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Syngas, or synthesis gas, is a mixture of
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 ...
and
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 ...
, in various ratios. The gas often contains some
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 ...
and
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ea ...
. It is principly used for producing
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 ...
or methanol. Syngas is combustible and can be used as a fuel. Historically, it has been used as a replacement for gasoline, when gasoline supply has been limited; for example,
wood gas Wood gas is a fuel gas that can be used for furnaces, stoves, and vehicles. During the production process, biomass or related carbon-containing materials are gasified within the oxygen-limited environment of a wood gas generator to produce a c ...
was used to power cars in Europe during
WWII World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
(in Germany alone half a million cars were built or rebuilt to run on wood gas).


Production

Syngas is produced by
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 ...
or
partial oxidation Partial oxidation (POX) is a type of chemical reaction. It occurs when a substoichiometric fuel-air mixture is partially combusted in a reformer, creating a hydrogen-rich syngas which can then be put to further use, for example in a fuel cell. A d ...
of natural gas or liquid hydrocarbons, or
coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal is formed when ...
gasification. Steam reforming of methane is an
endothermic reaction 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. p ...
requiring 206 kJ/mol of methane: : In principle, but rarely in practice, biomass and related hydrocarbon feedstocks could be used to generate
biogas Biogas is a mixture of gases, primarily consisting of methane, carbon dioxide and hydrogen sulphide, produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste and food waste. It is a ...
and
biochar Biochar is the lightweight black residue, made of carbon and ashes, remaining after the pyrolysis of biomass. Biochar is defined by the International Biochar Initiative as "the solid material obtained from the thermochemical conversion of ...
in
waste-to-energy Waste-to-energy (WtE) or energy-from-waste (EfW) is the process of generating energy in the form of electricity and/or heat from the primary treatment of waste, or the processing of waste into a fuel source. WtE is a form of energy recovery. Mo ...
gasification facilities. The gas generated (mostly methane and carbon dioxide) is sometimes described as ''syngas'' but its composition differs from syngas. Generation of conventional syngas (mostly H2 and CO) from waste biomass has been explored.


Composition, pathway for formation, and thermochemistry

The chemical composition of syngas varies based on the raw materials and the processes. Syngas produced by coal gasification generally is a mixture of 30 to 60% carbon monoxide, 25 to 30% hydrogen, 5 to 15% carbon dioxide, and 0 to 5% methane. It also contains lesser amount of other gases. Syngas has less than half the energy density of
natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbo ...
. The first reaction, between incandescent coke and steam, is strongly endothermic, producing carbon monoxide (CO), and hydrogen (
water gas Water gas is a kind of fuel gas, a mixture of carbon monoxide and hydrogen. It is produced by "alternately hot blowing a fuel layer okewith air and gasifying it with steam". The caloric yield of this is about 10% of a modern syngas plant. F ...
in older terminology). When the coke bed has cooled to a temperature at which the endothermic reaction can no longer proceed, the steam is then replaced by a blast of air. The second and third reactions then take place, producing an
exothermic reaction In thermochemistry, an exothermic reaction is a "reaction for which the overall standard enthalpy change Δ''H''⚬ is negative." Exothermic reactions usually release heat. The term is often confused with exergonic reaction, which IUPAC defines ...
—forming initially carbon dioxide and raising the temperature of the coke bed—followed by the second endothermic reaction, in which the latter is converted to carbon monoxide. The overall reaction is exothermic, forming "producer gas" (older terminology). Steam can then be re-injected, then air etc., to give an endless series of cycles until the coke is finally consumed. Producer gas has a much lower energy value, relative to water gas, due primarily to dilution with atmospheric nitrogen. Pure oxygen can be substituted for air to avoid the dilution effect, producing gas of much higher calorific value. In order to produce more hydrogen from this mixture, more steam is added and the water gas shift reaction is carried out: : The hydrogen can be separated from the by
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),
amine scrubbing Amine gas treating, also known as amine scrubbing, gas sweetening and acid gas removal, refers to a group of processes that use aqueous solutions of various alkylamines (commonly referred to simply as amines) to remove hydrogen sulfide (H2S) and c ...
, and
membrane reactor A membrane reactor is a physical device that combines a chemical conversion process with a membrane separation process to add reactants or remove products of the reaction. Chemical reactors making use of membranes are usually referred to as membr ...
s. A variety of alternative technologies have been investigated, but none are of commercial value. Some variations focus on new stoichiometries such as carbon dioxide plus methane or partial hydrogenation of carbon dioxide. Other research focuses on novel energy sources to drive the processes including electrolysis, solar energy, microwaves, and electric arcs. Electricity generated from
renewable sources A renewable resource, also known as a flow resource, is a natural resource which will replenish to replace the portion depleted by usage and consumption, either through natural reproduction or other recurring processes in a finite amount of ti ...
is also used to process carbon dioxide and water into syngas through
high-temperature electrolysis High-temperature electrolysis (also HTE or steam electrolysis) is a technology for producing hydrogen from water at high temperatures. Efficiency High temperature electrolysis is more efficient economically than traditional room-temperature elect ...
. This is an attempt to maintain
carbon neutrality Carbon neutrality is a state of net-zero carbon dioxide emissions. This can be achieved by balancing emissions of carbon dioxide with its removal (often through carbon offsetting) or by eliminating emissions from society (the transition to the "p ...
in the generation process.
Audi Audi AG () is a German automotive manufacturer of luxury vehicles headquartered in Ingolstadt, Bavaria, Germany. As a subsidiary of its parent company, the Volkswagen Group, Audi produces vehicles in nine production facilities worldwide. Th ...
, in partnership with company named Sunfire, opened a pilot plant in November 2014 to generate
e-diesel E-diesel is a synthetic diesel fuel created by Audi for use in automobiles. Currently, e-diesel is created by an Audi research facility in partnership with a company named Sunfire. The fuel is created from carbon dioxide, water, and electricity wit ...
using this process. Syngas that is not methanized typically has a lower heating value of 120 BTU/ scf . Untreated syngas can be run in hybrid turbines that allow for greater efficiency because of their lower operating temperatures, and extended part lifetime.


Uses

Syngas is used as a source of hydrogen as well as a fuel. it is also used to directly reduce iron ore to
sponge iron Direct reduced iron (DRI), also called sponge iron, is produced from the direct reduction of iron ore (in the form of lumps, pellets, or fines) into iron by a reducing gas or elemental carbon produced from natural gas or coal. Many ores are suit ...
. Chemical uses include the production of methanol which is a precursor to acetic acid and many acetates; liquid fuels and lubricants via the
Fischer–Tropsch process The Fischer–Tropsch process is a collection of chemical reactions that converts a mixture of carbon monoxide and hydrogen, known as syngas, into liquid hydrocarbons. These reactions occur in the presence of metal catalysts, typically at temperatu ...
and previously the Mobil
methanol to gasoline Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic Chemical compound, chemical and the simplest aliphatic Alcohol (chemistry), alcohol, with the Chemical formula, formula Carbon, CHydrogen, H3Oxygen, OHydro ...
process;
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 ...
via the
Haber process The Haber process, also called the Haber–Bosch process, is an artificial nitrogen fixation process and is the main industrial procedure for the production of ammonia today. It is named after its inventors, the German chemists Fritz Haber and ...
, which converts atmospheric nitrogen (N2) into ammonia which is used as a
fertilizer A fertilizer (American English) or fertiliser (British English; see spelling differences) is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply plant nutrients. Fertilizers may be distinct from ...
; and
oxo alcohol Oxo alcohols are alcohols In chemistry, an alcohol is a type of organic compound that carries at least one hydroxyl () functional group bound to a saturated carbon atom. The term ''alcohol'' originally referred to the primary alcohol ethanol ...
s via an intermediate aldehyde.


See also

*
Boudouard reaction The Boudouard reaction, named after Octave Leopold Boudouard, is the redox reaction of a chemical equilibrium mixture of carbon monoxide and carbon dioxide at a given temperature. It is the disproportionation of carbon monoxide into carbon dioxide ...
* Claus process * Coal gas * Industrial gas *
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 ...
*
Partial oxidation Partial oxidation (POX) is a type of chemical reaction. It occurs when a substoichiometric fuel-air mixture is partially combusted in a reformer, creating a hydrogen-rich syngas which can then be put to further use, for example in a fuel cell. A d ...
* Reformer sponge iron cycle * Syngas fermentation *
Underground coal gasification Underground coal gasification (UCG) is an industrial process which converts coal into product gas. UCG is an ''in-situ'' gasification process, carried out in non-mined coal seams using injection of oxidants and steam. The product gas is brought t ...


References


External links


"Sewage treatment plant smells success in synthetic gas trial" ARENA, accessed December 6 2020Fischer Tropsch archive
*https://www.technologyreview.com/s/508051/a-cheap-trick-enables-energy-efficient-carbon-capture/ {{Authority control Coal technology Fuel gas Waste treatment technology Synthetic fuels Industrial gases Synthetic fuel technologies Petrochemicals