Thermochemical production of hydrogen and sulfur: Development and modeling of a solar reactor for decomposition of sulfuric acid

Kurzfassung

Solar energy underlies significant diurnal and seasonal fluctuations and hence requires qualified storage in order to fully replace fossil sources. While thermal storage systems (e.g. molten salt tanks) can bridge short periods of discontinuity or intermittence of insolation during cloud passage or over night, chemical storage is needed to compensate for longer periods without solar radiation or to transport solar energy into regions without sufficient sunlight. Hydrogen produced by solar thermal water splitting is a clean energy carrier free of greenhouse gases and a promising candidate to substitute fossil fuels in combustion engines. Sulfur generated from sulfuric acid in a reversible solar driven process allows baseload operation of solar power plants. Thermochemical cycles are introduced to efficiently produce hydrogen and sulfur by concentrated solar radiation: the Hybrid Sulfur Cycle is a two step process to thermally split water at a manageable temperature level below 1000°C. In a variant of this cycle, sulfur can be generated form sulfuric acid by introducing a disproportionation reaction. Both cycles haven in common the decomposition of sulfuric acid by concentrated solar radiation. To technically realize this reaction, a two-chamber solar reactor has been developed and operated in extended experimental campaigns in the solar furnace of DLR in Cologne. In this receiver-reactor porous absorbers of siliconized silicon carbide (SiSiC) are heated by concentrated solar radiation while the reaction proceeds inside their open volume. Liquid sulfuric acid is injected into the SiSiC foam structure of the first chamber forming SO3 at 400°C. In the second chamber the SO3 is reduced in a catalytically activated SiSiC honeycomb structure to produce SO2 at 850°C. With different catalysts (i.e. Fe2O3, CuFe2O4, FeCr2O4) conversion of up to 80% was reached at a thermal efficiency of more than 25%. Both chambers have been modeled to gain an in-depth understanding of the process and optimize the operation of the reactor. Here the model of the second chamber for decomposition of SO3 will be presented. It consists of a set of sub-models of the solar radiation, the heat transfer and chemical reaction inside the absorber as well as the radiative and convective heat loss of the system. The model was implemented in Dymola using the Modelica Standard Library and validated by experimental results of steady-state and transient operation.