A critical step of sulfur based thermochemical cycles for hydrogen production from water is the endothermic decomposition of sulfuric acid. The necessary heat can be provided by concentrated solar radiation. A solar receiver-reactor has been developed and built to investigate this process. Experiments with the test reactor were carried out in the DLR solar furnace in Cologne and confirmed the technical feasibility of a receiver-reactor containing porous ceramics for the decomposition of sulfuric acid. The receiver-reactor and strategy of operation were iteratively optimized with respect to chemical conversion and reactor efficiency. Parametric studies were conducted with varying partial pressure of $SO3$, residence time, absorber temperature, the presence of catalyst, and the performance of different catalysts to quantify their influence on chemical conversion and reactor efficiency. The absorber temperature distribution was found to be the most crucial process parameter. Conversions close to the equilibrium—in some cases exceeding 90%—were achieved with a platinum catalyst. Thermal efficiencies of up to 35% for the foam vaporizer and 30% for the overall reactor were achieved. Enclosing the absorber in a cavity and using separate chambers for the evaporation and the $SO3$-decomposition were identified as potential measures to improve the reactor.

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