Sulfur Dioxide Disproportionation by Magnesium Sulfite as Intermediate

Solar-assisted thermochemical cycles can convert intermittent solar energy into storable chemical fuels. Within the European SULPHURREAL project, elemental sulfur is investigated as a long-term energy-storage medium in a cycle based on H2SO4, S, and SO2. This work investigates an indirect magnesium-mediated route for the disproportionation of SO2. The proposed cycle consists of three steps: aqueous reaction of SO2 with MgO to form sparingly soluble MgSO3; thermal decomposition of MgSO3 through competing pathways producing elemental sulfur, MgSO4, MgO, and SO2; and high-temperature decomposition of MgSO4 to regenerate MgO and produce sulfur oxides and oxygen. All three steps were experimentally investigated using laboratory-scale reactors, thermogravimetric analysis, X-ray diffraction, ion chromatography, and calorimetric measurements. The sulfur yield was approximately 25% of the theoretical maximum, corresponding to 8.3% relative to the initial SO2 amount. Complete MgSO4 conversion was achieved after 90 min at 1100 °C, at which temperature the SO2-forming pathway accounted for approximately 87% of the gaseous sulfur products. The experimental results were used to establish a preliminary mass and energy balance for the closed-loop process. The calculated gross heat requirement was 5349 kJ mol−1 of sulfur, corresponding to an energy efficiency of 5.5% when heat recovery was not considered. These results demonstrate the technical feasibility of the proposed magnesium-mediated route and provide a quantitative basis for its further development, identifying sulfur selectivity, high-temperature sulfate decomposition, quantitative product recovery, and heat integration as the main priorities for process optimisation.