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Effect of NaNO3 on MgO-CaCO3 absorbent for CO2 capture at warm temperature Xinfang Yang, Lifeng Zhao, and Yunhan Xiao Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 13 Nov 2013 Downloaded from http://pubs.acs.org on November 18, 2013

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Effect of NaNO3 on MgO-CaCO3 absorbent for CO2 capture at warm temperature Xinfang Yang a,b*, Lifeng Zhao a,b, Yunhan Xiao a,b

a

Key Laboratory of Advanced Energy and Power, Institute of Engineering

Thermophysics, Chinese Academy of Sciences, P.O. Box 2706, Beijing, 100190, China b

Research Center for Clean Energy and Power, Chinese Academy of Sciences, Lianyungang, Jiangsu 222069, China

Abstract:

The effect of NaNO3 on the activation of MgCO3-CaCO3 double salts to

form MgO-CaCO3 absorbent, and the cyclic CO2 absorption ability of the obtained absorbent have been investigated to analyze the affecting mechanism of NaNO3. The MgCO3-CaCO3 double salts were prepared by two methods, i.e. the co-precipitation of Mg(NO3)2, Ca(NO3)2 and Na2CO3, and the modification of dolomite by NaNO3. The results have indicated that NaNO3 has obvious effect on lowering the activation temperature of the MgCO3-CaCO3 double salts, and NaNO3 is the key component in facilitating the CO2 absorption of the MgO-CaCO3 absorbent. The affecting mechanism has also been analyzed based on the X-ray diffraction (XRD) characterization,

the

particle

diameter

distribution

analysis,

the

Brunauer–Emmett–Teller (BET) analysis, and the Scanning Electron Microscope 1 ACS Paragon Plus Environment

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(SEM) characterization of the absorbents. The results have indicated that the molten NaNO3 facilitates the decomposition of MgCO3 in MgCO3-CaCO3 double salts, affects the crystal lattice of activated absorbents and favors the CO2 absorption by improving the ion diffusion in absorbent.

Keywords: MgO-CaCO3 absorbent; CO2 capture; NaNO3; dolomite; MgCO3-CaCO3 double salts

1. Introduction In the system of integrated gasification combined cycle (IGCC), the removal of CO2 from coal gas not only reduces the greenhouse gas emissions, but also produces a large percentage of hydrogen via the water-gas shift reaction (Eqn.(1)). The key point in hydrogen production process is the optimal CO2 absorbent, which should satisfy the temperature window requirement for gas purification, and should also have high and stable CO2 absorption capacity.

CO + H2O ⇔ CO2 + H2

(1)

In IGCC system, in order to take advantage of the sensible heat of coal gas, the ideal temperature window for coal gas purification process (removal of dust and H2S) is higher than about 350oC 1. To accommodate the coal gas purification process, the development of CO2 absorbent for water-gas shift reaction process mainly focuses on two temperature windows. The higher temperature window is 450-700°C, and the absorbent is mainly CaO-based

2-4

. The lower temperature window is 200-500°C,

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which is consistent with the temperature window for the gas purification, and the absorbent is mainly MgO-based 5-8.

Studies on CaO-based absorbent have indicated

its high potential for CO2 capture, but the high regeneration temperature of 800-900°C to calcine the produced CaCO3 consumes large amount of heat and results in the problem of loss-in-capacity. Many research studies have been conducted to solve the problem

9-13

. For MgO-based absorbent, Duan et al.

14

indicated that MgO-based

absorbent had good absorption property at 327-427°C from the aspect of thermodynamics analysis, and could be regenerated at 350-500°C to reduce the heat consumption. MgO-based absorbent has attracted lots of attentions recently.

Pure MgO has a very low CO2 capture capacity (0.011 g CO2 / g sorbent)

15

, but

several MgO-based double slats absorbent have been reported to have obviously better CO2 absorption performance from both thermodynamic analysis and experimental investigation

5, 16-19

. MgO-based double salts absorbent has also been

described as alkali promoted MgO-based absorbent. Mayorga et al 5 indicated that the prepared K2CO3 promoted MgO-based absorbents (MgO-K2CO3) had an absorption capacity of 0.0484 - 0.568 g CO2/ g sorbent, which was closely related to the synthetic conditions. The absorption capacity could be stabilized at 0.484 g CO2/ g sorbent at 375°C. Xiao et al 16 reported that the highest CO2 absorption capacity of MgO-K2CO3 at 375°C was about 0.086 g CO2/ g sorbent in 100% CO2 under atmospheric pressure. Singh et al

17

indicated that the optimal temperature for CO2 absorption by

MgO-Na2CO3 absorbent was 375°C. Zhang et al

18

reported that the CO2 absorption

capacity of the prepared MgO-Na2CO3 could reach 0.150 g CO2/ g sorbent at 3 ACS Paragon Plus Environment

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300-470°C with NaNO3 present in the absorbent, and the absorption capacity was only 0.07 g CO2/ g sorbent if NaNO3 was absent. Li et al

19

reported that the CO2

absorption capacity of the prepared MgO-CaCO3 absorbent could be as high as 0.185 g CO2/ g sorbent at 300-500°C. Zhang et al

18

and Li et al

19

concluded that the

introduction of NaNO3 in absorbent preparation process has important influence on absorbent performance. Hassanzadeh et al

20

and Abbasi et al

21

indicated that the

introduction of K2CO3 into calcined dolomite (MgO-CaCO3) improved the absorption capacity and the reaction kinetics of MgO-CaCO3 absorbent obviously. Besides, the introduction of K2CO3 could also improve the CO2 absorption performance of the calcined hydrotalcite (MgO-Al2O3) 22-23.

The previous researches have indicated the potential of alkali promoted MgO-based absorbents for CO2 capture at 250-500°C. However, the absorption capacities of the prepared MgO-based absorbents by different researchers differ significantly, and the affecting factors are not clear. Furthermore, the affecting mechanism of NaNO3 on absorbent activity has not been clarified concretely. Therefore, the activity of MgO-based absorbent needs to be further investigated. Since the natural dolomite is popular, and the theoretical absorption capacity of MgO-CaCO3 absorbent is not lower than MgO-Na2CO3 or MgO-K2CO3 absorbent, the MgO-CaCO3 absorbent has been chosen in our investigation.

In this paper, the effect of NaNO3 on the absorption performance of MgO-CaCO3 absorbent has been investigated. The activity tests of MgO-CaCO3 absorbent prepared 4 ACS Paragon Plus Environment

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by Mg(NO3)2, Ca(NO3)2 and Na2CO3 , and dolomite modified with NaNO3 have also been conducted.

2. Experimental 2.1 Absorbent preparation The MgO-CaCO3 absorbents were prepared by two methods, i.e. the co-precipitation method and the dolomite modification method. The precipitated MgCO3-CaCO3 absorbent was prepared from Mg(NO3)2•6H2O, Ca(NO3)2•4H2O and Na2CO3•10H2O, with the molar ratio of 1 : 1 : 2. The powder was dissolved in de-ionized water at 80°C with the concentration of metal ions kept at 1 mol/L. The solutions were then mixed and stirred at 80°C for 15 minutes by magnetic stirrer. The mixture (white slurry) was filtered and washed by de-ionized water for different times, and then dried at 120°C for 2 h in oven to obtain the absorbent with different addition of NaNO3. The preparation of three precipitated MgCO3-CaCO3 samples (Pre-A, Pre-B and Pre-C) is described in Table 1. The NaNO3 modified dolomite was prepared by wet mixing method. The dolomite used in this research was bought from Jinan Quandong Standard Substance Institute in Shandong Province of China, which contained 44 wt% MgCO3, 54 wt% CaCO3, and small amount impurities. The dolomite was wet-impregnated by NaNO3 solutions with different concentrations, stirred for 15 minutes at 80°C, and then dried at 120°C for 2 h. The detailed description of the preparation process is presented in Table 2. 2.2 Sample Characterization The XRD phase components of the absorbent were investigated by Bruker D8 5 ACS Paragon Plus Environment

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advance X-ray diffractmeter. The diameter distributions of the samples were analyzed by a laser diffraction instrument (Malvern Mastersizer 2000). The BET specific surface area was measured by nitrogen adsorption using ASAP 2000. The Scanning Electron Microscope (SEM) images were taken by S-4800. The DTA analyses were conducted to investigate the amount of NaNO3 in absorbents using synchronous thermal analyzer (STA PT 1600) at nitrogen atmosphere (30 ml/min). 2.3 Cyclic carbonation / calcination tests The CO2 capture activity tests of the absorbent were carried out in a thermogravimetric analyzer (TGA, Thermax 500). For the TGA equipment, the diameter of the reactor tube was 38 mm, the diameter and wall height of the sample pan was 24 mm and 3 mm respectively. In order to ensure the sample dispersed uniformly in the sample pan, 30 mg of modified dolomite or 18 mg precipitated absorbent were loaded because of the density difference. During the tests, the absorbent was firstly activated under N2 atmosphere, and then carbonated / calcinated. The cyclic carbonation / calcination tests were conducted mainly at 400°C at atmospheric pressure. The carbonation atmosphere was 100% CO2 whilst the calcination atmosphere was 100% N2. The flow rate of the reaction gas was 600 ml/min controlled by mass flow meters. Except the reaction gas, the purge gas (protecting gas) and the furnace gas were both N2. The test consisted of 60 minutes carbonation and 30 minutes calcination. The mass of the solid material was recorded once per second by the data acquisition system. The increase in sample weight corresponds to carbonation, and the decrease in the sample weight corresponds to 6 ACS Paragon Plus Environment

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calcination. Before the cyclic carbonation / calcination tests, all the precipitated absorbents and the modified dolomites were calcined first so that magnesium existing in absorbents was in MgO form. The primary calcination is named as activation. The CO2 capture capacity of the absorbent was calculated by the weight gain during carbonation divided by the weight of absorbent after activation.

3. Results 3.1 Effect of NaNO3 on precipitated absorbent activation Figure 1 shows the DTA curves of the precipitated MgCO3-CaCO3 absorbents with different amount of NaNO3. The sharp endothermic valley around 303°C corresponds to NaNO3 molten in absorbent and the intensity of the valley is proportional to the amount of NaNO3. The preparation of the precipitated absorbents indicated that the Pre-A absorbent had the most amount of NaNO3 retained, while the Pre-C absorbent had the least. The relative intensity of the endothermic valley corresponding to the molten of NaNO3 matched the anticipative results. The DTA curves in Figure 1 also indicated that there were two kinds of decomposition during the activation. For Pre-A absorbent, the low temperature decomposition happened below 250°C, and the high temperature decomposition of the absorbent corresponding to the broad endothermic valley during the heating process was around 345°C and 385°C. For Pre-B absorbent, the broad endothermic valley corresponding to high temperature decomposition was around 400oC. For Pre-C absorbent, however, there was no obvious endothermic valley before 400°C. The DTA curves of the precipitated absorbents indicated that NaNO3 facilitated the decomposition of MgCO3 in the precipitated MgCO3-CaCO3 7 ACS Paragon Plus Environment

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double salts, and enabled the fast decomposition temperature of MgCO3 shift towards the even lower temperature. Moreover, the DTA curves of the precipitated absorbents also indicated that 400°C was high enough to activate the precipitated absorbents. Therefore, the precipitated absorbents were firstly activated at 400°C for 2 hours in the TGA equipment before the cyclic carbonation / calcination tests. Figure 2 shows the weight curves of the precipitated absorbents during activation. The increase of NaNO3 content in absorbent resulted in the faster and more weight decrease during activation. For Pre-A and Pre-B absorbents, the decomposition finished in the first 20 minutes, but the decomposition of the Pre-C did not finish until the end of the 2 hours’ activation process. XRD tests of Pre-A and Pre-C absorbents before and after activation were conducted to understand the two steps decomposition in activation process and the results were shown in Figure 3 and Figure 4. XRD results in Figure 3 indicated that Mg5(CO3)4(OH)2•4H2O, NaNO3, dolomite, and CaCO3 existed in Pre-A absorbent, and CaCO3 existed in the form of aragonite. After activation, Mg5(CO3)4(OH)2•4H2O and dolomite decomposed to produce MgO in periclase form, and CaCO3 in calcite form. XRD results in Figure 4 indicated that Mg5(CO3)4(OH)2•4H2O, dolomite and aragonite co-existed without obvious NaNO3 in Pre-C absorbent. After activation, the peaks representing dolomite also existed, and CaCO3 also existed in aragonite form, while Mg5(CO3)4(OH)2•4H2O decomposed to magnesium oxide. The XRD results comfirmed the two kinds of decomposition of Mg5(CO3)4(OH)2•4H2O and dolomite in the precipitated absorbents, and also suggested that NaNO3 in absorbent not only 8 ACS Paragon Plus Environment

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facilitated the absorbent decomposition, but also affected the crystal lattice of the activated absorbents. 3.2 Effect of NaNO3 on modified dolomite activation The DTA curves of natural and NaNO3 modified dolomites are shown in Figure 5. The intensity of the sharp endothermic valley around 303°C corresponding to the molten NaNO3 increased with the increasing amount of added NaNO3. The broad valley around 450°C corresponded to the decomposition of MgCO3 content in modified dolomite, which was higher than that of precipitated absorbents. For natural dolomite without NaNO3 addition, the broad valley was not found before 480°C, indicating that the addition of NaNO3 lowered the activation temperature of MgCO3 in dolomite. To address this observation, the modified dolomites were firstly activated at 500°C for 2 hours before the cyclic carbonation / calcinations tests in our study. Figure 6 shows the weight curves of natural and modified dolomites during the activation in TGA. Under the same activation condition, the weight loss of dolomite without NaNO3 was no more than 2%, the weight loss of the 0.10Na-dol absorbent was about 17.5%, the weight loss of the 0.15Na-dol absorbent was about 18.5%, and the weight loss of the 0.20Na-dol absorbent was about 14.6%. The results revealed that NaNO3 lowered the decomposition temperature of MgCO3 in dolomite. When the mass ratio of NaNO3 to dolomite increased from 0.10 to 0.15, the decomposition rate of MgCO3 in dolomite increased in the first 20 minutes, and then the decomposition rate of 0.10Na-dol gradually exceeded the decomposition rate of 0.15Na-dol in later activation process because of the more MgCO3 amount in 0.10Na-dol. The results 9 ACS Paragon Plus Environment

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indicated that the amount of NaNO3 introduced into dolomite should be proper during the activation process, and the weight loss could be the biggest in this activation process when the mass ratio of NaNO3 to dolomite was around 0.15. The XRD tests of NaNO3 modified dolomites and the activated 0.2Na-dol absorbent in a muffle furnace have also been carried out to understand the phase components of the absorbents. Figure 7 shows the XRD results of the 0.20Na-dol absorbent before and after activation. Before activation, the main components were dolomite and NaNO3. After activation, the main components were CaCO3 formed as calcite, MgO formed as periclase, and a bit of residual NaNO3. The phase components of modified dolomite were consistent with that of Pre-A samples, but different from that of Pre-C samples. 3.3 Effect of NaNO3 on cyclic carbonation / calcination behavior of precipitated absorbent Figure 8 shows the TGA curves of the first six carbonation / calcination cycles of Pre-A absorbent. The first carbonation started from room temperature to 400°C at the rate of 6°C/min and kept at 400°C for 1 hour, and then all the following cyclic calcination / carbonation started from 400°C. Figure 8 shows that, despite the first carbonation process had a little bit higher weight increase ratio of 22.8% (since it started from room temperature), the weight increase ratio of the following cycles all maintained around 20%, indicating the stability of CO2 absorption performance of Pre-A. Besides, about 90% of the absorption capacity, i.e. 0.20 g CO2/ g sorbent, is reached in the first 5 minutes, indicating the fast absorption rate of Pre-A absorbent. 10 ACS Paragon Plus Environment

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The cyclic carbonation performance of Pre-B and Pre-C were tested under the same reaction conditions. The results indicated that the CO2 capture ability of Pre-C absorbent without NaNO3 is poor, only around 0.01 g CO2/ g sorbent in the test cycles. The cyclic CO2 capture activity of Pre-B absorbent from the 2nd to the 15th cycles is shown in Figure 9. Its CO2 capture capacity remained around 0.18 g CO2/ g sorbent. For Pre-B absorbent, the capture capacity in the first 5 minutes was around 0.16 g CO2/ g sorbent, and the capture capacity in the last 55 minutes was only around 0.02 g CO2/ g sorbent. The Pre-C absorbent without NaNO3 showed no obvious CO2 absorption ability. The cyclic carbonation / calcination investigation of the precipitated absorbents indicated that Pre-A absorbent with a little more NaNO3 content than that in Pre-B absorbent had better CO2 capture activity, and their capture activity were both stable during the cyclic tests. Pre-C absorbent containing no obvious NaNO3 had poor CO2 capture activity. 3.4 Effect of NaNO3 on cyclic carbonation / calcination behavior of modified dolomite The cyclic carbonation / calcination behavior of the modified dolomites with different amount of NaNO3 has also been investigated to further analyze the NaNO3 impacts. Figure 10 shows the cyclic CO2 absorption capacity of 0.10Na-dol absorbent. Neglecting the first carbonation during the temperature rising stage, from the 2nd to the 4th carbonation cycle, the CO2 absorption capacity increased from 0.046 to 0.059 g CO2/ g sorbent, and then stabilized around 0.060 g CO2/ g sorbent. As the reaction cycle increases, the CO2 absorption capacity of the 0.10Na-dol absorbent in the first 5 11 ACS Paragon Plus Environment

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minutes increased from 0.027 to 0.038 g CO2/ g sorbent, and the absorption capacity in the last 55 minutes remained nearly constant around 0.02 g CO2/ g sorbent. Figure 11 shows the cyclic CO2 carbonation curves of 0.15Na-dol absorbent, which clearly indicated that the CO2 absorption capacity increased during the cyclic tests. From the 2nd to the 8th carbonation cycle, the CO2 absorption capacity increased from 0.056 to 0.091 g CO2/ g sorbent. From the 8th to the 11th carbonation cycle, the CO2 absorption capacity increased from 0.091 to 0.11 g CO2 / g sorbent. The increasing rate of the CO2 absorption capacity was gradually depressed from the 11th to the 15th cycle, and the CO2 absorption capacity gradually stabilized around 0.110 g CO2/ g sorbent. Besides, the increasing of the CO2 absorption capacity mainly happened in the first 5 minutes, which was the same as the 0.10Na-dol absorbent. Figure 12 shows the cyclic CO2 absorption capacity of 0.20Na-dol absorbent. The CO2 absorption capacity of 0.20Na-dol absorbent increased from 0.057 g CO2/ g sorbent in the 2nd cycle to 0.114 g CO2/ g sorbent in the 12th cycle, and then stabilized around 0.115 g CO2/ g sorbent when the cycle number increased from the 12th to the 15th. Most of the absorption capacity increase also happened in the first 5 minutes, and the absorption capacity in the last 55 minutes were around 0.02 g CO2/ g sorbent. Comparing the cyclic absorption capacity of the three NaNO3 modified dolomites, it was clear that, when the mass ratio of NaNO3 to dolomite increased from 0.10 to 0.15, the stabilized CO2 absorption capacity increased from 0.06 g CO2/ g sorbent to 0.11 g CO2/ g sorbent, and the increasing ratio was about 83%; when the mass ratio of 12 ACS Paragon Plus Environment

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NaNO3 to dolomite increased from 0.15 to 0.20, the stabilized CO2 absorption capacity increased from 0.110 g CO2/ g sorbent to 0.117 g CO2/ g sorbent, and the increasing ratio was about 6.4%. Therefore, under this activation and reaction conditions, the appropriate mass ratio of NaNO3 to dolomite could be 0.15 to 0.20. Dividing the gas-solid reaction into the fast kinetic reaction control stage and the slow diffusion control stage, the increase of the CO2 absorption capacity was mainly in the fast kinetic reaction control stage, which could be attributed to the improvement on the absorbent physical characteristics that enhanced the ions diffusion and shifted some grain reaction from diffusion control to kinetic reaction control. The increasing of CO2 absorption ability in the fast reaction control stage is positive for industrial application as the carbonation process could be shortened. 3.5 Characterization of precipitated absorbent and modified dolomite The investigation of the precipitated absorbents and the modified dolomites indicated that NaNO3 lowered the activation temperature of absorbents, affected the crystal lattice of the activated absorbents, and improved the cyclic CO2 absorption capacity. However, there were some differences between the precipitated absorbents and the modified dolomites: i) the activation temperature of the precipitated absorbents was lower than that of the modified dolomites, and the amount of NaNO3 in the precipitated absorbent was less than the amount of NaNO3 introduced into dolomite; ii) the CO2 absorption capacity of the precipitated absorbent was almost stable during the cyclic tests, but the CO2 absorption capacity of the modified dolomite increased in the first several cycles before stabilized; and iii) the stabilized CO2 absorption capacity of 13 ACS Paragon Plus Environment

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the precipitated absorbent was a little higher than that of the modified dolomite. In view of these differences, XRD, SEM and BET characterization and the particle diameter distribution analysis of the absorbents have been conducted. Figure 13 indicates that Pre-A absorbent and the NaNO3 modified dolomite absorbent after CO2 absorption had the same XRD components, and dolomite was the main component with a little MgO component existing in periclase form and a little CaCO3 component existing in calcite form.

However, for Pre-C absorbent without obvious

NaNO3, the main component was aragonite and magnesium oxide with little dolomite and calcite. Therefore, with NaNO3 present in the absorbent, the CaCO3 existed in calcite form after the activation of absorbent, and then the dolomite was produced after CO2 absorption. It is inferred that the molten NaNO3 facilitated the bond connection between MgO and CaCO3 to form MgO-CaCO3 absorbent.

The results

also verified that the CO2 absorption ability of MgO-based double salts absorbent improved a lot by CaCO3 forming MgO-CaCO3 absorbent. Figure 14 shows the particle diameter distribution of the precipitated absorbents and the NaNO3 modified dolomites. The particle diameters of the precipitated absorbents were mainly below 10µm, while the particle diameters of the NaNO3 modified dolomites were mainly around 50µm. The NaNO3 modified dolomites were bigger than that of the precipitated absorbents. Figure 15 shows the SEM of absorbents after activation and cyclic carbonation / calcination tests. Comparing SEM Micrograph (1), which is the SEM of the fresh activated Pre-A absorbent, and SEM Micrograph (2), which is the SEM of the 14 ACS Paragon Plus Environment

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activated Pre-A after 6 cyclic tests, the grain of the activated Pre-A after 6 cyclic tests seemed to be a little more uniform than that of the fresh activated Pre-A absorbent, and there was almost no big grains in Pre-A after 6 cyclic tests. The SEM micrograph of the activated Pre-C absorbent after 6 cyclic tests in SEM Micrograph (3) indicated that there were about three kinds of grains separated in pre-C, not as uniform as Pre-A. The different SEM micrographs of Pre-A and Pre-C after cyclic tests agreed with the XRD results that the contents of the activated Pre-A and Pre-C were different. In Figure 15, SEM Micrograph (4) shows the SEM of fresh activated 0.2Na-dol, SEM Micrograph (5) shows the SEM of the activated 0.2Na-dol after 1 cyclic test and SEM Micrograph (6) shows the SEM of the activated 0.2Na-dol after 15 cyclic tests. The SEM Micrograph (4), (5), and (6) have indicated that the surface of the fresh activated absorbent was covered by the molten and re-condensed NaNO3, the absorbent grains after the first cyclic carbonation / calcination test became smaller due to particle cracking, and the grains of the absorbent after 15 cyclic carbonation / calcination tests were uniformly distributed on coarse surface. The BET surface area of the fresh activated 0.2Na-dol absorbent was 1.520 m2/g, and increased to 1.655 m2/g after 1 cyclic test, and then increased to 7.946 m2/g after 15 cyclic tests. However, the BET surface area of Pre-A absorbent after activation and after cyclic tests kept around 11.629 m2/g. Therefore, the CO2 absorption activity of 0.2Na-dol increased during the first several cyclic tests, while the absorption ability of Pre-A absorbent remained stable during the tests. The SEM and the BET results also indicated that the grains of


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the 0.2Na-dol absorbent were obviously larger than those of the precipitated absorbent grains.

4 Discussions The above results have indicated that the molten NaNO3 facilitates the decomposition of MgCO3 in the MgCO3-CaCO3 double salts, affects the crystal lattice of activated absorbents and favors the CO2 absorption of MgO-CaCO3 absorbents. For the decomposition mechanism, previous researchers have investigated extensively on the half decomposition of dolomite. Hashimoto et al

24

suggested that CO2 was released

from the surface defects of dolomite, leaving oxygen ions O2-, and then, the neighboring Mg2+ might move towards the oxygen ions while Ca2+ ions migrate to the opposite direction 24. Galai et al

25

clarified the mechanism of the outward growth of

MgO fine particles and the inward growth of CaCO3 during dolomite partial decomposition, and indicated that the rate-limiting step of the growth process was the diffusion of Mg2+ from CaCO3/MgO interface to MgO/CO2. The previous research indicated the importance of ions migration during dolomite decomposition. Therefore, the introducing of molten NaNO3 provided a liquid channel for the diffusion of Mg2+ during the decomposition process of MgCO3-CaCO3 double salts, and reduced their activation temperature. The thermodynamic analysis of Li et al 19 already indicated that MgCO3-CaCO3 started to decompose to MgO-CaCO3 around 380oC in N2 atmosphere. Considering the different activation temperature for the precipitated absorbent and the modified dolomites, the absorbent decomposition might be mainly at the diffusion controlled stage, and the particle grain also affected the ions migration 16 ACS Paragon Plus Environment

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in the absorbent as depicted in Figure 16. Figure 16 indicates that small grain was good for Mg2+ and CO32- diffusion to grain surface, which could facilitate the activation of the absorbent. During the carbonation, it is also inferred that the molten NaNO3 penetrated MgO-CaCO3 grain boundaries and provided a liquid “channel” to accelerate CO2 or CO32- diffusion, and hence NaNO3 improved the CO2 absorption ability of the absorbent 18. The particle diameter distribution analysis, the SEM and BET results have all indicated that the grains of the precipitated absorbent were much smaller than the grains of dolomite, and thus the transfer of Mg2+ to grain surface is easier for the precipitated absorbent than that for dolomite. The activation of the precipitated absorbent could be carried out at 400°C, which was lower than the activation temperature of the modified dolomite around 500°C. The previous research has indicated that the incipient decomposition of NaNO3 in inert atmosphere was around 468°C

26

, and thus NaNO3 decomposed faster during the higher temperature

activation of modified dolomite. Therefore, the amount of NaNO3 introduced into the dolomite is a little more than the NaNO3 amount in the precipitated absorbent. For Pre-C absorbent with no obvious NaNO3, though it could also be activated around 400°C, the activated Pre-C absorbent showed no CO2 absorption activity. Combined with the XRD and SEM results, the molten NaNO3 could be beneficial to the integrating of MgO grains and CaCO3 grains to form MgO (periclase) and CaCO3 (calcite). Without NaNO3, MgO and CaCO3 might lack chemical bond combination,


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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 thus existed in the forms of MgO and aragonite which might be not active in CO2 absorption reaction. For the NaNO3 modified dolomite, as its grains were bigger than those of the precipitated absorbent, the transfer of Mg2+ and CO32- to particle surface became difficult, and hence, the activation of modified dolomite turned to be slower at the end of activation process. During the activation process, some inner dolomite might not decompose as shown in Figure 16. With MgO and MgCO3 molar volume changing during carbonation and calcination, grains cracked into smaller grains as SEM and BET results suggested. Therefore, grains became smaller and their surfaces became more irregular with increasing cyclic test numbers. The transfer of Mg2+ and CO32would become easier accordingly, and thus more dolomite could be activated into MgO-CaCO3 absorbent. This explanation agrees with the increases of weight loss in calcination and the increase of absorption ability in the first several cycles in carbonation (as shown in Figure 11). As the consequences, the CO2 absorption ability derived from the NaNO3 modified dolomite improved during the first several cyclic tests. For the precipitated absorbent, the decomposition of MgCO3 was completed during activation, and there was no more weight loss during the cyclic tests (as shown in Figure 8). The CO2 absorption ability of the precipitated absorbent was stable. 5 Conclusions The effects of NaNO3 on the activity of MgO-CaCO3 absorbent have been investigated via the activation tests of the precipitated MgCO3-CaCO3 double salts and the NaNO3 modified dolomites, and the CO2 absorption tests of the activated 18 ACS Paragon Plus Environment

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absorbents. The investigation has indicated that the effect of NaNO3 is mainly from the following three aspects: i) NaNO3 lowers the decomposition temperature of MgCO3 in MgCO3-CaCO3 double salts; ii) NaNO3 affects the crystal lattice of activated absorbents; and iii) NaNO3 is the key component for facilitating the CO2 absorption

ability

of

MgO-CaCO3

absorbents.

Considering

the

dolomite

decomposition and CO2 absorption mechanisms, it has been inferred that molten NaNO3 improves the absorbent activity and affects the crystal lattice of the activated absorbents by facilitating ion diffusions in absorbent.

Author Information Corresponding Author Fax: +86-10-82543102. E-mail: [email protected].

Acknowledgements The authors greatly thank the financial support from the research project “Sino-US Joint Research on CO2 Capture and Storage” (Project No.: 2013DFB60140-03). This study is also a part of Annex VI to the cooperation protocol in the field of energy technology development and utilization between the Ministry of Science and Technology of the People’s Republic of China and the Department of Energy of the United States of America in the area of advanced coal-based energy systems research, development and simulation. We would like to thank all the partners participated in this project for their supports and advices.


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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

Tables

Table 1 The precipitated MgCO3-CaCO3 samples Sample

Description of the slurry filtering and washing

Pre-A

The slurry was filtered without additional water washing

Pre-B

The slurry was filtered with one time rinsing by deionized water over the filter

Pre-C

The slurry was filtered with full rinsing by deionized water over the filter

Table 2 Dolomites modified with different contents of NaNO3 Sample

Description of the dolomite modified with NaNO3

0.1Na-dol

1 g dolomite was wet-impregnated in 20 ml NaNO3 solution containing 0.1 g NaNO3

0.15Na-dol


0.2Na-dol



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Figures 0.5 o

303 C

0.4

o

345 C 0.3

o

385 C Pre-A

0.2

µ

1 g m . V / A T D

0.1 Pre-B 0.0 Pre-C -0.1 100

150

200

250

Temperature / oC

300

350

400

Figure 1. DTA curves of the precipitated MgCO3-CaCO3 absorbents

500

100

400

90

300

o

95

Temperature / C

temperature Weight percent / %

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

Pre-C 85 200

Pre-B 80

Pre-A

100

75 0

20

40

60

80

100

120

140

t / min

Figure 2. Weight curves of the precipitated MgCO3-CaCO3 absorbents during activation


Energy & Fuels

2500

Mg5( CO3) 4(OH)2 4H 2O .

2000

Pre-A

△

NaNO 3

1500

△

☆

Intensity (A.U.)

1000

Dolmite CaCO 3, aragonite

☆

500

☆

△

☆

☆

☆

☆

☆

☆

△

△

☆

△

△

0 6000

NaNO 3

Pre-A activated

5000 4000

◇

CaCO 3, calcite, syn MgO, periclase, syn

3000 2000 ◇

1000

◇

◇

◇

◇

0 20

30

40

50

60

70

80

90

ο

( )

2θ

Figure 3. XRD results of the Pre-A absorbent before and after activation

2000

Mg5( CO3) 4(OH)2 4H 2O .

Pre-C △

1500

☆

1000

Intensity (A.U.)

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

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△

Dolmite

☆

CaCO3, aragonite

☆ ☆

☆

500

☆

☆ ☆△

☆

☆ ☆

☆

☆

△ ☆

△

△

△ △△

0 2000 ☆

☆

Pre-C activated

△

1500

☆

CaCO3, aragonite

○

MgO, magnisum oxide Dolmite

△

☆

1000

☆

☆

500

☆ △ ☆ ○ ☆ ☆☆

○

△

☆ ☆ ☆

△

☆

△ ☆

0 20

30

40

50

2θ

○

☆

☆☆

60

○

70

80

90

ο

( )

Figure 4. XRD results of the Pre-C absorbent before and after activation


Page 23 of 34

0.6

o

450 C o

0.10Na-dol

303 C

0.5

dolomite 0.4 0.15Na-dol

0.3

µ

1 g m . V / A T D

0.2 0.20Na-dol

0.1 0.0 -0.1 100

200

300

Temperature / oC

400

500

Figure 5. DTA curves of natural and NaNO3 modified dolomites

500

100 dolomite

450

95

350

90

300

o

400

Temperature / C

Weight percent / %

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

0.20Na-dol

85

0.10Na-dol 0.15Na-dol 80

250 200

0

30

60

90

120

150

t / min

Figure 6. Weight curves of natural and modified dolomites at N2 atmosphere


Energy & Fuels

△

6000

Before activated

△

5000

Intensity (A.U.)

△

△

Dolmite

△

3000 2000 1000

NaNO3

△

4000

△

△△

△

△

△

△ △

△

△ △ △△

0 6000

△△ △ △ △

△ △

NaNO3 ◇

3000

◇

2000

◇

1000 0 20

△

After activated

5000 4000

CaCO3, calcite, syn MgO, periclase, syn

◇

◇

30

◇

40

50

2θ

60

70

80

90

ο

( )

Figure 7. XRD results of the 0.20Na-dol absorbent before and after activation

25 400 20 300 15

Temperature/ o C

Weight increase/ decrease ratio/ %

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 24 of 34

10

200

5

100

0 0

100

200

300

400

500

0 600

t / min

Figure 8. Weight increase and decrease curves of the activated Pre-A absorbent during the first 6 carbonation/ calcinations cycles (Carbonation: 100% CO2; Calcination: 100% N2.)


Page 25 of 34

CO2 absorption capacity / g CO2/ g sorbent

0.25

the last 55 min the first 5 min

0.20

0.15

0.10

0.05

0.00 2

4

6

8

10

12

14

16

Cycle number

Figure 9. Cyclic CO2 absorption capacity of the activated Pre-B absorbent


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

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0.12

0.09

0.06

0.03

0.00 1

2

3

4

5

6

7

8

9

Cycle number

Figure 10. Cyclic CO2 absorption capacity of 0.10Na-dol absorbent


Energy & Fuels

0.12

Carbonation in the first 5 min CO2 absorption capacity g CO2/ g sorbent

15th 11th 8th

0.09

5th

0.06

2nd

0.03

0.00 0

10

20

30

40

50

60

t /min

Figure 11. Cyclic CO2 carbonation curves of the 0.15Na-dol absorbent


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 26 of 34


0.12

0.09

0.06

0.03

0.00 2

4

6

8

10

12

14

16

Cycle number

Figure 12. Cyclic CO2 absorption capacity of 0.20Na-dol absorbent


Page 27 of 34

Pre-A

4000 △

3000

△

Dolmite NaNO3 CaCO3, calcite, syn

◇

MgO, periclase, syn

2000 1000

△ △

Intensity (A.U.)

△

0 4000

△△

△

◇△

△△

△ △ ◇ △ ◇ △△ △ △ △ △ △

NaNO3 modified dolomite

△

△

3000 2000

◇

1000 △ △△

△

△ ◇

△ △

0 4000

△ △

Dolmite NaNO3 CaCO3, calcite, syn MgO, periclase, syn ◇

△ ◇△ △

△ △△

△

Pre-C 3000 2000 ☆ ○

1000

☆

0 20

☆

△ ☆

☆ ☆ ○ ☆ ☆☆ ☆ ☆ ☆☆ △☆ △☆

30

40

50

2θ

ο

☆

CaCO3, Aragonite

△

CaCO3, calcite, syn Dolmite

○

MgO, magnisum oxide

☆ ○

60

70

80

90

( )

Figure 13. XRD of the reaction products of precipitated absorbents and NaNO3 modified dolomite absorbent after CO2 absorption 6

Precipitated absorbent NaNO3 modified dolomite

5 Volume percent / %

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

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4 3 2 1 0 0.1

1

10 100 Particle diameter / µm

1000

Figure 14. Particle diameter distributions of the precipitated absorbent and the NaNO3 modified dolomite absorbent 27 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

Figure 15. SEM micrographs of the Pre-A, Pre-C and 0.2Na-dol absorbents ((1) the fresh activated Pre-A; (2) activated Pre-A after 6 cyclic tests; (3) activated Pre-C after 6 cyclic tests; (4) the fresh activated 0.2Na-dol; (5) activated 0.2Na-dol after 1 cyclic test; (6) activated 0.2Na-dol after 15 cyclic tests)

Figure 16. Sketch of the small and big grain after activation


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Figure Captions

Figure1. DTA curves of the precipitated MgCO3-CaCO3 absorbents Figure 2. Weight curves of the precipitated MgCO3-CaCO3 absorbents during activation Figure 3. XRD results of the Pre-A absorbent before and after activation Figure 4. XRD results of the Pre-C absorbent before and after activation Figure 5. DTA curves of natural and NaNO3 modified dolomites Figure 6. Weight curves of natural and modified dolomites at N2 atmosphere Figure 7. XRD results of the 0.20Na-dol absorbent before and after activation Figure 8. Weight increase and decrease curves of the activated Pre-A absorbent during the first 4 carbonation/ calcinations cycles (Carbonation: 100% CO2; Calcination: 100% N2.) Figure 9. Cyclic CO2 absorption capacity of the activated Pre-B absorbent Figure 10. Cyclic CO2 absorption capacity of 0.10Na-dol absorbent Figure 11. Cyclic CO2 carbonation curves of the 0.15Na-dol absorbent Figure 12. Cyclic CO2 absorption capacity of 0.20Na-dol absorbent Figure 13. XRD of the reaction products of precipitated absorbents and modified dolomite absorbent after CO2 absorption Figure 14. Particle diameter distributions of the precipitated absorbent and the NaNO3 modified dolomite absorbent


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 15. SEM micrographs of the Pre-A, Pre-C and 0.2Na-dol absorbents ((1) the fresh activated Pre-A; (2) activated Pre-A after 6 cyclic tests; (3) activated Pre-C after 6 cyclic tests; (4) the fresh activated 0.2Na-dol; (5) activated 0.2Na-dol after 1 cyclic test; (6) activated 0.2Na-dol after 15 cyclic tests) Figure 16. Sketch of the small and big grain after activation


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