Carbonation Characteristics of Dry Sodium-Based Sorbents for CO2

Jul 31, 2012 - *Telephone: +86-25-83793453. Fax: +86-25-83793453. E-mail: .... Low-temperature solid carbon dioxide carriers. Changsui Zhao , Chuanwen...
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Carbonation Characteristics of Dry Sodium-Based Sorbents for CO2 Capture Wei Dong, Xiaoping Chen,* Ye Wu, Chuanwen Zhao, and Cai LiangDaoying Liu Key Laboratory of Energy Thermal Conversion and Control, Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, People’s Republic of China ABSTRACT: Na2CO3 is one potential capture medium with the potential to decrease CO2 emission. Carbonation characteristics of four sodium-based sorbents were studied using thermogravimetric analysis (TGA), X-ray diffraction, scanning electron microscopy, and nitrogen adsorption apparatuses. Results show that, in TGA tests (carbonation atmosphere of 15% CO2 + 15% H2O(g) + 70% N2 and carbonation temperature of 333 K), sorbent Na2CO3 [analytical reagent (AR)] is impervious, while sorbent Na2CO3 calcined from NaHCO3 (AR) reacts, mostly forming Na5H3(CO3)4 instead of NaHCO3, corresponding to 72.87% conversion of Na2CO3 in about 140 min. With regard to the TGA tests of sorbent Na2CO3/Al2O3 and sorbent Na2CO3/ Al2O3 calcined from NaHCO3/Al2O3, only NaHCO3 is generated and the carbonation conversions are both above 85% in about 55 min. The microstructure changes were observed to analyze the reaction mechanisms and carbonation characteristics. It is confirmed that, by loading active components on Al2O3, the microscopic structures can be significantly improved, contributing to the great improvement of carbonation characteristics of the latter two sorbents. On the other hand, there is little improvement of carbonation characteristics using NaHCO3 as precursors for sorbents supported on Al2O3.

1. INTRODUCTION The Intergovernmental Panel on Climate Change (IPCC) fourth assessment report1 pointed out that the global surface temperature has increased by 0.74 °C since 1750, as a result of the massive emissions of greenhouse gases (GHGs). It was predicted that, if no measures are taken, the global surface temperature may rise 1.4−5.8 °C by 2100,2 which may have a profound impact on the global climate.3 Worldwide emission control and reduction of GHGs, especially CO2, have increasingly become a global consensus. Coal-fired power plants are the main CO2 emission sources. Therefore, CO2 removal from the flue gas of coal-fired power plants should be on schedule.4 During the past several years, significant attention has been paid to the technology of using alkali-metal-based dry sorbents for CO2 removal from flue gas. The relevant reaction is as follows: M 2CO3(s) + H 2O(g) + CO2 (g) ⇔ 2MHCO3(s)

TiO2, MgO, SiO2, and Al2O3, and studied the reaction characteristics of those sorbents using a fixed bed, bubbling fluidized bed, and circulating fluidized bed.18−33 In China, Zhao et al. from Southeast University impregnated K2CO3 on Al2O3 and studied its carbonation and regeneration reaction mechanism, multiple calcination−carbonation cycle characteristics, and abrasion resistance properties.34−38 It has been generally noted that potassium carbonate is superior to sodium carbonate in terms of both CO2 capacity and kinetics,18 and thus, most researchers have diverted their interest to the former. Nevertheless, the primary advantage of using sodium carbonate over potassium carbonate is its lower price, easier accessibility, and lower CO2 capture costs. If the reaction characteristics of sodium-based sorbents can be improved to high reactivity, high conversion rate, and short reaction time, this technology will have great market value and broad application prospect. The primary objective of this work is to develop a sodiumbased regenerable sorbent suitable for the CO2 absorption/ regeneration process at low temperatures. First, the weak reactivity of Na2CO3 [analytical reagent (AR)] is studied, and the reason is analyzed. Second, a precursor and two impregnation sorbents are chosen, and their carbonation characteristics and microstructure properties are also investigated. Essential data are given for the future study of dry sodium-based CO2 sorbents.

(1)

where M is alkali metal elements, Na, K, etc. The forward step is used for CO2 removal (carbonate reaction), while the reverse reaction is used for sorbent regeneration and CO2 release (regeneration reaction). Under the grant from the United States Department of Energy (DOE), the Research Triangle Institute, Church and Dwight Co., and Louisiana State University (LSU)5−17 investigated sodium-based dry sorbents for CO2 removal from flue gas and found that the effective carbonate reaction temperature of sodium carbonate is 333−353 K and the regeneration temperature is 393−473 K. The reaction mechanism was studied using the thermogravimetric analyzer and small-scale fixed-bed reactor, as well as 5 calcination− carbonation cycles. In South Korea, Ryu et al. impregnated K2CO3 on porous supports, such as activated carbon (AC), © 2012 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Samples Preparation. Analytical reagents Na2CO3 and NaHCO3 were provided by Shanghai Jiuyi Fine Chemical Co., Ltd. Na2CO3 and NaHCO3 are 99.8 and 99.5% pure, respectively. γ-Al2O3 Received: April 11, 2012 Revised: July 31, 2012 Published: July 31, 2012 6040

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was supplied by the Research Institute of Nanjing Chemical Industry Group, and the average particle size is 0.2 mm. The four sorbents used in this study were named Sorbs 1, 2, 3, and 4, respectively. Sorbs 1 and 2 were Na2CO3 (AR) and Na2CO3 calcined from NaHCO3 (AR), respectively. Sorb 3 was prepared by impregnating Na2CO3 on Al2O3. Sorb 4 was prepared by impregnating NaHCO3 on Al2O3. The preparation processes of Sorbs 3 and 4, were operated as follows: 75 g of support was added to aqueous solution containing 25 g of Na2CO3 and 39.62 g of NaHCO3, respectively, in 550 mL of deionized water. Then, they were mixed with a magnetic stirrer at ambient temperature for 10 h. Then, the two mixtures were dried in an oven at 383 K. The dried samples were calcined in a muffle furnace for 6 h at 473 K. Zhao et al.39 reported that NaHCO3 would decompose to Na2CO3 when the temperature was higher than 383 K; therefore, the active component of Sorb 4 was Na2CO3 after calcining, the same as that of Sorb 3. The amount of Na 2 CO 3 impregnated was determined by X-ray fluorescence (XRF). The properties of these sorbents are summarized in Table 1.

Figure 1. TGA test result of Sorb 1.

and ϕ is the Na2CO3 loading amount of sorbents. ϕ for Sorbs 1 and 2 is set as 100%. To predict and compare the carbonation conversions of different sorbents conveniently, dimensionless mass was used throughout this paper. It is defined as the ratio of the mass of the sorbents during the test to that of the initial sorbents.

Table 1. Sorbent Properties property

Sorb 1

Sorb 2

Sorb 3

Sorb 4

sorbent precursor pretreatment

Na2CO3 none

sorbent composition

Na2CO3

NaHCO3 calcined at 473 K Na2CO3

Na2CO3 calcined at 473 K Na2CO3/ Al2O3 10.85

NaHCO3 calcined at 473 K Na2CO3/ Al2O3 10.85

9.88

9.63

designed loading amount of Na (wt %) actual loading amount of Na (wt %)

dimensionless mass=

Figure 2. TGA test result of Sorb 2.

3. RESULTS AND DISCUSSION 3.1. TGA Test for Sorb 1. Result of a typical TGA test for Sorb 1 is shown in Figure 1. The carbonate reaction began at about 115 min. On the basis of the conversion of Na2CO3 to NaHCO3, the carbonation percent conversion η is calculated from [w(t ) − w(0)]M Na 2CO3 ϕw(0)[2M NaHCO3 − M Na 2CO3]

(3)

During the carbonate reaction of Sorb 1, the increment of dimensionless mass was just 0.005 27 within 60 min. The total carbonation percent conversion of Sorb 1 was only 0.913%. It indicated that analytical reagent Na2CO3 is hardly converted to NaHCO3. The result is quite inconsistent with the chemical reaction 1 listed above. In considering that the carbonate reaction belongs to the gas−solid reaction, the tests of particle morphology and microscopic structure were examined below to analyze the result. 3.2. TGA Test for Sorb 2. Liang et al.6 reported that the carbonation characteristics of Na2CO3 calcined from NaHCO3 were better than those of other sorbents, including analytical reagents Na2CO3 and Na2CO3·H2O. The TGA test of the analytical reagent NaHCO3 was processed, and the result is shown in Figure 2.

2.2. Apparatus and Procedure. Experimental studies were performed with a TherMax 500 high-pressure thermogravimetric apparatus in a simulated gas containing 15 vol % CO2 and 15 vol % H2O with a balance of 70 vol % N2. CO2 and N2 were fed from highpurity cylinders with mass flow controllers used to control gas flow, respectively. H2O was fed using a high-precision, high-pressure piston pump and was heated to ensure complete vaporization before mixing with other gases. The flow rate of the simulated gas was 500 mL/ min.34,36,41 The amount of sorbents used in each thermogravimetric analysis (TGA) test was about 100 mg. Because of the hydration of sorbents, TGA tests began with the increase of the temperature from ambient temperature to 423 K for the dehydration of sorbents. After dehydration was completed, the temperature was decreased to 333 K and the carbonate reaction was carried out. The structure of sorbents before and after the carbonate reaction was examined with an X-ray diffraction (XRD) system (Rigaku, model D/max-2500/PC). The microscopic shape of sorbents was observed with Hitachi s-4800 scanning electron microscopy (SEM). A micropore physisorption analyzer model ASAP 2020 with nitrogen adsorption−desorption was used for surface area and pore structure determination.

η=

w(t ) w(0)

The test began with the decomposition of NaHCO3. The furnace was heated from ambient temperature to 423 K in a N2 environment, and the decomposition was completed in 30 min. The final dimensionless mass of 0.634 was close to the theoretical value of 0.631, corresponding to the complete conversion of NaHCO3 to Na2CO3. The furnace was then cooled to the carbonation temperature, and the gas composition was changed to 15% CO2 and 15% H2O with a balance of N2 at a flow of 500 mL/min. During the carbonate reaction, the dimensionless mass increased from 0.634 to 0.819 in approximately 140 min. The XRD patterns of Sorb 2 before and after the carbonate reaction are shown in Figure 3. The XRD result before the

× 100% (2)

where t is the reaction time, w(t) is the mass of sorbents at time t, w(0) is the initial mass of sorbents, MNa2CO3 and MNaHCO3 are the molecular weights of Na2CO3 and NaHCO3, respectively, 6041

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Figure 4. TGA test result of Sorb 3. Figure 3. XRD patterns of Sorb 2 before/after the carbonate reaction: (1) before the reaction with CO2 and H2O at 333 K and (2) after the reaction with CO2 and H2O at 333 K. (▼) Na5H3(CO3)4, (○) NaHCO3, and (☆) Na2CO3.

carbonate reaction shows only one phase assigned to Na2CO3 in XRD pattern 1 of Figure 3. This result is in agreement with the TGA test result (Figure 2), in which NaHCO3 was entirely decomposed to Na2CO3. After the carbonate reaction, the XRD pattern contains three phases: Na2CO3, NaHCO3, and Na5H3(CO3)4. Na5H3(CO3)4 is known as Wegscheider’s salt or Wegscheiderite. Liang et al.6 claimed that the carbonate reaction product of Na2CO3 was favored to Wegscheider’s salt at reaction temperatures of 343 K and above, while NaHCO3 was favored at temperatures below 343 K. The relevant reaction is as follows: 5Na 2CO3 + 3H 2O + 3CO2 ⇔ 2Na5H3(CO3)4

Figure 5. XRD patterns of Sorb 3 before/after the carbonate reaction: (1) before the reaction with CO2 and H2O at 333 K and (2) after the reaction with CO2 and H2O at 333 K. (○) NaHCO3, (☆) Na2CO3, and (■) Al2O3.

(4)

phases. After CO2 absorption at 333 K, the XRD pattern shows three phases, including NaHCO3, Na2CO3, and Al2O3. This indicates that the carbonate reaction product of Sorb 3 is merely NaHCO3, and Na5H3(CO3)4 is not generated. The carbonate reaction time is greatly shortened to 55 min. The carbonation percent conversion is 85.47% according to eq 2. Especially, the carbonation percent conversion reaches 73% within the first 30 min. 3.4. TGA Test for Sorb 4. According to the report by Liang et al.6 and the result of the TGA test for Sorb 2, the carbonation characteristics can be improved using NaHCO3 as a precursor. Therefore, the TGA test for Sorb 4 was performed, and the result is shown in Figure 6. The whole carbonate reaction lasts about 56 min, and the dimensionless mass increases from 0.872 to 0.970.

6

Distinct from the claim by Liang et al., in our test, at the reaction temperature around 333 K, Na5H3(CO3)4 was still generated. After analysis by the XRD Rietveld refinement method,42 it was figured out that the quality ratio of NaHCO3, Na5H3(CO3)4, and residual Na2CO3 in the product was about 16.5, 62.5, and 21%, respectively. Therefore, the dimensionless mass of Na5H3(CO3)4 in the product was 0.512, and the dimensionless mass of Na2CO3 converted to Na5H3(CO3)4, according to reaction 4, was 0.378. Calculated in the same way, the dimensionless mass of Na2CO3 converted to NaHCO3 was 0.0853. Therefore, the total dimensionless mass of Na2CO3 participating in reactions was 0.464, and the carbonation percent conversion of Sorb 2 was 73.2%. Despite this, the amount of NaHCO3 generated was less than a quarter of that of Na5H3(CO3)4. On the other hand, the reaction lasts 140 min, which is still too long. Zhao et al.40 reported that the carbonation percent conversion of K2CO3 calcined from KHCO3 reached 83% in 25 min and the product generated was just KHCO3. The carbonation characteristics of Sorb 2 were worse than those of K2CO3 calcined from KHCO3, which was in agreement with previously reported results.12 3.3. TGA Test for Sorb 3. Activated alumina is a kind of porous matrix with high surface area. Zhao et al.34−38 reported that carbonation characteristics of their potassium-based sorbents were improved significantly by introducing activated Al2O3 as supports. Therefore, activated Al2O3 was also introduced into our sodium-based sorbents, and the result of the TGA test for Sorb 3 is shown in Figure 4. The whole carbonate reaction lasts about 55 min, and the dimensionless mass increases from 0.817 to 0.910. The structural change of sorbents before and after the carbonate reaction was examined by XRD, and the results are shown in Figure 5. The XRD result before the carbonate reaction shows two phases, such as the Na2CO3 and Al2O3

Figure 6. TGA test result of Sorb 4.

XRD patterns of Sorb 4 before and after the carbonate reaction are shown in Figure 7. The results are almost the same as those of Sorb 3. The carbonate reaction product of Sorb 4 is NaHCO3, and Na5H3(CO3)4 is not detected. Therefore, the carbonation percent conversion is 86.59% according to eq 2. The carbonation characteristics of Sorbs 3 and 4 are exactly 6042

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Figure 7. XRD patterns of Sorb 4 before/after the carbonate reaction: (1) before the reaction with CO2 and H2O at 333 K and (2) after the reaction with CO2 and H2O at 333 K. (○) NaHCO3, (☆) Na2CO3, and (■) Al2O3.

similar to each other. It indicates that the use of NaHCO3 as a precursor does not give any obvious improvement to the carbonation characteristics of Sorb 4. Lee et al.27 showed that the carbonation conversion of their sodium-based sorbents reaching 80% needs about 50−80 min. However, the detailed compositions were not given. Zhao et al.34 reported that the carbonation percent conversion of their potassium-based sorbents reaches about 95% in 24 min. The carbonation characteristics of Sorbs 3 and 4 are somewhat superior to the sorbents from Lee et al. Despite this, in comparison to the potassium-based sorbents from Zhao et al., our sodium-based sorbents still have obvious disadvantages, and there is a great potential for the improvement of the carbonation characteristics. 3.5. Particle Morphologies of the Sodium-Based Sorbents. As mentioned above, the carbonation behaviors of four sorbents are almost different. To explore the reasons lying behind the experimental results, the particle morphologies of four sorbents were examined via SEM, the examinations of which were made at magnifications of 2000× and 10 000×. The results are shown in Figure 8. The differences in particle morphologies of those sorbents are clearly visible. On the surface of Sorb 1, very few holes are observed, and its distribution is sparse and non-uniform. Many small granules are accumulated on the surface of Sorb 2, and numerous small round holes are observed. There are no small granules on the surface of Sorbs 3 and 4. Instead, the surface structure is loose and porous and has a well-developed network structure. 3.6. Analysis of Microscopic Structures. Nitrogen adsorption tests were performed to obtain adsorption and desorption isotherms of four sorbents and activated Al2O3. The results are shown in Figure 9. The initial part of adsorption isotherms of Sorbs 1 and 2 is nearly horizontal, which means that the micropores are soon filled up and almost no more adsorption takes place for the two sorbents. Then, the two adsorption isotherms rise rapidly when P/P0 is closed to 1, because of the generation of adsorbate agglomeration and multilayer adsorption. The desorption isotherms of Sorbs 1 and 2 belong to type H3 loops. The typical pore shapes are slit-shaped pores.43 The adsorption isotherms of Sorbs 3 and 4 and Al2O3 agree with the isotherms of type IV. Multilayer adsorption takes place on the surface, having a pore diameter of more than 5 nm.43 The desorption isotherms of Sorbs 3 and 4 and Al2O3 belong to type A loops. The typical pore shapes are mainly open-ended cylindrical pores lying in a narrow range of the radius.43

Figure 8. SEM images of different sorbents: (a) Sorb 1, (b) Sorb 2, (c) Sorb 3, and (d) Sorb 4.

The specific surface areas of four sorbents and activated Al2O3 were calculated according to the Brunauer−Emmett− Teller (BET) method. The specific pore volumes and pore size distributions were calculated using the Barrett−Joyner− Halenda (BJH) method. The results are shown in Figure 10. The surface area and pore volume for Sorb 1 are 1.12 m2/g and 6.05 mm3/g, respectively. The microscopic structure is extremely unfavorable to the occurrence of the carbonate reaction, which matches the result shown in Figure 1. After calcination of NaHCO3 in 423 K, the surface area and pore volume increase to 3.57 m2/g and 17.0 mm3/g for Sorb 2, respectively. The increase in the porosity of Sorb 2 is attributed to the generation and release of CO2 during the calcination process and, thus, leads to an increase in active sites. The possibility for effective gas−solid contact is provided. As a result, the carbonation percent conversion improves to 72.87%. However, the surface area and pore volume are still limited, as shown in Figures 8 and 10, leading to the insufficient spread and infiltrating of the reaction atmosphere. Therefore, a large number of byproducts of Na5H3(CO3)4 are generated, and the effective use of the active component of Na2CO3 is reduced. Sorbs 3 and 4 are both sodium-based sorbents supported on Al2O3. The surface areas are 89.7 and 93.4 m2/g, respectively, and the pore volumes are 241.22 and 252.00 mm3/g, 6043

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4. CONCLUSION In this paper, carbonation characteristics of four sodium-based CO2 sorbents were analyzed in TGA tests. For Sorb 1, virtually no carbonate reaction occurs because of its poor microscopic structure. Using NaHCO3 as a precursor, the microscopic structure as well as the carbonation characteristics of Sorb 2 receive limited improvement. However, the experiments reported in the paper confirmed that a majority of the products generated are Na5H3(CO3)4, instead of NaHCO3. In addition, the carbonate reaction lasts too long. Using activated alumina as porous matrixes, a significant improvement of microscopic structures of Sorbs 3 and 4 was observed, and thus, the carbonation characteristics improve remarkably. The products generated are merely NaHCO3. This means that the active component works more effectively when used with an appropriate inorganic matrix. On the other hand, there is no significant difference in microscopic structures as well as carbonation characteristics of Sorbs 3 and 4, which indicate that the improvement using NaHCO3 as precursors for sodiumbased sorbents supported on Al2O3 is not obvious enough. As alkali-metal-based sorbents, the carbonation percent conversion of sodium-based sorbents is not yet as high as that of potassium-based sorbents and the reaction time of sodium-based sorbents is unsatisfactory. Despite this, sodium carbonate is readily available and low in price. We believe that further improvement of the carbonation characteristics of Na2CO3/Al2O3 can be achieved by improving the microscopic structure or adding a certain catalyst. This suggests that the use of dry sodium-based sorbents has considerable potential for large-scale CO2 capture processes in the future.

Figure 9. N2 adsorption−desorption isotherm plots of four sorbents and Al2O3: (a) Sorb 1, (b) Sorb 2, (c) Sorb 3, (d) Sorb 4, and (e) Al2O3.

respectively. The values are about 81 and 41 times more than those of Sorb 1, respectively, as shown in Figure 10. Therefore, loading active components on porous supports can significantly improve the microscopic structures of sorbents, so that ample room for the effective and sufficient gas−solid contact is provided, as well as the active sites. The two carbonation percent conversions both rise above 85% without any byproduct generated. The carbonate reaction rates of Sorbs 3 and 4 are also increased. Moreover, the decrease of the BET surface areas and pore volumes of Sorbs 3 and 4 compared to those of Al2O3 suggests that Na2CO3 is located not only at the surface but also on the pores of Al2O3. On the other hand, for Sorbs 3 and 4, there is no significant difference in the microscopic structures and carbonation characteristics. It suggests that, for sodium-based sorbents supported on Al2O3, the improvement of microscopic structures and carbonation behaviors dominantly depends upon the supports and the improvement using NaHCO3 as a precursor is weak.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-25-83793453. Fax: +86-25-83793453. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National High Technology Research and Development Program of China (2009AA05Z311), the National Natural Science Foundation of China (50876021), and the Special Funds for Major State Basic Research Projects of China (2011CB707301) supported by the Foundation of State Key Laboratory of Coal Combustion (FSKLCC1006) is sincerely acknowledged.

Figure 10. Microscopic structures of sorbents and Al2O3: (a) Sorb 1, (b) Sorb 2, (c) Sorb 3, (d) Sorb 4, and (e) Al2O3. 6044

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NOTE ADDED AFTER ASAP PUBLICATION The version of this paper published August 8, 2012 had an incomplete author list. The correct version published August 14, 2012.

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