Thermochemical cycles, hydrogen

A special focus of the development regarding solar thermochemical processes is the high temperature endothermic decomposition of sulphuric acid. The solarisation of this step was part of the R D activities of the European project HYTHEC (HYdrogen THErmochemical Cycles) with an emphasis on components development and process improvement (Noglik, 2009). 

The decomposition of sulfuric acid at 850 900 °C [reaction (4.31)], a component of all the sulfur cycles, places severe specifications on the materials of plant construction due to the extremely corrosive nature of the species at high temperatures. This is one of the major issues that is being addressed by the European HYdrogen THErmochemical Cycles (HYTHEC) programme. The... 

Pickard, P., Sulfur-iodine thermochemical cycle, 2006 Annual Merit Review Proc., Hydrogen Production and Delivery, D. Nuclear Energy Initiative, http //www.hydrogen.energy.gov/ annual review06 delivery.html. 

High temperature nuclear thermochemical cycles, hydrogen production by, 13 847-849... 

Thermochemical cycles based on solar energy are another long-term option for hydrogen production in countries with favourable climatic conditions. 

Huang, C., Raissi, T.-A. 2005. Analysis of sulfur-iodine thermochemical cycle for solar hydrogen production. Part I decomposition of sulfuric acid. Solar Energy 78 632-646. 

If both MeOH and EtOH are present in the spectrometer vacuum system, then the reactions come to equilibrium, and the equilibrium constant can be determined, exactly as for proton transfer equilibria. Via a thermochemical cycle, the relative hydrogen bond strengths of the two ions present can be determined from that equilibrium constant and the conjugate acidities of the two bare ions involved, according to Equation (16),... 

Abanades S, Charvin P, Flamant G, Neveu P (2006) Screening of water-splitting thermochemical cycles potentially attractive for hydrogen production by concentrated solar energy. Energy 31 2805-2822... 

Sakurai M, Bilgen E, Tsutsumi A, Yoshida K (1996) Solar UT-3 thermochemical cycle for hydrogen production. Sol Energy 57 51-58... 

Sturzenegger M, Ganz J, Nuesch P, Schelling T (1999) Solar hydrogen from a manganese oxide based thermochemical cycle. J PhyS. IV JP 9 3-331... 

In situ formation and hydrolysis of Zn nanoparticles for H2 production by the 2-step ZnO/Zn water-splitting thermochemical cycle. Int J Hydrogen Energy 31 55-61... 

Steinfeld A (2002) Solar hydrogen production via a two-step water-splitting thermochemical cycle based on Zn/ZnO redox reactions. Int J Hydrogen Energy 27 611-619... 

Hydrogen fluoride in aqueous solution is a weak acid, characterized by its pKa value of 3.2. By comparison, the other hydrogen halides are extremely strong acids in aqueous solution all three are fully dissociated in dilute solution, and their pA", values may be estimated by thermochemical cycle calculations. The thermochemical cycle shown in Figure 3.1 represents the various processes as the aqueous hydrogen halide, HX, is converted to a solution containing hydrated protons and hydrated halide ions. The enthalpy of acid dissociation of the HX(aq) compound is given by ... 

M—H bond dissociation energies, 1, 287 photochemistry, 1, 251 single crystal neutron diffraction, 1, 578 stability toward disproportionation, 1, 301 Metal—hydrogen bonds bond dissociation energy in 1,2-dichloroethane, 1, 289 stable metal hydrides in acetonitrile, 1, 287 thermochemical cycle, 1, 286 in THF and dichloromethane, 1, 289 olefin insertion thermodynamics, 1, 629 in Zr(IV) bis-Cp complexes, 4, 878 Metal—hydrogen hydricity data, 1, 292... 

Session 4 focused on recent advances in the thermochemical copper chloride and calcium bromide cycles. Much of the current research on thermochemical cycles for hydrogen production involves the sulphur cycles (sulphur-iodine, hybrid sulphur), however, these cycles require very high temperatures ( 800-900°C) to drive the acid decomposition step. The interest in the Cu-Cl and Ca-Br cycles is due to the lower peak temperature requirements of these cycles. The peak temperature requirement for the Cu-Cl cycle is about 550°C, which would allow this cycle to be used with lower temperature reactors, such as sodium- or lead-cooled reactors, or possibly supercritical water reactors. Ca-Br requires peak temperatures of about 760°C. Both of these cycles are projected to have good efficiencies, in the range of 40%. Work on Cu-Cl is ongoing in France, Canada and the United States. Work on Ca-Br has been done primarily in Japan and the US, with the more recent work being done in the US at ANL. The papers presented in this session summarised the recent advances in these cycles. 

The thermochemical cycles (S-I > 850°C) or hybrid cycles (S-electrolysis > 850°C) still feature many uncertainties in terms of feasibility and performances. Uncertainties still exist in parts of the flow sheet and technologies needed to provide high temperature heat whether from solar or nuclear nature. Potential assets of thermochemical cycles lie in a theoretical potential for a global efficiency above 35% and a scaling law of the hydrogen plant after the volume of reactants instead of the total surface of electrolytic cells. In return, their practical feasibility and economic viability have to be entirely demonstrated. Especially, a global efficiency above 30% is to be demonstrated to compete with alkaline electrolysis. Moreover, the safety of co-located nuclear and chemical plants has to be demonstrated. 

Doizi, D., et al. (2007), Total and Partial Pressure Measurements for the Sulphur-iodine Thermochemical Cycle , Int. J. Hydrogen Energy, 32 (9), 1183-1191. 

The target cost of nuclear hydrogen production must be cheaper than that of the conventional electrolysis subtracting the distribution cost. Hydrogen production efficiency of EED-aided SI thermochemical cycle is expected around 43% with current knowledge (Cho, 2009). 

---