Solar Reduction of Cobalt Oxide Particles: Rotary Kiln Reactor Model and Experimental Results

A solar rotary kiln reactor was analyzed numerically to determine how efficiently it utilizes concentrated solar energy to reduce Co3O4 to CoO as a function of reactor operational parameters, including the rotation rate, the feed rate of Co3O4, and the solar power. The solar thermal efficiency, defined as the fraction of solar energy used to drive the reduction reaction, is calculated using an axisymmetric, finite-volume model of the rotary kiln reactor. The model iteratively solves the nonlinear, coupled energy and species equations accounting for conduction heat transfer, volumetric and surface radiation heat transfer, and cobalt oxide reduction kinetics within cloud of cobalt oxide particles that moves through the reactor in a plug flow. Radiation is simulated using Monte Carlo Ray Tracing, and the reduction kinetics follow the shrinking core model. For a cloud of 15 micrometer-diameter particles with a volume fraction of 10-5, we show an optimum solar thermal efficiency of 27% with a Co3O4 feed rate of 3.6 kilograms per hour and 3.5 kilowatts of solar power. At this optimum operating point, we show the temperature and conversion fields within the reactor. Furthermore, the results of a preliminary experiment are shown and provide experimental evidence of the promise of the solar rotary kiln reactor: 18% of the Co3O4 was reduced to CoO

Model of the solar-driven reduction of cobalt oxide in a particle suspension reactor

We develop a model to investigate the impact of volume fraction and extent of mixing on the thermal efficiency with which a cavity-based particle suspension solar reactor reduces Co3O4 into CoO and O2. Thermal efficiency is defined as the fraction of solar energy used to drive the endothermic reduction. In the model, particles move continuously through the reactor in either mixed or plug flow—mixing conditions that give rise to isothermal and nonisothermal suspensions, respectively—and reduce at a rate governed by shrinking core kinetics, the parameters of which are determined using thermogravimetric data. Radiation is simulated using the Monte Carlo Ray Tracing technique. The model is applied to a reactor heated by 4 kW of point-focused solar radiation with a mean concentration ratio of 1400 suns containing monodisperse suspensions of 40 μm diameter particles with volume fractions between 1×10⁻⁵ and 1×10⁻². Thermal efficiency is insensitive to mixing for the two conditions considered. The maximum thermal efficiency obtained for mixed flow with an isothermal suspension is 34.1% at 102 g min⁻¹ and a volume fraction of 2×10⁻⁴. At the same volume fraction, the maximum thermal efficiency for nonisothermal plug flow is 33.2% at 94 g min⁻¹. Thermal efficiency is more sensitive to the volume fraction, but only below a threshold value of 2×10⁻⁴. Thus, from the perspective of coupling heat transfer to the chemical reaction, design and operational efforts of particle suspension reactors for the reduction of cobalt oxide should focus on generating suspensions of at least this threshold value rather than on mixing the particles within the suspension

Model of a Rotary Kiln Solar Reactor for the Reduction of Cobalt Oxide Particles in a Two-step, Hybrid Thermochemical Water Splitting Cycle

Thermochemical water splitting cycles remain a promising approach to produce hydrogen from water using concentrated sunlight, due to their high theoretical solar-to-hydrogen conversion efficiency. One promising cycle is a hybrid cycle based on cobalt oxide. Hydrogen is produced in two chemical steps. In one step, concentrated sunlight is used to reduce cobalt oxide from Co3O4 to CoO near 1000 °C. In the second step, the CoO is integrated into the anode of an electrolysis cell and oxidized back to Co3O4 during the electrolysis of water near room temperature to produce hydrogen. The Co3O4 is recycled, and a fraction of the hydrogen produced is fed to a fuel cell in order to provide the small electrical input for electrolysis, such that the net effect of the cycle is the splitting of water using concentrated sunlight. The ideal solar-to-hydrogen conversion efficiency is 38%. One advantage of this approach is that the fuel production step is decoupled from the solar step and proceeds at room temperature; it can be carried out where water is readily available. The cycle brings the sun to the water rather than the water to the sun. At Valparaiso University, we have been developing a rotary kiln solar reactor for the reduction of Co3O4 particles to CoO. The defining feature of this reactor is its ability to disperse the Co3O4 particles into a “cloud” spread over the volume of the reactor. The hypothesis is that this cloud enhances the direct absorption and distribution of the concentrated solar input to reaction sites, and thereby increases the thermal efficiency of the reactor. To determine the impact of the cloud of Co3O4 particles on reactor performance, we developed a numerical model that couples the radiative and non-radiative heat transfer within the cloud to the cobalt oxide reduction kinetics in order to calculate the reactor temperature and the rate of reduction of Co3O4. Radiation is simulated using Monte Carlo ray tracing, and the reduction kinetics follow the shrinking core model. Several cases of particle motion were investigated, including plug flow and mixed flow. In this presentation, we show the results of the modeling effort. Reactor thermal efficiency, which is defined as the fraction of solar energy used to drive the reduction reaction, is discussed as a function of the feed rate of Co3O4, the solar power, and the volume fraction of Co3O4 in the cloud