Energy Fuels 2011, 25, 456–458 Published on Web 12/10/2010
: DOI:10.1021/ef101364c
High-Capacity and Low-Cost Carbon-Based “Molecular Basket” Sorbent for CO2 Capture from Flue Gas Dongxiang Wang, Cigdem Sentorun-Shalaby, Xiaoliang Ma,* and Chunshan Song* EMS Energy Institute and Department of Energy and Mineral Engineering, Pennsylvania State University, 209 Academic Projects Building, University Park, Pennsylvania 16802, United States Received October 7, 2010. Revised Manuscript Received December 5, 2010 MBSs were the mesoporous silica molecular sieves, such as MCM-41, MCM-48, SBA-15, and KIT-6. These materials are commercially unavailable right now, and the preparation cost of them is very high. Consequently, it is impractical to use the current MBSs in mass CO2 capture from the coal power plants, which produce about 1500 megatons of CO2/year in the U.S. According to our preliminary evaluation, the cost of the support materials accounts for more than 90% of the total MBS preparation cost, indicating that reducing the cost of the support materials can significantly reduce the cost of the sorbent preparation. On the other hand, the mesoporous silica molecular sieves usually show poor hydrothermal stability at even 100 °C,16 which may result in the degradation of the sorbent in the cycles. Therefore, it is necessary to develop a new generation of MBS with high CO2 sorption capacity, low material cost, and high hydrothermal stability. With this in mind, the carbon-based support materials have attracted our great attention, because many carbon-based porous materials with a well-developed pore structure, wide range of pore sizes, and large total pore volume are commercially available17,18 and easy to be prepared from the widely available and low-cost feedstocks, such as coal or petroleum pitch. In addition, the porous structure and surface chemistry of carbon-based materials can be tailored and modified easily. Some studies on the preparation of CO2 sorbents by loading PEI on the carbon materials have been reported in the literature. Arenillas et al. reported the preparation of a sorbent by loading 60 wt % PEI on a fly-ash-derived activated carbon, which showed a CO2 capacity of 40 mg of CO2/g of sorbent.19 Maroto-Valer et al. reported the preparation of the sorbents by loading 33.5 wt % PEI on the activated anthracites. The measured sorption capacity of the sorbent was 26.3 mg of CO2/g of sorbent at 75 °C.20 Recently, Maroto-Valer et al. reported the loading 39 wt % PEI on a fly ash to prepare a CO2 sorbent with the measured capacity of 49.8 mg of CO2/g of sorbent at 70 °C.21 Plaza et al. loaded PEI on a commercial activated carbon to prepare a sorbent for CO2 capture but found no positive effect of PEI loading on the improvement of CO2 sorption capacity.22 Up to date, all sorbents prepared by
The continuous rise of the atmospheric CO2 concentration and its linkage with climate change demand an urgent technological solution to reduce CO2 emissions.1 Carbon capture and sequestration (CCS) have been considered as one of the key options for mitigating CO2 emissions.2 On the basis of the current technology (amine scrubbing), the CCS cost is very high, in which the CO2 capture from the sources was estimated to be two-thirds or even more of the total costs for CCS.3,4 Consequently, many research approaches have been carried out for the development of novel technologies to reduce the cost for the CO2 capture. Among all of these research efforts, the CO2 capture by adsorption/sorption on the immobilized amine sorbents has been considered as one of the most promising approaches.4-9 In our previous studies for CO2 capture, we have developed the novel sorbents, called as the “molecular basket” sorbents (MBSs), which were prepared by immobilizing CO2-philic polyethylenimine (PEI) on silica mesoporous molecular sieves.10-12 The second generation of MBS (MBS-2) prepared by loading 50 wt % PEI on SBA-15 showed a CO2 capacity as high as 140 mg of CO2/g of sorbent at a CO2 partial pressure of 15 kPa, because the MBS increases the total density of the accessible amine functional groups on/ in the sorbent.13-15 In addition, the MBS has also some other significant potential advantages, including high selectivity for CO2, no or less corrosion problem, high sorption/desorption rate because of high gas-sorbent interface area (∼80 m2/g), positive effect of moisture on the MBS performance, and lower energy consumption during regeneration. However, the support materials currently used in the preparation of the *To whom correspondence should be addressed. E-mail: mxx2@psu. edu (X.M.);
[email protected] (C.S.). (1) Khatri, R. A.; Chuang, S. S. C.; Soong, Y. Energy Fuels 2006, 20, 1514–1520. (2) Song, C. S. Catal. Today 2006, 115, 2–32. (3) Haszeldine, R. S. Science 2009, 325, 1647–1652. (4) Liu, W.; King, D.; Liu, J.; Johnson, B.; Wang, Y.; Yang, Z. G. JOM 2009, 61 (4), 36–44. (5) Bai, H. L.; Yeh, A. C. Ind. Eng. Chem. Res. 1997, 36, 2490–2493. (6) Yeh, A. C.; Bai, H. L. Sci. Total Environ. 1999, 228, 121–133. (7) Rao, A. B.; Rubin, E. S. Environ. Sci. Technol. 2002, 36, 4467– 4475. (8) Olajire, A. A. Energy 2010, 35, 2610–2628. (9) Choi, S.; Drese, J. H.; Jones, C. W. ChemSusChem 2009, 2 796–854. (10) Xu, X. C.; Song, C. S.; Andresen, J. M.; Miller, B. G.; Scaroni, A. W. Energy Fuels 2002, 16, 1463–1468. (11) Xu, X. C.; Song, C. S.; Andresen, J. M.; Miller, B. G.; Scaroni, A. W. Microporous Mesoporous Mater. 2003, 62, 29–45. (12) Xu, X. C.; Song, C. S.; Miller, B. G.; Scaroni, A. W. Ind. Eng. Chem. Res. 2005, 44, 8113–8119. (13) Wang, X. X.; Schwartz, V.; Clark, J. C.; Ma, X. L.; Overbury, S.; Xu, X. C.; Song, C. S. J. Phys. Chem. 2009, 113 (17), 7260–7268. (14) Wang, X. X.; Ma, X. L.; Sun, L.; Song, C. S. Top. Catal. 2008, 49, 108–117. (15) Ma, X. L.; Wang, X. X.; Song, C. S. J. Am. Chem. Soc. 2009, 131, 5777–5783. r 2010 American Chemical Society
(16) Jeong, H. S.; Jeong, S. Y.; Lee, J. M.; Yim, D. J.; Ryu, S. K. J. Ind. Eng. Chem. 1999, 5, 245–252. (17) Siriwardane, R. V.; Shen, M.; Fisher, E. P.; Poston, J. Energy Fuels 2001, 15, 279–284. (18) Burchell, T. D.; Judkins, R. R.; Rogers, M. R.; Williams, A. M. Carbon 1997, 35, 1279–1294. (19) Arenillas, A.; Smith, K. M.; Drage, T. C. Fuel 2005, 84, 2204– 2210. (20) Maroto-Valer, M. M.; Tang, Z.; Zhang, Y. Fuel Process. Technol. 2005, 86, 1487–1502. (21) Maroto-Valer, M. M.; Lu, Z.; Zhang, Y. Z. Waste Manage. 2008, 28, 2320–2328. (22) Plaza, M. G.; Pevida, C.; Arenillas, A.; Rubiera, F.; Pis, J. J. Fuel 2007, 86, 2204–2212.
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: DOI:10.1021/ef101364c capacity of the sorbents, measured in this study, should be the same as those measured using a real flue gas. Both C4 and C5 are the commercial carbon blacks, and C4 has a SBET value of 1486 m2/g and a total pore volume (Vtotal) of 2.93 mL/g. As is well-known, the carbon black has a hierarchy of structures consisting of particles covalently bound into aggregates, which in turn associate by weak interactions into agglomerates.23 Interestingly, although the porous structures of C4 and C5 are quite different from that of SBA-15, which has an ordered mesoporous structure, the sorption performances of PEI(50)/C4 and PEI(50)/C5 are similar to that of PEI(50)/SBA-15, indicating that the porous material with a structure of the aggregative particles can also be used as a good support for preparing MBS with a high CO2 sorption capacity. It was also found that the performances of PEI(50)/C1, PEI(50)/C2, and PEI(50)/C3 were much poorer than those of PEI(50)/C4 and PEI(50)/C5. C1, C2, and C3 are the commercial activated carbons, which are characterized by their slit-shaped pore structures. Although the SBET values of these activated carbons are similar to or even higher than those of C4 and C5, their total pore volume is less than 1.7 mL/g, which may be one of the reasons why PEI(50)/C1, PEI(50)/ C2, and PEI(50)/C3 showed poorer capacity. The preliminary correlation between the porous properties and the CO2 sorption capacity of the prepared sorbents indicates that neither the BET surface area nor the microporous volume but the mesoporous volume plays a more important role in determining the sorption performance of the sorbents. The detailed correlation between the performance and the textural and porous structure of their support materials is underway in our laboratory for clarifying how the porous structure of the supports affect the performance of CB-MBS. Interestingly, it was further found that the volume-based capacity of PEI(50)/C4 is even higher than that of MBS-2 by 57% because of the higher packing density (0.35 g/mL) of the former than that of the latter. It will significantly reduce the volume of the sorbent bed and, thus, reduce the cost for equipment investment for the mass CO2 capture. The regenerability of PEI(50)/C4 was also evaluated by conducting the CO2 sorption/desorption cycles using a TGA. The CO2 sorption capacity as a function of the cycle number is shown in Figure 2. A slight reduction of the CO2 sorption capacity was observed with an increasing sorption/desorption cycle number, but such a drop trend becomes insignificant with an increasing cycle number. About 92% of the initial CO2 sorption capacity can be recovered after 10 cycles. As is well-known, the flue gas also contains moisture and O2. The effects of both moisture and O2 were also examined. It was found that the presence of O2 had almost no effect on the sorption performance of PEI(50)/C4, while the presence of moisture in the model gas had even a positive effect on the sorption capacity, as the same as that found in our previous study for MBS-2.12 Further investigations in finding a major reason for the sorbent degradation and how to improve the stability of CB-MBS are necessary. The preparation cost of PEI(50)/C4 sorbent, which includes the costs for PEI, the support carbon material, the consumed solvent, and the operation cost for the sorbent preparation at an industrial scale, was estimated in comparison to that of MBS-2. The estimated preparation cost of PEI(50)/C4 is about $44/kg, while the estimated preparation cost of MBS-2
Figure 1. CO2 sorption capacity of the prepared CB-MBS samples in comparison to MBS-1 [PEI(50)/MCM-41] and MBS-2 [PEI(50)/ SBA-15]. Sorption condition: 100% CO2 at 75 °C.
loading PEI on the carbon materials, reported in the literature, showed much lower CO2 capacity in comparison to those by loading PEI on the mesoporous silica molecular sieves.9,15 The objective of the present study is to develop an inexpensive MBS with high CO2 sorption capacity by loading PEI on the carbon-based porous materials, instead of the expensive mesoporous silica molecular sieves. The ultimate purpose is to substantially reduce the MBS preparation cost and, thus, allow the CO2 capture to be conducted more cost-effectively. A series of commercial carbon-based materials with different pore properties and structures were selected as the supports for the preparation of carbon-based MBS (CB-MBS) in this study. The Brunauer-Emmett-Teller (BET) surface area (SBET) of the carbon samples changes in a range from 1151 to 2320 m2/g, and the total pore volume was in a range from 0.64 to 2.93 mL/g. CB-MBSs were prepared by loading 50 wt % PEI on the carbon samples using the wet impregnation method that was reported in our previous paper.7 The prepared CB-MBS samples were designated as PEI(X)/Y, where X is the weight percentage of the loaded PEI in the sorbent and Y indicates the support of the carbon material. The CO2 sorption performance of the CB-MBS samples was evaluated in a thermogravimetric analyzer (TGA). The CO2 sorption tests were carried out using high-purity CO2 (99.99%) gas with a flow rate of 100 mL/min at 75 °C. Ultrahigh-purity N2 (99.99%) was used as a carrier gas for the cleanup of the sample at 100 °C before the CO2 sorption and for desorption of the sorbent saturated by CO2. The CO2 sorption capacity was calculated on the basis of the changes in the sample mass. Figure 1 shows the CO2 sorption capacity of the CB-MBS samples in comparison to those of MBS-1 [PEI(50)/MCM-41] and MBS-2 [PEI(50)/SBA-15], which are the first and second generation of MBSs developed in our laboratory.10,13,15 In all of the CB-MBS samples, PEI(50)/ C4 gave the highest CO2 sorption capacity, 135 mg of CO2/g of sorbent, which is higher than the best carbon-based sorbent reported in the literature21 at the compatible test conditions by a factor of 2.7. This value is also higher than that of MBS-1 (110 mg of CO2/g of sorbent, measured at the same conditions) by 19% and is almost the same as that of MBS-2 (138 mg of CO2/g of sorbent, measured at the same conditions). The measured sorption capacity of PEI(50)/C5 is slightly lower than that of PEI(50)/C4 but significantly higher than other carbon materials. It needs to be mentioned that the CO2 capacity measured using a TGA with the pure CO2 gas may be overestimated in comparison to those measured using a real flue gas with about 14 vol % CO2. However, the relative
(23) Hjelm, R. P.; Wampler, W.; Gerspacher, M. Proc. SPIE 1997, 2867, 144–147.
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Energy Fuels 2011, 25, 456–458
: DOI:10.1021/ef101364c Table 1. Comparison of PEI(50)/C4 and PEI(50)/SBA-15 (MBS-2) PEI(50)/C4 mass-based capacity (mg of CO2/g of sorbent) volume-based capacity (mg of CO2/mL of sorbent) SBET (m2/g) packing density (g/mL) MBS preparation cost ($/kg)
PEI(50)/SBA-15
135
138
47
30
37 0.35 ∼44
80 0.22 ∼760
available carbon black. The major properties and CO2 sorption capacity of PEI(50)/C4 in comparison to MBS-2 are listed in Table 1. The mass-based CO2 sorption capacity of PEI(50)/C4 (135 mg of CO2/g of sorbent) is similar to that of MBS-2 developed in our previous study,15 while the volumebased CO2 capacity (47 mg of CO2/mL of sorbent) is higher than that of MBS-2 by 57% because of the higher packing density of the former than that of the latter. In addition, the estimated cost for sorbent preparation is significantly reduced from ∼$760/kg for MBS-2 to ∼$44/kg for PEI(50)/C4. The results indicate that the carbon-based MBS is a promising sorbent for cost-efficient CO2 capture from flue gas.
Figure 2. CO2 sorption capacity as a function of the cycle number.
is about $760/kg. The preparation cost of the former is only about 6% of that of the latter. This is because the preparation cost of SBA-15 accounts for more than 95% of the total PEI(50)/SBA-15 preparation cost. Thus, using a cheaper support, such as C4, instead of SBA-15, for the preparation of MBS can reduce the sorbent preparation cost by more than 90%, resulting in a significant decrease in the CO2 capture cost. In summary, a novel carbon-based MBS [PEI(50)/C4] has been developed by loading 50 wt % PEI on a commercially
Acknowledgment. This work was supported by the U.S. Department of Energy, National Energy Technology Laboratory through the NETL contract DE-FE0000458 and the Consortium for Premium Carbon Products from Coal (CPCPC) under the contract DE-FC26-03NT41874.
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