Energy Fuels 2009, 23, 4683–4687 Published on Web 08/18/2009
: DOI:10.1021/ef900182d
CO2 Absorption Using Dry Potassium-Based Sorbents with Different Supports Chuanwen Zhao, Xiaoping Chen,* and Changsui Zhao School of Energy and Environment, Southeast University, Nanjing 210096, China Received March 3, 2009. Revised Manuscript Received July 27, 2009
The CO2 capture characteristics of dry potassium-based sorbents were investigated with thermogravimetric analysis (TGA) and a bubbling fluidized-bed reactor. Potassium-based sorbents were prepared by impregnation with potassium carbonate on supports such as coconut activated charcoal (AC1), coal active carbon (AC2), silica gel (SG), and activated alumina (Al2O3). Sorbents such as K2CO3/AC1, K2CO3/AC2, and K2CO3/Al2O3 showed excellent carbonation capacity; The total conversion rates of those sorbents were 97.2, 95.9, and 95.2%, respectively in the TG test, and 89.2, 87.9, and 87.6%, respectively, in the fluidized-bed test. However, K2CO3/SG showed poor carbonation capacity, the total conversion rates were only 34.5 and 18.8%, respectively, in TG and fluidized-bed tests. The differences in carbonation capacity of those sorbents were analyzed by studying the microscopic structure and crystal structure of the supports and the sorbents with X-ray diffraction, scanning electron microscopy, and N2 adsorption tests.
for CO2 recovery.6-19 CO2 capture using a dry sodium-based sorbent was reported.6-9 However, when CO2 reacted with Na2CO3, the global carbonation reaction rate was rather slow. The CO2 absorption and regeneration of potassium-based sorbents such as K2CO3/activated carbon, K2CO3/TiO2, K2CO3/MgO, K2CO3/ZrO2, and K2CO3/Al2O3 were studied in a fixed-bed reactor using multiple tests.10-13 The mechanism was mainly investigated from the changes in chemical constituents of sorbents before/after the carbonation reaction. However, the effect of the microscopic structure of sorbents for carbonation was not mentioned. The kinetics of CO2 sorption by K2CO3 in a porous matrix was studied in a fixed-bed reactor.14,15 As we all know, the proper reactor for this process to treat flue gases from fossil-fuel fired power plants is a fluidized-bed or a transport reactor.16-19 Some results were obtained from a fluidized-bed reactor16,18 or two fluidized-bed reactors,19 and the main sorbent was sodiumbased sorbent. K2CO3, with a hexagonal crystal structure, was reported to have excellent carbonation capacity.20-22 This paper studies CO2 capture characteristics of dry potassium-based sorbents that are prepared by impregnating K2CO3 with structure of hexagonal crystal on several porous matrix in TGA and a bubbling fluidized-bed reactor. In addition, the mechanism is
1. Introduction Global warming is emerging as the important environmental issue of the 21st century. Since CO2 is the principal greenhouse gas of interest, and fossil-fuel fired power plants are the largest stationary sources of CO2 emissions,1 capturing CO2 from these sources is of critical importance. Various CO2 capture options are available, such as precombustion decarbonization,2 O2 combustion with CO2 recycle,3 chemical looping combustion,4 and postcombustion capture.5 However, a process which is inexpensive, has a low energy demand, and a high carbonation reactivity, is still being investigated. Recently, dry alkali metal-based sorbents for capturing CO2 from flue gas were investigated as an innovative concept *To whom correspondence should be addressed. Telephone and Fax: þ86 25 83793453. E-mail:
[email protected]. (1) Berger, A. The Effect of Greenhouse Gases on Climate. In: 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. In: Proceedings of the 6th International Conference on Greenhouse Gas Control Technologies, Kyoto, Japan, 2002. (3) Jordal, K.; Anheden, M.; Yan, J.; Stromberg, L. Oxyfuel Combustion for Coal-Fired Power Generation with CO2 Capture ; Opportunities and Challenges. In: 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. (6) Hoffman, J. S.; Pennline, H.W. J. Energy Environ. Res. 2001, 1, 90–100. (7) Liang, Y.; Harrison, D. P.; Gupta, R. P.; Green, D. A.; McMichael, W. J. Energ. Fuel. 2004, 18, 569–575. (8) 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. (9) Seo, Y. W.; Jo, S. H.; Ryu, C. K.; Yi, C. K. Chemosphere. 2007, 69, 712–718. (10) Lee, S. C.; Choi, B. Y.; Lee, T. J.; Ryu, C. K.; Ahn, Y. S.; Kim, J. C. Catal. Today 2006, 111, 385–390. (11) 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. (12) Lee, S. C.; Kim, J. C. Catal. Surv. Asia. 2007, 11, 171–185. (13) Lee, S. C.; Chae, H. J.; Lee, S. J.; Park, Y. H.; Ryu, C. K.; Yi, C. K.; Kim, J. C. J. Mol. Catal., B 2009, 56, 179–184. r 2009 American Chemical Society
(14) Sharonov, V. E.; Okunev, A. G.; Aristov, Y. I. React. Kinet. Catal. L. 2004, 82, 363–369. (15) Okunev, A. G.; Sharonov, V. E.; Aristov, Y. I.; Parmon, V. N. React. Kinet. Catal. L. 2000, 71, 355–362. (16) 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. (17) Cho, K. C.; Lee, E. Y.; Yoo, J. S.; Choung, Y. H.; Park, S. W.; Oh, K. J. Stud. Surf. Sci. Catal. 2006, 159, 549–552. (18) Yi, C. K.; Jo, S. H.; Ryu, H. J.; Yoo, Y. W.; Lee, J. B.; Ryu, C. K. Greenhouse Gas Control Technol. 2005, 7, 1765–1769. (19) Yi, C. K.; Jo, S. H.; Seo, Y. W.; Lee, J. B.; Ryu, C. K. Int. J. Greenhouse Gas Control 2007, 1, 31–36. (20) Zhao, C. W.; Chen, X. P.; Zhao, C. S. J. Chem. Ind. Eng. (China) 2008, 59, 2328–2333. (21) Zhao, C. W.; Chen, X. P.; Zhao, C. S.; Liu, Y. K. Energy Fuels 2009, 23, 1766–1769. (22) Zhao, C. W.; Chen, X. P.; Zhao, C. S. Chemosphere 2009, 75, 1401–1404.
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Energy Fuels 2009, 23, 4683–4687
: DOI:10.1021/ef900182d
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analyzed from the differences in microscopic structure, particle morphology, and chemical constituents of those sorbents. 2. Experimental Section Sorbents Preparation. The potassium-based sorbents used in this study were prepared by impregnating K2CO3 with a hexagonal crystal structure on supports such as coconut activated charcoal (AC1), coal active carbon (AC2), silica gel (SG), and activated alumina (Al2O3). A 150 g portion of supports was added to an aqueous solution containing 50 g of K2CO3 in 500 mL of deionized water. Then, it was mixed with a magnetic stirrer at room temperature for 10 h. After being stirred, the mixture was dried in a oven at 105 °C. The dried samples were calcined in a muffle furnace for 2 h at 300 °C. The amount of K2CO3 impregnated was determined by X-ray fluorescence (XRF), and the range of particle sizes for sorbents were 400-600 μm. Apparatus and Procedure. A TherMax 500 high pressure thermogravimetric analysis (TGA) was used for the thermogravimetry (TG) test. The amount of sorbent used in the tests is 100 mg. The carbonation temperature was 60 °C, and the carbonation reaction was processed in the gas composition of 15% CO2, 15% H2O, and 70% N2 at 500 mL min-1. Figure 1 shows the schematic diagram of the experimental apparatus for the fluidized-bed test. The apparatus consists of a gas injection part (CO2 and N2 were obtained from high purity cylinders with mass flow controllers to control flow. H2O was fed using a high-precision piston pump, then heated to ensure completely vaporization), a bubbling fluidized-bed reactor (with an inner diameter of 0.05 m and a height of 1.0 m, made of stainless steel, and placed inside of electric heating. A cooling jacket was used to maintain the temperature constant. The temperature was controlled by a temperature controller, and measured by a precision digital thermometric instrument), gas post-treatment part (a filter to remove dust, and a condenser to remove H2O), and gas analyzer (analyzes CO2 every 2 s). A 0.2 kg portion of the sorbent was placed into the reactor; to simulate real flue gas composition, a gas mixture of 15% CO2, 70% N2, and 15% H2O at 1.8 m3 h-1 (based on the critical fluidization velocity of these sorbents) was used for carbonation reactions. The carbonation temperature was maintained at 60 °C. After the carbonation reaction completed, the gas composition was changed to 100% N2 at 1.8 m3 h-1. When the CO2 concentration decreased to zero, the temperature was increased to 200 °C. Calcination was carried out. An accelerated surface area and porosimetry 2020 micropore physisorption analyzer was used for BET surface area and porosity determinations. The microscopic shape of sorbents was observed with a JSM-5610LV-VANTAGE SEM, and the structural change of sorbents before and after the carbonation reactions was examined with a D/max 2500 VL/PC XRD.
Figure 1. The schematic diagram of experimental apparatus for the fluidized-bed test.
Microscopic Structure of Those Sorbents. The N2 adsorption isotherm plots of those sorbents were shown in Figure 3. The isotherm of K2CO3/AC1 and K2CO3/AC2 belong to type II isotherm, K2CO3/SG and K2CO3/Al2O3 are type IV isotherms. Analyzed from Figure 3, the pores of K2CO3/AC1 and K2CO3/AC2 are mainly microporous, and the pore shapes are capillary space between parallel plates or open slit-shaped capillaries. K2CO3/SG and K2CO3/Al2O3 possess the characteristics of mesopores, and pore shapes are mainly open-ended cylindrical pores lying in a narrow range of radius.23 The surface areas of the sorbents were measured according to the BET method, and the pore volumes of the samples were calculated using the Dubinin-Radushkevich (DR) method, and are shown in Figure 4. The surface areas for K2CO3/AC1 is maximal, and for K2CO3/SG is minimal. The pore volumes for K2CO3/SG is maximal, and for K2CO3/AC2 is minimal. The pore size distribution of those sorbents were obtained using the Barrett-Joyner-Halenda method, and is depicted in Figure 5. As shown in Figure 5, 90% of pores of K2CO3/AC1 and K2CO3/AC2 are distributed from 0 to 4 nm, whereas 90% pores of K2CO3/Al2O3 are distributed from 2 to 15 nm. There are two pore distribution ranges for K2CO3/SG, one is 0-5 nm and another is 10-100 nm. The average pore size of K2CO3/SG is 36.35 nm. TGA Test Results for These Sorbents. The important reactions involved in the capture of CO2 using these potassiumbased sorbents are: K2 CO3 þ H2 O þ CO2 T 2KHCO3 ΔHr0 ¼ -141kJ=mol K2 CO3 Results from a typical TG test for K2CO3/AC1 are shown in Figure 6, where sorbents mass and temperature are plotted versus time. As the water absorption capacity of these sorbents is strong, the sorbents were heated from ambient to 200 °C for dehydration at the beginning, and after the mass maintained constant, the carbonation was performed. On the basis of the conversion of K2CO3 to KHCO3, the percent conversion η is calculated from MK2 CO3 ðwðtÞ - wð0ÞÞ 100% η ¼ Rwð0Þð2MKHCO3 - MK2 CO3 Þ
3. Experimental Results K2CO3 Loading Amount and Particle Morphologies of These Potassium-Based Sorbents. The actual values of K2CO3 loading amount for K2CO3/AC1, K2CO3/AC2, K2CO3/SG and K2CO3/ Al2O3 are 24.3, 23.7, 22.8, and 24.5%, respectively, while the theoretical values that all K2CO3 was loaded on support for all sorbents are 25%. The particle morphologies of those sorbents were shown in Figure 2. The differences of those sorbents in particle morphologies are clearly visible. The surfaces of K2CO3/AC1, K2CO3/AC2, and K2CO3/Al2O3 are uneven, and a lot of small pores and cracks distribute on the surface. On the contrary, the surface of K2CO3/ SG is smooth, and the pores are hardly seen using a magnification of 1000.
Where, t is the reaction time, w(t) is the weight of sorbent at time t, w(0) is the sorbent weight at the beginning, R is the K2CO3 loading amount of those sorbents, and MK2CO3 and MKHCO3 are the molecular weight of K2CO3 and KHCO3. The percent conversion of K2CO3/AC1 and other sorbents is plotted versus reaction time in Figure 7. (23) Gregg, S. J.; Sing K. S. Adsorption, Surface Area and Porosity, 2nd ed; Academic Press: London, 1982.
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Figure 2. The SEM images of different sorbents: A: K2CO3/AC1; B: K2CO3/AC2; C: K2CO3/SG; D: K2CO3/Al2O3; I: the whole particle morphology; II: Instrumental magnification 1000.
Figure 3. The N2 adsorption-desorption isotherm plots of those sorbents.
All the reactions of those sorbents were complete within 25 min. The total percent carbonation of K2CO3/AC1,
K2CO3/AC2, and K2CO3/Al2O3 are about 97.2% in 20.5 min, 95.9% in 21.5 min, and 95.2% in 23.8 min, respectively. It is 4685
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Figure 4. The surface areas and the pore volumes of those sorbents No. 1: K2CO3/AC1; No. 2: K2CO3/AC2; No. 3: K2CO3/SG; No. 4: K2CO3/Al2O3.
Figure 7. The percent conversion for different sorbents.
Figure 5. The pore size distribution of those sorbents No. 1: K2CO3/ AC1; No. 2: K2CO3/AC2; No. 3: K2CO3/SG; No. 4: K2CO3/Al2O3.
Figure 8. Typical test result of K2CO3/AC1 during a single carbonation-regeneration cycle.
Figure 6. Typical TG test result of K2CO3/AC1.
indicated that K2CO3 was nearly completely converted to KHCO3 for these sorbents. However, The total percent carbonation of K2CO3/SG is only 34.5% in 23 min. Fluidized-Bed Test Results for These Sorbents. A typical test result during carbonation-regeneration cycle using K2CO3/ AC1 in a bubbling fluidized-bed reactor is shown in Figure 8, where the CO2 concentration is plotted versus time. The left part of the figure represents the carbonation reaction, and the right part represents the regeneration reaction.The CO2 concentration was nearly zero at the beginning. After several minutes, CO2 concentration rapidly increased to 15%, and was maintained constantly. Then calcination was carried out. CO2 was reached a maximum, and then decreased to zero, indicating that calcination was complete. The CO2 removal of those sorbents is shown in Figure 9. On the basis of the conversion of K2CO3 to KHCO3, good material balance closure was obtained with numerical integration of the area under the curve. The percent conversion (ηC) is calculated from: nCO2 MK2 CO3 100% ηC ¼ 200R
Figure 9. The amount of CO2 removed by carbonation.
K2CO3/AC1, K2CO3/AC2, and K2CO3/Al2O3 are about 0.314 mol, 89.2%; 0.302 mol, 87.9%; and 0.311 mol, 87.6%, respectively for a single test cycle. The CO2 capture capacity is very higher. Lee et al. reported that K2CO3/AC and K2CO3/Al2O3 showed high CO2 capture capacity in fixed-bed reactor test.10-13 However, the result of K2CO3/SG are only 0.062 mol and 18.8%, respectively. Okunev et al. reported that CO2 capture capacity of K2CO3/SG was poor.14 XRD Tests Results of Those Sorbents. The structural change of those sorbents before/after carbonation reaction in TG test was examined by XRD, and the XRD patterns are shown in Figure 10. The XRD result shows that the main constituents of K2CO3/ AC1, K2CO3/AC2, and K2CO3/Al2O3 are K2CO3, besides SiO2 or Al2O3. After the carbonation reaction, K2CO3 is completely converted to KHCO3 (Figure 10, panels a, b, and d). Lee et al. showed that SiO2 was not observed in the case of K2CO3/AC, and a new phase of KAl(CO3)2(OH)2 was observed in the case of K2CO3/Al2O3.10,11 We attribute this to the difference of support materials and sorbent preparation process.
Where nCO2 is the amount of CO2 removed by carbonation. The total amount of CO2 removal and percent conversions of 4686
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Figure 10. The XRD patterns of those sorbents before/after carbonation reactions: (a) K2CO3/AC1; (b) K2CO3/AC2; (c) K2CO3/SG; (d) K2CO3/Al2O3; (I) fresh; (II) after carbonation reaction; ( -SiO2; 3-K2CO3 3 1.5H2O; b-K2CO3; 0-KHCO3; 2-Al2O3).
Then, effected by its microscopic structure, K2CO3 3 1.5H2O is quickly converted to KHCO3.
The XRD of K2CO3/SG shows that besides K2CO3 and SiO2, K2CO3 3 1.5H2O appears in Figure 10c-I. After carbonation reaction, the main product is K2CO3 3 1.5H2O (Figure 10cII). It is reported that a new phase of K2SiO3 was formed by reaction between silica gel and K2CO3.14 It is not observed in this paper. Reaction Principles of Different Sorbents Analyzed. The excellent CO2 capture capacities of K2CO3/AC1 and K2CO3/AC2 are attributed to their extensive micropore structures (judged by their high surface areas and pore size distribution). A great deal of CO2 and H2O were easily adsorbed on the sorbents, and quickly reacted with active components. Combined with XRD patterns, it can be deduced that the carbonation reaction of K2CO3/AC1 and K2CO3/AC2 was successfully carried out, and KHCO3 was produced. Combined with the SEM and N2 adsorption tests results, we can deduce that the pores of K2CO3/SG are mainly deep pores lying in the narrow range of 10-100 nm. This character of microscopic structure decreases CO2 capture capacity. The hydroscopicity of SG is strong, so H2O can be easily adsorbed. On the contrary, the CO2 adsorption of SG is weak. K2CO3 can easily react with H2O to produce K2CO3 3 1.5H2O. This conclusion can be obtained from XRD results and the calculation of TG test results (based on the conversion of K2CO3 to KHCO3, the percent conversion was 34.5%. However, based on the conversion of K2CO3 to K2CO3 3 1.5H2O, the percent conversion was 79.2%). Zhao et al. reported that the carbonation reactivity of K2CO3 3 1.5H2O was weak.20-22 The extensively mesopores structures of K2CO3/Al2O3 were found from SEM and N2 adsorption tests, and this character can improve the CO2 adsorption. Zhao et al. reported that the carbonation reactivity of K2CO3 3 1.5H2O was improved by loading on Al2O3. It can be deduced that K2CO3 is quickly converted to K2CO3 3 1.5H2O for the K2CO3/Al2O3 sorbent.
4. Conclusions The CO2 capture characteristics of dry potassium-based sorbents such as K2CO3/AC1, K2CO3/AC2, K2CO3/SG, and K2CO3/Al2O3 were studied in TGA and a bubbling fluidizedbed reactor. K2CO3/AC1, K2CO3/AC2, and K2CO3/Al2O3 showed excellent carbonation capacity. However, K2CO3/SG showed poor carbonation capacity. The reason was analyzed from the microscopic structures, the crystal structures and the structural changes before/after carbonation reaction for those sorbents. As the microscopic structure of K2CO3/SG is poor, and the main product is K2CO3 3 1.5H2O. The CO2 capture capacity of K2CO3/SG is poor. SG is not a proper choice as the sorbent support either. Because of its poor antiattrition character, K2CO3/AC could not be used in fluidized bed. K2CO3/Al2O3 could be used as the sorbent that has the potential for a large scale CO2 capture process with two fluidized-bed reactors. To investigate the sorbent durability, CO2 capture capacity of K2CO3/Al2O3 as a function of cycle number in fluidized bed reactor should be reported later. Acknowledgment. Financial support from the National High Technology Research and Development Program of China (No. 2009AA05Z311), the National Natural Science Foundation (No. 50876021), the Foundation of Graduate Creative Program of Jiangsu Province (No. CX08B_141Z) and the Scientific Research Foundation of Graduate School of Southeast University (YBJJ0825) are sincerely acknowledged.
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