Energy & Fuels 2006, 20, 2417-2420
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Overcoming the Problem of Loss-in-Capacity of Calcium Oxide in CO2 Capture Bo Feng,* Wenqiang Liu, Xiang Li, and Hui An School of Engineering, The UniVersity of Queensland, St Lucia, Queensland 4072, Australia ReceiVed June 6, 2006. ReVised Manuscript ReceiVed August 11, 2006
Calcium oxide has been identified to be one of the best candidates for CO2 capture in zero-emission powergeneration systems. However, it suffers a well-known problem of loss-in-capacity (i.e., its capacity of CO2 capture decreases after it undergoes cycles of carbonation/decarbonation). This problem is a potential obstacle to the adoption of the new technologies. This paper proposes a method of fabricating a CaO-based adsorbent without the problem of loss-in-capacity. An adsorbent was fabricated using the method and tested on a thermogravimetric analyzer. It was shown that the sorbent attained a utilization efficiency of more than 90% after 9 cycles of carbonation/decarbonation.
Introduction Recently, a few novel coal gasification processes have been proposed in the world, which promise to deliver high plant efficiency (over 50%) and, simultaneously, zero emissions 1. These include the Japanese HyPr-RING process,2-4 General Electric’s unmixed fuel processing technology,5 and the ZeroEmission Coal Alliance’s (ZECA) zero-emission coal technology (ZEC).6 The improvements of energy efficiency in these processes are achieved by the use of CO2 adsorbing materials (e.g., calcium oxide) in the coal gasifier or steam methanereforming reactor. The efficiency of the above processes with CaO is high because the expensive processes of oxygen separation and CO2 separation in a conventional system are eliminated, and the processes of coal gasification, water gas shift reaction, and CO2 capture (through the carbonation reaction CaO + CO2 f CaCO3) are combined in a single reactor.4,7 The reacted CaO will be regenerated through the decarbonation reaction (CaCO3 f CO2 + CaO) in a separate reactor and recycled back to the gasifier, and the generated pure CO2 stream is sent to storage, ready for sequestration. The chemical looping process (carbonation/decarbonation cycle) of calcium oxide plays an important role in these processes. Although many materials could be potentially used in the zero-emission power-generation systems for the carbonation reaction, calcium oxide has been identified to be the best candidate. 1 However, calcium oxide suffers from a well-known * Corresponding author. Phone: 61 7 3346 9193. Fax: 61 7 3365 4799. Email:
[email protected] (1) Feng, B.; An, H.; Tan, E. Presented at the Sino-Australia Symposium on Advanced Coal Technologies, 12-14 July 2006, Wuhan, China; Energy Fuels 2006, submitted. (2) Lin, S. Y.; Harada, M.; Suzuki, Y.; Hatano, H. Fuel 2002, 81, 20792085. (3) Lin, S. Y.; Harada, M.; Suzuki, Y.; Hatano, H. Energy Fuels 2003, 17, 602-607. (4) Lin, S. Y.; Harada, M.; Suzuki, Y.; Hatano, H. Energy ConVers. Manage. 2004, 46 (6), 869-880. (5) Zamansky V. A. Clean Coal Day in Japan, September 2002, Tokyo, Japan. (6) Zero Emission Coal Alliance. http://www.zeca.org/. (7) Deiana, P. Presented at the Second International Conference on Clean Coal Technologies for our Future, May 2005, Sardinia, Italy.
problem of loss-in-capacity (i.e., its capacity of CO2 adsorption decreases significantly after cycles of carbonation/decarbonation 1,8-13, as shown in Figure 1 a). The degree of reaction (i.e., the mole fraction of reacted CaO) decreases to only about 20% after 9 cycles. The problem of loss-in-capacity, an indication of poor reversibility of the CaO-CO2 reaction cycle, is potentially an obstacle to the adoption of the novel processes and thus needs to be solved. This paper proposes a method of overcoming the problem. The phenomenon of loss-in-capacity was observed a long time ago by Barker,8 who noticed that although the decomposition of CaCO3 is always 100%, the reactivity of CaO carbonation fell dramatically after a rapid initial reaction. He proposed that two main reasons are the loss of pore volume of the porous calcium oxide and sintering. The former resulted in a rapid decreases of reversibility in the first few cycles, while the latter resulted in slower decreases of reversibility during thefollowing cycles. Bhatia and Perlmutter 9 also investigated this reaction, and they, particularly, examined the latter stages of carbonation, characterized by very low carbonation rates, and found that this period was controlled by diffusion in the product CaCO3 layer. Mess et al. 10 also observed nonporous particles of CaO during carbonation and noticed that the product layer consists of crystalline grains, whose diameters continued to increase to the approximate dimension of the particle. Abanades and co-workers 11-13 also proposed that the decrease in maximum capacity is a result of the loss of suitable pore volume during every calcination (or decarbonation) step. Abanades and co-workers 12 calculated a 0.1 µm increase of pore radius and a thickness layer of 0.22 µm as a consequence of carbonation. Few efforts have been made to overcome the problem. Silaban et al.14 suggested using calcined dolomite as a sorbent to calcined (8) Barker, R. J. Appl. Chem. Biotechnol. 1973, 23, 733-742. (9) Bhatia, S. K.; Perlmutter, D. D. AIChE J. 1981, 29, 9 (1), 79-86. (10) Mess, D.; Sarofim, A. F.; Longwell, J. P. Energy Fuels 1999, 13, 999-1005. (11) Abanades, J. C. Chem. Eng. J. 2002, 90, 303-306. (12) Abanades, J. C.; Alvarez, D. Energy Fuels 2003, 17, 308-315. (13) Salvador, C.; Lu, D.; Anthony, E. J.; Abanades, J. C. Chem. Eng. J. 2003, 96, 187-195. (14) Silaban, A.; Narcida, P.; Harrison, D. P. Chem. Eng. Comm. 1996, 146, 149-162.
10.1021/ef060258w CCC: $33.50 © 2006 American Chemical Society Published on Web 10/05/2006
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Feng et al. component for CO2 capture. The procedure of fabricating the adsorbent is as follows. Wet Impregnation. A saturated CaCl2 solution was prepared at 353 K using 75 mL of distilled water. The sample of γ-alumina was weighed (about 5 g) and subsequently added into the solution. The mixture was stirred continuously for 12 h, while the temperature of the solution was maintained at 353 K. Drying. The mixture was filtered, and the collected particles were dried in an oven at 373 K for 1 h. Calcination. The dry particles were calcined in a furnace at 823 K in air for 6 h to convert calcium hydroxide (precipitated on the surfaces of the alumina) into calcium oxide. The sorbent was ready for testing after calcination. The sample was weighed and compared with the initial weight to determine the amount of CaO coating (which was found to be 2.95 wt % after the first coating process). The above process was repeated once to increase the amount of coating to 4.30 wt %. 2.2. Reversibility Testing. The sorbent was subsequently tested using a Cahn thermogravimetric analyzer (model TG-121). The procedure was as follows. About 5 mg of sorbent was added into the sample holder which was placed in a glass reactor. The reactor was then flushed with pure nitrogen (ultrahigh purity of 99.999%) for 30 min. The reactor was heated to 323 K at a heating rate of 10 K/min in nitrogen of 100 mL/min, and it was maintained at 323 K for 5 min. Subsequently, the reactor was heated to 1273 K at 10 K/min, and the nitrogen is switched to pure carbon dioxide (food grade) of 100 mL/min. When the temperature was reached, the reactor was maintained at 1273 K for 10 min, before the gas was switched back to pure nitrogen and the furnace was cooled to 323 K. One cycle was completed when the temperature reached 323 K. The above process was repeated 8 times to obtain the result of 9 cycles. The experiment was also repeated to test the repeatability of the result. Almost exactly the same result was obtained. The degree of reaction of the metal oxides was calculated using the equation
Figure 1. Degree of reaction of (a) the raw CaO particles and (b) CaO in the new adsorbent. The experimental procedure was exactly the same, as described in the text.
CaCO3. Salvador et al.13 tested the addition of NaCl and Na2CO3 to enhance the CaO performance and achieved limited success. This paper proposes the use of sufficiently fine particles of CaO to achieve good reversibility, on the basis of the early work of Barker,8,15 who observed that when the CaO of particle size was 10 nm, the conversion level reached as high as 93% after 30 reaction cycles at 902 K. It is hypothesized that the distribution of fine CaO particles in an inert porous framework should have no problem of loss-in-capacity. To prove this hypothesis, a calcium-based adsorbent was fabricated using porous alumina as the inert support, and the testing result confirmed that the problem of loss-in-capacity did not exist for this adsorbent. 2. Experimental Section 2.1. Fabrication of Sorbent. The sorbent was fabricated by application of a wet impregnation method and the use of granular γ-alumina and CaCl2 as the raw materials. γ-Alumina (as received from Sigma) was used as the support material providing a high surface area and a rigid framework, and CaCl2 was used as received from Sigma as the precursor for CaO particles impregnated on the pore surface of the support material. The wet impregnation method was chosen because it is easy to use in the laboratory. Because the pore size of γ-alumina is small, CaO particles of a small size will be formed in the pore space, and they are expected to be the active (15) Barker, R. J. Appl. Chem. Biotechnol. 1974, 24, 221-227.
φ)
(m - m0)/44 m0 /56
where φ (mol/mol) is the molar ratio of the reacted CaO over the initial CaO, m (g) is the mass of the solid products during reaction, and m0 is the initial mass of CaO. The maximum value of the degree of reaction, φ, is unity when CaO is fully converted. 2.3. Scanning Electron Microscopy and Transmission Electron Microscopy. The morphologies of the adsorbents, before and after cycles of reaction, were also observed under a scanning electron microscope (SEM, JEOL JSM 6400F) and a transmission electron microscope (TEM, JEOL 2010). Both instruments were available from the Center for Microscopy and Microanalysis of the University of Queensland.
3. Results and Discussion 3.1. Reversibility of CaO Carbonation. The degree of reaction of CaO in the newly prepared adsorbent is shown in Figure 1b during 9 cycles of carbonation/decarbonation. It is clear that the maximum capacity of CaO adsorption of the sorbent does not decrease after cycles, unlike the raw CaO in Figure 1a. Also, a high conversion of over 90% was maintained after 9 cycles, compared with about 20% for the raw CaO in Figure 1a. The figure clearly demonstrates that full reversibility of the CaO-CO2 reaction has been achieved for the new adsorbent. The evolution of the maximum carbonation capacity with the number of carbonation/ decarbonation cycles, obtained from the data in Figure 1, is shown in Figure 2, in comparison with the literature data.11 The trend for the raw CaO particles is similar to that reported in the literature, although the experimental conditions in this work differ from the others. The maximum
Loss-in-Capacity of Calcium Oxide in CO2 Capture
Figure 2. Evolution with the number of carbonation/calcination cycles of the maximum carbonation capacity of CaO from different authors.1,8,14,16-18 The reaction conditions have been discussed in Abanades.11
carbonation capacity of the new adsorbent is much higher than that of the raw CaO and remains high after the cycles. 3.2. SEM Pictures. The SEM pictures are shown in Figure 3 for the new adsorbent before and after 9 cycles of carbonation/ decarbonation. It can be observed that CaO in the new adsorbent showed no pronounced change in shape or size. The structure of the alumina in the new adsorbent appears to have changed (broken), probably, because of the volume increase of CaO when it becomes calcium carbonate. 3.3. Discussions. The results in this paper clearly show that the reversibility of the CaO-CO2 reaction can be improved to a satisfactorily high level (>90%). This is attributed to the high
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reactivity of CaO particles of fine sizes. Theoretically, if the particle size of CaO is as small as the thickness of the product layer that can form on a single CaO crystal under chemically controlled conditions, the capacity of the particle could be 100%. This particle size can be defined as the critical particle size under a certain reaction condition under which the CaO particles of below the critical particle size can achieve high conversion (or degree of reaction) without the problem of loss-in-capacity. This is because a typical multicrystal CaO particle will eventually turn into a single-crystal particle after an infinite number of cycles (calcination), which has the slowest reaction rate because of the smallest surface area. Apparently, if the reaction rate of a single-crystal particle is sufficiently fast, there will not be observable decrease in the reaction rate of the particle. This is the case when the particle size is sufficiently small (i.e., smaller than the critical particle size). The thickness of the product layer on a large single CaO crystal (less than 100% conversion) can be calculated, by considering the particle radius change resulting from the reaction.12 A CaCO3 layer of 220 nm thick surrounding a single crystal was found at 600 °C and 1 atm (from the experimental data of Mess et al.).10 In other words, under the experimental conditions of Mess et al., CaO particles of less than 220 nm could achieve high conversion (or degree of reaction), and if the particles are not in contact with each other, there would be no problem of lossin-capacity as well. Nevertheless, it should be pointed out that the critical particle size is sensitive to the reaction conditions, as suggested by the experimental data of Mess et al. More detailed experimental work to determine the critical particle size under various conditions is needed. The particle size of the CaO in the newly prepared adsorbent could not be determined confidently because it was difficult to distinguish CaO from alumina and obtain clear pictures of CaO
Figure 3. SEM photos of the new adsorbent, before (a and b) and after (c and d) 9 cycles of carbonation/decarbonation.
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under TEM. The particle size could not be determined from the pore size distribution change of the adsorbent after CaO coating either, because CaO could deposit on any pore surface. The γ-alumina support has a BET surface area of 153 m2/g and an average pore size of about 7 nm by the BJH method. It is thus considered safe to assume that the particle size is smaller than the critical particle size. Therefore, it is confirmed that the distribution of CaO particles of fine size on inert porous material can eliminate the problem of loss-in-capacity. The results obtained in this paper can potentially be used in the development of commercial CO2-adsorbing material (CAM) for zero-emission power-generation systems. Because the CAM is to be repeatedly used in a coal gasifier, it must attain the following chemical and physical properties under the conditions of coal gasification (high temperature of 500-1000 °C and high pressure of 1-110 bar):1 (1) high selectivity and adsorption capacity for carbon dioxide, (2) adequate adsorption/desorption kinetics for carbon dioxide, (3) stable adsorption capacity of carbon dioxide after repeated adsorption/desorption cycles (or good reversibility), (4) adequate mechanical strength and attrition resistance of sorbent particles after cyclic exposure to high-pressure/temperature wet streams, and (5) good regenerability. Natural minerals (including CaO) do not meet all the above requirements. Therefore, it appears that synthesized materials would need to be developed, in particular, CaO-based materials because of the good properties that CaO has already shown. The adsorbent tested in this work meets requirements 2-5. Also the problem of eutectic melting of the sorbent because of the effect of ash 19 seems to be avoidable because, in the new adsorbent, there is no direct contact between CaO and ash.
Feng et al.
However, because the content of CaO in the adsorbent tested in this work is relatively low (4.3%), the overall capacity of the adsorbent is not high. Nevertheless, the work demonstrates that it is possible to fabricate an adsorbent that meets all the above requirements, if the overall capacity can be enhanced. Therefore a sol-gel process is being examined in our laboratory to fabricate the adsorbents with much higher content of CaO of fine particle size. The critical particle size of CaO under various conditions is also under investigation. 4. Conclusions A CO2 adsorbent was fabricated via application of a wet impregnation method and the use of γ-alumina and CaCl2 as the raw materials. It was found that this adsorbent did not have the loss-in-capacity problem that the raw CaO particles suffered. The results imply that a commercial CO2 adsorbent could be developed by distributing fine CaO particles on inert porous support. Acknowledgment. The project was supported by the University of Queensland. EF060258W (16) Curran, G. P.; Fink, C. E.; Gorin, E. AdV. Chem. Ser. 1967, 69, 141-165. (17) Shimizu, T.; Hirama, T.; Hosoda, H.; Kitano, K.; Inagaki, M.; Tejima, K. Trans. Inst. Chem. Eng. 1999, 77, 62-68. (18) Aihara, M.; Nagai, T.; Matsushita, J.; Negishi, Y.; Ohya, H. Appl. Energy 2001, 69, 225-238. (19) Lin, S. Y.; Harada, M.; Suzuki, Y.; Hatano, H. Fuel 2006, 85, 11431150.
