A sulfur based thermochemical energy storage cycle for baseload power generation is being developed under the support of US DOE Sunshot program. Solar heat is stored in elemental sulfur via thermal decomposition of sulfuric acid and disproportionation of sulfur dioxide into elemental sulfur and sulfuric acid. Heat energy is recovered upon sulfur combustion. On-sun decomposition of sulfuric acid in a solar furnace has been demonstrated between 650 and 850°C. Near equilibrium conversion was obtained at high temperature but conversion was reduced due to catalyst poisoning at the lower temperatures. Sulfur dioxide disproportionation modeling showed the reaction driving force is maximized at the high system pressure and low system temperature. The effect of system pressure was validated experimentally. However, the disproportionation rate was found to increase with system temperature as a result of increased reaction kinetics. Homogenous iodide catalysts were used to further enhance the degree of disproportionation and the reaction rate. The process steps required to recover the catalyst for reuse have been verified.

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