High thermochemical decomposition water

Westinghouse A proposed thermochemical process for decomposing water to oxygen and hydrogen by electrolysis, coupled with the high-temperature decomposition of sulfuric acid ... 

High temperature (often exceeding 1000 K) drives the endothermic chemical reactions. Multistep cycles for water splitting are used because very high temperatures are required before an appreciable amount of water decomposes in single-step cycles. Thus, in one or more subsequent chemical reactions, the intermediary compounds can be recovered to the original substance, which is used repeatedly. The thermochemical water decomposition steps involve the following five principal reactions ... 

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. 

Thermochemical splitting of water involves heating water to a high temperature and separating the hydrogen from the equilibrium mixture. Unfortunately the decomposition of water does not proceed until temperatures around 2500 K are reached. This and other thermal routes are discussed in Chapter 5. Solar thermal processes are handicapped by the Carnot efficiency limits. On the other hand, solar photonic processes are limited by fundamental considerations associated with band-gap excitation these have been reviewed in Refs.32 and 33. 

In this context, the CEA has chosen to work on biomass decomposition, free CO2 processes and bioprocesses. In this paper, we focus only on CO free processes, i.e. on the splitting of water by high-temperature electrolysis or by thermochemical processes. These processes do rely on a high-temperature nuclear reactor, but can also be studied in relation with gcothcnnic or solar sources of energy. 

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