(688g) Synthesis and Characterization of Aerosol Particles and Reactions for the Solar Driven Hybrid-Sulfur-Ammonia Water-Splitting Cycle

Thermochemical water-splitting cycles are some of the most promising and renewable methods for hydrogen production. The Hybrid-Sulfur-Ammonia (HySA) cycle is one such cycle that ideally utilises the whole spectrum of solar irradiation by combining one photochemical with three thermochemical steps. All steps have been studied in the past, but there are still open challenges and opportunities to improve the reported solar-H₂ efficiency of around 25%. One of the advantages of the HySA cycle is the fluid-only thermochemical steps, i.e. no crystallisation and solidification of the employed sulfate-pyrosulfate molten salts. However, to achieve this requirement, large quantities of the pyrosulfate salt, e.g. 10:1 for potassium, are required in order to maintain the binary solid-solution in the liquid state. Consequently, such large quantities lead to large process streams, excess heating energy, and massive storage and reactor units.

Earlier our group presented the concept of an approach that addresses this issue by conducting all thermochemical steps with aerosol particles, i.e. solid-gas states instead of liquid-gas states. Also, aerosol phase reactors could utilize direct solar heating with the potential for higher solar to chemical energy efficiency compared to the indirect ones. Currently, there are multiple studies on particle reactors such as moving and fluidization beds but very few on aerosol reactors. On the other hand, aerosol phase processes are very similar to solid fuel combustion and gasification, and free-board reactions. Therefore, in this study, we investigated the potential and efficiency of the aerosol phase reactors in the HySA cycle experimentally and as an extension in other similar thermochemical processes. Particles were synthesized via evaporation, crystallization, and drying of precursor aqueous solutions at various potassium sulfate, potassium pyrosulfate, and ammonium sulfate ratios. Ammonium sulfate is a product of earlier steps that participates in the thermochemical steps. The exact mechanism, onset temperatures, and reaction energies were defined at bench-scale with a combination of thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier transformation infrared spectroscopy (FTIR), and mass spectroscopy (MS). For a selected number of particle compositions and later reference, reactions were studied at the “bulk” phase (fixed bed) in a high-temperature tube reactor while evolved gases were continuously monitored via an MS. Finally, the same were studied in an aerosol phase reactor, where particles were fed using a powder feeder in a countercurrent flow configuration. Overall, reaction kinetic rates in the aerosol reactor were similar with the bench scale experiments, but conversion efficiency was lower although significantly higher than in the bulk phase. As a next step, the solar to chemical energy efficiency will be investigated utilizing a high-flux solar simulator and the presented aerosol reactor.