Water decomposition thermochemical process

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

Although H2S is at present a by-product of the desulfurization of fossil fuels on a large scale, only the recovery of free sulfur is carried out by the Claus treatment. On the other hand, the application of H2S decomposition as an H2 evolution method is proposed for use in the thermochemical process undertaken for water splitting. Thus, the thermochemical decomposition of H2S has wide-spread applications in various field. 

Hwang, G.J. and Onuki, K., Simulation study ou the catalytic decomposition of hydrogen iodide in a membrane reactor with a silica membrane for the thermochemical water splitting IS process. Journal of Membrane Science, 194, 207, 2001. 

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. 

The net result of the two reactions is the decomposition of water into hydrogen and oxygen. Since the chemistry involves only sulfur, oxygen and hydrogen compounds, many of the development issues associated with more complex thermochemical processes, such as cross-contamination and halide-induced stress corrosion cracking, are eliminated. 

In this study, the silica membranes to apply for HI decomposition reaction was investigated, and prepared by the sol-gel and the thermal chemical vapor deposition (CVD) methods. The objective of this work is to study the characteristics of the silica membrane preparation and the hydrogen permselectivity of the membrane reactor used for HI decomposition in the thermochemical water splitting IS process. 

Alternative methods of hydrogen production are thermochemical water decomposition, photoconversions, photobiological processes, production from biomass, and various industrial processes where it is a by-product. 

Electrolysis, and thermochemical and photochemical decomposition of water followed by purification through diffusion methods are expensive processes to produce hydrogen. 

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. 

Because these temperatures are impractical, the thermochemical water-splitting cycles achieve the same result (i.e., separation of water into hydrogen and oxygen) at lower temperatures. A thermochemical water-splitting cycle is a series of chemical reactions tliat sum to the decomposition of water. To be useful, each reaction must be spontaneous and clean. Chemicals are chosen to create a closed loop where water can be fed to the process, oxygen and hydrogen gas are collected, and all other reactants are regenerated and recycled [2]. 

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