100% N2 for 20 min; second and third cal: 80% N2 + 20% H2O for 20 min; car: 80% CO2 + 20% H2O for 30 min.).

was conducted in 100% N2 without H2O, whereas the second and third calcinations were with 80% N2 and 20% H2O. All the carbonation tests were conducted with 80% CO2 and 20% H2O. The CO2 absorption ability of the sorbent for the first carbonation was ∼38 wt %, whereas that for the second and third carbonations was ∼29 wt %. During the cyclic tests with H2O, the CO2 absorption ability decreased from ∼38 to ∼29 wt %. Comparing the cyclic calcination curves in Figure 7, the decomposition of MgCO3 in the first cycle in N2 was slow, whereas that in the second and third cycle was fast and thorough in the presence of H2O. The different decomposition extents of MgCO3 during calcinations were one possible factor that resulted in the difference in CO2 absorption ability between the three carbonations in the presence of H2O. In addition, the cyclic tests without H2O were also conducted, and the carbonation and calcination curves had good reproducibility (shown as Figure S2). To indicate the different carbonation rates more clearly, the cyclic carbonation curves of 0.5Na-MgCO3 under 80% CO2 +

Figure 9. H2O absorption and desorption over activated 0.5NaMgCO3 sorbents.

was placed under 60% H2O and 40% N2 at 350 °C to do the H2O absorption for 20 min, and then in N2 atmosphere to do the decomposition with the temperature rising from 350 to 500 °C at 10 °C/min before staying at 500 °C for 5 min. During H2O absorption, the weight increased by ∼3 wt %, then the H2O desorption over MgO began around 480 °C, which was reported for calcined dolomite previously.25 Therefore, during carbonation and calcination between 350 and 400 °C, formed Mg-(OH)2 moieties would not decompose, and then the weight increased for Mg-(OH)2 carbonation would be less than that for MgO carbonation. The formed Mg-(OH)2 in the presence of H2O would be a reason that resulted in the less F

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Industrial & Engineering Chemistry Research weight increase in the second and third carbonations. However, the impact of H2O absorption was very small and can be ignored, since the weight increased by H2O absorption was only 3 wt %.

4. DISCUSSION The effects of alkali or alkali earth metal carbonates on the CO2 absorption performance of NaNO3 modified MgO sorbents were resulted from the shift of the thermodynamic reaction temperature (Tt) to the higher temperature. For Tt = ΔH/ΔS (ΔH indicates the enthalpy change of the carbonation reaction, ΔS indicates the entropy change of the reaction), the shift of Tt to a higher temperature could be resulted by the increase of ΔH or the decrease of ΔS through the introduction of alkali or alkali earth metal carbonates. Compared to NaNO3 modified MgO, the introduction of CaCO3 could result in small changes in ΔS. However, when the CaCO3 was introduced to MgCO3, the CaCO3 could have strong interaction with MgCO3, even to form CaMg(CO3)2 that was more stable than pure MgCO3, which is due to the increased bonding sites caused by the alkaline or alkali earth metal carbonates.29 Therefore, the introduction of alkali or alkali earth metal carbonates stabilize the carbonation products to increase the ΔH and have the higher Tt, which has also been calculated by Duan et al.9 The above discussions elaborated that alkali or alkali earth metal carbonates can shift the thermodynamic equilibrium temperature of NaNO3 modified MgO sorbents to the higher temperature range, and reduce the required equilibrium partial pressure of CO2 (Ceq) at a certain temperature. Therefore, the carbonation rate of NaNO3 modified MgO sorbents with CaCO3 promoting at 350−400 °C can be facilitated kinetically by lowering the Ceq. Based on this promoting theory, the investigation on the carbonation procedure and regeneration extent verified that both carbonation of MgO at a lower temperature and the incomplete decomposition of MgCO3 during regeneration facilitated the subsequent carbonation of NaNO3 modified MgO sorbents at the higher temperature. The Raman results also indicated that only MgCO3 was formed during the different carbonation stages, which verified the promoting effects of the formed MgCO3 on the subsequent high temperature carbonation. The carbonation at high temperature favored the kinetic reactions of the obtained MgCO3 promoted MgO-based sorbents, which is crucial for industrial use of NaNO3 modified MgO sorbents. In addition, the increase of CO2 partial pressure to 0.3 MPa can promote the carbonation of NaNO3 modified MgO sorbents at high temperature. The existence of H2O enhanced both the carbonation and regeneration rates. To recognize the effect of H2O more clearly, the BET surface area of 0.5Na-MgCO3 after calcined with/ without H2O was characterized. The surface area of 0.5NaMgCO3 after calcination without H2O was 20.13 m2/g, whereas that after calcination with H2O was 21.59 m2/g, which agrees with the results that the 0.5Na-MgCO3 decomposed more completely with H2O than that without H2O, and hence, produced a little more surface area. The increased surface area would facilitate the fast kinetic reaction of 0.5Na-MgCO3 sorbent, but the not very big increase of the surface area indicated the existing of other acting path of H2O. The Raman spectra of the sorbent 0.5Na-MgCO3 regenerated in the presence of H2O or not were compared, and the results are shown in Figure 10. Figure 10 indicated that an added broad vibration band (Eg(R)) from 900−1200 appeared in the Raman

Figure 10. Raman spectra of the obtained 0.5Na-MgCO3 sorbents after the regeneration with H2O present or not.

spectra of the obtained 0.5Na-MgCO3 sorbent after regenerated in the presence of H2O, compared to the spectra of the obtained 0.5Na-MgCO3 sorbent after regenerated without H2O. The added broad vibration band (Eg(R)) corresponded to the vibration of OH− in Mg-(OH)2.30,31 Therefore, it was inferred that the introduction of H2O induced the switchover of surface oxide to hydroxide over MgO particles, and coincidently produced Mg-(OH)2, whose lattice bonds might be easier to be broken than that of MgO, for the easier transferring ability of OH− than that of O2−.32 Based on the promoting effects of OH−, the H2O can enhance the carbonation rate of NaNO3 modified MgO sorbents effectively. For the decrease of the CO2 absorption ability during the cyclic tests with H2O, it can be analyzed from the following two aspects. On one hand, H2O induced the switchover of surface oxide to hydroxide over MgO particles, and coincidently produced Mg-(OH)2 that would not decompose until the temperature was higher than 480 °C. Because 480 °C is higher than the regeneration temperature of 400 °C, some Mg-(OH)2 still exists after sorbent regeneration in the presence of H2O. The theoretical weight increase of MgO sorbent for CO2 absorption was 110 wt %, whereas that of Mg-(OH)2 was 76 wt %. Therefore, the presence of H2O results in decrease of the weight increase during carbonation. The decrease in the weight increase during carbonation does not mean the decrease of CO2 absorption ability, because the amount of CO2 captured did not reduce significantly at this condition. On the other hand, the introduction of H2O made the regeneration of MgCO3 faster and more complete. The residual MgCO3 promoted the high temperature carbonation of NaNO3 modified MgO sorbents. Based on this promoting theory, the advantage of solid MgO sorbents with some residual MgCO3 granules will be analyzed. Figure 11 presents a schematic description of one kind of pores for MgO sorbents with some residual MgCO3 granules. With the promoting of MgCO3, the carbonation of MgO close to MgCO3 granules goes quickly, and CO2 and H2O can diffuse quickly through portals A and B to react with the inner MgO sorbents. Thus, the reaction rate and the CO2 absorption ability increased (shown as Figures 7 and 8). During the carbonation process, the newly formed MgCO3 near portals A and B will plug up the pores, and hinder the further diffusion of CO2 and H2O. Therefore, the lower carbonation rate of the inner MgO sorbents results in the lower CO2 absorption ability. This indicates that the complete G

DOI: 10.1021/acs.iecr.6b03909 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (51406198) is gratefully acknowledged.

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Figure 11. Schematic description of solid MgO sorbents with some residual MgCO3 granule.

decomposition of MgCO3 during regeneration may be a key factor causing the decrease of CO2 absorption ability during cyclic carbonation/calcination in the presence of H2O. Therefore, the decrease of the CO2 absorption ability after the regeneration with H2O might mainly be caused by the complete decomposition of MgCO3, which also indicates the promoting effect of residual MgCO3 indirectly. At this condition, the increase of CO2 partial pressure can decrease the impact of H2O on the fully regenerated MgO sorbents.

5. CONCLUSIONS The study has verified the promoting effect of alkali and alkali earth carbonates on NaNO3 modified MgO sorbents at high temperature (350−400 °C), which means the alkali and alkali earth carbonates can reduce the required equilibrium partial pressure of CO2 at a certain temperature. When the CO2 partial pressure is not high enough for the high temperature carbonation of NaNO3 modified MgO sorbents, the subsequent carbonation can be facilitated by the alkali earth carbonates MgCO3, which can be produced by the adoption of procedure carbonation with stage heating, or preserved by the control of the regeneration extent. The study confirms that increasing CO2 partial pressure improves the carbonation rate, and forecasts the potential of NaNO3 modified MgO sorbents during the water gas shift reaction. The introduction of H2O accelerates both the carbonation and regeneration rates of NaNO3 modified MgO sorbents, although the complete regeneration of MgCO3 in the presence of H2O results in a slight decrease in the cyclic CO2 absorption ability.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b03909. Carbonation curves of NaNO3 modified MgO sorbents with different H2O contents (Figure S1), and cyclic CO2 absorption and desorption curves of activated 0.5NaMgCO3 at 0.3 MPa under 100% CO2 (Figure S2) (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*X. Yang. Fax: +86-10-82543071. E-mail: yangxinfang@mail. etp.ac.cn. ORCID

Xinfang Yang: 0000-0002-2948-7970 Notes

The authors declare no competing financial interest. H

DOI: 10.1021/acs.iecr.6b03909 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.6b03909 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX