Kinetic analysis of Bunsen reaction with HI existence in the thermochemical sulfur–iodine cycle for hydrogen production

Highlights: • The detailed kinetics of Bunsen reaction with HI recycled was studied. • Effects of temperature and initial HI on the overall reaction rate were explored. • A ${\text{SO}}_4^{2-}$ production rate model for the Bunsen reaction was proposed. • The activation energy for kinetics-controlled reactions (4) and (5) were determined. - Abstract: The sulfur–iodine (SI) cycle is considered as a promising method for large-scale and high-efficiency hydrogen production. The Bunsen reaction in the cycle dominates the following HI and H₂SO₄ decomposition section and even the whole SI flowsheet. In this work, the detailed kinetics of the Bunsen reaction at practical condition with HI recycled was studied. The effects of operating temperature and initial HI concentration on the overall reaction rate were explored. A ${\text{SO}}_4^{2-}$ production rate model was proposed based on three elementary reactions, and well described the kinetic process of Bunsen reaction. Both theoretical and experimental results indicated that increasing the initial HI concentration (0–3.08 mol/kg_H2O) or operating temperature (303–358 K) enhanced the reaction rate, including the earlier appearance of liquid separation and thermodynamic equilibrium. But the recycled HI should be controlled within 6.17 mol/kg_H2O, so as to make the separation of H₂SO₄ and HIx phase possible. The calculated apparent activation energy for kinetics-controlled reactions (4) and (5) were 8.536 and 21.516 kJ/mol, respectively. These results may contribute to the further optimization and reactor design of the Bunsen reaction in the SI cycle.