Sulfur-iodine cycle

The sulfur-iodine cycle is a series of thermochemical processes 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.

1. I₂ + SO₂ + 2 H₂O → 2 HI + H₂SO₄ (120°C)
   • The HI is then separated by distillation. Note that concentrated H₂SO₄ may react with HI, giving I₂, SO₂ and H₂O (backward reaction). Many chemical processes are reversible reactions, such as ammonia production from N₂ and H₂, but removing the desired product will shift equilibrium to the right.
2. 2 H₂SO₄ → 2 SO₂ + 2 H₂O + O₂ (830°C)
   • The water, SO₂ and residual H₂SO₄ must be separated from the oxygen byproduct by condensation. See Sulfur dioxide#Temperature dependence of aqueous solubility for temperatures.
3. 2 HI → I₂ + H₂ (450°C)
   • Iodine and any accompanying water or SO₂ are separated by condensation, and the hydrogen product remains as a gas.

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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 the cycle and the heat leaving the cycle exits the cycle in the form of the heat of combustion of the hydrogen produced.

Research

The S-I cycle was invented at General Atomics in the 1970s. The Japan Atomic Energy Agency (JAEA) has conducted successful experiments with the S-I cycle with the intent of using nuclear high-temperature generation IV reactors to produce hydrogen. (For some reason the Japanese refer to the cycle as the IS cycle.) Plans have been made to test larger-scale automated systems for hydrogen production. Under an International Nuclear Energy Research Initiative (INERI) agreement, the French CEA, General Atomics and Sandia National Laboratories are jointly developing the sulfur-iodine process. Additional research is taking place at the Idaho National Laboratory, in Canada, Korea and Italy.

Hydrogen economy

The sulfur-iodine cycle has been proposed as a way to supply hydrogen for a hydrogen-based economy. With an efficiency of around 50% it is more efficient than electrolysis, and it does not require hydrocarbons like current methods of steam reforming but requires heat from combustion, nuclear reactions, or solar heat concentrators. Considerable additional research must occur before the sulfur-iodine cycle can become a viable source of hydrogen. The first commercial generation IV reactors are expected around 2030.