A thermodynamic analysis
of continuous fuel production by redox
cycling of ceria in a single solar reactor under isothermal conditions
is presented. Ceria is partially reduced in a sweep gas flow of purified
nitrogen and reoxidized with either steam or carbon dioxide to produce
hydrogen or carbon monoxide, respectively. The sweep gas and oxidizer
flows are preheated by the product gases. The influence of selected
process parameters, including operating temperature, pressure, and
the effectiveness of heat recovery, on the solar-to-fuel conversion
efficiency is determined. For a solar concentration ratio of 3000,
typical of state-of-the-art solar dish concentrators, and operating
temperature of 1773 K, 95.5% of the available gas-phase heat must
be recovered to reach conversion efficiencies of 10% and 18% for hydrogen
and carbon monoxide production, respectively, assuming the flow rate
of inert sweep gas is equivalent to that in a counter-current flow
arrangement of gas and ceria. The efficiency depends strongly on the
gas-phase heat recovery effectiveness and the sweep gas flow rate.
Introducing a temperature swing of 150 K between reduction and oxidation
steps strongly reduces the sweep gas flow rate and increases the efficiency
from 10% to 31.6% for hydrogen production.
Heat transfer is predicted for a solid-solid heat recuperation system employed in a novel directly-irradiated solar thermochemical reactor realizing a metal oxide based nonstoichiometric redox cycle for production of synthesis gas from water and carbon dioxide. The system is designed for continuous operation with heat recuperation from a rotating hollow cylinder of a porous reactive material to a counter-rotating inert solid cylinder via radiative transfer. A transient heat transfer model coupling conduction, convection, and radiation heat transfer predicts temperatures, rates of heat transfer, and the effectiveness of heat recovery. Heat recovery effectiveness of over 50% is attained within a parametric study of geometric and material parameters corresponding to the design of a two-step solar thermochemical reactor.
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.
The design procedure for a 3 kWth prototype solar thermochemical reactor to implement isothermal redox cycling of ceria for CO2 splitting is presented. The reactor uses beds of mm-sized porous ceria particles contained in the annulus of concentric alumina tube assemblies that line the cylindrical wall of a solar cavity receiver. The porous particle beds provide high surface area for the heterogeneous reactions, rapid heat and mass transfer, and low pressure drop. Redox cycling is accomplished by alternating flows of inert sweep gas and CO2 through the bed. The gas flow rates and cycle step durations are selected by scaling the results from small-scale experiments. Thermal and thermo-mechanical models of the reactor and reactive element tubes are developed to predict the steady-state temperature and stress distributions for nominal operating conditions. The simulation results indicate that the target temperature of 1773 K will be reached in the prototype reactor and that the Mohr–Coulomb static factor of safety is above two everywhere in the tubes, indicating that thermo-mechanical stresses in the tubes remain acceptably low.
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