Thermochemical sulphur cycle for long-term storage of concentrated solar thermal energy: Process implementation of a particle heated reactor concept and techno-economic optimization

Kurzfassung

The solid sulphur cycle process utilizes elemental sulphur as a thermochemical energy storage medium for concentrated solar thermal energy. The produced sulphur can be transported to a different location to produce electricity from thermal energy released in a chemical process. In this thesis a process was designed which also includes sulphuric acid recycling. A detailed simulation model of a novel particle heated reactor concept was developed. It is used for a numerical techno-economic optimization based on an Aspen Plus process simulation. Improvements in reactor design were derived from the simulation model. The techno-economic optimum was found for a large reactor that converts a large share (98.65 %) of the educt SO3. An optimal thermal energy supply temperature close to 1000 °C was determined. This is the maximum output temperature of the concentrated solar thermal energy system. Particle temperature at the outlet of the reactor was at the design maximum. Reactor performance was improved by increasing its heat transfer capabilities. It was achieved by decreasing the tube distance in the reactor and increasing the fluid velocity. The result of the techno-economic analysis shows that the optimized process is not economically feasible under the assumed boundary conditions. The main cost factor is the transportation of diluted sulphuric acid for recycling between both locations of the process. High revenues are created by the recycling process of sulphuric acid. Should the sulphuric acid recycling prices increase from 50 €/t to 96.21 €/t the process will become economically feasible. A high conversion of the SO3 decomposition reaction is important for an economic process design. An increased maximum concentrated solar thermal energy system output temperature would increase the economic performance. Higher temperature differences between particles are desirable if the reactor can withstand them. Further energy integration might yield and economically feasible process. The high spent acid treatment revenues and transportation costs show the potential for on-site sulphuric acid recycling powered by concentrated solar thermal energy.