Development of direct resistive heating method for SO3 decomposition in the S–I cycle for hydrogen production

The Sulfur–Iodine (S–I) cycle has been considered as one of the efficient and promising thermochemical water-splitting cycles for hydrogen production using nuclear energy. However, the catalytic SO3 decomposition process in the S–I cycle demands high temperature heat (>800°C). Existing nuclear reactors cannot provide such heat for SO3 decomposition. AECL proposed a direct resistive heating concept to compensate for the requirement of high temperature heat. An experimental program was established at AECL to demonstrate the concept and to develop reliable catalyst structures for SO3 decomposition. Due to the high temperature and harsh chemical environment, Hastelloy C-276 was selected as the material for the heating element and reactor. The catalyst was directly applied on the surface of an electrical heating element. SO3 was produced online from H2SO4 in a pre-heated vessel. The SO3 decomposition percentage was determined using the measured O2 concentration in the exit gas stream. The results showed that SO3 decomposition can be successfully achieved with the direct resistive heating method. As much as 90% of the initial SO3 was decomposed under the experimental conditions explored. The Pt-based catalyst performed better than the Fe-based catalyst in the low temperature region (<700°C). The effect of carrier gas flow on SO3 decomposition was also considered.