Solar Carbo-Thermal and Methano-Thermal Reduction of MgO and ZnO for Metallic Powder and Syngas Production by Green Extractive Metallurgy

The solar carbo-thermal and methano-thermal reduction of both MgO and ZnO were performed in a flexible solar reactor operated at low pressure through both batch and continuous operations. The pyro-metallurgical process is an attractive sustainable pathway to convert and store concentrated solar energy into high-value metal commodities and fuels. Substituting fossil fuel combustion with solar energy when providing high-temperature process heat is a relevant option for green extractive metallurgy. In this study, a thermodynamic equilibrium analysis was first performed to compare the thermochemical reduction of MgO and ZnO with solid carbon or gaseous methane, and to determine the product distribution as a function of the operating conditions. The carbo-thermal and methano-thermal reduction of the MgO and ZnO volatile oxides was then experimentally assessed and compared using a directly irradiated cavity-type solar reactor under different operating conditions, varying the type of carbon-based reducing agent (either solid carbon or methane), temperature (in the range 765–1167 °C for ZnO and 991–1550 °C for MgO), total pressure (including both reduced 0.10–0.15 bar and atmospheric ~0.90 bar pressures), and processing mode (batch and continuous operations). The carbo-thermal and methano-thermal reduction reactions yielded gaseous metal species (Mg and Zn) which were recovered at the reactor outlet as fine and reactive metal powders. Reducing the total pressure favored the conversion of both MgO and ZnO and increased the yields of Mg and Zn. However, a decrease in the total pressure also promoted CO2 production because of a shortened gas residence time, especially in the case of ZnO reduction, whereas CO2 formation was negligible in the case of MgO reduction, whatever the conditions. Continuous reactant co-feeding (corresponding to the mixture of metal oxide and carbon or methane) was also performed during the solar reactor operation, revealing an increase in both gas production yields and reaction extent while increasing the reactant feeding rate. The type of carbon reducer influenced the reaction extent, since a higher conversion of both MgO and ZnO was reached when using carbon with a highly available specific surface area for the reactions. The continuous solar process yielded high-purity magnesium and zinc content in the solar-produced metallic powders, thus confirming the reliability, flexibility, and robustness of the solar reactor and demonstrating a promising solar metallurgical process for the clean conversion of both metal oxides and concentrated solar light to value-added chemicals.

NuFIT: Three-Flavour Global Analyses of Neutrino Oscillation Experiments

In this contribution, we summarise the determination of neutrino masses and mixing arising from global analysis of data from atmospheric, solar, reactor, and accelerator neutrino experiments performed in the framework of three-neutrino mixing and obtained in the context of the NuFIT collaboration. Apart from presenting the latest status as of autumn 2021, we discuss the evolution of global-fit results over the last 10 years, and mention various pending issues (and their resolution) that occurred during that period in the global analyses.

Effect of Carbon Particle Feeding as Radiant Absorbent for Enhanced Heat Transfer

Carbon particles can be used as catalyst in solar reactors where they serve as radiant absorbent and nucleation sites for the heterogeneous decomposition reaction. Unlike commonly used metal catalysts, carbon catalyst does not have durability problem and high cost. However, in order to achieve sustainable catalytic decomposition of feedstock over carbon catalysts at elevated temperatures, the surface area of the carbon particles must be maintained. A subsequent treatment of deactivated carbon samples with CO2 at about 1000 °C would increase the surface and would recover the original activity as catalyst. In a windowed solar reactor, carbon particles are directly exposed to the high flux irradiation providing efficient radiation heat transfer directly to the reaction site. Therefore, one of the key parameters to achieve higher conversion efficiencies in a solar reactor is the presence and transport of carbon particles. In this paper, a transient one-dimensional model is presented to describe effect of carbon particle feeding on energy transport and temperature profile of a cavity-type solar receiver. The model was developed by dividing the receiver into several control volumes and formulating energy balance equations for gas phase, particles, and cavity walls within each control volume. Monte Carlo ray tracing (MCRT) method was used to determine the solar heat absorbed by particles and cavity walls, as well as the radiative exchange between particles and cavity walls. Model accuracy was verified by experimental work using a solar receiver where carbon particles were injected uniformly. Comparison of simulation results with the experimentally measured temperatures at three different locations on cavity receiver wall showed an average deviation of 3.81%. The model was then used to study the effect of carbon particle size and feeding rate on the heat transfer, temperature profile, and energy absorption of the solar receiver. Based on the simulation results, it was found that injection of carbon particles with a size bigger than 500 µm has no significant influence on heat transfer of the system. However, by reducing the particle size lower than 500 µm, temperature uniformity and energy absorption were enhanced.

Complete Benzothiazole elimination by solar photo-Fenton in aqueous and β-cyclodextrin solutions

Benzothiazole (BTH), a ubiquitous contaminant, has been degraded by solar photo-Fenton in a solar reactor with a cylindrical-parabolic solar collector. Due to the low solubility of BTH, a β-cyclodextrin (β-CD)...

Reversible Molten Catalytic Methane Cracking Applied to Commercial Solar-Thermal Receivers

When driven by sunlight, molten catalytic methane cracking can produce clean hydrogen fuel from natural gas without greenhouse emissions. To design solar methane crackers, a canonical plug flow reactor model was developed that spanned industrially relevant temperatures and pressures (1150–1350 Kelvin and 2–200 atmospheres). This model was then validated against published methane cracking data and used to screen power tower and beam-down reactor designs based on “Solar Two,” a renewables technology demonstrator from the 1990s. Overall, catalytic molten methane cracking is likely feasible in commercial beam-down solar reactors, but not power towers. The best beam-down reactor design was 9% efficient in the capture of sunlight as fungible hydrogen fuel, which approaches photovoltaic efficiencies. Conversely, the best discovered tower methane cracker was only 1.7% efficient. Thus, a beam-down reactor is likely tractable for solar methane cracking, whereas power tower configurations appear infeasible. However, the best simulated commercial reactors were heat transfer limited, not reaction limited. Efficiencies could be higher if heat bottlenecks are removed from solar methane cracker designs. This work sets benchmark conditions and performance for future solar reactor improvement via design innovation and multiphysics simulation.

Solar-Driven Chemical Looping Methane Reforming Using ZnO Oxygen Carrier for Syngas and Zn Production in a Cavity-Type Solar Reactor

Converting sunlight into chemical fuels and metal commodities, via solar thermochemical conversion processes, is an attractive prospect for the long-term storage of renewable energy. In this study, the combined methane reforming and ZnO reduction in a single reaction for co-production of hydrogen-rich syngas and metallic Zn was demonstrated in a flexible solar thermochemical reactor prototype, driven by highly concentrated sunlight. Using solar energy as the process heat source in chemical-looping methane reforming with the ZnO/Zn oxygen carrier is a means to reduce the dependence on conventional energy resources and to reduce emissions of CO2 and other pollutants, while upgrading the calorific value of the feedstock for the production of energy-intensive and high-value chemical fuels and materials. On-sun experiments were carried out with different operating parameters including operating temperatures (800–1000 °C), inlet methane flow-rates (0.1–0.4 NL/min), and inlet ZnO feeding-rates (0.5–1.0 g/min) both in batch and continuous modes under reduced (0.15 and 0.45 bar) and atmospheric pressures (0.90 bar), thereby demonstrating solar reactor flexibility and reliability. As a result, increasing the temperature promoted net ZnO conversion at the expense of favored methane cracking reaction, which can be lowered by decreasing pressure to vacuum conditions. Diminishing total pressure improved the net ZnO conversion but favored CO2 yield due to insufficient gas residence time. Rising ZnO feeding rate under a constant over-stoichiometric CH4/ZnO molar ratio of 1.5 enhanced ZnO and methane consumption rates, which promoted Zn and syngas yields. However, an excessively high ZnO feeding rate may be detrimental, as ZnO could accumulate when the ZnO feeding rate is higher than the ZnO consumption rate. In comparison, continuous operation demonstrated greater performance regarding higher ZnO conversion (XZnO) and lower methane cracking than batch operation. High-purity metallic Zn with a well-crystallized structure and of micrometric size was produced from both batch and continuous tests under vacuum and atmospheric pressures, demonstrating suitable reactor performance for the solar thermochemical methane-driven ZnO reduction process. The produced Zn metal can be further re-oxidized with H2O or CO2 in an exothermic reaction to produce pure H2 or CO by chemical-looping.