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.

I₂ + SO₂ + 2 H₂O → 2 HI + H₂SO₄ (120°C) (Reaction 1)

2 H₂SO₄ → 2 SO₂ + 2 H₂O + O₂ (830°C) (Reaction 2)

2 HI → I₂ + H₂ (320°C) (Reaction 3)

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 temperatue 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 1970's. 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 Japaneese 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. Additonal research is taking place at the Idaho National Laboratory, in Korea and in 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 uranium 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.

External links

• Hydrogen: Our Future made with Nuclear (in MPR Profile issue 9)
• The Hydrogen Economy (World Nuclear Association)
• JAERI press release (August 26, 2004)
• Thermodynamics of the Sulfur-Iodine Cycle for Thermochemical Hydrogen Production (PDF)