Hydrogen Production Via Sulphur Based Thermochemical Cycles: Development And Assessment of Key Components of the Process

HycycleS was a cooperation of nine European partners and further non-European partners and aimed at the qualification and enhancement of materials and components for key steps of solar and nuclear powered thermochemical cycles for hydrogen generation from water. The focus of HycycleS was the decomposition of sulphuric acid which is the central step of the sulphur-based family of those processes, especially the hybrid sulphur cycle (HyS) and the sulphur-iodine cycle (SI). Emphasis was put on materials and components for sulphuric acid evaporation, decomposition, and sulphur dioxide separation. The suitability of materials and components was demonstrated by decomposing sulphuric acid and separating its decomposition products in scalable prototypes. Silicon Carbide (SiC) turned out as the material of choice for the components facing the most corrosive environment of the process: the sulphuric acid evaporator and decomposer. Candidate catalysts for the high temperature reduction of sulphur trioxide have been screened and analysed – mainly considering oxides of transition metals. Their catalytic activity and their durability has been investigated and quantified experimentally. It was concluded that a Cr-Fe mixed oxide (Fe0.7Cr1.3O3) is the most promising material among the ones examined. Based on the use of the highlighted construction and catalyst materials prototype decomposers have been developed and tested. The successful fabrication and testing of a large size heat exchanger/reactor prototype composed of SiC plates shows promise with respect to its use for sulphuric acid decomposition in the SI and HyS cycle. A solar specific decomposer prototype was developed, realised and tested on sun in a solar furnace. It has been designed for a direct coupling of concentrated solar radiation into the process and makes use of a two-chamber reactor with volumetric absorbers which allows performance of both solar evaporation of liquid H2SO4 and subsequent catalytic decomposition of SO3. Design activities and experimental series are accompanied and supported by different methods of reactor and process simulation. A novel approach of using dense oxygen transport membranes, made from novel and complex ceramics, for oxygen removal from the H2SO4 decomposition product in order to shift the equilibrium in favour of increased decomposition was investigated. The membranes stability and suitability for carrying out this separation was investigated experimentally. Parallel to this, a conventional oxygen separator, a low-temperature wet scrubbing system, was investigated as well. Finally, scale-up scenarios of individual components and of the process have been addressed.