Process Modeling and Thermochemical Experimental Analysis of a Solar Sulfur Ammonia Hydrogen Production Cycle

A solar thermochemical water-splitting cycle which utilizes the electrolytic oxidation of aqueous ammonium sulfite as the hydrogen production half cycle, and an all-liquid potassium sulfate/potassium pyrosulfate salt solution for the oxygen production half cycle was modeled using Aspen Plus. The thermochemistry of various molten salt mixtures for the oxygen production half cycle was studied. A robust Aspen Plus chemical plant model represents the thermodynamic characteristics of the cycle, as verified by experiment. The laboratory experiments showed that the melting point of the salt may be lowered by 100 °C with the addition of sodium sulfate/pyrosulfate to the potassium sulfate/pyrosulfate and that the viscosities of the mixtures are low enough for pumping. There is a temperature separation of ∼25 °C between evolution of ammonia and SO3 gases during molten salt decomposition.