1766
Energy & Fuels 2009, 23, 1766–1769
Carbonation and Hydration Characteristics of Dry Potassium-Based Sorbents for CO2 Capture Chuanwen Zhao,† Xiaoping Chen,*,† Changsui Zhao,† and Yakun Liu‡ Thermoenergy Engineering Research Institute, Southeast UniVersity, Nanjing 210096, China, and College of EnVironmental and Chemical Engineering, Shanghai UniVersity, Shanghai 200072, China ReceiVed October 14, 2008. ReVised Manuscript ReceiVed December 26, 2008
Thermogravimetric apparatus (TGA) and X-ray diffraction (XRD) have been used to study the characteristics of potassium-based sorbents for CO2 capture. The carbonation reactivity of K2CO3 · 1.5H2O and K2CO3 dehydrated from K2CO3 · 1.5H2O was weak. However, K2CO3 calcined from KHCO3 showed excellent carbonation capacity and no deactivation of sorbents during multiple cycles. The XRD results showed that the sample dehydrated from K2CO3 · 1.5H2O was K2CO3 with structure of monoclinic crystal (PC#1). The carbonation products of PC#1 included K2CO3 · 1.5H2O and KHCO3, and K2CO3 · 1.5H2O was the main product. Correspondingly, K2CO3 with structure of hexagonal crystal (PC#2) was the product calcined from KHCO3, and the main carbonation product of PC#2 was KHCO3. The byproduct of K4H2(CO3)3 · 1.5H2O for PC#2 would affect the carbonation processes. Hydration tests confirmed the two hypotheses: the hydration reaction will first occur for K2CO3 with structure of monoclinic crystal, and the carbonation reaction will first occur for K2CO3 with structure of hexagonal crystal. The reaction principles were analyzed by product and the relevant reactions. This investigation can be used as basic data for dry potassium-based sorbents capturing CO2 from flue gas.
1. Introduction Global warming is emerging as the important environmental issue of the 21st century. Researchers estimate that if uncontrolled greenhouse gas emissions continue, from 1990 to 2100, the average global temperature will increase by 1.4 to 5.8 °C, the sea level will rise by 0.09 to 0.88 m,1 and droughts, expanding deserts, heat waves, and ecosystem disruption will appear. CO2 is the principal greenhouse gas of interest, due to its large current greenhouse forcing, and its long persistence in the atmosphere. Because fossil-fuel fired power plants are the largest stationary sources of CO2 emissions, capturing CO2 from fossil-fuel fired power plants is of critical importance. The various CO2 capture options include precombustion decarbonization,2 O2 combustion with CO2 recycle,3 chemical looping combustion,4 and postcombustion capture.5 Every option above holds interest by investigators around the world. However, a process that is inexpensive, of low energy demand, and has high activity with CO2 is still under development. Dry alkali metal-based sorbents for capturing CO2 from flue gas is considered to be one of the more promising technologies. * To whom correspondence should be addressed. Telephone/fax: +86 25 83793453. E-mail:
[email protected]. † Institute for Thermal Power Engineering of Southeast University. ‡ Shanghai University. (1) Berger, A. The effect of greenhouse gases on climate. Proceedings of the Conference on Future Energy Systems and Technology for Abatement; Antwerp, Belgium, 2002. (2) Freund, P.; Haines, M. R. Precombustion decarbonisation for power generation. Proceedings of the 6th International Conference on Greenhouse Gas Control Technologies; Kyoto, Japan, 2002. (3) Jordal, K.; Anheden, M.; Yan, J.; Stro¨mberg, L. Oxyfuel combustion for coal-fired power generation with CO2 capture-Opportunities and challenges. Proceedings of the 7th International Conference on Greenhouse Gas Control Technologies; Vancouver, Canada, 2004. (4) Hossain, M. M.; Lasa, H. I. Chem. Eng. Sci. 2008, 63, 4433–4451. (5) Oexmann, J.; Hensel, C.; Kather, A. Int. J. Greenhouse Gas Control 2008, 2, 539–552.
Recently, this technology has been investigated as an innovative concept for CO2 capture.6-20 Liang11 reported preliminary results on a sodium-based sorbent process. Lee12 studied the capacity and regeneration property of several potassium-based sorbents in a fixed-bed reactor using multicycle test. Seo13,14 presented some results from a bubbling fluidized-bed reactor. Yi15 focused on the performance of solid sorbent in a fast fluidized-bed reactor. Yi16 investigated CO2 reaction characteristics of dry sorbent in the continuous solid circulating mode between a fast (6) Hoffman, J. S.; Pennline, H. W. J. Energy EnViron. Res. 2001, 1, 90–100. (7) Lee, S. C.; Choi, B. Y.; Lee, T. J.; Ryu, C. K.; Ahn, Y. S.; Kim, J. C. Korean J. Chem. Eng. 2006, 23, 374–379. (8) Sharonov, V. E.; Okunev, A. G.; Aristov, Y. I. React. Kinet. Catal. Lett. 2004, 82, 363–369. (9) Lee, S. C.; Chae, H. J.; Lee, S. J.; Choi, B. Y.; Yi, C. K.; Lee, J. B.; Ryu, C. K.; Kim, J. C. EnViron. Sci. Technol. 2008, 42, 2736–3741. (10) Lee, J. B.; Ryu, C. K.; Baek, J.; Lee, J. H.; Eom, T. H.; Kim, S. H. Ind. Eng. Chem. Res. 2008, 47, 4465–4472. (11) Liang, Y.; Harrison, D. P.; Gupta, R. P.; Green, D. A.; McMichael, W. J. Energy Fuels 2004, 18, 569–575. (12) Lee, S. C.; Choi, B. Y.; Lee, T. J.; Ryu, C. K.; Ahn, Y. S.; Kim, J. C. Catal. Today 2006, 111, 385–390. (13) Seo, Y. W.; Jo, S. H.; Ryu, C. K.; Yi, C. K. Korean Chem. Eng. Res. 2005, 43, 537–541. (14) Seo, Y. W.; Jo, S. H.; Ryu, C. K.; Yi, C. K. Chemosphere 2007, 69, 712–718. (15) Yi, C. K.; Jo, S. H.; Seo, Y.; Park, S. D.; Moon, K. H.; Yoo, J. S.; Lee, J. B.; Ryu, C. K. Stud. Surf. Sci. Catal. 2006, 159, 501–504. (16) Yi, C. K.; Jo, S. H.; Seo, Y. W.; Lee, J. B.; Ryu, C. K. Int. J. Greenhouse Gas Control 2007, 1, 31–36. (17) Liang, Y. Carbon Dioxide Capture from Flue Gas Using Regenerable Sodium-Based Sorbents; Louisiana State University, Baton Rouge, LA, 2003. (18) Lee, S. C.; Kim, J. C. Catal. SurV. Asia 2007, 11, 171–185. (19) Okunev, A. G.; Sharonov, V. E.; Gubar, A. V.; Danilova, I. G.; Paukshtis, E. A.; Moroz, E. M.; Kriger, T. A.; Malakhov, V. V.; Aistov, Y. I. Russ. Chem. Bull. Int. Ed. 2003, 52, 359–363. (20) Zhao, C. W.; Chen, X. P.; Zhao, C. S. J. Chem. Ind. Eng. 2008, 59, 2328–2333.
10.1021/ef800889m CCC: $40.75 2009 American Chemical Society Published on Web 02/06/2009
Dry Potassium-Based Sorbents for CO2 Capture
Energy & Fuels, Vol. 23, 2009 1767
Figure 1. First TGA test result of K2CO3 · 1.5H2O.
Figure 2. Second TGA test result of K2CO3 · 1.5H2O.
fluidized-bed carbonator and a bubbling fluidized-bed regenerator. However, all of the investigations met the same problems. The global carbonation reaction rate was rather slow, when CO2 reacted with Na2CO3 or K2CO3. To solve this problem, this Article focuses on the carbonation characteristics of K2CO3 samples, analyzes the carbonation reaction principles, and determines that K2CO3 with structure of hexagonal crystal showed excellent carbonation capacity. The carbonation conversion reached 70% in 10 min. This investigation can be used as basic data for dry potassium-based sorbents capturing CO2 from flue gas.
Figure 3. TGA test result of KHCO3.
2. Experimental Section Samples. Analytical reagents K2CO3 · 1.5H2O/KHCO3 were provided by Shanghai Jiuyi fine Chemical Co., Ltd. K2CO3 · 1.5H2O and KHCO3 were 99% and 99.5% pure, respectively. Both samples were prepared with the same average particle sizes of 300 µm. Apparatus and Procedure. Carbonation reactions of these potassium-based sorbents were studied with the TherMax 500 high pressure TGA. CO2 and N2 were obtained from high-purity cylinders with mass flow controllers used to control flow. H2O was fed using a high-precision, high-pressure piston pump and was heated to ensure complete vaporization before mixing with other gases. The calcination tests of K2CO3 · 1.5H2O and KHCO3 samples were in the atmosphere of pure N2 at 200 mL min-1. The reactor was heated at 20 K min-1 from 293 K to the final temperature. The carbonation tests of K2CO3 samples were processed in the gas composition of CO2, H2O, and N2 at a flow of 500 mL min-1 and at a temperature of 333 K. The structural change of sorbents before and after the reactions was examined with a D/max2500 VL/PC X-ray diffractometer.
3. Experimental Results TGA Tests for Analytical Reagent K2CO3 · 1.5H2O. The XRD results show that K2CO3 · 1.5H2O (22257-ICSD) is the main composition of the samples. As Seo13 mentioned, the important reaction involved in the capture of CO2 using potassium-based sorbents was: K2CO3 · 1.5H2O + CO2 f 2KHCO3 + 0.5H2O
(1)
To confirm this conclusion, the TGA test was performed in the gas composition of 15 mol % CO2 with a balance of N2 at a flow of 500 mL min-1 and at a temperature of 333 K. The results are shown in Figure 1. It can be seen from Figure 1 that the dimensionless weight maintained constant at 0.97 when the gas composition was CO2 and N2. The gas composition was then changed to 15 mol % CO2, 15 mol % H2O with a balance of N2. The dimensionless weight increased from 0.97 to 1.01 in 22 min. This value is only equal to 20% of the theoretical value increment of 0.20 corresponding to the complete conversion of K2CO3 · 1.5H2O
to KHCO3. It is clear that reaction 1 was impossible in this carbonation condition and confirmed that the carbonation reactivity of K2CO3 · 1.5H2O was weak. As carbonation reactivity of K2CO3 · 1.5H2O was weak, K2CO3 dehydrated from K2CO3 · 1.5H2O was considered to be an excellent sorbent for CO2 capture. So another TGA test beginning with the dehydration of K2CO3 · 1.5H2O was processed. The results are shown in Figure 2. K2CO3 · 1.5H2O was heated from ambient to 573 K in an N2 gas environment. Dehydration of K2CO3 · 1.5H2O was complete in 7 min. The final dimensionless weight of 0.88 was near the theoretical value of 0.84 corresponding to the complete conversion of K2CO3 · 1.5H2O to K2CO3. The system was then cooled to the carbonation temperature, and the gas composition was changed to 15 mol % CO2, 15 mol % H2O with a balance of N2 at a flow of 500 mL min-1. The dimensionless weight increased from 0.88 to 1.03 in 78 min. This value is equal to 44% of the theoretical value increment of 0.34 corresponding to the complete conversion of K2CO3 to KHCO3. These results prove that the carbonation reactivity of K2CO3 dehydrated from K2CO3 · 1.5H2O was also weak. TGA Tests for Analytical Reagent KHCO3. Liang17 reported that the carbonation of Na2CO3 calcinated from NaHCO3 was better than that of other sorbents including analytical reagent Na2CO3 and Na2CO3 · H2O. KHCO3 may have similar character. The TGA test of the analytical reagent KHCO3 sample was processed, and the results are shown in Figure 3. KHCO3 was heated from ambient to 473 K in an N2 gas environment. Calcination of KHCO3 was completed in 10 min. The final dimensionless weight of 0.65 is near the theoretical value of 0.69, corresponding to the complete conversion of KHCO3 to K2CO3. The system was then cooled to the carbonation temperature, and the gas composition was changed to 15 mol % CO2, 15 mol % H2O with a balance of N2 at a flow of 500 mL min-1. The dimensionless weight increased from 0.65 to 0.89 in 25 min. This value corresponded to 83% conversion of K2CO3 to KHCO3. It can be seen the carbonation conversion reached 70% in 10 min. Liang11 reported that the carbonation conversion rate of Na2CO3 calcinated from NaHCO3 reached 65% in 100 min. Lee9,18 and Lee10 showed that the
1768 Energy & Fuels, Vol. 23, 2009
Figure 4. Multicycle carbonation/calcination testing result.
Zhao et al.
Figure 6. Hydration and carbonation for K2CO3 with structure of monoclinic crystal.
Figure 7. Hydration and carbonation for K2CO3 with structure of hexagonal crystal.
Figure 5. The XRD patterns of K2CO3 before/after carbonation reactions: (a) dehydrated from K2CO3 · 1.5H2O; (b) calcined from KHCO3; (I) fresh; (II) after reaction with CO2 and H2O at 333 K; (×, K2CO3 with structure of monoclinic crystal; b, K2CO3 · 1.5H2O; 3, KHCO3; 9, K2CO3 with structure of hexagonal crystal; 2, K4H2(CO3)3 · 1.5H2O).
carbonation conversion rate of their potassium-based sorbents or sodium-based sorbents reaching 65% needs 50-100 min. The carbonation character of K2CO3 calcinated from KHCO3 was better than those sorbents. To find whether the activity and the capacity of K2CO3 calcinated from KHCO3 would decrease when subjected to repeat calcinations and carbonation cycles, a five-cycle test was processed, and the results are shown in Figure 4. It is shown that there is no change in the activity or reactivity of this material. Another five-cycle test was processed, and the same result was found. Comparison of Carbonation of Different K2CO3 Samples by XRD. As was mentioned previously, as compared to K2CO3 dehydrated from K2CO3 · 1.5H2O, K2CO3 calcined from KHCO3 showed high carbonation reaction activity and no deactivation during multiple cycles. To investigate these properties, the structural change of sorbents before and after carbonation reaction was examined by XRD. The XRD patterns of K2CO3 dehydrated from K2CO3 · 1.5H2O and K2CO3 calcined from KHCO3 before and after carbonation reactions are shown in Figure 5. The XRD results of fresh sorbent, which was obtained from dehydration of K2CO3 · 1.5H2O at 573 K under N2, show only one phase in Figure 5a: I. The diffraction maxima appear with 2θ of 26.21, 30.04, 31.65, 32.0, 32.09, 34.13, 37.68, 38.12,
38.94, 41.11, and 42.84, which are assigned to the K2CO3 with structure of monoclinic crystal phase (66943-ICSD). After carbonation reaction at 333 K, the XRD pattern of this sorbent shows two phases including K2CO3 · 1.5H2O (22257ICSD) and KHCO3 (81619-ICSD) in Figure 5a: II. The intensity of the K2CO3 · 1.5H2O peak is stronger than that of KHCO3. It is deduced that K2CO3 with structure of monoclinic crystal could first react with H2O in the gas of CO2, H2O, and balanced N2 to produce K2CO3 · 1.5H2O. The carbonation reaction was prevented. In this way, it could explain the reason why the increment was far from the theoretical increment corresponding to the complete conversion of K2CO3 to KHCO3, which was shown in Figure 2. Figure 5b: I shows the XRD patterns of K2CO3 calcined from KHCO3 before carbonation reactions. Unlike Figure 5a: I, the diffraction maximums appear with 2θ of 25.06, 31.08, 31.59, 33.56, 38.84, and 44.91, which are assigned to the K2CO3 with structure of hexagonal crystal (52535-ICSD). As compared to Figure 5a: II, b: II shows that the intensity of KHCO3 peak was the strongest peak, and the intensity of K2CO3 · 1.5H2O was quite weak. The new diffraction maxima appear with 2θ of 23.15, 27.43, 30.76, 30.61, and 36.73, which are assigned to the K4H2(CO3)3 · 1.5H2O phase (401721-ICSD). It is deduced that K2CO3 with structure of hexagonal crystal could first produce KHCO3. The hydration reaction was prevented. The new production of K4H2(CO3)3 · 1.5H2O will affect the carbonation processes. Hydration Tests for Different K2CO3 Samples. To confirm the effect of hydration for different K2CO3 samples, TGA tests of hydration and carbonation for K2CO3 samples were processed. The results are shown in Figures 6 and 7. As shown in Figure 6, K2CO3 · 1.5H2O was first converted into K2CO3 while dehydrated in an N2 gas environment at 573 K. The temperature was then dropped to 333 K, and the gas composition was changed to 85 mol % N2 and 15 mol % steam. The dimensionless weight increased from 0.83 to 0.98 in 82 min. This value corresponded to the 92% conversion of K2CO3 to K2CO3 · 1.5H2O. The gas composition was then changed to
Dry Potassium-Based Sorbents for CO2 Capture
Energy & Fuels, Vol. 23, 2009 1769
15 mol % CO2, 15 mol % H2O with a balance of N2. The dimensionless weight increased from 0.98 to 1.01 in 100 min. This value is only equal to 16% of the theoretical value increment of 0.19 corresponding to the complete conversion of K2CO3 · 1.5H2O to KHCO3. However, this value corresponded to complete conversion of the residual K2CO3 to KHCO3. As Figure 7 shows, KHCO3 was calcined at 473 K in an N2 gas environment. The temperature was then decreased to 333 K and exposed to the gas composed of 85 mol % N2 and 15 mol % H2O. The dimensionless weight of the sample did not change in 17 min, and then the dimensionless weight increased from 0.70 to 0.77 in 88 min. This value corresponded to 51% conversion of K2CO3 to K2CO3 · 1.5H2O. The gas composition was then changed to 15 mol % CO2, 15 mol % H2O with a balance of N2. The dimensionless weight increased from 0.77 to 0.91 in 50 min. This value corresponded to 89% conversion of the residual K2CO3 to KHCO3. It is confirmed that the hydration reaction will first occur for K2CO3 with structure of monoclinic crystal, and the carbonation reaction will first occur for K2CO3 with structure of hexagonal crystal. Reaction Principles of Different K2CO3 Samples Analyzed. On the basis of the primary product of KHCO3, and the byproduct of K2CO3 · 1.5H2O and K4H2(CO3)3 · 1.5H2O, the carbonation of K2CO3 was analyzed, and the relevant reactions are as follows: 3 K2CO3(s) + H2O(g) T K2CO3 · 1.5H2O(s) 2
(2)
K2CO3(s) + CO2(g) + H2O(g) T 2KHCO3(s)
(3)
5 2K2CO3(s) + CO2(g) + H2O(g) T K4H2(CO3)3 · 1.5H2O(s) 2 (4) K4H2(CO3)3 · 1.5H2O(s) + CO2(g) T 4KHCO3(s) + 0.5H2O(g) (5) For the carbonation of K2CO3 with structure of monoclinic crystal (PC#1), only reactions 2 and 3 appeared, because only K2CO3 · 1.5H2O and KHCO3 were produced. As was mentioned previously, K2CO3 · 1.5H2O is the main product for the above
process, so it can be deduced that the reaction capability of reaction 2 is better than that of reaction 3 for PC#1. For the carbonation of K2CO3 with structure of hexagonal crystal (PC#2), reactions 2-5 may all exist. Reaction 2 did not play an important role in this system, because KHCO3 was the main product. There are two probable processes. For the one, reaction 3 would take the leading role, while the other reactions are insignificant. For the other, reactions 4 and 5 would be the main reactions. It should be analyzed later with the method of chemical kinetics to determine which probability was correct. 4. Conclusions The characteristics of potassium-based sorbents for CO2 capture were investigated with TGA and XRD. In carbonation condition, the carbonation reactivity of K2CO3 · 1.5H2O and K2CO3 dehydrated from K2CO3 · 1.5H2O was weak, but K2CO3 calcined from KHCO3 showed excellent carbonation capacity and reproducibility. The conversion of K2CO3 to KHCO3 reached 82% in 25 min, and there was no deactivation of sorbents during five cycles. The XRD results showed that the sorbent dehydrated from K2CO3 · 1.5H2O was K2CO3 with structure of monoclinic crystal (PC#1). The main carbonation product of PC#1 was K2CO3 · 1.5H2O. Correspondingly, K2CO3 with structure of hexagonal crystal was the composition of sorbent calcined from KHCO3, and the main carbonation product of this sorbent was KHCO3. The product of K4H2(CO3)3 · 1.5H2O would affect the carbonation processes. The hypothesis that the hydration reaction will first occur for K2CO3 with structure of monoclinic crystal, and the carbonation reaction will first occur for K2CO3 with structure of hexagonal crystal, was confirmed by hydration tests. Acknowledgment. Financial support from National Natural Science Foundation (No. 50876021), the National Key Program of Basic research of China (No. 2006CB705806), the Foundation of Graduate Creative Program of Jiangsu Province (No. CX08B_141Z), and the Scientific Research Foundation of Graduate School of Southeast University is sincerely acknowledged. EF800889M
