Final Report Summary - HYTHEC (High Temperature Thermochemical Cycles) The HYTHEC project focused on the two major thermochemical cycles, sulfur-iodine (S_I) cycle and hybrid-sulfur (HyS) cycle, also known as Westinghouse cycle, addressing both fundamental and industrial scale aspects. The approach is multi-faceted, involving modelling, chemical analysis, process flow-sheeting, experimental investigations on the critical high temperature and hydrogen production steps, capabilities of industrial scale-up (50 to 600 MWth) including the coupling to a high temperature heat source and the related safety aspects, and economic evaluation. The experimental work was focused on the high temperature H2SO4 decomposition reaction step common to both cycles, which is performed with a solar primary energy source, and on the S_I cycle hydrogen production step, both to increase the knowledge of the chemical system and to improve the efficiency by the use of alternative, low energy separation techniques such as membranes.To reach the overall HYTHEC objectives, he work has been broken down in seven sub-projects:- SP1: Project management - Organisation of the work (management committees, progress reviews and communications), relations with the European Commission, dissemination and contacts with other European projects, progress reports and regular progress meetings.- SP2: The optimisation of the whole S_I-S_I cycle flow-sheets issued at various times of the project, to give theoretical descriptions of the process, that will be used for the industrial scale-up activities (coupling to reactor, component sizing and cost analysis), taking into account the theoretical and experimental improvements found during the project.- SP3: Vapour liquid equilibrium (VLE) analysis - Improvement of the vapour liquid equilibrium model of the HIx (HI/I2/H2O) system, for a better knowledge and improvement of the H2 production part of the S_I cycle production rate of hydrogen (partial pressure measurements of gases needed, mainly by the mean of mass or optical spectrometry methods).- SP4: Review of membrane separation techniques - Exploratory research into alternative, low energy separation techniques relevant to the S_I process, to be used in the HIx separation steps; membrane distillation and pervapouration, already used to separate azeotropes successfully, will be assessed as a technique to concentrate the HI solution.- SP5: Experimental study of membrane distillation and pervaporation of HIx - Experiments at a laboratory scale, evaluation of the separation performances of selected membranes (partial pressures in the vapour will be measured by optical means, measurements similar to those of SP3).- SP6: Sulphuric acid decomposition - Direct decomposition of H2SO4 in a solar furnace located in Cologne (up to 25 kW), at temperatures up to 1 100 - 1 200 degrees Celsius; indirect heating in tube type reactor using a catalyst will also be performed, applying VHTR nuclear reactor temperature (850-900 degrees Celsius). Process simulations and assessment of the feasibility of scale-up at a commercial scale (including safety and costs evaluations).- SP7: Assessment of the Westinghouse cycle (WH) - Study of the technical feasibility of a solar and hybrid (solar and nuclear) operation of the WH process (thermo-chemical and electrolysis steps), including industrial scale-up, safety aspects during normal and transient operation and economic potential, in comparison with the S_I process.An overall analysis of the S_I cycle, based upon a reference flow-sheet used for the coupling to a VHTR, the major components' sizing and costs, and an overall cycle economic evaluation, has been carried out by the mean of a fruitful interplay between different activities and partners. Different possibilities to update the flow-sheet have been given, with the related impact on the efficiency: increase by the use of membranes (about + 3%), new flash in section II, heat exchange rearrangements in section III; but unavoidable uncertainties and constraints due to the connection to the heat source (safety considerations) may have a negative effect. Anyway, the fundamentals (mainly section III modelling) could not change yet. The coupling and safety evaluations have been reinforced by connections with other European projects (RAPHAEL, HYSAFE). The first S_I reference cost evaluation was found higher than the usual target (5,3 EUR/kg H2), but the strong impact of the interplay between the heat (and electricity) source coupling, and the component sizing has been shown, leading to the existence of an optimal value (5,3 EUR/kg H2 [ 4,2 EUR/kg H2) that doesn't correspond to the best efficiency. Costs are strongly dependent on technical sensitivities (HPHE, plant rearrangement), and therefore may be lowered to 3,1 EUR/kg. Economic hypothesis (maintenances, discount rates, availability, plant life) have a strong impact too.A successful progressive methodology has been applied to study the liquid vapour equilibria of ternary HI-H2O-I2 mixtures. Optical analytical techniques (FTIR and UV visible) characterised the speciation of the vapour phase. Up to 2 bars and 130 degrees Celsius, an important set of liquid vapour equilibrium data gave [HI] contents in good agreement with literature on the left of the azeotrope (more on the right side), and enabled a more precise evaluation of the H2 produced purity: the specification [(I2+HI)/H2 < 10-5] is reached.A membranes' database dedicated to HIx section enrichment in HI has been given. Flow-sheet modelling to identify the best process conditions showed that feed dewatering, then under low temperature and pressure conditions, is the most efficient: maximum 3 % increase in efficiency obtained at a dewatering of 8,25 %, the impact of the permeate side pressure has been given too. The stability of Nafion 212 and 115 membranes in azeotropic HI solution has been shown. Flux and selectivity data have been obtained, in particular very high separation factors (Nafion 117, Nafion 212): between 0,5 and almost 1,35 Kg/h/m2. A Raman scattering study has provided a self-consistent picture of the chemical nature of the HIx phase of the sulphur iodine cycle, particularly in respect to the I2-containing species present. Raman scattering measurements have been done on the two aqueous binary systems, I2/H2O and HI/H2O, and on the ternary system, HI/I2/H2O, which are each closely related to HIx, with an analysis of a wide range of previously published work on aqueous iodide-I2 solutions and solid polyiodides. Feasibility and successful test of solar H2SO4 splitting has been proved : conversion rates up to 90 %, at direct 'solar' decomposition high temperatures (circa 1 200 degrees Celsius maximum; 1 000 degrees Celsius average), in a Pt coated SiSiC absorber. The reactor efficiency is up to 40 %. Efficiencies dependent on several process parameters; trends show that the maximum values could rather been obtained at temperatures lower than 1 000 degrees Celsius, due to re-radiation losses. Pure SiSiC is catalytic active, with a strong effect (up to the possible maximum achievable) even at 850 degrees Celsius, in a packed bed tube-type reactor (strongly dependent on pretreatment and surface characteristics).Similarly to what has been done on the S_I cycle, an overall analysis of the HyS cycle, based upon a reference flow-sheet used for the coupling to a solar source, and / or a VHTR, has been done. The major components' have been sized, and overall economics evaluated. Operational strategies need to be defined (including calculation of time-dependent storages of SO2 and H2SO4), and it seems that a self-sustaining concept is not desirable for the pure nuclear coupling. For the solar only case, a 50 MWth (140 MWth installed) seems to be close to the optimal; tower heights of 230 m for 50 MW (470 m for 300 MW) were found to be economically optimal. Solar components have been sized, and the costs evaluated; the major chemical cycle components have been sized similarly to the S_I cycle ones (except electrolyser), and the overall economics evaluations showed that the costs are in the range 3 - 6 EUR/kg. For small plant sizes (50 MWth) solar only plants are advantageous, while for large scale production (more than 500 MWth) purely nuclear energy lead to the most economic results. In the range in between, hybrid plant have the lowest hydrogen production costs. Related documents Final Report - HYTHEC (High Temperature Thermochemical Cycles)
