(644a) A Thermodynamic and Process Analysis of the Thermochemical Steps of the Hybrid Photo-Thermal Sulfur-Ammonia Water Splitting Cycle

Solar driven hybrid sulfur-ammonia water splitting cycle (HySA) T-Raissi et al. (2006) is a promising technology for energy and environmental applications. It is a state-of-the-art process that integrates a solar-photocatalytic hydrogen production step (driven by the photonic portion of solar irradiance) with a high-temperature solar thermochemical oxygen evolution step (driven from the thermal portion) and efficient thermal energy storage as part of the cycle operation. Water splitting is achieved at a photochemical oxidation of ammonium sulfite with a simultaneous release of hydrogen. Then, ammonium sulfate decomposes to ammonia, sulfur dioxide and oxygen through three thermochemical steps via successive temperature increases. In the first step, a sulfate is employed to restrain the release of sulfur trioxide by forming a pyrosulfate, which separates it from ammonia and the remaining water. At a higher temperature, the formed pyrosulfate decomposes back to the sulfate. Finally, sulfur trioxide decomposes in to sulfur dioxide and oxygen while later both gasses are separated using an ammonia absorber.

The present work investigates in depth the oxygen sub-cycle and more specifically the use of alkali sulfates/pyrosulfates solid-solutions. It provides an updated assessment (improving upon previous approximate thermodynamic calculations of the oxygen cycle) and discussion of the related developments and analyzes the thermodynamics and implications of the reactions involved. The analysis is based on extensive literature research, on thermochemical experiments, and on the use of advanced thermodynamic numerical tools (Aspen Plus and FactSAGE) and comprehensive thermodynamic databases (e.g. FACT and DIPPR project 801). These tools facilitated the consideration of new materials and processes into the HySA.

The starting point for our thermodynamic analysis is the work of Littlefield (2012) and Luc (2013), who studied a molten salt potassium sulfate/ pyrosulfate pair sub-cycle for oxygen evolution step. As it has been described earlier, the use of a sulfate is critical for the separation of SO₃ and NH₃. An important aspect is the state (phase) of the mixture. Potassium sulfate and the like (i.e. sodium) have a melting point of close to 1000 ^oC, which is an inhibiting factor for most chemical processes. Extensive experimental studies have shown that they form binary and reciprocal mixtures with quite lower melting points. Both aforementioned studies used Aspen Plus process simulation software to analyze the oxygen-subcycle. Instead of using the experimentally derived thermodynamic properties directly (Lindberg, 2006), they applied a series of hard-coded specifications to constrain the mixture composition within the liquid phase (melt). This limits the applicability of such studies to specific temperature ranges and composition. Instead, we incorporated into Aspen Plus the proper thermodynamic properties, which allows for a wider range of conditions. We also used FactSage equilibrium calculator to check the literature thermodynamic properties and perform a detailed thermodynamic analysis.

Finally, the thermodynamic analysis of the thermochemical steps demonstrates the importance of using specialized thermodynamic tools to screen the selected materials and optimize the operating conditions. Nevertheless, the analysis confirmed Littlefield’s and Luc’s results on the increase in efficiency with the use of higher than one ratio of pyrosulfate to sulfate. Further analysis (numerical and experimental) and improvements are needed in order to optimize the integrated cycle.