Sorption enhanced water gas shift (SEWGS) is a technology that combines a pre-combustion

carbon capture Carbon capture may refer to: * Carbon capture and utilization, where the captured carbon dioxide is used * Carbon sequestration, where the captured carbon dioxide is stored ** Carbon capture and storage, referring to carbon sequestration from point ...

process with the

water gas shift reaction Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a s ...

(WGS) in order to produce a

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-toxic ...

rich stream from the syngas fed to the SEWGS reactor. The

converts the

carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...

into carbon dioxide, according to the following chemical reaction: : CO + H₂O CO₂ + H₂ While

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...

is captured and removed through an

adsorption Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which ...

process. The in-situ CO₂ adsorption and removal shifts the water gas shift reaction to the right-hand side, thereby completely converting the CO and maximizing the production of high pressure hydrogen. Since the beginning of the second decade of the 21st century this technology has started gaining attention, as it shows advantages over carbon capture conventional technologies and because hydrogen is considered the energy carrier of the future.

Process

The SEWGS technology is the combination of the water gas shift reaction with the adsorption of carbon dioxide on a solid material. Typical temperature and pressure ranges are 350-550 °C and 20-30 bar. The inlet gas of SEWGS reactors is typically a mixture of hydrogen, CO and CO₂, where steam is added to convert CO into CO₂. The conversion of carbon monoxide into carbon dioxide is enhanced by shifting the reaction equilibrium through CO₂ adsorption and removal, the latter being one the produced species. The SEWGS technology is based on a multi-bed

pressure swing adsorption Pressure swing adsorption (PSA) is a technique used to separate some gas species from a mixture of gases (typically air) under pressure according to the species' molecular characteristics and affinity for an adsorbent material. It operates at ne ...

(PSA) unit in which the vessels are filled with the water gas shift

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 ...

and the CO₂ adsorbent material. Each vessel is subjected to a series of processes. In the sorption/reaction step, a high pressure hydrogen-rich stream is produced, while during sorbent regeneration a CO₂ rich stream is generated. The process starts feeding syngas to the SEWGS reactor, where CO₂ is adsorbed and a hydrogen-rich stream is produced. The regeneration of the first vessel starts when the sorbent material is saturated by CO₂, directing the feed stream to another vessel. After the regeneration, the vessels are re-pressurized. A multibed configuration is necessary to guarantee a continuous production of hydrogen and carbon dioxide. The optimal number of beds usually varies between 6 and 8.

Water gas shift reaction

The water gas shift reaction is the reaction between carbon monoxide and steam to form hydrogen and carbon dioxide: : CO + H₂O CO₂ + H₂ This reaction was discovered by

Felice Fontana Abbé Gasparo Ferdinando Felice Fontana (15 April 1730 – 9 March 1805) was an Italian polymath who contributed to experimental studies in physiology, toxicology, and physics. As a physicist he discovered the water gas shift reaction in 1780. He ...

and nowadays is adopted in a wide range of industrial applications, such as in the production process of

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...

hydrocarbon 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 ...

s, methanol, hydrogen and other chemicals. In the industrial practice two water gas shift sections are necessary, one at high temperature and one at low temperature, with an intersystem cooling.

Adsorption process

Adsorption is the phenomenon of

sorption Sorption is a physical and chemical process by which one substance becomes attached to another. Specific cases of sorption are treated in the following articles: ; Absorption: "the incorporation of a substance in one state into another of a d ...

of gases or

solutes In chemistry, a solution is a special type of homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. If the attractive forces between the solvent ...

on solid or liquid surfaces. Adsorption on solid surface occurs when some substances collide with the solid surface creating bonds with the atoms or the molecules of the solid surface. There are two main adsorption processes: physical adsorption and chemical adsorption. The first one is the result of the interaction of intermolecular forces. Since weak bonds are formed, the adsorbed substance can be easily separated. In chemical adsorption, chemical bonds are formed, meaning that the absorption or release of adsorption heat and the activation energy are larger with respect to physical adsorption. These two processes often take place simultaneously. The adsorbent material is then regenerated through

desorption Desorption is the physical process where a previously adsorbed substance is released from a surface. This happens when a molecule gains enough energy to overcome the activation barrier of the bounding energy that keeps it in the surface. There ...

, which is the opposite phenomenon of sorption, releasing the captured substance from the adsorbent material. In SEWGS technology the pressure swing adsorption (PSA) process is employed to regenerate the adsorbent material and produce a CO₂ rich stream. The process is similar to the one conventionally used for air separation, hydrogen purification and other gas separations.

Conventional technology for carbon dioxide removal

The industrially used technology for

carbon dioxide removal Carbon dioxide removal (CDR), also known as negative emissions, is a process in which carbon dioxide gas () is removed from the atmosphere and sequestered for long periods of time. Similarly, greenhouse gas removal (GGR) or negative greenho ...

is called

amine In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen Hydrogen is the chemical element wi ...

washing technology and is based on chemical absorption of carbon dioxide. In chemical absorption, reactions between the absorbed substance (CO₂) and the

solvent A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...

occur and produce a rich liquid. Then, the rich liquid enters the desorption column where carbon dioxide is separated from the sorbent which is reused for CO₂ absorption.

Ethanolamine Ethanolamine (2-aminoethanol, monoethanolamine, ETA, or MEA) is an organic chemical compound with the formula or . The molecule is bifunctional, containing both a primary amine and a primary alcohol. Ethanolamine is a colorless, viscous liquid w ...

(C₂H₇NO),

diethanolamine Diethanolamine, often abbreviated as DEA or DEOA, is an organic compound with the formula HN(CH2CH2OH)2. Pure diethanolamine is a white solid at room temperature, but its tendencies to absorb water and to supercool meaning that it is often encoun ...

(C₄H₁₁NO₂),

triethanolamine Triethanolamine, or TEA is a viscous organic compound that is both a tertiary amine and a triol. A triol is a molecule with three alcohol groups. Approximately 150,000 tonnes were produced in 1999. It is a colourless compound although samples m ...

(C₆H₁₅NO₃) mono-ethanolamine (C₂H₇NO) and methyl-diethanolamine (C₅H₁₃NO₂) are commonly used for the removal of CO₂.

Advantages of SEWGS over conventional technologies

SEWGS technology shows some advantages in comparison with traditional technologies adoptable for pre-combustion removal of carbon dioxide. Traditional technologies require employing two water gas shift reactors (a high temperature and a low temperature stage) in order to get high conversions of carbon monoxide into carbon dioxide with an intermediate cooling stage between the two reactors. In addition, another cooling stage is necessary at the outlet of the second WGS reactor for the CO₂ capture with a solvent. Furthermore, the hydrogen rich stream at the outlet of SEWGS section can be directly fed into a gas turbine, while the hydrogen rich stream produced by the traditional route needs a further heating stage.

Applications

The importance of this technology is directly related to the problem of

global warming In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...

and the mitigation of the carbon dioxide emissions. In

hydrogen economy The hydrogen economy is using hydrogen to decarbonize economic sectors which are hard to electrify, essentially, the "hard-to-abate" sectors such as cement, steel, long-haul transport etc. In order to phase out fossil fuels and limit climate ch ...

hydrogen is considered a clean energy carrier with high energy content and is expected to replace fossil fuels and other energy sources associated with pollution issues. For these reasons, since the beginning of second decade of the 21st century this technology attracted the public interest. The SEWGS technology enables producing high-purity hydrogen without need for further purification processes. It furthermore finds potential application in a wide range of industrial processes, such as in the production of electricity from fossil fuels or in the iron and steel industry. The integration of the SEWGS process in

natural gas combined cycle A combined cycle power plant is an assembly of heat engines that work in tandem from the same source of heat, converting it into mechanical energy. On land, when used to make electricity the most common type is called a combined cycle gas turb ...

(NGCC) and

integrated gasification combined cycle integrated gasification combined cycle (IGCC) is a technology using a high pressure gasifier to turn coal and other carbon based fuels into pressurized gas—synthesis gas (syngas). It can then remove impurities from the syngas prior to the electr ...

(IGCC) power plants has been investigated as a possible way to produce electricity from natural gas or coal with almost-zero emissions. In NGCC power plant the carbon capture achieved is around 95% with a CO₂ purity over 99%, while in IGCC power plants the carbon capture ratio is around 90% with a CO₂ purity of 99%. The investigation of SEWGS integration in steel mills started during the second decade of 21st century. The goal is to reduce the carbon footprint of this industrial process that is responsible of the 6% of total global CO₂ emissions and 16% of the emissions generated by industrial processes. The captured and removed CO₂ can be then stored or used for the production of high value chemical products.

Sorbents for SEWGS process

The reactor vessels are loaded with sorbent pellets. Sorbent must have the following features: * high CO₂ capacity and selectivity over H₂ * low H₂O adsorption * low specific cost * mechanical stability under pressure and temperature variation * chemical stability in the presence of impurities * easy regeneration by steam Different sorbent materials have been investigated to the purpose of being employed in SEWGS. Some examples include: * K₂CO₃-promoted hydrotalcite * potassium promoted alumina * Na–Mg double salt *CaO{{cite journal , last1=Zivkovic , first1=Luka A. , last2=Pohar , first2=Andrej , last3=Likozar , first3=Blaz , last4=Nikacevic , first4=Nikola M. , title=Kinetics and reactor modeling for CaO sorption-enhanced high-temperature water–gas shift (SE–WGS) reaction for hydrogen production , journal=Applied Energy , date=2016-09-15 , volume=178 , pages=844–855 , doi=10.1016/j.apenergy.2016.06.071 , url=http://dx.doi.org/10.1016/j.apenergy.2016.06.071 , language=en , issn=0306-2619 Potassium promoted hydrotalcite is the most studied sorbent material for SEWGS application. Its principal features are listed below: * low cost * sufficiently high CO₂ cyclic working capacity * fast adsorption kinetics * good mechanical stability

References

External links

Projects in which SEWGS technology is investigated:
Web page of STEPWISE projectWeb page of C4U project
Chemical processes Hydrogen production Industrial gases