Syngas, or synthesis gas, is a mixture of hydrogen and carbon monoxide, [1] 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 a fuel. [2] [3] [4] Historically, it has been used as a replacement for gasoline, when gasoline supply has been limited; for example, wood gas was used to power cars in Europe during WWII (in Germany alone half a million cars were built or rebuilt to run on wood gas). [5]
Syngas is produced by steam reforming or partial oxidation of natural gas or liquid hydrocarbons, or coal gasification. [6]
C + H2O → CO + H2 [1]
CO + H2O → CO2 + H2 [1]
C + CO2 → 2CO [1]
Steam reforming of methane is an endothermic reaction requiring 206 kJ/mol of methane:
In principle, but rarely in practice, biomass and related hydrocarbon feedstocks could be used to generate biogas and biochar in waste-to-energy gasification facilities. [7] 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. [8] [9]
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. [10] Syngas has less than half the energy density of natural gas. [11]
The first reaction, between incandescent coke and steam, is strongly endothermic, producing carbon monoxide (CO), and hydrogen H
2 (water gas 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—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 CO2 by pressure swing adsorption (PSA), amine scrubbing, and membrane reactors. A variety of alternative technologies have been investigated, but none are of commercial value. [12] Some variations focus on new stoichiometries such as carbon dioxide plus methane [13] [14] 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. [15] [16] [17] [18] [19] [20]
Electricity generated from renewable sources is also used to process carbon dioxide and water into syngas through high-temperature electrolysis. This is an attempt to maintain carbon neutrality in the generation process. Audi, in partnership with company named Sunfire, opened a pilot plant in November 2014 to generate e-diesel using this process. [21]
Syngas that is not methanized typically has a lower heating value of 120 BTU/scf . [22] Untreated syngas can be run in hybrid turbines that allow for greater efficiency because of their lower operating temperatures, and extended part lifetime. [22]
Syngas is used as a source of hydrogen as well as a fuel. [12] It is also used to directly reduce iron ore to sponge iron. [23] 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 and previously the Mobil methanol to gasoline process; ammonia via the Haber process, which converts atmospheric nitrogen (N2) into ammonia which is used as a fertilizer; and oxo alcohols via an intermediate aldehyde.
Producer gas is fuel gas that is manufactured by blowing through a coke or coal fire with air and steam simultaneously. It mainly consists of carbon monoxide (CO), hydrogen (H2), as well as substantial amounts of nitrogen (N2). The caloric value of the producer gas is low (mainly because of its high nitrogen content), and the technology is obsolete. Improvements over producer gas, also obsolete, include water gas where the solid fuel is treated intermittently with air and steam and, far more efficiently synthesis gas where the solid fuel is replaced with methane.
Gasification is a process that converts biomass- or fossil fuel-based carbonaceous materials into gases, including as the largest fractions: nitrogen (N2), carbon monoxide (CO), hydrogen (H2), and carbon dioxide (CO2). This is achieved by reacting the feedstock material at high temperatures (typically >700 °C), without combustion, via controlling the amount of oxygen and/or steam present in the reaction. The resulting gas mixture is called syngas (from synthesis gas) or producer gas and is itself a fuel due to the flammability of the H2 and CO of which the gas is largely composed. Power can be derived from the subsequent combustion of the resultant gas, and is considered to be a source of renewable energy if the gasified compounds were obtained from biomass feedstock.
Coal gas is a flammable gaseous fuel made from coal and supplied to the user via a piped distribution system. It is produced when coal is heated strongly in the absence of air. Town gas is a more general term referring to manufactured gaseous fuels produced for sale to consumers and municipalities.
The Fischer–Tropsch process (FT) 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 temperatures of 150–300 °C (302–572 °F) and pressures of one to several tens of atmospheres. The Fischer–Tropsch process is an important reaction in both coal liquefaction and gas to liquids technology for producing liquid hydrocarbons.
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 production. The reaction is represented by this equilibrium:
In industrial chemistry, coal gasification is the process of producing syngas—a mixture consisting primarily of carbon monoxide (CO), hydrogen, carbon dioxide, methane, and water vapour —from coal and water, air and/or oxygen.
The Sabatier reaction or Sabatier process produces methane and water from a reaction of hydrogen with carbon dioxide at elevated temperatures and pressures in the presence of a nickel catalyst. It was discovered by the French chemists Paul Sabatier and Jean-Baptiste Senderens in 1897. Optionally, ruthenium on alumina makes a more efficient catalyst. It is described by the following exothermic reaction:
Coal liquefaction is a process of converting coal into liquid hydrocarbons: liquid fuels and petrochemicals. This process is often known as "Coal to X" or "Carbon to X", where X can be many different hydrocarbon-based products. However, the most common process chain is "Coal to Liquid Fuels" (CTL).
Synthetic fuel or synfuel is a liquid fuel, or sometimes gaseous fuel, obtained from syngas, a mixture of carbon monoxide and hydrogen, in which the syngas was derived from gasification of solid feedstocks such as coal or biomass or by reforming of natural gas.
Gas to liquids (GTL) is a refinery process to convert natural gas or other gaseous hydrocarbons into longer-chain hydrocarbons, such as gasoline or diesel fuel. Methane-rich gases are converted into liquid synthetic fuels. Two general strategies exist: (i) direct partial combustion of methane to methanol and (ii) Fischer–Tropsch-like processes that convert carbon monoxide and hydrogen into hydrocarbons. Strategy ii is followed by diverse methods to convert the hydrogen-carbon monoxide mixtures to liquids. Direct partial combustion has been demonstrated in nature but not replicated commercially. Technologies reliant on partial combustion have been commercialized mainly in regions where natural gas is inexpensive.
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 [coke] with air and gasifying it with steam". The caloric yield of this is about 10% of a modern syngas plant. Further making this technology unattractive, its precursor coke is expensive, whereas syngas uses cheaper precursor, mainly methane from natural gas.
Hydrogen gas is produced by several industrial methods. In 2022 less than 1% of hydrogen production was low-carbon. Fossil fuels are the dominant source of hydrogen, for example by steam reforming of natural gas. Other methods of hydrogen production include biomass gasification and methane pyrolysis. Methane pyrolysis and water electrolysis can use any source of electricity including renewable energy. Underground hydrogen is extracted.
A methane reformer is a device based on steam reforming, autothermal reforming or partial oxidation and is a type of chemical synthesis which can produce pure hydrogen gas from methane using a catalyst. There are multiple types of reformers in development but the most common in industry are autothermal reforming (ATR) and steam methane reforming (SMR). Most methods work by exposing methane to a catalyst at high temperature and pressure.
Carbon dioxide reforming is a method of producing synthesis gas from the reaction of carbon dioxide with hydrocarbons such as methane with the aid of noble metal catalysts. Synthesis gas is conventionally produced via the steam reforming reaction or coal gasification. In recent years, increased concerns on the contribution of greenhouse gases to global warming have increased interest in the replacement of steam as reactant with carbon dioxide.
The first time a catalyst was used in the industry was in 1746 by J. Roebuck in the manufacture of lead chamber sulfuric acid. Since then catalysts have been in use in a large portion of the chemical industry. In the start only pure components were used as catalysts, but after the year 1900 multicomponent catalysts were studied and are now commonly used in the industry.
The Kim reformer is a type of syngas plant invented by Hyun Yong Kim. It is a high temperature furnace, filled with steam and/or carbon dioxide gas and maintaining a thermal equilibrium at a temperature just above 1200 °C, in which the reforming reaction is at its thermodynamic equilibrium and carbonaceous substance is reformed with the highest efficiency.
Coal gasification is a process whereby a hydrocarbon feedstock (coal) is converted into gaseous components by applying heat under pressure in the presence of steam. Rather than burning, most of the carbon-containing feedstock is broken apart by chemical reactions that produce "syngas." Syngas is primarily hydrogen and carbon monoxide, but the exact composition can vary. In Integrated Gasification Combined Cycle (IGCC) systems, the syngas is cleaned and burned as fuel in a combustion turbine which then drives an electric generator. Exhaust heat from the combustion turbine is recovered and used to create steam for a steam turbine-generator. The use of these two types of turbines in combination is one reason why gasification-based power systems can achieve high power generation efficiencies. Currently, commercially available gasification-based systems can operate at around 40% efficiencies. Syngas, however, emits more greenhouse gases than natural gas, and almost twice as much carbon as a coal plant. Coal gasification is also water-intensive.
Lower-temperature fuel cell types such as the proton exchange membrane fuel cell, phosphoric acid fuel cell, and alkaline fuel cell require pure hydrogen as fuel, typically produced from external reforming of natural gas. However, fuels cells operating at high temperature such as the solid oxide fuel cell (SOFC) are not poisoned by carbon monoxide and carbon dioxide, and in fact can accept hydrogen, carbon monoxide, carbon dioxide, steam, and methane mixtures as fuel directly, because of their internal shift and reforming capabilities. This opens up the possibility of efficient fuel cell-based power cycles consuming solid fuels such as coal and biomass, the gasification of which results in syngas containing mostly hydrogen, carbon monoxide and methane which can be cleaned and fed directly to the SOFCs without the added cost and complexity of methane reforming, water gas shifting and hydrogen separation operations which would otherwise be needed to isolate pure hydrogen as fuel. A power cycle based on gasification of solid fuel and SOFCs is called an Integrated Gasification Fuel Cell (IGFC) cycle; the IGFC power plant is analogous to an integrated gasification combined cycle power plant, but with the gas turbine power generation unit replaced with a fuel cell power generation unit. By taking advantage of intrinsically high energy efficiency of SOFCs and process integration, exceptionally high power plant efficiencies are possible. Furthermore, SOFCs in the IGFC cycle can be operated so as to isolate a carbon dioxide-rich anodic exhaust stream, allowing efficient carbon capture to address greenhouse gas emissions concerns of coal-based power generation.
Chemical looping reforming (CLR) and gasification (CLG) are the operations that involve the use of gaseous carbonaceous feedstock and solid carbonaceous feedstock, respectively, in their conversion to syngas in the chemical looping scheme. The typical gaseous carbonaceous feedstocks used are natural gas and reducing tail gas, while the typical solid carbonaceous feedstocks used are coal and biomass. The feedstocks are partially oxidized to generate syngas using metal oxide oxygen carriers as the oxidant. The reduced metal oxide is then oxidized in the regeneration step using air. The syngas is an important intermediate for generation of such diverse products as electricity, chemicals, hydrogen, and liquid fuels.
Sorption enhanced water gas shift (SEWGS) is a technology that combines a pre-combustion carbon capture process with the water gas shift reaction (WGS) in order to produce a hydrogen rich stream from the syngas fed to the SEWGS reactor.
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