Energy & Fuels 2004, 18, 569-575
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Carbon Dioxide Capture Using Dry Sodium-Based Sorbents Y. Liang and D. P. Harrison* Gordon A. and Mary Cain Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803
R. P. Gupta, D. A. Green, and W. J. McMichael Research Triangle Institute, P.O. Box 12194, Research Triangle Park, North Carolina 27709 Received September 11, 2003
Electrobalance and fixed-bed reactors have been used to study the capture of CO2 from simulated flue gas using a regenerable Na2CO3 sorbent. CO2 capture was effective in the temperature range of 60-70 °C, while regeneration occurred in the range of 120-200 °C, depending on the partial pressure of CO2 in the regeneration gas. Equal molar quantities of CO2 and H2O are produced during sorbent regeneration, and pure CO2 suitable for use or sequestration is available after condensation of the H2O. Capture of as much as 90% of the CO2 was possible at appropriate reaction conditions, and little or no reduction in either carbonation rate or sorbent capacity was observed in limited multicycle tests. The concept is potentially applicable to the capture of CO2 from existing fossil fuel-fired power plants, where amine scrubbing is the only CO2 capture process currently available.
Introduction Global warming, increasingly thought to be associated with the atmospheric emission of greenhouse gases, principally CO2, is emerging as the key environmental issue of the early 21st century. Since the beginning of the industrial revolution in about 1850, the average atmospheric concentration of CO2 has increased from 280 ppmv to 370 ppmv while the average global temperature has increased between 0.6 °C and 1 °C in the same time period.1 Continued uncontrolled greenhouse gas emissions may, in the future, contribute to increases in sea level and increased frequency and intensity of climatic extremes such as hurricanes and floods. Annual CO2 emissions in the year 2000 in the United States were about 5.9 billion metric tonnes, roughly equally divided between the transportation, commercial and residential, and industrial sectors.2 Initial efforts to limit CO2 emissions will no doubt focus on large stationary sources, with fossil fuel-fired power plants obvious prime targets. A number of new power generation concepts that may result in CO2 control are being developed. These include O2 combustion with CO2 recycle,3 precombustion decarbonization,4 and chemical * Corresponding author. Tel: (225) 578-3066. Fax: (225) 578-1476. E-mail:
[email protected]. (1) Berger, A. The Effect of Greenhouse Gases on Climate. Proceedings of the Conference on Future Energy Systems and Technology for CO2 Abatement, Antwerp, Belgium, Nov. 18-19, 2002; pp 1-18. (2) Energy Information Administration, 2003, U.S. Department of Energy, www.eia.doe.gov. (3) Douglas, M.; Zheng, L.; Bulut, D.; Tan, Y.; Thambimuthu, K.; Jamal, A.; Berruti, A.; McArthur, J.; Curran, K. Oxy-Combustion Field Demonstration Project. Proceedings of the Second Annual Conference on Carbon Sequestration, Alexandria, VA, May 5-8, 2003. http:// www.carbonsq.com/papers.cfm.
looping combustion.5,6 However, in addition to being costly and energy intensive, these processes cannot generally be retrofitted to the large number of existing power plants. The only currently available process for capturing CO2 from flue gas that is also capable of being retrofitted to existing plants is based on amine scrubbing. For example, the Econamine FG Plus process,7 which uses a solvent of monoethanolamine with an oxidation inhibitor, has been used commercially in a number of plants for CO2 recovery. However, scrubbing processes are also costly and energy intensive because of the large volume of gas to be treated, the low partial pressure of CO2 in the flue gas, the presence of contaminants that may be detrimental to the solvent, and the energy demand associated with solvent regeneration. Solid sorbent processes for CO2 capture are also under study. This paper reports preliminary results on a dry, regenerable, sodium-based sorbent process that is potentially applicable to existing as well as new power plants. CO2 is removed from the flue gas by reaction (4) Doctor, R.; Molburg, J.; Brockmeier, N.; Manfredo, L.; Gorokhov, V.; Ramezan, M.; Stiegal, G.; Life-Cycle Analysis of a Shell GasificationBased Multi-Product System with CO2 Recovery. Proceedings of the First Annual Conference on Carbon Sequestration, Washington, DC, May 14-17, 2001. http://www.netl.gov/publications/proceedings/01/ carbon_sq/4b1.pdf. (5) Hurst, P.; Boden, J.; Wilkinson, M.; Simmonds, M. Chemical Looping Combustion for CO2 Capture. Proceedings of the Second Annual Conference on Carbon Sequestration, Alexandria, VA, May 5-8, 2003. http://www.carbonsq.com/papers.cfm. (6) Yu, J.; Corripio, A.; Copeland, R.; Harrison, D. Adv. Environ. Sci. 2003, 7, 335-345. (7) Reddy, S.; Scherffius, J.; Freguia, S.; Roberts, C. Fluor’s Econamine FG PlusSM Technology. Proceedings of the Second Annual Conference on Carbon Sequestration, Alexandria, VA, May 5-8, 2003. http://www.carbonsq.com/papers.cfm.
10.1021/ef030158f CCC: $27.50 © 2004 American Chemical Society Published on Web 02/17/2004
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with Na2CO3 while sorbent regeneration produces an off-gas containing only CO2 and H2O. Condensation of H2O produces a pure CO2 stream suitable for subsequent use or sequestration. Chemistry and Thermodynamics The important reactions involved in the capture of CO2 using Na2CO3 are
Na2CO3(s) + CO2(g) + H2O(g) T 2NaHCO3(s) (1) ∆Hr° ) -135 kJ/mol Na2CO3 Na2CO3(s) + 0.6CO2(g) + 0.6H2O(g) T 0.4[Na2CO3‚3NaHCO3](s) (2) ∆Hr° ) -82 kJ/mol Na2CO3 The product of the second reaction is known as Wegscheider’s salt or Wegscheiderite. Both reactions are reversible and highly exothermic so that energy management will be an important consideration in a commercial system. Other potential reaction products, such as sodium sesquicarbonate, Na2CO3‚NaHCO3‚2H2O, and the hydrate, NaHCO3‚H2O, do not appear to be important at the reaction conditions of interest. Contaminants in flue gas such as SO2 and HCl will react irreversibly with Na2CO3 and must therefore be reduced to low levels prior to CO2 capture. Equilibrium analysis using HSC Chemistry8 suggests that NaHCO3 should be the only product formed at the reaction conditions studied. However, phase diagrams supplied by Church and Dwight, Inc.,9 the supplier of the sorbent precursor used in the experimental study, indicate that Wegscheider’s salt is favored at reaction temperatures of 70 °C and above at most of the H2O and CO2 partial pressures studied. X-ray diffraction analysis of the product from a fixed-bed reactor test confirmed the formation of Wegscheider’s salt. The discussion and analysis of electrobalance experimental results in the following sections are based on the formation of Wegscheiders’s salt at carbonation temperatures of 70 °C and above, and NaHCO3 at temperatures below 70 °C. The fixed-bed reactor product is treated as Wegscheider’s salt even in tests using a nominal 60 °C carbonation temperature because of the highly exothermic heat of reaction. Experimental Section Experimental studies were performed using both an electrobalance (TGA) and a small-scale fixed-bed reactor. The electrobalance was used to study both the carbonation and regeneration reactions as a function of temperature and gas composition and to perform limited multicycle tests. Helium was substituted for N2 and O2 to increase the electrobalance sensitivity. CO2 and He were obtained from high purity cylinders and their flow was monitored using calibrated rotameters and needle valves. H2O was fed using a syringe pump and the feed lines were heat traced to ensure complete vaporization before mixing with the permanent gases. (8) Roine, A. HSC Chemistry for Windows, Version 4.0, User’s Guide, Outokumpu Research Oy, Pori, Finland, 1999. (9) Church and Dwight Co. Personal communication, 2000.
Figure 1. Typical electrobalance response in a single calcination-carbonation cycle. Table 1: Selected Properties of Church and Dwight Grade #3 NaHCO3 composition
NaHCO3
>99%
sulfate chloride heavy metals
