Development And test of a solar decomposer of sulphuric acid for thermochemical hydrogen production

Kurzfassung

Decomposition of sulphuric acid is a key step of sulphur based thermochemical cycles for thermal splitting of water. These processes lower the temperature required for thermal water splitting from more than 2000°C to a technically manageable values below 1000°C. The Hybrid Sulphur Cycle (HyS) consisting of two reaction steps is considered as one of the most promising cycles: Firstly, sulphuric acid is decomposed by high temperature heat of 800-1200°C forming sulphur dioxide (SO2), which in a second step is used to electrochemically split water. Compared to conventional water electrolysis only about a tenth of the theoretical voltage is required making the HyS one of the most efficient processes to produce hydrogen by concentrated solar radiation. As a result, this thermochemical cycle has the potential to significantly reduce the amount of energy required for water splitting and to efficiently generate hydrogen free of carbon dioxide emissions. The European research project HycycleS (Materials and components for Hydrogen production by sulphur based thermochemical cycles) aims at a technical realisation of the Hybrid Sulphur Cycle. One objective of the project is to develop and qualify a solar interface, meaning a device to couple concentrated solar radiation into the endothermal steps of the chemical process. Therefore a test reactor for decomposition of sulphuric acid by concentrated solar radiation was developed and tested in the solar furnace of DLR in Cologne. Based on the preceding project HYTHEC (HYdrogen THErmochemical Cycles) an optimised reactor concept with two chambers was constructed: In the evaporator liquid sulphuric acid is vaporised on a solar absorber made of a foam structure of siliconized silicon carbide (SiSiC) at a temperature of about 400°C. The product gas mainly consisting of sulphur trioxide (SO3) and water vapour is subsequently conveyed to a second chamber, the decomposer, in which SO3 is reduced on a catalytically activated SiSiC honeycomb structure at a design temperature of 850°C. A UV/Vis spectrometer in the exhaust section of the reactor analyses the amount of SO2 produced during decomposition. Tests in concentrated solar radiation were carried out for temperatures of the honeycomb of up to 950°C decomposing sulphuric acid of 50 and 96 weight-percent. Mass and energy flow of the process were calculated in able to determine energy efficiency and chemical conversion. The influence of process parameters like temperature, flow rates and space velocity on chemical conversion and reactor efficiency was analysed in detail. If catalysts like iron oxide (Fe2O3) and mixed oxides (i.e. CuFe2O4) were used a conversion of SO3 to SO2 of more than 80% at a thermal efficiency of over 25% could be reached.