Nuclear Production of Hydrogen, Fourth Information Exchange ...

CEA ASSESSMENT OF THE SULPHUR-IODINE CYCLE FOR HYDROGEN PRODUCTION
Apart from energy and raw material costs, discussed separately below, operation and maintenance
costs were calculated with the following main assumptions:
• the plant would be operated by a team of about 30 operators;
• a maintenance rate of 7% of the installed real investment cost per annum was taken into
account, but no specific provision for the replacement of any particular equipment was made;
• tax and insurance, as well as plant overhead costs, were also included.
Input water was taken at EUR 10 m 3 , which directly leads to a EUR 0.1 kg H2 contribution. Oxygen
by-product valorisation was discarded. Finally, somewhat optimistic values were taken for the cost of
thermal energy from the V/HTR (EUR 20 MWh) and for electricity (EUR 40 MWh).
With all these assumptions, the hydrogen production cost from the sulphur-iodine cycle is
estimated to be slightly less than EUR 10 kg H2 . This value is much higher than the estimated hydrogen
production cost from alkaline electrolysis at the same production scale, about EUR 3 to 4 kg H2 for
comparable energy unit costs.
Conclusion
CEA has conducted a wide-ranging assessment of the sulphur-iodine cycle for massive hydrogen
production. In particular, the hydrogen production cost was estimated, and found to be higher than
what was anticipated. Although uncertainties do remain and areas for process improvements have
been identified, the sulphur-iodine cycle competitiveness appears to require breakthroughs in efficiency
increase and investment cost reduction.
The efficiency of the cycle was evaluated to be close to 39%, a value which is not far from what
would be expected from alkaline electrolysis using the 50% efficiency electricity produced by a V/HTR.
There may be some room for possible process improvements, especially in terms of Bunsen section
optimisation, but recent thermodynamic data will more likely lead to a revision of the efficiency to a
lower value. Furthermore, Figure 3 demonstrates that efficiency is not the dominant factor for cycle
competitiveness.
The feasibility of the sulphur-iodine cycle under industrially realistic conditions has not been
fully demonstrated yet, but, provided long life catalysts can be developed, it does not appear to be the
major source of concern.
The main problem for the sulphur-iodine cycle competitiveness is the very high level of the
investment which appears to be required to build a production plant. This high level arises from two
factors:
• the large size of the components of the plant, due to the large amount of chemicals that need
to be processed, and the large internal heat exchanges required to reduce the outside energy
consumption;
• the very corrosive nature of the chemicals handled in the process, which leads to the use of
costly materials.
The first point is closely related to flow sheet optimisation, a subject which has already attracted
much effort throughout the world. However, recent experimental findings indicate at least two
directions for further developments:
• exploring the feasibility of low pressure/low temperature reactive distillation, with the aim of
being able to construct the reactive distillation column in cheap glass-lined steel;
• examining the conditions for possible liquid phase HI decomposition, which could significantly
reduce the size of the reactive distillation column.
As per material cost reductions, it would be beneficial to develop low thickness coating technologies
that would allow significant reductions of the amount of expensive materials used. Silicon carbide
also appears to be a promising candidate in terms of corrosion resistance for all sections, and the
development of low cost SiC technologies could change the economic perspectives of the cycle.
NUCLEAR PRODUCTION OF HYDROGEN – © OECD/NEA 2010 175