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CEA ASSESSMENT OF THE SULPHUR-IODINE CYCLE FOR HYDROGEN PRODUCTION
The first one is related to the maximum temperature of the cycle. As stated above, the maximum
chemical fluid temperature used in our flow sheet (in the SO 3 decomposition reactor) is around 850°C.
Quite interestingly, increasing this maximum temperature does not lead to reduced heat requirements
(Buckingham, 2009). It is therefore not necessary to have the hottest possible heat source, which is
good in terms of feasibility. The V/HTR operation temperature just needs to be compatible with a 850°C
fluid temperature, which implies an outlet temperature of about 950°C if 50°C pinches are assumed
(between the primary coolant and the secondary loop, and between the secondary loop and the
chemical fluids).
Another feasibility-related point is the availability of efficient catalysts for SO 3 and HI
decompositions.
• For SO 3 decomposition, Pt has proven its efficiency, but it is subject to sintering at high
temperatures, which makes its long-term efficiency problematic and uneconomical. Fe 2 O 3
appears as an interesting option for high temperatures, but it suffers from sulphate formation
at low temperatures. A combination of these catalysts, using Pt at low temperatures and Fe 2 O 3
at higher temperatures, may look promising (Leybros, 2009), but remains to be demonstrated
under actual operation.
• For HI decomposition, Pt is also expected to be an efficient catalyst. However, it must be kept
in mind that, even though HI decomposition reaction is assumed to take place in the gas
phase, liquid condensation on the catalyst is not unlikely to occur in a reactive distillation
column, with possible negative impact on the catalyst lifetime.
More generally, reactive distillation was never fully demonstrated, and CEA has undertaken a
collaborative effort with the Indian BARC to demonstrate its feasibility. Catalyst stability is one of the
major challenges to be met, and design efforts will be dedicated to providing the best possible
conditions for this stability.
Chemicals handled in the sulphur-iodine cycle are known to be quite corrosive, especially at the
high temperatures and pressures that are used in the flow sheets for better efficiency. However,
corrosion-resistant materials do exist for each environment, and some, such as Ta or SiC, seem to
present an excellent overall behaviour, which will be an asset to resist actual operating conditions,
with cross-contamination between the different sections.
The feasibility of a sulphur-iodine cycle, although still not fully demonstrated under actual plant
conditions, therefore does not appear to be a show-stopper, even if catalyst long-term performance
remains, to a certain extent, an unknown.
Hydrogen production cost
Plant capital cost
As a first element required for hydrogen production cost assessment, CEA has evaluated the investment
cost for a sulphur-iodine cycle based hydrogen production plant. The scale of the plant was matched
to a heat generation dedicated 600 MW th V/HTR, with all required electricity taken from another
electricity source (most likely another nuclear reactor to avoid greenhouse gas emissions). With the
energy consumption figures given above, it leads to a 1 000 mol/s plant (2 kg H2 /s, or about 170 t H2 /day).
The first step for the cost assessment is to size the components that appear in the flow sheet.
Standard technologies; such as shell and tube heat exchangers, were assumed to be used. Problems
were encountered with the sizing of the Bunsen reactor (where heat removal must be ensured) and
the reactive distillation column (which appeared to be too large to build as a single component).
To solve these problems, it was decided to split the whole plant into 10 parallel 100 mol/s shops, which
was enough to bring back the major component dimensions into acceptable industrial standards.
Furthermore, this division of the hydrogen plant into 10 smaller size shops introduces more flexibility
in the operation of the plant and reduces the size of the required emergency heat sink, two features
that appear to be favourable for the coupling to the nuclear reactor.
NUCLEAR PRODUCTION OF HYDROGEN – © OECD/NEA 2010 173