Sulfur-iodine cycle
Encyclopedia
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 reaction
s 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:
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.
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 reactor
s 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.
Some materials suggested include tantalum alloys, niobium alloys, noble metals, high-silicon steels, several nickel-based superalloys, mullite
, silicon carbide
(SiC), glass, silicon nitride
(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.
. 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.
Thermochemical cycle
Thermochemical cycles combine solely heat sources 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 these processes are continuously recycled.If work is partially used...
used to produce hydrogen
Hydrogen production
Hydrogen production is the family of industrial methods for generating hydrogen. Currently the dominant technology for direct production is steam reforming from hydrocarbons. Many other methods are known including electrolysis and thermolysis...
.
The S–I cycle consists of three chemical reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
s whose net reactant is water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
and whose net products are hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
. All other chemicals are recycled. The S–I process requires an efficient source of heat.
Process description
| H2O | ½O2 | |||||
| ↓ | ↑ | |||||
| I2 | → | Reaction 1 | ← | SO2+H2O | ← | Separate |
| ↑ | ↓ | ↑ | ||||
| 2HI | ← | Separate | → | H2SO4 | → | Reaction 2 |
| ↓ | ||||||
| H2 |
The three reactions that produce hydrogen are as follows:
- I2IodineIodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
+ SO2 + 2 H2O → 2 HI + H2SO4 (120°C)- The HI is then separated by distillationDistillationDistillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....
. Note that concentrated H2SO4 may react with HI, giving I2, SO2 and H2O (backward reaction). Many chemical processChemical processIn a "scientific" sense, a chemical process is a method or means of somehow changing one or more chemicals or chemical compounds. Such a chemical process can occur by itself or be caused by somebody. Such a chemical process commonly involves a chemical reaction of some sort...
es are reversible reactionReversible reactionA reversible reaction is a chemical reaction that results in an equilibrium mixture of reactants and products. For a reaction involving two reactants and two products this can be expressed symbolically as...
s, such as ammonia productionAmmonia productionBecause of its many uses, ammonia is one of the most highly-produced inorganic chemicals. There are numerous large-scale ammonia production plants worldwide, producing a total of 131,000,000 metric tons of ammonia in 2010. China produced 32.1% of the worldwide production, followed by India with...
from N2 and H2, but removing the desired product will shift equilibriumDynamic equilibriumA dynamic equilibrium exists once a reversible reaction ceases to change its ratio of reactants/products, but substances move between the chemicals at an equal rate, meaning there is no net change. It is a particular example of a system in a steady state...
to the right. This reaction is sometimes referred to as Bunsen reactionBunsen ReactionThe Bunsen Reaction is a chemical reaction that describes water, sulfur dioxide, and iodine reacting to form sulfuric acid and hydrogen iodide. This reaction is the first step in the Sulfur-iodine cycle to produce hydrogen....
.
- The HI is then separated by distillation
- 2 H2SO4 → 2 SO2Sulfur dioxideSulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
+ 2 H2O + O2OxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
(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 + H2HydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
(450°C)- Iodine and any accompanying water or SO2 are separated by condensationCondensationCondensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....
, and the hydrogen product remains as a gas.
- Iodine and any accompanying water or SO2 are separated by condensation
- Net reaction: 2 H2O → 2 H2 + O2
The sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
and iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
compounds are recovered and reused, hence the consideration of the process as a cycle. This S–I process is a chemical heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
engine. Heat enters the cycle in high temperature endothermic
Endothermic
In thermodynamics, the word endothermic describes a process or reaction in which the system absorbs energy from the surroundings in the form of heat. Its etymology stems from the prefix endo- and the Greek word thermasi,...
chemical reactions 2 and 3, and heat exits the cycle in the low temperature exothermic
Exothermic
In thermodynamics, the term exothermic describes a process or reaction that releases energy from the system, usually in the form of heat, but also in the form of light , electricity , or sound...
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
Heat of combustion
The heat of combustion is the energy released as heat when a compound undergoes complete combustion with oxygen under standard conditions. The chemical reaction is typically a hydrocarbon reacting with oxygen to form carbon dioxide, water and heat...
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 AtomicsGeneral Atomics
General Atomics is a nuclear physics and defense contractor headquartered in San Diego, California. General Atomics’ research into fission and fusion matured into competencies in related technologies, allowing the company to expand into other fields of research...
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 reactor
Generation IV reactor
Generation IV reactors are a set of theoretical nuclear reactor designs currently being researched. Most of these designs are generally not expected to be available for commercial construction before 2030...
s 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
Commissariat à l'Énergie Atomique
The Commissariat à l'énergie atomique et aux énergies alternatives or CEA, is a French “public establishment related to industrial and commercial activities” whose mission is to develop all applications of nuclear power, both civilian and military...
, General Atomics and Sandia National Laboratories
Sandia National Laboratories
The Sandia National Laboratories, managed and operated by the Sandia Corporation , are two major United States Department of Energy research and development national laboratories....
are jointly developing the sulfur-iodine process. Additional research is taking place at the Idaho National Laboratory
Idaho National Laboratory
Idaho National Laboratory is an complex located in the high desert of eastern Idaho, between the town of Arco to the west and the cities of Idaho Falls and Blackfoot to the east. It lies within Butte, Bingham, Bonneville and Jefferson counties...
, in Canada, Korea and Italy.
Material challenge
The S–I cycle involves operations with corrosive chemical at temperatures up to about 1000 °C (1,832 °F). 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
Mullite
Mullite or porcelainite is a rare silicate mineral of post-clay genesis. It can form two stoichiometric forms 3Al2O32SiO2 or 2Al2O3 SiO2. Unusually, mullite has no charge balancing cations present...
, silicon carbide
Silicon carbide
Silicon carbide , also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...
(SiC), glass, silicon nitride
Silicon nitride
Silicon nitride is a chemical compound of silicon and nitrogen. If powdered silicon is heated between 1300° and 1400°C in an atmosphere of nitrogen, trisilicon tetranitride, Si3N4, is formed. The silicon sample weight increases progressively due to the chemical combination of silicon and nitrogen...
(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.
Hydrogen economy
The sulfur-iodine cycle has been proposed as a way to supply hydrogen for a hydrogen-based economyHydrogen economy
The hydrogen economy is a proposed system of delivering energy using hydrogen. The term hydrogen economy was coined by John Bockris during a talk he gave in 1970 at General Motors Technical Center....
. With an efficiency of around 50% it is more efficient than electrolysis
Electrolysis
In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...
, and it does not require hydrocarbons like current methods of steam reforming
Steam reforming
Fossil fuel reforming is a method of producing hydrogen or other useful products from fossil fuels such as natural gas. This is achieved in a processing device called a reformer which reacts steam at high temperature with the fossil fuel. The steam methane reformer is widely used in industry to...
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.
See also
- Cerium(IV) oxide–cerium(III) oxide cycle
- Copper–chlorine cycle
- Hybrid sulfur cycleHybrid sulfur cycleThe hybrid sulfur cycle is a two-step water-splitting process intended to be used for hydrogen production. Based on sulfur oxidation and reduction, it is classified as a hybrid thermochemical cycle because it uses an electrochemical reaction for one of the two steps...
- High-temperature electrolysisHigh-temperature electrolysisHigh-temperature electrolysis is a method currently being investigated for the production of hydrogen from water with oxygen as a by-product.-Efficiency:...
- Iron oxide cycleIron oxide cycleThe iron oxide cycle is a two-step thermochemical cycle proposed for use for hydrogen production.-Process description:The thermochemical two-step water splitting process uses redox systems...
- Zinc–zinc oxide cycle
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)