The Sulphur-Iodine Cycle
The sulphur-iodine (SI) cycle is characterised by the following three basic reactions:
xI 2(l )  SO2( g )  2 H 2 O(l )  2 HI x ( aq)  H 2 SO4( aq)
(1)
H 2 SO4( g )  H 2 O( g )  SO2( g )  1 O2( g )
2
2 HI x ( g )  H 2 ( g )  I 2 ( g )
(2)
(3)
where x in the reactions represents the average of several polyiodides that are formed.
Equation (1) is the Bunsen reaction. It is an exothermic reaction that is spontaneous in
the range 20oC<T<100oC. At certain reactant concentrations, involving an excess of
iodine, a phase separation occurs between the two acid products leading to a H2SO4 phase
principally devoid of HI and vice versa.
Equation (2) is the sulphuric acid decomposition step. This is an endothermic 2-stage
reaction:
H 2 SO4  H 2O  SO3
(4)
2SO3  2SO2  O2
(5)
The first stage, as seen in equation (4), occurs at a temperature of 400-500oC, whereas the
second stage, given by equation (5), occurs at 800oC in the presence of a solid catalyst.
Equation (3) is the hydriodic acid decomposition reaction. This is a slightly endothermic
reaction and can be conducted in the liquid or gas phase.
Figure 1 shows a flowsheet of the SI process. The only input is water and the only
products are hydrogen and oxygen.
The HI processing stage is very important. The simple option would be to distill the HIx
(HI-H2O-I2) solution, followed by the gas phase thermal decomposition of HI. However,
due to the presence of the azeotropic composition in HI-H2O (which occurs at a molar
ratio of 1:5), this distillation requires a large amount of thermal energy. Also, the low
equilibrium decomposition ratio of HI of about 20% imposes a large HI circulation which
leads to an increased thermal requirement and therefore a decreased thermal efficiency.
Much research and effort have been directed at improving the efficiency of the HI
decomposition section.
In the late 1990's GA restarted investigating thermochemical cycles. They screened 115
cycles and as the SI cycle had the highest predicted efficiency and the greatest potential
for further improvement they selected it for further research. The helium gas cooled
reactor was chosen as the most suitable for coupling to the cycle as it has high
temperature potential and is sufficiently developed for nuclear hydrogen production to be
possible with essentially no further development. An intermediate helium loop between
the reactor coolant loop and the hydrogen production system is used to ensure that any
leakage from the reactor will not contaminate the hydrogen. Using an assumed peak
process temperature of 827oC (i.e. a reactor temperature of 850oC) the process efficiency
was estimated to be 42%. If the maximum process temperature and reactor temperature
are increased to 900oC and 950oC respectively, an efficiency of 52% can be achieved.