Sulfur-iodine cycle

The sulfur-iodine cycle (S-I 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. All other chemicals are recycled. The S-I process requires an efficient source of heat.

Process Description

The three reactions that produce hydrogen are as follows:
#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. This reaction is sometimes referred to as Bunsen Reaction.
#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.
#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.:: 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 the cycle and the heat leaving the cycle exits the cycle in the form of the heat of combustion of the hydrogen produced.

Advantages and disadvantages

The characteristics of the S-I process can be described as follows:
* All fluid (liquids, gases) process, therefore well suited for continuous operation;
* High utilization of heat predicted (about 50%), but very high temperatures required (at least 850 deg C);
* Completely closed system without byproducts or effluents (besides hydrogen and oxygen);
* Corrosive reagents used as intermediaries (iodine, sulfur dioxide, hydriodic acid, sulfuric acid); therefore, advanced materials needed for construction of process apparatus;
* Suitable for application with solar, nuclear, and hybrid (e.g., solar-fossil) sources of heat;
* More developed than competitive thermochemical processes (but still requiring significant development to be feasible on large scale).

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. (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.

Material challenge

The S-I cycle involves operations with corrosive chemical at temperatures up to about 1000 °C. The selection of materials with sufficient corrosion resistance under the process conditions is of key importance to the economic viability of this process. The materials suggested include the following classes: refractory metals, reactive metals, superalloys, ceramics, polymers, and coatings. [ [http://www.hydrogen.energy.gov/pdfs/review05/pd27_pickard.pdf Paul Pickard, Sulfur-Iodine Thermochemical Cycle 2005 DOE Hydrogen Program Review] ] [B. Wonga, R.T. Buckingham, L.C. Brown, B.E. Russ, G.E. Besenbruch, A. Kaiparambil, R. Santhanakrishnan and Ajit Roy, "Construction materials development in sulfur–iodine thermochemical water-splitting process for hydrogen production", International Journal of Hydrogen EnergyVolume 32, Issue 4, March 2007, Pages 497-504.] Some materials suggested include tantalum alloys, niobium alloys, noble metals, high-silicon steels [ [http://www.akerkvaerner.com/NR/rdonlyres/064EE95D-F672-46E4-8551-20E4E17D18FA/12159/Saramet.pdf Saramet info sheet] ] , several nickel-based superalloys, mullite , silicon carbide SiC, glass, silicon nitride Si₃N₄, and others.

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.

ee also

*Hybrid sulfur cycle
*High-temperature electrolysis

External links

* [http://www.mpr.com/pubs/profile/pf9_hydrogen.html Hydrogen: Our Future made with Nuclear] (in "MPR Profile" issue 9)

References

* Paul M. Mathias and Lloyd C. Brown "Thermodynamics of the Sulfur-Iodine Cycle for Thermochemical Hydrogen Production", presented at the 68 th Annual Meeting of the Society of Chemical Engineers, Japan 23 March 2003. [http://www.aspentech.com/publication_files/TP51.pdf (PDF)] .

* Atsuhiko TERADA; Jin IWATSUKI, Shuichi ISHIKURA, Hiroki NOGUCHI, Shinji KUBO, Hiroyuki OKUDA, Seiji KASAHARA, Nobuyuki TANAKA, Hiroyuki OTA, Kaoru ONUKI and Ryutaro HINO, "Development of Hydrogen Production Technology by Thermochemical Water Splitting IS Process Pilot Test Plan", Journal of Nuclear Science and Technology, Vol.44, No.3, p.477-482 (2007). [http://www.jstage.jst.go.jp/article/jnst/44/3/477/_pdf (PDF)] .

Footnotes