Energy & Fuels 2005, 19, 1447-1452
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Synthesis, Experimental Studies, and Analysis of a New Calcium-Based Carbon Dioxide Absorbent Zhen-shan Li, Ning-sheng Cai,* Yu-yu Huang, and Hai-jin Han Department of Thermal Engineering, Tsinghua University, Beijing 100084, China Received December 10, 2004. Revised Manuscript Received March 23, 2005
A new kind of Ca-based regenerable CO2 absorbent, CaO/ Ca12Al14O33, was synthesized on the basis of the integration of CaO, as solid reactant, with a composite metal oxide Ca12Al14O33, as a binder, for applying it to repeated calcination/carbonation cycles. The carbonation reaction can be applied in many industrial processes, and it is important for practical calcination/carbonation processes to have absorbents with high performance. The cyclic carbonation reactivity of the new absorbent was investigated by TGA (thermogravimetric analysis). The effects of the ratio of active material to binder in the new absorbent, the mechanics for preparation, and the reaction process of the high-reactivity CaO/Ca12Al14O33 absorbent have been analyzed. The results obtained here indicate that the new absorbent, CaO/Ca12Al14O33, has a significantly improved CO2 absorption capacity and cyclic reaction stability compared with other Ca-based CO2 absorbents. These results suggest that this new absorbent is promising in the application of calcination/ carbonation reactions.
1. Introduction The carbonation reaction of CaO with CO2 is very important in many industrial processes such as CO2 absorption gasification,1 hydrogen production from integrated coal gasification,2-4 methane steam reforming process,5,6 CO2 separation from flue gas7 or syngas,8 and chemical heat pump9 and energy storage system.10 The carbonation process is a gas-solid reaction, and Cabased absorbents are repeatedly used; in other words, after the carbonation reaction, the absorbents must be regenerated in the subsequent cyclic reaction. Therefore, the carbonation/calcination characteristics of absorbents are very important for practical application. The most common Ca-based absorbents suitable for the carbonation/calcination process are either calcium-containing natural materials, such as dolomite, or prepared materials of calcium oxide deposited on a substrate. Some absorbents, such as limestone, undergo a decay of CO2 absorption capacity and physical breakdown.11,12 High* Author to whom correspondence should be addressed. Tel/Fax: 8610-62789955. E-mail:
[email protected]. (1) Squires, A. M. Adv. Chem. Ser. 1967, 69, 205-229. (2) Ziock, H. J.; Lackner, K. S.; Harrison, D. P. Zero emission coal power, a new concept, Los Alamos Report, LA-UR-01-2214, 2001; http:// www.zeca.org/ docs.html. (3) Lyon, R. K.; Cole, J. A. Combust. Flame 2000, 121 (1-2), 249261. (4) Lin, S.-Y.; Suzuki, Y.; Hatano, H.; Harada, M. Energy Convers. Manage. 2002, 43, 1283-1290. (5) Balasubramanian, B.; Lopez, A.; Kaytakoglu, S.; Harrison, D. P. Chem. Eng. Sci. 1999, 54, 3543-3552. (6) Kato, Y.; Ando, K.; Yoshizawa, Y. J. Chem. Eng. Jpn. 2003, 36, 860-866. (7) Gupta, H.; Fan, L. S. Ind. Eng. Chem. Res. 2002, 41, 4035. (8) Kim, J. K.; Yoo, K. S.; Park, T. J.; Song, B. H.; Lee, J. G.; Kim, J. H.; Han, C. J. Korean Inst. Chem. Eng. 2002, 40 (5), 582. (9) Kyaw, K.; Kanamori, M.; Matsuda, H.; Hasatani, M. J. Chem. Eng. Jpn. 1996, 29 (1), 112. (10) Kato, Y.; Saku, D.; Harada, N.; Yoshizawa, Y. J. Chem. Eng. Jpn. 1997, 30 (6), 1013. (11) Barker, R. J. Appl. Chem. Biotechnol. 1973, 23, 733-742.
performance absorbents should have not only a large calcium oxide content but also a high cyclic stability. Through the experiments on 10-µm CaO powder, Barker11 demonstrated a drop in the absorption capacity from 59 wt % in the first carbonation cycle to 8 wt % at the end of the 25th cycle, and suggested that, because of the formation of a 22-nm-thick product layer, particles smaller than 22 nm in diameter should be able to achieve stoichiometric conversion. Barker13 later proved this hypothesis by obtaining repeated 93% conversion of 10-nm CaO particles over 30 cycles with a carbonation time of 24 h under 100% CO2 at 850 K. Harrison and co-workers developed an absorption-enhanced hydrogen production process from the steam methane reforming reaction by removing CO2 from the gas mixture through the carbonation of CaO, but the absorption capacity of absorbents decreased rapidly with the increase of cyclic carbonation number.14,15 Abanades and Alvarez,16 on the basis of the changing microporosity within grains and the mesoporosity surrounding them due to sintering, hypothesized the explanation for the drop in capture capacity over multiple carbonation/calcination cycles. In the application of Ca-based chemical heat pump process, CaO showed a drop in CO2 capture from 53 wt % in the first cycle to 27.5 wt % by the fifth cycle.17 A lithium zirconate (Li2ZrO3)-based sorbent provided 20 wt % capacity over two cycles.18 In another study, researchers at Toshiba Corp. (New York) observed that the reactivity of lithium orthosilicate (Li4SiO4) was better than that (12) Abanades, J. C. Chem. Eng. J. 2002, 90, 303-306. (13) Barker, R. J. Appl. Chem. Biotechnol. 1974, 24, 221-227. (14) Han, C.; Harrison, D. P. Chem. Eng. Sci. 1994, 49, 5875-5883. (15) Ortiz, A. L.; Harrison, D. P. Ind. Eng. Chem. Res. 2001, 40, 5102-5109. (16) Abanades, J. C.; Alvarez, D. Energy Fuels 2003, 17, 308-315. (17) Kato, Y.; Harada, N.; Yoshizawa, Y. Appl. Therm. Eng. 1999, 19, 239-254. (18) Ida, J.-I.; Lin, Y. S. Environ. Sci. Technol. 2003, 37, 1999-2004.
10.1021/ef0496799 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/12/2005
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Energy & Fuels, Vol. 19, No. 4, 2005
Li et al. Table 1. Chemical Composition of Three Dolomites
sample dolomite I dolomite II dolomite III
Figure 1. Simplified flow diagram of the experimental setup used for the multiple carbonation and calcination reactions.
of lithium zirconate.19,20 Extended cyclical studies performed on lithium orthosilicate samples attained 26.5 wt % sorption capacity over 50 cycles without any change in the reactivity.21 In this study, a new Ca-based absorbent material has been synthesized on the basis of the integration of a new binder, Ca12Al14O33. It has high reactivity and stability under the conditions of the multiple carbonation/ calcination cycles. Furthermore, we have focused on several other kinds of natural Ca-based absorbents by studying the decay of the CO2 absorption capacity and the effect of the inert MgO in dolomite on the cyclic stability during carbonation/calcination cycles. Finally, the comparison of the CO2 absorption capacity and the cyclic stability for these absorbents are given. 2. Experiments 2.1. Experimental System. A Dupont 951 TGA (TA Instrument 1200) was used to study the carbonation reaction of Ca-based CO2 absorbents. The TGA consists of a quartz tube placed in an oven which can be operated at temperatures up to 1473 K. A computer continuously records reaction temperature and sample weight. The reacting gas contains CO2 and N2, and their proportion can be adjusted by mass flow controllers. The simplified TGA schematic diagram is shown in Figure 1. A small amount of the absorbent (about
