Synthesis of CaO-Based Sorbents for CO2 Capture by a Spray-Drying

Article pubs.acs.org/est

Synthesis of CaO-Based Sorbents for CO2 Capture by a Spray-Drying Technique Wenqiang Liu,*,† Junjun Yin,‡ Changlei Qin,‡ Bo Feng,*,‡ and Minghou Xu† †

State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China School of Mechanical and Mining Engineering, The University of Queensland, St. Lucia, Queensland 4072, Australia

‡

S Supporting Information *

ABSTRACT: Highly effective and durable CO2 sorbents were synthesized with different calcium and support precursors using a spray-drying technique. It was found that spray-drying could be a useful technique for producing sorbents with enhanced cyclic performance, especially when Dgluconic acids of calcium and magnesium were used. Seven sorbents were synthesized with five calcium precursors and three inert solid precursors, and the sorbent made from calcium D-gluconte monohydrate and magnesium D-gluconate hydrate with 75 wt % CaO content achieved a high CO2 sorption capacity of 0.46 g of CO2/g of calcined sorbent at the 44th cycle of carbonation and calcination.

1. INTRODUCTION Carbon capture and storage (CCS), as one of the major solutions to the global warming issue, has attracted great attention. The major barrier in CCS has been identified as the process of CO2 capture, which accounts for the majority of the total cost.1 There are many capture technologies, including chemical absorption, physical adsorption, membrane separation, cryogenic separation, oxy-fuel combustion, chemical looping process, and calcium looping process1 In comparison to the high cost of capture with the commercial first-generation solvent technology,1 the cost of capture could be much lower with the technologies being actively developed. Particularly, the calcium looping process (CLP) showed the potential of achieving a cost of ∼US$15 per ton of CO2 captured based on economic analyses, in comparison to ∼US$24 per ton for a standard oxy-fuel circulating fluidized bed combustion (CFBC) process (best estimate).2 Therefore, CLP has attracted great attention in the world recently. Currently, a few postcombustion capture pilot plants based on CLP are being demonstrated in Canada,3 Spain,4 UK,5 and Germany,6 showing the commercialization potential of the technology. The calcium looping process is based on the reversible reaction CaO + CO2⇔CaCO3. The forward reaction is a twostage carbonation reaction including the initial chemical reaction-controlled stage followed by the diffusion-controlled stage.7,8 The backward reaction is a calcination reaction that decomposes CaCO3 back to CaO for the subsequent cycle of carbonation. The use of this reversible reaction for CO2 capture has some attractive aspects: (i) the backward calcination reaction has been found to be fast and complete under appropriate operating conditions, (ii) the CO2 carrying capacity of CaO is very high (up to 0.7857 g of CO2/g of CaO), and (iii) the source of calcium sorbent such as limestone is abundant and cheap. However, some problems remain © 2012 American Chemical Society

unsolved, such as the efficient heat recuperation and integration with the steam cycle, the management of ash and other impurities, the acceptable attrition resistance of CaO, and maintaining stable capacity of CaO with the number of cycles. This paper concerns with the last problem. It was known that the CO2 capture capacity of CaO decays quickly with the cycle number of carbonation/calcination due to the sintering of CaO and CaCO3 crystals and grains during the cyclic process, resulting in the loss of reaction surface area and pore volumes.8,9 The activity decay problem could be overcome through two major strategies, i.e., enhancing the performance of natural minerals and synthesizing sintering resistant sorbents. It is worth noting that synthetic sorbents are unlikely to be able to operate in coal-based postcombustion calcium looping systems when considering the increased cost of materials and the necessary purge of CaSO4 and coal ash from the system to prevent their accumulations. However, these material could be still used in natural gas based systems such as sorptionenhanced reforming10 and chemical loops of Ca and redox chemical looping.11 Therefore, a lot of efforts have been put into the synthesis of sorbents, and a range of synthesis methods have been reported so far, such as (i) the utilization of potentially sintering-resistant calcium precursors, (ii) doping with metal salts, and (iii) the dispersion of CaO particles into an inert matrix. A recent review12 summarized more than 10 methods that have been used to disperse fine CaO into inert materials, such as Ca12Al14O33,13−17 Ca9Al6O18,18 CaTiO3,19,20 MgO,14,21 MgAl2O4,22 SiO2,23 SBA-15,24 and cement,25,26 to slow down Received: Revised: Accepted: Published: 11267

May 4, August August August

2012 31, 2012 31, 2012 31, 2012

dx.doi.org/10.1021/es301783b | Environ. Sci. Technol. 2012, 46, 11267−11272

Environmental Science & Technology

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sintering occurring inside the particles. Most methods were able to improve the cyclic CO2 capture capacity of CaO. In particular, Liu et al.18 reported a simple but effective wet mixing method to produce a range of sorbents using different calcium precursors and inert materials precursors, all exhibiting sustained high capacity over many cycles (up to 50 cycles). The general steps for most synthesis methods in the literature (except physical mixing) include (i) mixing calcium precursors and inert precursors with water, (ii) using different methods to improve the dispersion of calcium precursors with inert precursors, and some researchers also used different treatments to ensure better physical properties, (iii) drying the mixtures to obtain dried solids, and (iv) calcining the dried solids to obtain final sorbents. The third step in the literature generally used an oven that consumed a lot of energy and is slow. Therefore, there is a need to reduce the energy cost and increase the rate of drying for all the synthesis methods in the literature. Spray-drying, which is a commercial method of quick drying to transform liquid or slurry to dry powders, has been widely used in the food and pharmaceutical industries.27 In the process of spray-drying, the liquid or slurry is pumped, atomized, and fed into a chamber to form a spray, which is evaporated to form dry powders by mixing with hot gas.27 The spray-drying process is fast and much more efficient than an oven-drying process. It is envisaged that the spraying process could be used for drying the CO2 sorbent, but so far there are no results reported in the literature. The objective of this work is to find out whether the spray-drying technique can be used to replace the conventional drying methods and produce sintering-resistant sorbents with high cyclic capacity.

Table 1. Spray-Drying Operating Conditions for Sorbents and the Components in Final Sorbentsa sample name

inlet temp (C)

aspirator setting (m3/h)

pump settings (mL/min)

nozzle air flow (l/h)

CA-MA-75 CL-MA-75 CG-MG-75 CG-MG-75-1 CG-MG-75-2 CaO160 nm-CE-75 CH-CE-75

200 200 200 200 200 200 200

38 38 38 38 38 38 38

7.5 7.5 7.5 4.5 1.5 7.5 7.5

421 421 421 421 421 541 541

a

Notations: CA, calcium acetate hydrate (∼99%, Sigma-Aldrich); CL, calcium L-lactate hydrate (98%, Fluka); CG, calcium D-gluconate monohydrate (98%, Sigma); CH, calcium hydroxide (96%, Ajax Finechem); CaO160 nm, (99%, Amresco); MG, magnesium Dgluconate hydrate (98%, Sigma); 75, the weight fraction of CaO in the sorbent. Two more sorbents (CG-MG-75-1 and CG-MG-75-2) were also made from CG and MG with different pump settings.

900 °C for 30 min in air in a furnace, using different calcium and inert solid precursors (Table 1). The notations for the synthesized sorbents are summarized in Table 1. For example, the notation CA-MA-75 means that the sorbent with 75 wt % CaO was synthesized from calcium acetate hydrate and magnesium acetate tetrahydrate. In addition, two more samples (CG-MG-75-1 and CG-MG-75-2) were produced to investigate the effect of spray-drying operating conditions (pump settings). 2.2. Sorbent Performance Tests. In a thermogravimetric analyzer (Cahn TG121), carbonation was conducted at 650 °C for 30 min in 15% CO2, while calcination was conducted at 900 °C for 10 min in 100% N2. The weight and temperature of the sample were continuously monitored and recorded during the entire process. The capture capacity of CO2 (g of CO2/g of calcined sorbent) and conversion of CaO (g of reacted CaO/g of CaO) in the sample were calculated from the weight change and used as indicators of CO2 capture performance. The detailed calculations of carbonation conversion of CaO in sorbents and the sorption capacity of sorbents can be found in our recent paper.21 2.3. Sample Characterization: X-ray Diffraction (XRD) Patterns. The diffraction patterns of samples were measured in a Bruker D8 Advance X-ray diffractometer equipped with a copper tube, graphite monochromator, and scintillation counter.

2. EXPERIMENTAL SECTION 2.1. Preparation of Sorbents. The strategy for sorbent preparation was to produce ultrafine CaO particles separated by inert solid particles. Recently, we have screened suitable calcium precursors for the synthesis of calcium-based sorbent.28 In this work, we selected calcium hydroxide, calcium Dgluconate monohydrate, calcium L-lactate hydrate, calcium acetate hydrate, and nanosized calcium oxide ( CA-MA-75 ∼ CL-MA-75 > CaO160 nm-CE-75 > CH-CE-75. However, the CO2 capture capacity ranking for sorbents carbonated after 30 min was changed to CL-MA-75 > CaO160 nm-CE-75 > CA-MA-75 > CG-MG-75 > CH-CE-75. The differences between the sorbents may be due to the variances of the nucleation rates of precursors during the calcination step of synthesis process, which was also used to explain the differences in the carbonation conversions between the CaOs calcined from different precursors.28 3.3.2. Multiple Cycles of Carbonation and Regeneration. To be practically used in CLP, the sorbents should also have good reactivity during multiple cycles of carbonation and regeneration, which is one of the major criteria to judge the performance of sorbents. Figure 4 shows the profiles of 30-min conversions of various sorbents when cyclically tested in the simulated conditions of CLP. It was found that all sorbents

Figure 4. CaO conversion of multiple cycles of carbonation and calcination of CaO sorbents produced using the spray-drying technique.

exhibited a decay of conversion with the cycle number of carbonation and calcination, except the sorbent CG-MG-75, which maintained a conversion of >83% after 22 cycles. It is interesting to note that an initial increase of conversion occurs for the sorbent CaO160 nm-CE-75, which exhibited an increase in conversion for the first two cycles. The increase of conversion could be due to a self-reactivation phenomenon that has also been reported by other researchers.31−33 Although a pore−skeleton model32 and long carbonation times33 were used to explain this phenomenon for highly sintered sorbent, the mechanism for our sorbents, which are not highly sintered, is unclear. The sorbent CL-MA-75 also shows initial self11270

dx.doi.org/10.1021/es301783b | Environ. Sci. Technol. 2012, 46, 11267−11272

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reactivation and high conversions of >90% over the first six cycles but later its conversion decays to only 55.3% at the 20th carbonation cycle. It should be also noted that the reason for the lower conversion of CH-CE-75 even from the initial cycles might be due to the reaction of CaO with CaAl2O4 to form Ca12Al14O33, therefore decreasing the actual CaO fraction in the sorbents.21 Figure 4 also summarized the 30-min conversions after each cycle of carbonation of various sorbents. It is clear that sorbent CG-MG-75, which was produced from calcium D-gluconate monohydrate and magnesium D-gluconate hydrate, shows the highest cyclic capacity in comparison to other sorbents. Therefore, a prolonged cycle of carbonation and regeneration was conduced to further investigate its performance. Due to the experimental limit, the maximum number of cycles for each run was 22. In this work, two runs were conducted showing the performance of the sorbent over 44 cycles, as shown in Figure 5. There was a time interval of overnight between each run and

the morphology of the sorbents is changing from an irregular structure to a spherical structure (Figure S4). A detailed analysis is needed to explain the changing trend in the future. A review of experimental results available using large-scale calcium looping reactors revealed that the particle residence time is on the order of 1−5 min;34 therefore, shorter residence time (3-min carbonation conversions) for all samples are summarized in Figure S3 (Supporting Information) and Figure 5B. It was found that although the cyclic conversion is lower than that of 30-min carbonation owing to the shorter carbonation time, the 3-min conversion of sorbent CG-MG75 still achieved a high conversion of 63.5% (0.374 g of CO2/g of calcined sorbent) at the 44th cycle, as shown in Figure 5B & C. 3.4. Discussion. Within the tested cycles, the spray-drying technique investigated in this work can produce highly durable capacity sorbents. Using this method, we were able to produce sorbents quickly, simultaneously maintaining a reasonable mixing of calcium and inert solid. However, except the sorbent CG-MG-75, the reactivity of the sorbents is generally not as good as the ones produced using the wet mixing method, which is probably due to the less uniform distribution of the active calcium oxide and inert solid in comparison to drying in the oven. Further work should be conducted to investigate the optimum operating conditions of the spray-dryer. In addition, the spray-drying technique is producing sorbents in the form of powders; however, in practical cases the sorbents are required to be in the form of particles. We proposed that the spraydrying in this work can be used as the first stage of manufacturing sorbents in the form of particles, and the sorbents produced from D-gluconic acids of calcium and magnesium, which were found to be able to react quickly and maintained high CO2 capture capacity with the number of cycles, can readily be manufactured in the form of particles, for example, using a screw extruder.25 The cost of the sorbents could be reduced by utilizing waste materials containing CG.21

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ASSOCIATED CONTENT

S Supporting Information *

Sample characterization including sorbents preparation, porosimetric analysis, particle size distribution, and morphologies changes with cycles; one table showing specific BET surface areas of uncalcined sorbents collected after spraying drying; four figures showing isotherm linear plot and BJH pore size distributions for the uncalcined sorbent collected from the cyclone prepared under different spray-drying conditions; particle size distributions of three calcined sorbents prepared under different spray-drying conditions; 3-min conversion under multicycles of carbonation and calcination of sorbents; and SEM images of sorbent CG-MG-75 before cycles and after 44 cycles. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 5. Multiple cycles of carbonation and regeneration of sorbent CG-MG-75 produced using the spray-drying technique, in comparison to the conversion of CaO produced directly from calcium D-gluconate (CG-CaO) and conventional calcium carbonate (CC-CaO), both adapted from Liu et al.28 (A, CaO conversion profile; B, CaO conversion; C, 30-min CO2 capture capacity).

the conversion decreases from 83.3% to 79.5% during the first carbonation of the second run. The reason for the decrease needs further investigation. However, the conversion increases quickly back to 85%. It was found that the sorbent exhibits excellent cyclic capacity and achieves a high CO2 sorption capacity of 0.46 g of CO2/g of calcined sorbent (78.7% CaO conversion) at the 44th cycle of carbonation and calcination. In comparison, the CaO sorbent produced from conventional calcium carbonate decreased to 18.9% at the 24th cycle, even CaO sorbent directly produced from calcium D-gluconate only achieved a conversion of 44% at the 44th cycle. This indicates that the MgO in the sorbent is acting as an inert solid matrix preventing the sintering between CaO particles during the cyclic process of carbonation and calcination. Figure S4 (Supporting Information) shows the SEM images of sorbent CG-MG-75 before cycles and after 44 cycles. It was found that

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to give special thanks to Professor Tim Langrish from the University of Sydney for his assistance on Buchi B-290 11271

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spray dryer. The authors are grateful for the financial support of the Department of Resources, Energy and Tourism under the Australia−China Joint Coordination Group on Clean Coal Technology grant scheme. The project was also supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51021065).

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(20) Aihara, M.; Nagai, T.; Matsushita, J.; Negishi, Y.; Ohya, H. Development of porous solid reactant for thermal-energy storage and temperature upgrade using carbonation/decarbonation reaction. Appl. Energy 2001, 69 (3), 225−238. (21) Qin, C.; Liu, W.; An, H.; Yin, J.; Feng, B. Fabrication of CaObased sorbents for CO2 capture by a mixing method. Environ. Sci. Technol. 2012, 46 (3), 1932−1939. (22) Li, L.; King, D. L.; Nie, Z.; Li, X. S.; Howard, C. MgAl2O4 spinel-stabilized calcium oxide absorbents with improved durability for high-temperature CO2 capture. Energy Fuels 2010, 24 (6), 3698−3703. (23) Wang, M. H.; Lee, C. G.; Ryu, C. K. CO2 sorption and desorption efficiency of Ca2SiO4. Int. J. Hydrogen Energy 2008, 33 (21), 6368−6372. (24) Huang, C. H.; Chang, K. P.; Yu, C. T.; Chiang, P. C.; Wang, C. F. Development of high-temperature CO2 sorbents made of CaObased mesoporous silica. Chem. Eng. J. 2010, 161 (1−2), 129−135. (25) Qin, C.; Yin, J.; An, H.; Liu, W.; Feng, B. Performance of extruded particles from calcium hydroxide and cement for CO2 capture. Energy Fuels 2012, 26 (1), 154−161. (26) Manovic, V.; Anthony, E. J. Long-term behavior of CaO-based pellets supported by calcium auminate cements in a long series of CO2 capture cycles. Ind. Eng. Chem. Res. 2009, 48 (19), 8906−8912. (27) Goula, A. M.; Adamopoulos, K. G. A new technique for spray drying orange juice concentrate. Innovative Food Sci. Emerging Technol. 2010, 11 (2), 342−351. (28) Liu, W.; Low, N. W. L.; Feng, B.; Wang, G.; Diniz da Costa, C. Calcium precursors for the production of CaO sorbents for multicycle CO2 capture. Environ. Sci. Technol. 2010, 44 (2), 841−847. (29) Islam, M. I. U.; Langrish, T. A. G. An investigation into lactose crystallization under high temperature conditions during spray drying. Food Res. Int. 2010, 43 (1), 46−56. (30) Mess, D.; Sarofim, A. F.; Longwell, J. P. Product layer diffusion during the reaction of calcium oxide with carbon dioxide. Energy Fuels 1999, 13 (5), 999−1005. (31) Manovic, V.; Anthony, E. J.; Grasa, G.; Abanades, J. C. CO2 looping cycle performance of a high-purity limestone after thermal activation/doping. Energy Fuels 2008, 22 (5), 3258−3264. (32) Manovic, V.; Anthony, E. J. Thermal activation of CaO-based sorbent and self-reactivation during CO2 capture looping cycles. Environ. Sci. Technol. 2008, 42 (11), 4170−4174. (33) Arias, B.; Abanades, J. C.; Anthony, E. J. Model for selfreactivation of highly sintered CaO particles during CO2 capture looping cycles. Energy Fuels 2011, 25 (4), 1926−1930. (34) Charitos, A.; Rodríguez, N.; Hawthorne, C.; Alonso, M. n.; Zieba, M.; Arias, B.; Kopanakis, G.; Scheffknecht, G. n.; Abanades, J. C. Experimental validation of the calcium looping CO2 capture process with two circulating fluidized bed carbonator reactors. Ind. Eng. Chem. Res. 2011, 50 (16), 9685−9695.

REFERENCES

(1) Bert, M.; Ogunlade, D.; Heleen, d. C.; Manuela, L.; Leo, M. IPCC special report on carbon dioxide capture and storage; The Intergovernmental Panel on Climate Change; 2005. (2) Abanades, J. C.; Grasa, G.; Alonso, M.; Rodriguez, N.; Anthony, E. J.; Romeo, L. M. Cost structure of a postcombustion CO2 capture system using CaO. Environ. Sci. Technol. 2007, 41 (15), 5523−5527. (3) Lu, D. Y.; Hughes, R. W.; Anthony, E. J. Ca-based sorbent looping combustion for CO2 capture in pilot-scale dual fluidized beds. Fuel Process. Technol. 2008, 89 (12), 1386−1395. (4) Abanades, J. C.; Alonso, M.; Rodriguez, N.; González, B.; Fuentes, F.; Grasa, G. Capture of CO2 from flue gases with CaO: Results from a 30 kW interconnected fluidized bed facility. In In-situ Carbon Removal 4; Imperial College: London, 2008. (5) Mašek, O.; Bosoaga, A.; Oakey, J. E. Progress in Ca-based carbon oxide capture research at Cranfield University. In In-situ Carbon Removal 4; Imperial College: London, 2008. (6) Hawthorne, C.; Charitos, A.; Perez-Pulido, C. A.; Bing, Z.; Scheffknecht, G. Design of a dual fluidised bed system for the postcombustion removal of CO2 using CaO. Part I: CFB carbonator reactor model. In 9th International Conference on Circulating Fluidized Beds; Hamburg, Germany, 2008. (7) Abanades, J. C.; Alvarez, D. Conversion limits in the reaction of CO2 with lime. Energy Fuels 2003, 17 (2), 308−315. (8) Barker, R. The reversibility of the reaction CaCO3↔CaO+CO2. J. Appl. Chem. Biotechnol. 1973, 23 (10), 733−742. (9) Alvarez, D.; Abanades, J. C. Determination of the critical product layer thickness in the reaction of CaO with CO2. Ind. Eng. Chem. Res. 2005, 44 (15), 5608−5615. (10) Harrison, D. P. Sorption-enhanced hydrogen production: A review. Ind. Eng. Chem. Res. 2008, 47 (17), 6486−6501. (11) Abanades, J. C.; Murillo, R.; Fernandez, J. R.; Grasa, G.; Martínez, I. New CO2 capture process for hydrogen production combining Ca and Cu chemical loops. Environ. Sci. Technol. 2010, 44 (17), 6901−6904. (12) Liu, W.; An, H.; Qin, C.; Yin, J.; Wang, G. G. X.; Feng, B.; Xu, M., Performance enhancement of calcium oxide sorbents for cyclic CO2 captureA review. Energy Fuels 2012. (13) Li, Z. S.; Cai, N. S.; Huang, Y. Y. Effect of preparation temperature on cyclic CO2 capture and multiple carbonation− calcination cycles for a new Ca-based CO2 sorbent. Ind. Eng. Chem. Res. 2006, 45 (6), 1911−1917. (14) Pacciani, R.; Müller, C. R.; Davidson, J. F.; Dennis, J. S.; Hayhurst, A. N. Synthetic Ca-based solid sorbents suitable for capturing CO2 in a fluidized bed. Can. J. Chem. Eng. 2008, 86 (3), 356−366. (15) Martavaltzi, C. S.; Lemonidou, A. A. Development of new CaO based sorbent materials for CO2 removal at high temperature. Microporous Mesoporous Mater. 2008, 110 (1), 119−127. (16) Luo, C.; Zheng, Y.; Ding, N.; Zheng, C. Enhanced cyclic stability of CO2 adsorption capacity of CaO-based sorbents using La2O3 or Ca12Al14O33 as additives. Kor. J. Chem. Eng. 2011, 28 (4), 1042−1046. (17) Florin, N.; Fennell, P. Synthetic CaO-based sorbent for CO2 capture. Energy Procedia 2011, 4 (0), 830−838. (18) Liu, W.; Feng, B.; Wu, Y.; Wang, G.; Barry, J.; Diniz da Costa, J. C. Synthesis of sintering-resistant sorbents for CO2 capture. Environ. Sci. Technol. 2010, 44 (8), 3093−3097. (19) Wu, S. F.; Zhu, Y. Q. Behavior of CaTiO3/Nano-CaO as a CO2 reactive adsorbent. Ind. Eng. Chem. Res. 2010, 49 (6), 2701−2706. 11272

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