(315g) Reactor Design and Evaluation for the Solar Photo-Thermochemical Processing of Carbon Dioxide and Methane

Solar chemical processing of low-value feedstock gases like carbon dioxide (CO₂) and methane (CH₄) is an appealing approach to mitigate environmental emissions while at the same time store solar energy in the form of chemical fuels and produce valuable products like carbon black in a sustainable manner. This study focuses on the design and evaluation of a direct solar receiver reactor for the catalytic photo-thermochemical reduction of CO₂ and CH₄at potentially lower temperatures than established solar thermochemical approaches.

The reactor consists of a single chamber and solar flux aperture enclosing a catalytic monolith and fed by two types of gas injection ports. The reactor chamber has a mirror finish interior surface designed to act as an optical cavity to increase the photon absorption probability within the catalytic monolith. A quartz layer over the reaction chamber interior is used to mitigate surface reactions and preserve reactor integrity. The reactor is equipped with gas injection ports to generate vortex and radial flow. The vortex flow is produced adjacent to the reactor’s aperture in order to prevent carbon deposition on the quartz window, and to supplement the radial gas flow while increasing gas residence time inside the reactor.

Three types of catalytic monoliths, each of different material and structure, namely: copper mesh, zirconia foam, and quartz tube-bundle, are tested, with and without photo-catalytic coatings. The copper and zirconia catalytic monoliths demonstrated to be more suitable for thermochemical processing as they rapidly absorb the influx solar radiation to convert it into heat. The tube-bundle monolith consists of a set of quartz tubes placed together in a configuration parallel to the reactor axis. This structure has low attenuation coefficient, allows better light transmission throughout the reaction chamber, and also provides larger surface area to enhance photochemistry. The different catalytic monoliths are tested bare and with different TiO₂-metal doped photo-catalytic coatings. The coatings were applied by dip-coating the catalytic monoliths in a solution prepared using sol-gel processes.

Experimental and computational results using the different catalytic monoliths are presented. The effect of change in experimental parameters such as feed rate, ratio of CO₂ to CH₄, and radiation influx from a high-flux solar simulator are analyzed. The performance of the photo-catalysts (TiO₂) with different concentration of metal loading for processing of CO₂ and CO₂/CH₄ mixture are studied. Experimental runs with just CO₂ yielded carbon monoxide (CO) while the CO₂/CH₄ mixtures produced carbon black and CO as major constituents. These results are analyzed using different analytical techniques such as gas chromatography (GC), energy dispersive spectroscopy (EDS), X-ray powder diffraction (XRD) and field emission scanning electron microscopy (FESEM). A computational fluid dynamics (CFD) model is used to analyze the reactor design and to complement experimental evaluations. The model describes the gas flow throughout the chamber, together with radiation transport and heterogeneous chemistry across the catalytic monolith. The flow model includes transport through porous media for the copper mesh and zirconia foam monoliths, and a fully resolved geometric model of the tube-bundle quartz monolith. The obtained results indicate that the reactor design for the synergistic use of high-temperature photo- and thermo-catalysts can lead to the solar processing of gases like CO₂ and CH₄ at significantly lower temperature than solely solar thermochemical process.