Sulfur Synthesis by Auto-Catalytic Bisulfite Disproportionation for Solar Thermochemical Fuel Production: Experimental Investigation

A solar-assisted thermochemical cycle to store concentrated solar energy in solid elemental sulfur via the reversible interconversion of sulfuric acid and sulfur is being developed within the SULPHURREAL project. This process enables long-term, transportable energy storage through internal recycling of sulfur oxides. A central objective is to integrate SO2 capture and conversion in separation-friendly steps that support closed-loop operation with minimal additives and limited downstream purification, while remaining compatible with industrial sulfuric acid and sulfur feedstocks. The method presented in this paper can also be feasible for SO2 removal from fossil fuels and industrial emissions. With this purpose, indirect SO2 conversion via bisulfite disproportionation was investigated using elemental sulfur as a heterogeneous auto-catalyst. Batch tests were performed in a pressurized, Teflon-lined autoclave with concentrated bisulfite solutions (3 M) at 140–180 °C for 3 h. Sodium bisulfite showed no conversions at 140–160 °C, whereas sulfur auto-catalysis was observed at T ≥ 170 °C. Ammonium bisulfite was also evaluated as a separable SO2-capture intermediate; due to thermal instability, operation was limited to 170 °C, where sulfur formation remained detectable. For loop closure, NH3 and H2SO4 must be recovered from the produced sulfate. This was addressed by reacting (NH4)2SO4 with metal oxides in a tubular furnace at 500 °C. The evolved NH3 was trapped in acid and quantified by ion chromatography. Near-quantitative NH3 recovery (≈92–98%) was achieved with MgO and ZnO, and the corresponding metal sulfates were identified by XRD. These results support integrated process development and motivate kinetic and mass-balance studies toward continuous operation and scale-up.