Study of Hybrid-Sulfur thermochemical cycle for the water splitting powered by Concentrated solar Energy

Hydrogen production through water splitting thermochemical cycles powered with renewable energy represents the concrete possibility of obtaining an actually carbon free fuel. While since the 80’s several thermochemical cycles have been proposed in the literature, only few of them appear today as industrially feasible. Among these, the Hybrid Sulfur (HyS) cycle is considered as particularly promising. Such cycle involves two main steps. In a first low-temperature (below 100 °C) step, H2O and SO2 are electrochemically reacted to produce H2 and H2SO4 in a process called Sulfur Dioxide Depolarized Electrolysis (SDE). The standard cell potential of SDE is about 85% lower than that of liquid water electrolysis, which may entail a significant reduction of the electrical power required. In the high-temperature thermochemical step, H2SO4 is thermally vaporized and decomposed into SO3 and H2O; subsequently,SO3 is thermally decomposed (above 800 °C) in the presence of a catalyst to produce O2 and SO2, which is recycled to SDE to close the cycle. The HyS cycle was initially conceived to be powered with high temperature nuclear heat and several technological and process challenges must be faced in order to replace the continuous nuclear source with solar energy, which is characterized by natural fluctuations. SOL2HY2 is a research project ended in November 2016 and co-funded by the Fuel Cells and Hydrogen Joint Undertaking (FCH JU) within the Seventh Framework Programme for Research and Technological Development (FP7). The project focused on applied bottle-neck solving, materials research, development and demonstration of the relevant key components of the solar-powered HyS water splitting cycle, i.e. the sulfur depolarized electrolyzer and the reactor for solar-powered SO3 decomposition. The project also aimed at providing tools for the simulation and optimization of the solar HyS cycle as well as developing a techno-economic assessment of the whole process. The PhD project was funded by ENEA Italian National Agency for New Technologies, Energy and Sustainable Economic Development. within the framework of the SOL2HY2 project and is focused on the study of the solar HyS cycle. After a preliminary review of hydrogen production processes, in particular of water splitting thermochemical cycles, the PhD work has been articulated in two main parts: a first part focused on the theoretical and experimental study of the high-temperature section of the cycle, and a second one focused on the process analysis and techno-economic assessment of the whole Solar-powered Hybrid-Sulfur process. The first part is particularly devoted to the selection and development of the catalyst for the SO3 decomposition reaction. The choice of the catalyst has taken into account the features required for the use in a high temperature adiabatic reactor subjected to daily thermal cycles, as expected for the specific application considered. Compared to other catalysts suggested in the literature, Fe2O3 supported by gamma-Al2O3 pellets has exhibited, in the operating conditions of interest, better or similar performance in terms of stability and activity with a greater cost-effectiveness, lower toxicity, easier and cheaper large-scale production. Such catalyst was tested in a laboratory reactor under realistic operating conditions (temperature 925-1000 °C, pressure 1-3 atm, WHSV (Weight Hourly Space Velocity) is defined as the ratio of the inlet sulfuric acid mass flowrate divided by the catalyst mass. 22-33 h^-1 and feed sulfuric acid concentration 50 % w/w and has shown a notable catalytic activity (about 70% SO3 conversion at 1000°C and 1 atm with 50% sulfuric acid ), good stability over a total of more than 100 hours on stream, even including several stand-by phases in which the catalyst was subjected to thermal cycling from operating to room temperature. The results obtained have been processed to analyze the reaction kinetics. In order to determine the kinetic parameters of the reaction, a simple reactor model was used and the reaction rate was evaluated by assuming the SO3 decomposition as an elementary equilibrium reaction, which led to a calculated activation energy of about 148 kJ/mol. Since the catalyst developed can be used as-is in a prototype or full scale reactor in the same range of operating conditions explored, the kinetic expression obtained is useful to design and predict the performance of an actual SO3 decomposition reactor. Indeed, such expression was used by DLR German Aerospace Center. to design the demo SO3 decomposition reactor developed within the SOL2HY2 project. Moreover, the catalyst production procedure was upscaled in order to provide all the catalyst used in the demo reactor (about 20 kg); in so doing, the procedure has been carefully adjusted to ensure a good support coating, and therefore catalytic activity, without being costly and time expensive in case of larger-scale production. The PhD project also involved active participation (for a period of three months) to the latest phases of the installation of the demo plant at DLR's solar tower in Juelich, Germany. During this period, some preliminary on-sun tests were also performed in cooperation with DLR's staff. Such tests were aimed at the thermal characterization of the solar receiver-reactor system for sulfuric acid decomposition and were carried out without catalyst in the system. The obtained results were crucial to identify issues in the original plant configuration and were used to suggested modifications to the plant layout, which were implemented by DLR in a subsequent experimental stage. The second part of the PhD work has been focused on the process analysis and techno-economic assessment of the Solar-powered Hybrid-Sulfur cycle. In this context, the HyS process was studied block-wise in order to identify the best process options for each part of the plant. AspenPlus\textregistered flowsheets were developed for all process blocks and used to simulate the plant operation in order to determine the energy required for hydrogen production. The total energy consumption of the proposed process resulted equal to 870 kJ/mol of produced hydrogen. In particular, three energy types are required by the process: power (42%), high-temperature heat (23%) and medium-temperature heat (35%), which gives a thermochemical efficiency in excess of 19%. Furthermore, the sizing and costing of the main equipment employed in the process has been carried out in order to allow the final economical evaluation of process. Different options for the integration the HyS plant with Concentrating Solar Thermal (CST) systems have been identified. The possibility to integrate other energy sources, such as heat obtained through sulfur burning and Photovoltaic (PV) technologies, has also been considered. The economic performance of the solar HyS plant was assessed in several scenarios including different plant locations and combinations of (renewable and conventional) energy sources. Plant capacities in the range of 2.6 to 7.6 t/d of hydrogen were considered. Hydrogen production costs in the range 8 €/kg to about 13 €/kg have been found, with the highest values corresponding to 100% renewable hydrogen. The lowest prices are obtained with the synergistic use of PV, heat from sulfur burning process and CST technology, which leads to a promising performance of the SOL2HY2 cycle. The costs obtained are still high compared to conventional hydrogen production processes like methane steam reforming, but are in line with the current targets set by the Fuel Cells and Hydrogen Joint Undertaking (FCH-JU) for the hydrogen production by high temperature water splitting. Furthermore the cost analysis carried out is based on some conservative hypotheses. Firstly, valorization of oxygen co-produced with hydrogen is not considered. Furthermore, SO2 and S from external sources required in some process configurations are assumed to be bought at market prices, while for some plant locations such chemicals could be available at a much lower or even at no cost.