The sulfur–iodine cycle (S–I cycle) is a three-step

thermochemical cycle Thermochemical cycles combine solely heat sources (''thermo'') with ''chemical'' reactions to split water into its hydrogen and oxygen components. The term ''cycle'' is used because aside of water, hydrogen and oxygen, the chemical compounds used in ...

used to produce hydrogen. The S–I cycle consists of three

chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking ...

s whose net reactant is water and whose net products are

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...

and

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

. 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 ();

Bunsen reaction The Bunsen reaction is a chemical reaction that describes water, sulfur dioxide, and iodine reacting to form sulfuric acid and hydrogen iodide: : 2H2O + SO2 + I2 → H2SO4 + 2HI This reaction is the first step in the sulfur-iodine cycle to produ ...

#*The HI is then separated by

distillation Distillation, or classical distillation, is the process of separating the components or substances from a liquid mixture by using selective boiling and condensation, usually inside an apparatus known as a still. Dry distillation is the he ...

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 Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapo ...

, and the hydrogen product remains as a gas. : : Net reaction: 2 H₂O → 2 H₂ + O₂ The

sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...

and

iodine Iodine is a chemical element with the Symbol (chemistry), symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , ...

compounds are recovered and reused, hence the consideration of the process as a cycle. This S–I process is a chemical

heat engine In thermodynamics and engineering, a heat engine is a system that converts heat to mechanical energy, which can then be used to do mechanical work. It does this by bringing a working substance from a higher state temperature to a lower stat ...

. Heat enters the cycle in high-temperature

endothermic In thermochemistry, an endothermic process () is any thermodynamic process with an increase in the enthalpy (or internal energy ) of the system.Oxtoby, D. W; Gillis, H.P., Butler, L. J. (2015).''Principle of Modern Chemistry'', Brooks Cole. ...

chemical reactions 2 and 3, and heat exits the cycle in the low-temperature

exothermic In thermodynamics, an exothermic process () is a thermodynamic process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity ...

reaction 1. The difference between the heat entering and leaving the cycle exits the cycle in the form of the

heat of combustion The heating value (or energy value or calorific value) of a substance, usually a fuel or food (see food energy), is the amount of heat released during the combustion of a specified amount of it. The ''calorific value'' is the total energy rele ...

of the hydrogen produced.

Characteristics

Advantages

* All fluid (liquids, gases) process, therefore well suited for

continuous production Continuous production is a flow production method used to manufacture, produce, or process materials without interruption. Continuous production is called a continuous process or a continuous flow process because the materials, either dry bulk ...

* High

thermal efficiency In thermodynamics, the thermal efficiency (\eta_) is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, steam turbine, steam engine, boiler, furnace, refrigerator, ACs etc. For a ...

predicted (about 50%) * Completely closed system without byproducts or effluents (besides hydrogen and oxygen) * Suitable for application with solar,

nuclear Nuclear may refer to: Physics Relating to the nucleus of the atom: *Nuclear engineering *Nuclear physics *Nuclear power *Nuclear reactor *Nuclear weapon *Nuclear medicine *Radiation therapy *Nuclear warfare Mathematics *Nuclear space *Nuclear ...

, and hybrid (e.g., solar-fossil) sources of heat - if high enough temperatures can be achieved * More developed than competing thermochemical processes *

Scalable Scalability is the property of a system to handle a growing amount of work by adding resources to the system. In an economic context, a scalable business model implies that a company can increase sales given increased resources. For example, a ...

from relatively small scale to huge applications * No need for expensive or toxic

catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

s or additives * More efficient than

electrolysis of water Electrolysis of water, also known as electrochemical water splitting, is the process of using electricity to decompose water into oxygen and hydrogen gas by electrolysis. Hydrogen gas released in this way can be used as hydrogen fuel, or remi ...

(~70-80% efficiency) using electricity derived from a

thermal power plant A thermal power station is a type of power station in which heat energy is converted to electrical energy. In a steam-generating cycle heat is used to boil water in a large pressure vessel to produce high-pressure steam, which drives a steam ...

(~30-60% efficiency) combining to ~21-48% efficiency *

Waste heat Waste heat is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing work. All such processes give off some waste heat as a fundamental result of the laws of thermodynamics. Waste heat has lower utilit ...

suitable for

district heating District heating (also known as heat networks or teleheating) is a system for distributing heat generated in a centralized location through a system of insulated pipes for residential and commercial heating requirements such as space heating ...

cogeneration Cogeneration or combined heat and power (CHP) is the use of a heat engine or power station to generate electricity and useful heat at the same time. Cogeneration is a more efficient use of fuel or heat, because otherwise- wasted heat from elec ...

is desired

Disadvantages

* Very high temperatures required (at least ) - unachievable or difficult to achieve with current

pressurized water reactor A pressurized water reactor (PWR) is a type of light-water nuclear reactor. PWRs constitute the large majority of the world's nuclear power plants (with notable exceptions being the UK, Japan and Canada). In a PWR, the primary coolant (water) i ...

s or

concentrated solar power Concentrated solar power (CSP, also known as concentrating solar power, concentrated solar thermal) systems generate solar power by using mirrors or lenses to concentrate a large area of sunlight into a receiver. Electricity is generated when ...

* Corrosive reagents used as intermediaries (iodine, sulfur dioxide, hydriodic acid, sulfuric acid); therefore, advanced materials needed for construction of process apparatus * Significant further development required to be feasible on large scale * At the proposed temperature range advanced thermal power plants can achieve efficiencies (electric output per heat input) in excess of 50% somewhat negating the efficiency advantage * In case of leakage corrosive and somewhat toxic substances are released to the environment - among them volatile iodine and

hydroiodic acid Hydroiodic acid (or hydriodic acid) is an aqueous solution of hydrogen iodide (HI). It is a strong acid, one that is ionized completely in an aqueous solution. It is colorless. Concentrated solutions are usually 48% to 57% HI. Reactions Hy ...

* If hydrogen is to be used for process heat the required high temperatures make the benefits compared to direct utilization of heat questionable * Unable to use non-thermal or low-grade thermal energy sources such as hydropower, wind power or most currently available geothermal power

Research

The S–I cycle was invented at

General Atomics General Atomics is an American energy and defense corporation headquartered in San Diego, California, specializing in research and technology development. This includes physics research in support of nuclear fission and nuclear fusion energy. Th ...

in the 1970s. The Japan Atomic Energy Agency (JAEA) has conducted successful experiments with the S–I cycle in the Helium cooled High Temperature Test Reactor, a reactor which reached first criticality in 1998, JAEA have the aspiration of using further nuclear very high-temperature

generation IV reactor Generation IV reactors (Gen IV) are six nuclear reactor designs recognized by the Generation IV International Forum. The designs target improved safety, sustainability, efficiency, and cost. The most developed Gen IV reactor design is the sodium ...

s ( VHTR) to produce industrial scale quantities of 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 Sandia National Laboratories (SNL), also known as Sandia, is one of three research and development laboratories of the United States Department of Energy's National Nuclear Security Administration (NNSA). Headquartered in Kirtland Air Force Bas ...

are jointly developing the sulfur-iodine process. Additional research is taking place at the

Idaho National Laboratory Idaho National Laboratory (INL) is one of the national laboratories of the United States Department of Energy and is managed by the Battelle Energy Alliance. While the laboratory does other research, historically it has been involved with nu ...

, in Canada, Korea and Italy.

Material challenge

The S–I cycle involves operations with corrosive chemicals at temperatures up to about . 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. Some materials suggested include tantalum alloys, niobium alloys, noble metals, high-silicon steels, several nickel-based superalloys, mullite,

silicon carbide Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder and crystal s ...

(SiC), glass,

silicon nitride Silicon nitride is a chemical compound of the elements silicon and nitrogen. is the most thermodynamically stable and commercially important of the silicon nitrides, and the term "silicon nitride" commonly refers to this specific composition. It ...

(Si₃N₄), and others. Recent research on scaled prototyping suggests that new tantalum surface technologies may be a technically and economically feasible way to make larger scale installations.T. Drake, B. E. Russ, L. Brown, G. Besenbruch, "Tantalum Applications For Use In Scale Sulfur-Iodine Experiments", AIChE 2007 Fall Annual Meeting, 566a.
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Hydrogen economy

The sulfur-iodine cycle has been proposed as a way to supply hydrogen for a hydrogen-based economy. It does not require

hydrocarbons In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...

like current methods of

steam reforming Steam reforming or steam methane reforming (SMR) is a method for producing syngas (hydrogen and carbon monoxide) by reaction of hydrocarbons with water. Commonly natural gas is the feedstock. The main purpose of this technology is hydrogen product ...

but requires heat from combustion, nuclear reactions, or solar heat concentrators.

Footnotes

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
(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)
(PDF)

External links

Hydrogen: Our Future made with Nuclear
(in ''MPR Profile'' issue 9)
Use of the modular helium reactor for hydrogen production
(''World Nuclear Association Symposium'' 2003) {{DEFAULTSORT:Sulfur-iodine cycle Chemical reactions Hydrogen production