(196a) Effective Heat and Mass Transport Properties of Porous Ceria for Solar-Thermal Fuel Generation | AIChE

(196a) Effective Heat and Mass Transport Properties of Porous Ceria for Solar-Thermal Fuel Generation

Authors 

Haussener, S. - Presenter, Swiss Federal Institute of Technology Zurich, ETHZ


Thermochemical cycles for splitting H2O and CO2 via ceria-based redox reaction are considered using highly concentrated solar process heat [1,2]. The closed material cycle consists of the solar-driven reduction of ceria at 1700 K, followed by oxidation of the ceria by CO2 and/or H2O at 1000 K, generating CO and/or H2; the oxide is then recycled to the first step. For implementation in a solar reactor, the reactive oxide is preferably utilized as a porous structure that is directly exposed to high-flux solar irradiation while being subjected to the reacting gas flows. In such a configuration, the optimisation of the solar reactor for maximum solar-to-fuel energy conversion efficiency will rely on the accurate determination of the effective heat and mass transport properties of the porous material. Accordingly, candidate porous samples have been prepared and imaged using high-resolution X-ray tomography to obtain their exact 3D geometrical configuration. Direct pore-level simulations were performed on the exact geometry to calculate the morphological and effective transport properties [3-4], namely: porosity, specific surface area, pore size distribution, extinction coefficient, scattering function, conductivity, convective heat transfer coefficient, permeability, Dupuit-Forchheimer coefficient, tortuosity, and residence time. Since the porous ceria is produced via the sacrificial poreformer route, its morphology is well described by artificial media composed of a ceria matrix with transparent inclusions of spherical shape. Artificial ceria samples are used for process optimization by adjusting the morphologies and, consequently, the transport properties to the specific process needs. The effective properties can then be applied in volume-averaged governing equations for modelling and optimization of the solar reactor configuration [5].

References: [1] A. Steinfeld, Solar Thermochemical Production of Hydrogen ? A Review, Solar Energy, 78, pp. 603-615, 2005. [2] W. C. Chueh, and S. M. Haile, ?A Thermochemical Study of Ceria: Exploiting an Old Material for New Modes of Energy Conversion and CO2 Mitigation,? Phil. Trans. (accepted); W.C. Chueh and S. M. Haile, ?Ceria as a Thermochemical Reaction Medium for Selectively Generating Syngas or Methane from H2O and CO2,? ChemSusChem, 2, 735-769 (2009) [3] S. Haussener, P. Coray, W. Lipinski, P. Wyss, and A. Steinfeld, Tomography-Based Heat and Mass Transfer Characterization of Reticulate Porous Ceramics for High-Temperature Processing, ASME Journal of Heat Transfer, 132, 023305, pp. 1-9, 2010. [4] J. Petrasch, P. Wyss, R. Stämpfli, and A. Steinfeld, Tomography-based Multi-scale analyses of the 3D geometrical morphology of reticulated porous ceramics, Journal of the American Ceramic Society, 91, pp. 2659-2665, 2008. [5] S. Whitaker, The Method of Volume Averaging. In J. Bear, editor, Theory and Applications of Transport in Porous Media, Volume 13. Kluwer Academic Publishers, 1999.

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