A Model of Transient Heat and Mass Transfer in a Heterogeneous Medium of Ceria Undergoing Nonstoichiometric Reduction

Abstract

The redox chemistry of nonstoichiometric metal oxides can be used to produce chemical fuels by harnessing concentrated solar energy to split water and/or carbon dioxide. In such a process, it is desirable to use a porous reactive substrate for increased surface area and improved gas transport. The present study develops a macroscopic-scale model of porous ceria undergoing thermal reduction. The model captures the coupled interactions between the heat and mass transfer and the heterogeneous chemistry using a local thermal nonequilibrium (LTNE) formulation of the volume-averaged conservation of mass and energy equations in an axisymmetric cylindrical domain. The results of a representative test case simulation demonstrate strong coupling between gas phase mass transfer and the chemical kinetics as well as the pronounced impact of optical thickness on the temperature distribution and thus global solar-to-chemical energy conversion.

Modelling of solar thermochemical reaction systems

“…Keene et al derives an expression for adsorption and desorption of oxygen into and out of partially reduced porous ceria structures in terms of the local temperature, oxygen partial pressure, and non-stoichiometry (Keene et al, 2013). It appears in a general form as…”

“…Dense fluidised reactor Stationary reactor Material study CeO 2 (Oles and Jackson, 2015) (Groehn et al, 2016) (Bader et al, 2015;Bala Chandran et al, 2015b) (Bala Chandran et al, 2015aKeene et al, 2013Keene et al, , 2014Lapp et al, 2013;Lapp and Lipiń ski, 2014) t (Wheeler et al, 2014) (Suter et al, 2014) (Dombrovsky et al, 2012;Ganesan et al, 2013a,b) * , , (Ackermann et al, 2014(Ackermann et al, , 2017 ZnO/Zn Geometry only (Akolkar and Petrasch, 2012;Jörg Petrasch, Meier, et al, 2008;Petrasch et al, 2007) Other (Lu et al, 2016) …”

“…These studies were refined by including the relationship between pressure and velocity in Darcy's law which allowed the velocity field to be eliminated altogether and replaced by pressure in (Yue and Lipiń ski, 2015a,b,c). The approach was adopted from a study of reacting porous ceria in (Keene et al, 2013(Keene et al, , 2014. Ströhle et al used Ergun's equation-a form of Darcy's law with the Forchheimer correction-for the pressure drop through a packed bed of spheres (Ströhle et al, 2014).…”

“…Even at low densities of particles, a separate temperature for the solid phase has been used Saade et al, 2012). Homogenised equations for high-temperature heat and mass transfer in a porous system of reducing and oxidising ceria were developed by Keene et al (2013Keene et al ( , 2014 assuming LTNE between the phases and found that the temperature difference between them was only as high as 2%. Later studies that utilise this model assumed LTE (Bader et al, 2015;Bala Chandran et al, 2015a;Yue and Lipiń ski, 2015a,b,c).…”

“…Inclusion of enthalpy change due to mass diffusion for individual species has also been considered (Keene et al, 2013;Wang et al, 2014a;Yue and Lipiń ski, 2015a) as described by…”