(618g) Characterization and Performance of Alumina Supported Rare Earth Oxides for Hot Gas Desulfurization of Syngas

During the production of syngas from coal gasification, a number of gas phase contaminants are produced, such as particulates, tars, hydrogen sulfide and metallic vapors. In order to prevent fouling of downstream processes, these contaminants must be removed from the syngas prior to its ultimate use. The current project is focused on the removal of H₂S, which is in concentrations of 1700 – 14,000 ppmv for syngas produced by coal gasification [1,2]. The permissible limit of H₂S is dependent upon the ultimate application but typically lies in the low ppmv range. Current technologies exist for the removal of H₂S from syngas, but a low temperature adsorption process is utilized, which incurs an efficiency penalty for any process requiring high temperature syngas[3]. For an integrated gasification combined cycle plant, approximately a 10% increase in overall efficiency can be obtained through the use of hot gas desulfurization[3,4]. Previous studies of sorbents for high-temperature desulfurization have focused on Cu-, Fe-, Mn- and Zn-based regenerable sorbents, with studies of rare-earth based sorbents becoming more prevalent in recent years [1,5].  Due to the vapor pressure of metallic Zn and the equilibrium limitations of metallic Cu, Mn oxide, and Fe oxide, these materials are unsuitable for desulfurization above 600°C to produce a high purity syngas. Rare-earth oxide sorbents are stable in reducing atmospheres and have favorable equilibrium constants for desulfurization.  In addition, it has been shown that alumina supported rare-earth oxides form a highly dispersed rare-earth aluminate when the loading is kept below a critical limit[6].  This high dispersion of the rare earth elements minimizes the required loading of these costly materials.

         Alumina-supported rare earth oxides, prepared at different weight loadings (1-10%) using impregnation, have been characterized with volumetric chemisorption, Temperature Programmed Desorption (TPD), and Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) to investigate the binding between the adsorbate and the oxide. Due to the similarities in the binding sites between CO₂ and H₂S, CO₂ has been used as a probe molecule. DRIFTS data suggests that the addition of lanthanum to alumina results in formation of a La₂O₃ domain that retains the adsorption characteristics of unsupported La₂O₃. Using this information, the CO₂adsorption behaviour has been related to the hot gas desulfurization performance. In addition, the characterization and performance of other supported rare earth oxides including cerium oxide, samarium oxide and europium oxide will be discussed.

[1] Meng, X., de Jong, W., Pal, R., & Verkooijen, A. H. Fuel Process. Technol., 2010. 91(8), 964.

[2] Meng, X.M., De Jong, W., Verkooijen A.H.M. Environ. Prog. Sustainable Energy. 2009. 28(3), 360 [3] Gangwal, S., Portzer, J., Roberts, G., & Kozup, S. Engineering Evaluation of Hot-Gas Desulfurization with Sulfur Recovery. Tech. rep., Research Triangle Institute, 1998.

[4] Gasper-Galvin, L. D., Atimtay, A. T., & Gupta, R. P. Ind. Eng. Chem. Res., 1998. 37(10), 4157.

[5] Cheah, S., Carpenter, D.L., and Magrini-Bair, K.A. Energy Fuels, 2009. 23. 5291.

[6] Bettman, M., Chase, R. E., Otto, K., & Weber, W. H. J. Catal., 1989. 117(2), 447.