Steam reforming

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Illustrating inputs and outputs of steam reforming of natural gas, a process to produce hydrogen and CO2 greenhouse gas that may be captured with CCS SMR+WGS-1.png
Illustrating inputs and outputs of steam reforming of natural gas, a process to produce hydrogen and CO2 greenhouse gas that may be captured with CCS

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: [1]

Contents

The reaction is strongly endothermic (ΔHSR = 206 kJ/mol).

Hydrogen produced by steam reforming is termed 'grey' hydrogen when the waste carbon dioxide is released to the atmosphere and 'blue' hydrogen when the carbon dioxide is (mostly) captured and stored geologically—see carbon capture and storage. Zero carbon 'green' hydrogen is produced by thermochemical water splitting, using solar thermal, low- or zero-carbon electricity or waste heat, [2] or electrolysis, using low- or zero-carbon electricity. Zero carbon emissions 'turquoise' hydrogen is produced by one-step methane pyrolysis of natural gas. [3]

Steam reforming of natural gas produces most of the world's hydrogen. Hydrogen is used in the industrial synthesis of ammonia and other chemicals. [4]

Reactions

Steam reforming reaction kinetics, in particular using nickel-alumina catalysts, have been studied in detail since the 1950s. [5] [6] [7]

Pre-reforming

Depiction of the general process flow of a typical steam reforming plant. (PSA = Pressure swing adsorption, NG = Natural gas) Steam Reforming (SMR) Process Flow.jpg
Depiction of the general process flow of a typical steam reforming plant. (PSA = Pressure swing adsorption, NG = Natural gas)

The purpose of pre-reforming is to break down higher hydrocarbons such as propane, butane or naphtha into methane (CH4), which allows for more efficient reforming downstream.

Steam reforming

The name-giving reaction is the steam reforming (SR) reaction and is expressed by the equation:

Via the water-gas shift reaction (WGSR), additional hydrogen is released by reaction of water with the carbon monoxide generated according to equation [1]:

Some additional reactions occurring within steam reforming processes have been studied. [6] [7] Commonly the direct steam reforming (DSR) reaction is also included:

As these reactions by themselves are highly endothermic (apart from WGSR, which is mildly exothermic), a large amount of heat needs to be added to the reactor to keep a constant temperature. Optimal SMR reactor operating conditions lie within a temperature range of 800 °C to 900 °C at medium pressures of 20-30 bar. [8] High excess of steam is required, expressed by the (molar) steam-to-carbon (S/C) ratio. Typical S/C ratio values lie within the range 2.5:1 - 3:1. [8]

Industrial practice

Global Hydrogen Production by Method Global Hydrogen Production by Method.png
Global Hydrogen Production by Method

The reaction is conducted in multitubular packed bed reactors, a subtype of the plug flow reactor category. These reactors consist of an array of long and narrow tubes [10] which are situated within the combustion chamber of a large industrial furnace, providing the necessary energy to keep the reactor at a constant temperature during operation. Furnace designs vary, depending on the burner configuration they are typically categorized into: top-fired, bottom-fired, and side-fired. A notable design is the Foster-Wheeler terrace wall reformer.

Inside the tubes, a mixture of steam and methane are put into contact with a nickel catalyst. [10] Catalysts with high surface-area-to-volume ratio are preferred because of diffusion limitations due to high operating temperature. Examples of catalyst shapes used are spoked wheels, gear wheels, and rings with holes (see: Raschig rings). Additionally, these shapes have a low pressure drop which is advantageous for this application. [11]

Steam reforming of natural gas is 65–75% efficient. [12]

The United States produces 9–10 million tons of hydrogen per year, mostly with steam reforming of natural gas. [13] The worldwide ammonia production, using hydrogen derived from steam reforming, was 144 million tonnes in 2018. [14] The energy consumption has been reduced from 100 GJ/tonne of ammonia in 1920 to 27 GJ by 2019. [15]

Globally, almost 50% of hydrogen is produced via steam reforming. [9] It is currently the least expensive method for hydrogen production available in terms of its capital cost. [16]

In an effort to decarbonise hydrogen production, carbon capture and storage (CCS) methods are being implemented within the industry, which have the potential to remove up to 90% of CO2 produced from the process. [16] Despite this, implementation of this technology remains problematic, costly, and increases the price of the produced hydrogen significantly. [16] [17]

Autothermal reforming

Autothermal reforming (ATR) uses oxygen and carbon dioxide or steam in a reaction with methane to form syngas. The reaction takes place in a single chamber where the methane is partially oxidized. The reaction is exothermic. When the ATR uses carbon dioxide, the H2:CO ratio produced is 1:1; when the ATR uses steam, the H2:CO ratio produced is 2.5:1. The outlet temperature of the syngas is between 950–1100 °C and outlet pressure can be as high as 100 bar. [18]

In addition to reactions [1] – [3], ATR introduces the following reaction: [19]

The main difference between SMR and ATR is that SMR only uses air for combustion as a heat source to create steam, while ATR uses purified oxygen. The advantage of ATR is that the H2:CO ratio can be varied, which can be useful for producing specialty products. Due to the exothermic nature of some of the additional reactions occurring within ATR, the process can essentially be performed at a net enthalpy of zero (ΔH = 0). [20]

Partial oxidation

Partial oxidation (POX) occurs when a sub-stoichiometric fuel-air mixture is partially combusted in a reformer creating hydrogen-rich syngas. POX is typically much faster than steam reforming and requires a smaller reactor vessel. POX produces less hydrogen per unit of the input fuel than steam reforming of the same fuel. [21]

Steam reforming at small scale

The capital cost of steam reforming plants is considered prohibitive for small to medium size applications. The costs for these elaborate facilities do not scale down well. Conventional steam reforming plants operate at pressures between 200 and 600 psi (14–40 bar) with outlet temperatures in the range of 815 to 925 °C.

For combustion engines

Flared gas and vented volatile organic compounds (VOCs) are known problems in the offshore industry and in the on-shore oil and gas industry, since both release greenhouse gases into the atmosphere. [22] Reforming for combustion engines utilizes steam reforming technology for converting waste gases into a source of energy. [23]

Reforming for combustion engines is based on steam reforming, where non-methane hydrocarbons (NMHCs) of low quality gases are converted to synthesis gas (H2 + CO) and finally to methane (CH4), carbon dioxide (CO2) and hydrogen (H2) - thereby improving the fuel gas quality (methane number). [24]

For fuel cells

There is also interest in the development of much smaller units based on similar technology to produce hydrogen as a feedstock for fuel cells. [25] Small-scale steam reforming units to supply fuel cells are currently the subject of research and development, typically involving the reforming of methanol, but other fuels are also being considered such as propane, gasoline, autogas, diesel fuel, and ethanol. [26] [27]

Disadvantages

The reformer– the fuel-cell system is still being researched but in the near term, systems would continue to run on existing fuels, such as natural gas or gasoline or diesel. However, there is an active debate about whether using these fuels to make hydrogen is beneficial while global warming is an issue. Fossil fuel reforming does not eliminate carbon dioxide release into the atmosphere but reduces the carbon dioxide emissions and nearly eliminates carbon monoxide emissions as compared to the burning of conventional fuels due to increased efficiency and fuel cell characteristics. [28] However, by turning the release of carbon dioxide into a point source rather than distributed release, carbon capture and storage becomes a possibility, which would prevent the release of carbon dioxide to the atmosphere, while adding to the cost of the process.

The cost of hydrogen production by reforming fossil fuels depends on the scale at which it is done, the capital cost of the reformer, and the efficiency of the unit, so that whilst it may cost only a few dollars per kilogram of hydrogen at an industrial scale, it could be more expensive at the smaller scale needed for fuel cells. [29] [ self-published source? ]

Challenges with reformers supplying fuel cells

There are several challenges associated with this technology:

  • The reforming reaction takes place at high temperatures, making it slow to start up and requiring costly high-temperature materials.
  • Sulfur compounds in the fuel will poison certain catalysts, making it difficult to run this type of system from ordinary gasoline. Some new technologies have overcome this challenge with sulfur-tolerant catalysts.
  • Coking would be another cause of catalyst deactivation during steam reforming. High reaction temperatures, low steam-to-carbon ratio (S/C), and the complex nature of sulfur-containing commercial hydrocarbon fuels make coking especially favorable. Olefins, typically ethylene, and aromatics are well-known carbon-precursors, hence their formation must be reduced during steam reforming. Additionally, catalysts with lower acidity were reported to be less prone to coking by suppressing dehydrogenation reactions. H2S, the main product in the reforming of organic sulfur, can bind to all transition metal catalysts to form metal–sulfur bonds and subsequently reduce catalyst activity by inhibiting the chemisorption of reforming reactants. Meanwhile, the adsorbed sulfur species increases the catalyst acidity, and hence indirectly promotes coking. Precious metal catalysts such as Rh and Pt have lower tendencies to form bulk sulfides than other metal catalysts such as Ni. Rh and Pt are less prone to sulfur poisoning by only chemisorbing sulfur rather than forming metal sulfides. [30]
  • Low temperature polymer fuel cell membranes can be poisoned by the carbon monoxide (CO) produced by the reactor, making it necessary to include complex CO-removal systems. Solid oxide fuel cells (SOFC) and molten carbonate fuel cells (MCFC) do not have this problem, but operate at higher temperatures, slowing start-up time, and requiring costly materials and bulky insulation.
  • The thermodynamic efficiency of the process is between 70% and 85% (LHV basis) depending on the purity of the hydrogen product.

See also

Related Research Articles

<span class="mw-page-title-main">Haber process</span> Industrial process for ammonia production

The Haber process, also called the Haber–Bosch process, is the main industrial procedure for the production of ammonia. It converts atmospheric nitrogen (N2) to ammonia (NH3) by a reaction with hydrogen (H2) using a finely divided iron metal catalyst:

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 a fuel. 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.

<span class="mw-page-title-main">Gasification</span> Form of energy conversion

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.

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.

<span class="mw-page-title-main">Sabatier reaction</span> Methanation process of carbon dioxide with hydrogen

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:

The water–gas shift reaction (WGSR) describes the reaction of carbon monoxide and water vapor to form carbon dioxide and hydrogen:

<span class="mw-page-title-main">Gas to liquids</span> Conversion of natural gas to liquid petroleum products

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.

Hydrogen gas is produced by several industrial methods. Nearly all of the world's current supply of hydrogen is created from fossil fuels. Most hydrogen is gray hydrogen made through steam methane reforming. In this process, hydrogen is produced from a chemical reaction between steam and methane, the main component of natural gas. Producing one tonne of hydrogen through this process emits 6.6–9.3 tonnes of carbon dioxide. When carbon capture and storage is used to remove a large fraction of these emissions, the product is known as blue hydrogen.

A methanol reformer is a device used in chemical engineering, especially in the area of fuel cell technology, which can produce pure hydrogen gas and carbon dioxide by reacting a methanol and water (steam) mixture.

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.

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 distinction is made between thermal partial oxidation (TPOX) and catalytic partial oxidation (CPOX).

Methanation is the conversion of carbon monoxide and carbon dioxide (COx) to methane (CH4) through hydrogenation. The methanation reactions of COx were first discovered by Sabatier and Senderens in 1902.

PROX is an acronym for PReferential OXidation, that refers to the preferential oxidation of carbon monoxide in a gas mixture by a catalyst. It is intended to remove trace amounts of CO from H2/CO/CO2 mixtures produced by steam reforming and water-gas shift. An ideal PROX catalyst preferentially oxidizes carbon monoxide (CO) using a heterogeneous catalyst placed upon a ceramic support. Catalysts include metals such as platinum, platinum/iron, platinum/ruthenium, gold nanoparticles as well as novel copper oxide/ceramic conglomerate catalysts.

The Glossary of fuel cell terms lists the definitions of many terms used within the fuel cell industry. The terms in this fuel cell glossary may be used by fuel cell industry associations, in education material and fuel cell codes and standards to name but a few.

Hydromethanation, [hahy-droh- meth-uh-ney-shuhn] is the process by which methane is produced through the combination of steam, carbonaceous solids and a catalyst in a fluidized bed reactor. The process, developed over the past 60 years by multiple research groups, enables the highly efficient conversion of coal, petroleum coke and biomass into clean, pipeline quality methane.

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.

<span class="mw-page-title-main">Kim reformer</span>

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.

A small stationary reformer is an on-site device used for the production of hydrogen from hydrocarbons on a small scale.

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.

<span class="mw-page-title-main">Reversible solid oxide cell</span>

A reversible solid oxide cell (rSOC) is a solid-state electrochemical device that is operated alternatively as a solid oxide fuel cell (SOFC) and a solid oxide electrolysis cell (SOEC). Similarly to SOFCs, rSOCs are made of a dense electrolyte sandwiched between two porous electrodes. Their operating temperature ranges from 600°C to 900°C, hence they benefit from enhanced kinetics of the reactions and increased efficiency with respect to low-temperature electrochemical technologies.

References

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  2. ↑ Safari, Farid; Dincer, Ibrahim (2020). "A review and comparative evaluation of thermochemical water splitting cycles for hydrogen production". Energy Conversion and Management. 205: 112182. doi:10.1016/j.enconman.2019.112182. S2CID   214089650.
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  11. ↑ Reimert, Rainer; Marschner, Friedemann; Renner, Hans-Joachim; Boll, Walter; Supp, Emil; Brejc, Miron; Liebner, Waldemar; Schaub, Georg (2011). "Gas Production, 2. Processes". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.o12_o01. ISBN   978-3-527-30673-2.
  12. ↑ "Hydrogen Production – Steam Methane Reforming (SMR)" (PDF), Hydrogen Fact Sheet, archived from the original (PDF) on 4 February 2006, retrieved 28 August 2014
  13. ↑ "Fact of the Month May 2018: 10 Million Metric Tons of Hydrogen Produced Annually in the United States". Energy.gov.
  14. ↑ Nitrogen (Fixed)—Ammonia (PDF) (Report). United States Geological Survey. January 2020.
  15. ↑ Ramskov, Jens (16 December 2019). "Vinder af VIDENSKABENS TOP 5: Hydrogen og methanol uden energifrås". Ingeniøren (in Danish).
  16. 1 2 3 Velazquez Abad, A.; Dodds, P. E. (2017-01-01), "Production of Hydrogen", in Abraham, Martin A. (ed.), Encyclopedia of Sustainable Technologies, Oxford: Elsevier, pp. 293–304, doi:10.1016/b978-0-12-409548-9.10117-4, ISBN   978-0-12-804792-7 , retrieved 2021-11-16
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  18. ↑ Topsoe ATR
  19. ↑ Blumberg, Timo; Morosuk, Tatiana; Tsatsaronis, George (December 2017). "A Comparative Exergoeconomic Evaluation of the Synthesis Routes for Methanol Production from Natural Gas". Applied Sciences. 7 (12): 1213. doi: 10.3390/app7121213 .
  20. ↑ Semelsberger, T. A. (2009-01-01), "FUELS – HYDROGEN STORAGE | Chemical Carriers", in Garche, Jürgen (ed.), Encyclopedia of Electrochemical Power Sources, Amsterdam: Elsevier, pp. 504–518, doi:10.1016/b978-044452745-5.00331-2, ISBN   978-0-444-52745-5 , retrieved 2021-11-16
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  22. ↑ "Atmospheric Emissions". Archived from the original on 2013-09-26.
  23. ↑ "Wärtsilä Launches GasReformer Product For Turning Oil Production Gas Into Energy". Marine Insight. 18 March 2013. Archived from the original on 2015-05-11.
  24. ↑ "Method of operating a gas engine plant and fuel feeding system of a gas engine".
  25. ↑ "Fossil fuel processor". 2000-10-04.
  26. ↑ Wyszynski, Miroslaw L.; Megaritis, Thanos; Lehrle, Roy S. (2001). Hydrogen from Exhaust Gas Fuel Reforming: Greener, Leaner and Smoother Engines (PDF) (Technical report). Future Power Systems Group, The University of Birmingham.
  27. ↑ "Commonly used fuel reforming today". 2000-10-04.
  28. ↑ Fossil fuel reforming not eliminating any carbon dioxides
  29. ↑ Doty, F. David (2004). "A Realistic Look at Hydrogen Price Projections". CiteSeerX   10.1.1.538.3537 .{{cite journal}}: Cite journal requires |journal= (help)
  30. ↑ Zheng, Qinghe; Janke, Christiane; Farrauto, Robert (2014). "Steam reforming of sulfur-containing dodecane on a Rh–Pt catalyst: Influence of process parameters on catalyst stability and coke structure". Applied Catalysis B: Environmental. 160–161: 525–533. doi:10.1016/j.apcatb.2014.05.044.
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