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

carbon capture Carbon capture may refer to: * Carbon capture and storage Carbon capture and storage (CCS) is a process by which carbon dioxide (CO2) from industrial installations is separated before it is released into the atmosphere, then transported to a l ...

process with the

water gas shift reaction Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known living organisms ( ...

(WGS) in order to produce a

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

rich stream from the

syngas Syngas, or synthesis gas, is a mixture of hydrogen and carbon monoxide in various ratios. The gas often contains some carbon dioxide and methane. It is principally used for producing ammonia or methanol. Syngas is combustible and can be used as ...

fed to the SEWGS reactor. The

converts the

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

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

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

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 a ...

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 Steam is water vapor, often mixed with air or an aerosol of liquid water droplets. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporization. Saturated or superheated steam is inv ...

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 increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...

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 chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

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 Hydrophobe, hydrophobic; their odor is usually fain ...

methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical compound and the simplest aliphatic Alcohol (chemistry), alcohol, with the chemical formula (a methyl group linked to a hydroxyl group, often ab ...

, 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 defined by IUPAC as "A liquid or solid phase containing more than one substance, when for convenience one (or more) substance, which is called the solvent, is treated differently from the other substances, which are ...

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 Adsorption, adsorbed atoms or molecules are released from a surface into the surrounding vacuum or fluid. This occurs when a molecule gains enough energy to overcome the activation barrier and the binding e ...

, 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) is a process in which carbon dioxide () is removed from the atmosphere by deliberate human activities and durably stored in geological, terrestrial, or ocean reservoirs, or in products.IPCC, 2021:Annex VII: Glossar ...

is called

amine In chemistry, amines (, ) are organic compounds that contain carbon-nitrogen bonds. Amines are formed when one or more hydrogen atoms in ammonia are replaced by alkyl or aryl groups. The nitrogen atom in an amine possesses a lone pair of elec ...

washing technology and is based on chemical

absorption Absorption may refer to: Chemistry and biology *Absorption (biology), digestion **Absorption (small intestine) *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which su ...

of carbon dioxide. In chemical absorption, reactions between the absorbed substance (CO₂) and the

solvent A solvent (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a gas ...

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 a naturally occurring organic chemical compound with the formula or . The molecule is bifunctional, containing both a primary amine and a primary alcohol. Ethanolamine is a colorl ...

(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 often results in it being foun ...

(C₄H₁₁NO₂),

triethanolamine Triethanolamine, or TEOA, is an organic compound with the chemical formula . It is a colourless, viscous liquid. It is both a tertiary amine and a triol. A triol is a molecule with three alcohol groups. Approximately 150,000 tonnes were produ ...

(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 Present-day climate change includes both global warming—the ongoing increase in global average temperature—and its wider effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes ...

and the mitigation of the carbon dioxide emissions. In

hydrogen economy The hydrogen economy is an umbrella term for the roles hydrogen can play alongside low-carbon electricity to reduce emissions of greenhouse gases. The aim is to reduce emissions where cheaper and more energy-efficient clean solutions are not ava ...

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 synthesis gas. This enables removal of impurities from the fuel prior to generating electrici ...

(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 A carbon footprint (or greenhouse gas footprint) is a calculated value or index that makes it possible to compare the total amount of greenhouse gases that an activity, product, company or country Greenhouse gas emissions, adds to the atmospher ...

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 , bibcode=2016ApEn..178..844Z , url=http://dx.doi.org/10.1016/j.apenergy.2016.06.071 , language=en , issn=0306-2619, url-access=subscription 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