Among the thermo chemical cycles conceivable for hydrogen massive production coupled with a nuclear power source, two are currently considered and compared at the Nuclear Energy Division of the French CEA: the iodine-sulfur cycle (based on the Bunsen reaction) and the hybrid sulfur cycle based on depolarized SO2 oxidation (the Westinghouse cycle).

While prospects on the iodine-sulfur cycle has led to the realization of a joint SNL-GA-CEA test plant, which is today operated in San Diego [ref1]; our studies regarding the Westinghouse process were mainly theoretical and prospective.

In this scope, the electrochemical step has been modeled considering a filter-press technology; and, although few literature data were available, contributions of the major electrochemical and fluid mechanics factors to the overall cell voltage have been analyzed by numerical simulation [ref2].

Despite their different degrees of achievement, the investigations led on both cycles within this exploratory phase have enabled their respective key issues identification and the evaluation of their energy requirements. The hybrid cycle relevance was emphasized.

This assessment is nevertheless based on generic materials and technologies. A test plant dedicated to electrolysis cells operating has therefore been designed and is running in Marcoule Laboratory since April'08 [ref3].

Thanks to this versatile tool, hydrogen production resulting from depolarized SO2 oxidation is currently studied under a wide range of operating conditions (electrolytes flow rates, concentrations and temperatures). Cell voltages measured under galvanostatic conditions are analyzed and compared with model predictions, thus allowing a better understanding of the observed overall performances.

At last, model development status and needs are discussed. Indeed, a more detailed description of i) the electrochemical process, ii) species transport and membrane exchange, iii) phases equilibriums, iv) hydrogen bubbles behavior, is required in order to reach a sufficient degree of understanding of the electrolytic cell functioning, and therefore achieving the design and optimization of an industrial-scale process.

References:

[ref1] J. Leybros et al., GLOBAL Meeting, Boise, Sept. 2007.

[ref2] F. Jomard et al., Journal of Applied Electrochemistry, 38, 2008, pp.297.

[ref3] P. Rivalier et al., 16th ICONE Meeting, Orlando, May 2008.