Sulfur–iodine cycle

The sulfur–iodine cycle (S–I cycle) is a three-step thermochemical cycle used to produce hydrogen. The S–I cycle consists of three chemical reactions whose net reactant is water and whose net products are hydrogen and oxygen. All other chemicals are recycled. The S–I process requires an efficient source of heat. The three reactions that produce hydrogen are as follows: I2 + SO2 + 2 H2O 2 HI + H2SO4 (); Bunsen reaction The HI is then separated by distillation or liquid/liquid gravitic separation. 2 H2SO4 2 SO2 + 2 H2O + O2 () The water, SO2 and residual H2SO4 must be separated from the oxygen byproduct by condensation. 2 HI I2 + H2 () Iodine and any accompanying water or SO2 are separated by condensation, and the hydrogen product remains as a gas. Net reaction: 2 H2O → 2 H2 + O2 The sulfur and iodine compounds are recovered and reused, hence the consideration of the process as a cycle. This S–I process is a chemical heat engine. Heat enters the cycle in high-temperature endothermic chemical reactions 2 and 3, and heat exits the cycle in the low-temperature exothermic reaction 1. The difference between the heat entering and leaving the cycle exits the cycle in the form of the heat of combustion of the hydrogen produced. All fluid (liquids, gases) process, therefore well suited for continuous production High thermal efficiency predicted (about 50%) Completely closed system without byproducts or effluents (besides hydrogen and oxygen) Suitable for application with solar, nuclear, and hybrid (e.g.