View source for Steam reforming

{{Short description|Method for producing hydrogen and carbon monoxide from hydrocarbon fuels}}
{{distinguish|catalytic reforming}}
[[File:SMR+WGS-1.png|thumb|Illustrating inputs and outputs of steam reforming of natural gas, a process to produce hydrogen and CO<sub>2</sub> greenhouse gas that may be captured with CCS|330x330px]]'''Steam reforming''' or '''steam methane reforming (SMR)''' is a method for producing [[syngas]] ([[hydrogen]] and [[carbon monoxide]]) by reaction of [[hydrocarbon]]s with water. Commonly, [[natural gas]] is the feedstock. The main purpose of this technology is often [[hydrogen production]], although syngas has multiple other uses such as production of [[Ammonia production|ammonia]] or [[methanol]]. The reaction is represented by this equilibrium:<ref>{{cite book|doi=10.1002/9780470561256|title=Hydrogen and Syngas Production and Purification Technologies|year=2009|isbn=9780470561256|editor1-last=Liu|editor1-first=Ke|editor2-last=Song|editor2-first=Chunshan|editor3-last=Subramani|editor3-first=Velu}}</ref>

:<chem>CH4 + H2O <=> CO + 3 H2</chem>

The reaction is strongly [[endothermic]] (Δ''H''<sub>SR</sub> = 206 kJ/mol).

Hydrogen produced by steam reforming is termed as [[grey hydrogen|'grey' hydrogen]] when the waste carbon dioxide is released to the atmosphere and [[blue hydrogen|'blue' hydrogen]] when the carbon dioxide is (mostly) captured and stored geologically—see [[carbon capture and storage]]. Zero carbon [[green hydrogen|'green' hydrogen]] is produced by [[Thermochemical cycle|thermochemical water splitting]], using solar thermal, low- or zero-carbon electricity or waste heat,<ref>{{Cite journal|url=https://www.sciencedirect.com/science/article/abs/pii/S0196890419311884|doi = 10.1016/j.enconman.2019.112182|title = A review and comparative evaluation of thermochemical water splitting cycles for hydrogen production|year = 2020|last1 = Safari|first1 = Farid|last2 = Dincer|first2 = Ibrahim|journal = Energy Conversion and Management|volume = 205|page = 112182|s2cid = 214089650}}</ref> or [[electrolysis]], using low- or zero-carbon electricity. Zero carbon emissions 'turquoise' hydrogen is produced by one-step [[Pyrolysis#Methane pyrolysis for hydrogen|methane pyrolysis]] of natural gas.<ref>{{cite journal |last1=Lumbers |first1=Brock |title=Mathematical modelling and simulation of the thermo-catalytic decomposition of methane for economically improved hydrogen production |url=https://www.sciencedirect.com/science/article/abs/pii/S0360319921044438 |journal=International Journal of Hydrogen Energy |year=2022 |volume=47 |issue=7 |pages=4265–4283 |doi=10.1016/j.ijhydene.2021.11.057 |s2cid=244814932 |access-date=16 March 2022}}</ref>

Steam reforming of the [[natural gas]] produces most of the world's hydrogen. Hydrogen is used in the [[Ammonia production|industrial synthesis of ammonia]] and other chemicals.<ref>{{cite tech report|url=http://www.me.ncu.edu.tw/energy/CleanEnergyTechnology/The%20Hydrogen%20Economy_Addition.pdf |date=2004 |first1=George W. |author-link1=George Crabtree|last1=Crabtree |first2=Mildred S. |last2=Dresselhaus |author-link2=Mildred Dresselhaus|first3=Michelle V. |last3=Buchanan |title=The Hydrogen Economy}}</ref>

== Reactions ==
Steam reforming reaction kinetics, in particular using [[nickel]]-[[Aluminium oxide|alumina]] catalysts, have been studied in detail since the 1950s.<ref>{{Cite journal|last1=Akers|first1=W. W.|last2=Camp|first2=D. P.|date=1955|title=Kinetics of the methane-steam reaction|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/aic.690010415|journal=AIChE Journal|language=en|volume=1|issue=4|pages=471–475|doi=10.1002/aic.690010415|issn=1547-5905}}</ref><ref name=":0">{{Cite journal|last1=Xu|first1=Jianguo|last2=Froment|first2=Gilbert F.|date=1989|title=Methane steam reforming, methanation and water-gas shift: I. Intrinsic kinetics|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/aic.690350109|journal=AIChE Journal|language=en|volume=35|issue=1|pages=88–96|doi=10.1002/aic.690350109|issn=1547-5905}}</ref><ref name=":1">{{Cite journal|last1=Hou|first1=Kaihu|last2=Hughes|first2=Ronald|date=2001-03-15|title=The kinetics of methane steam reforming over a Ni/α-Al2O catalyst|url=https://www.sciencedirect.com/science/article/pii/S1385894700003673|journal=Chemical Engineering Journal|series=FRONTIERS IN CHEMICAL REACTION ENGINEERING|language=en|volume=82|issue=1|pages=311–328|doi=10.1016/S1385-8947(00)00367-3|issn=1385-8947}}</ref>

=== Pre-reforming ===
[[File:Steam Reforming (SMR) Process Flow.jpg|alt=Depiction of the general process flow of a typical steam reforming plant. From left to right: Desulphurisation, pre-reforming, steam reforming, shift conversion, and pressure-swing-adsorption. |thumb|616x616px|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]] (CH<sub>4</sub>), 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:

<math>[1]\qquad \mathrm{CH}_4 + \mathrm{H}_2\mathrm{O} \rightleftharpoons \mathrm{CO} + 3\,\mathrm{H}_2
\qquad \Delta H_{SR} = 206\ \mathrm{kJ/mol}</math>

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

<math>[2]\qquad \mathrm{CO} + \mathrm{H}_2\mathrm{O} \rightleftharpoons \mathrm{CO}_2 + \mathrm{H}_2
\qquad \Delta H_{WGSR} = -41\ \mathrm{kJ/mol}</math>

Some additional reactions occurring within steam reforming processes have been studied.<ref name=":0" /><ref name=":1" /> Commonly the direct steam reforming (DSR) reaction is also included:

<math>[3]\qquad \mathrm{CH}_4 + 2\,\mathrm{H}_2\mathrm{O} \rightleftharpoons \mathrm{CO}_2 + 4\,\mathrm{H}_2
\qquad \Delta H_{DSR} = 165\ \mathrm{kJ/mol}</math>

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&nbsp;°C to 900&nbsp;°C at medium pressures of 20-30 bar.<ref name=":2">{{Cite book|last=Speight|first=James G.|url=https://www.worldcat.org/oclc/1179046717|title=The refinery of the future|date=2020|publisher=Gulf Professional Publishing|isbn=978-0-12-816995-7|edition=2nd|location=Cambridge, MA|oclc=1179046717}}</ref> 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.<ref name=":2" />

== Industrial practice==
[[File:Global Hydrogen Production by Method.png|thumb|231x231px|Global Hydrogen Production by Method<ref name=":3">{{Cite journal|last1=Dincer|first1=Ibrahim|last2=Acar|first2=Canan|date=2015-09-14|title=Review and evaluation of hydrogen production methods for better sustainability|url=https://www.sciencedirect.com/science/article/pii/S0360319914034119|journal=International Journal of Hydrogen Energy|language=en|volume=40|issue=34|pages=11096|doi=10.1016/j.ijhydene.2014.12.035|issn=0360-3199}}</ref>]]
The reaction is conducted in multitubular [[packed bed]] reactors, a subtype of the [[Plug flow reactor model|plug flow reactor]] category. These reactors consist of an array of long and narrow tubes<ref name=":4">{{Cite book|last=Speight|first=James G.|url=https://www.worldcat.org/oclc/1129385226|title=Handbook of industrial hydrocarbon processes|date=2020|isbn=9780128099230|edition=Second|location=Cambridge, MA|oclc=1129385226}}</ref> 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|Foster-Wheeler]] terrace wall reformer.

Inside the tubes, a mixture of steam and [[methane]] are put into contact with a [[nickel]] catalyst.<ref name=":4" /> 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 ring|''Raschig rings'']]). Additionally, these shapes have a low [[pressure drop]] which is advantageous for this application.<ref>{{cite book |doi=10.1002/14356007.o12_o01 |chapter-url={{Google books|qBknDwAAQBAJ|page=429|plainurl=yes}} |chapter=Gas Production, 2. Processes |title=Ullmann's Encyclopedia of Industrial Chemistry |year=2011 |last1=Reimert |first1=Rainer |last2=Marschner |first2=Friedemann |last3=Renner |first3=Hans-Joachim |last4=Boll |first4=Walter |last5=Supp |first5=Emil |last6=Brejc |first6=Miron |last7=Liebner |first7=Waldemar |last8=Schaub |first8=Georg |isbn=978-3-527-30673-2 }}</ref>

Steam reforming of natural gas is 65–75% efficient.<ref>{{citation|title=Hydrogen Production – Steam Methane Reforming (SMR)|url= https://www.amiqweb.es/app/download/9343795/6hydrogenproductionsteammethanereforming.pdf|work=Hydrogen Fact Sheet|archive-url=https://web.archive.org/web/20060204211916/http://www.getenergysmart.org/Files/HydrogenEducation/6HydrogenProductionSteamMethaneReforming.pdf|access-date=28 August 2014|archive-date=4 February 2006}}</ref>

The [[United States]] produces 9–10 million tons of hydrogen per year, mostly with steam reforming of natural gas.<ref>{{cite web|title=Fact of the Month May 2018: 10 Million Metric Tons of Hydrogen Produced Annually in the United States|url=https://www.energy.gov/eere/fuelcells/fact-month-may-2018-10-million-metric-tons-hydrogen-produced-annually-united-states|website=Energy.gov|language=en}}</ref> The worldwide ammonia production, using hydrogen derived from steam reforming, was 144 million tonnes in 2018.<ref name="USGS">{{cite report|url=https://pubs.usgs.gov/periodicals/mcs2020/mcs2020-nitrogen.pdf|title=Nitrogen (Fixed)—Ammonia|date=January 2020|publisher=United States Geological Survey}}</ref> The energy consumption has been reduced from 100 GJ/tonne of ammonia in 1920 to 27 GJ by 2019.<ref name="ram2019">{{cite web|last1=Ramskov|first1=Jens|date=16 December 2019|title=Vinder af VIDENSKABENS TOP 5: Hydrogen og methanol uden energifrås|url=https://ing.dk/artikel/vinder-videnskabens-top-5-hydrogen-methanol-uden-energifraas-230864|website=[[Ingeniøren]]|language=da}}</ref>

Globally, almost 50% of hydrogen is produced via steam reforming.<ref name=":3" /> It is currently the least expensive method for hydrogen production available in terms of its capital cost.<ref name=":5">{{Citation|last1=Velazquez Abad|first1=A.|title=Production of Hydrogen|date=2017-01-01|url=https://www.sciencedirect.com/science/article/pii/B9780124095489101174|encyclopedia=Encyclopedia of Sustainable Technologies|pages=293–304|editor-last=Abraham|editor-first=Martin A.|place=Oxford|publisher=Elsevier|language=en|doi=10.1016/b978-0-12-409548-9.10117-4|isbn=978-0-12-804792-7|access-date=2021-11-16|last2=Dodds|first2=P. E.}}</ref>

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 CO<sub>2</sub> produced from the process.<ref name=":5" /> Despite this, implementation of this technology remains problematic, costly, and increases the price of the produced hydrogen significantly.<ref name=":5" /><ref>{{cite journal |last1=Abdulla |first1=Ahmed |last2=Hanna |first2=Ryan |last3=Schell |first3=Kristen R |last4=Babacan |first4=Oytun |last5=Victor |first5=David G |title=Explaining successful and failed investments in U.S. carbon capture and storage using empirical and expert assessments |journal=Environmental Research Letters |date=29 December 2020 |volume=16 |issue=1 |page=014036 |doi=10.1088/1748-9326/abd19e |s2cid=234429781 |doi-access=free }}</ref>

== 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 H<sub>2</sub>:CO ratio produced is 1:1; when the ATR uses steam, the H<sub>2</sub>:CO ratio produced is 2.5:1. The outlet temperature of the syngas is between 950–1100&nbsp;°C and outlet pressure can be as high as 100 bar.<ref>[http://www.topsoe.com/business_areas/methanol/Processes/AutothermalReforming.aspx Topsoe ATR]</ref>

In addition to reactions [1] – [3], ATR introduces the following reaction:<ref>{{Cite journal|last1=Blumberg|first1=Timo|last2=Morosuk|first2=Tatiana|last3=Tsatsaronis|first3=George|date=December 2017|title=A Comparative Exergoeconomic Evaluation of the Synthesis Routes for Methanol Production from Natural Gas|journal=Applied Sciences|language=en|volume=7|issue=12|pages=1213|doi=10.3390/app7121213|doi-access=free}}</ref>

<math>[4]\qquad \mathrm{CH}_4 + 0.5\,\mathrm{O}_2 \rightleftharpoons \mathrm{CO} + 2\,\mathrm{H}_2
\qquad \Delta H_{R} = -24.5\ \mathrm{kJ/mol}</math>

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 H<sub>2</sub>: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 (Δ''H'' = 0).<ref>{{Citation|last=Semelsberger|first=T. A.|title=FUELS – HYDROGEN STORAGE {{!}} Chemical Carriers|date=2009-01-01|url=https://www.sciencedirect.com/science/article/pii/B9780444527455003312|encyclopedia=Encyclopedia of Electrochemical Power Sources|pages=504–518|editor-last=Garche|editor-first=Jürgen|place=Amsterdam|publisher=Elsevier|language=en|doi=10.1016/b978-044452745-5.00331-2|isbn=978-0-444-52745-5|access-date=2021-11-16}}</ref>

== Partial oxidation ==
{{main|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.<ref>{{Cite web | url=https://www.energy.gov/eere/fuelcells/hydrogen-production-natural-gas-reforming |title = Hydrogen Production: Natural Gas Reforming}}</ref>

==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&nbsp;°C.

===For combustion engines ===
[[Gas flare|Flared gas]] and vented [[volatile organic compound]]s (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.<ref>{{cite web|title=Atmospheric Emissions|url=http://www.oilandgasuk.co.uk/knowledgecentre/atmospheric_emissions.cfm|url-status=dead|archive-url=https://web.archive.org/web/20130926063455/http://www.oilandgasuk.co.uk/knowledgecentre/atmospheric_emissions.cfm|archive-date=2013-09-26}}</ref> Reforming for combustion engines utilizes steam reforming technology for converting waste gases into a source of energy.<ref>{{cite news|title=Wärtsilä Launches GasReformer Product For Turning Oil Production Gas Into Energy|url=http://www.marineinsight.com/shipping-news/wartsila-launches-gasreformer-product-for-turning-oil-production-gas-into-energy/|newspaper=Marine Insight|date=18 March 2013|archive-url=https://web.archive.org/web/20150511023948/https://www.marineinsight.com/shipping-news/wartsila-launches-gasreformer-product-for-turning-oil-production-gas-into-energy/|archive-date=2015-05-11}}</ref>

Reforming for combustion engines is based on steam reforming, where non-methane hydrocarbons ([[NMHC]]s) of low quality gases are converted to [[Syngas|synthesis gas]] (H<sub>2</sub> + CO) and finally to [[methane]] (CH<sub>4</sub>), [[carbon dioxide]] (CO<sub>2</sub>) and [[hydrogen]] (H<sub>2</sub>) - thereby improving the fuel gas quality (methane number).<ref>{{cite web|title=Method of operating a gas engine plant and fuel feeding system of a gas engine|url=http://worldwide.espacenet.com/publicationDetails/biblio?CC=EP&NR=1861610B1&KC=B1&FT=D}}</ref>

===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]].<ref>{{cite web|url=http://auto.howstuffworks.com/fuel-processor.htm |title=Fossil fuel processor|date=2000-10-04}}</ref> 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]].<ref>{{cite tech report|url=http://www.h2net.org.uk/PDFs/RN_1/HydrogenMLW.pdf |title=Hydrogen from Exhaust Gas Fuel Reforming: Greener, Leaner and Smoother Engines |first1=Miroslaw L. |last1=Wyszynski |first2=Thanos |last2=Megaritis |first3=Roy S. |last3=Lehrle |date=2001 |institution=Future Power Systems Group, The [[University of Birmingham]]}}</ref><ref>{{cite web|url=http://auto.howstuffworks.com/fuel-processor2.htm |title=Commonly used fuel reforming today|date=2000-10-04}}</ref>

====Disadvantages====
The reformer&ndash; 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.<ref>[http://www.howstuffworks.com/hydrogen-economy4.htm Fossil fuel reforming not eliminating any carbon dioxides]</ref> 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.<ref>{{cite journal |citeseerx=10.1.1.538.3537 |first1=F. David |last1=Doty |year=2004 |title=A Realistic Look at Hydrogen Price Projections }}</ref>{{self-published inline|date=January 2019}}

====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. H<sub>2</sub>S, 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.<ref>{{cite journal |doi=10.1016/j.apcatb.2014.05.044 |title=Steam reforming of sulfur-containing dodecane on a Rh–Pt catalyst: Influence of process parameters on catalyst stability and coke structure |journal=Applied Catalysis B: Environmental |volume=160-161 |pages=525–533 |year=2014 |last1=Zheng |first1=Qinghe |last2=Janke |first2=Christiane |last3=Farrauto |first3=Robert }}</ref>
* 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 cell]]s (SOFC) and [[molten carbonate fuel cell]]s (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% ([[Lower heating value|LHV basis]]) depending on the purity of the hydrogen product.

== See also ==
* [[Biogas]]
* [[Boudouard reaction]]
* [[Catalytic reforming]]
* [[Chemical looping reforming and gasification]]
* [[Cracking (chemistry)]]
* [[Hydrogen pinch]]
* [[Hydrogen technologies]]
* [[Industrial gas]]
* [[Lane hydrogen producer]]
* [[Methane pyrolysis]] (for Hydrogen)
* [[Partial oxidation]]
* [[PROX]]
* [[Reformed methanol fuel cell]]
* [[Reformer sponge iron cycle]]
* [[Syngas]]
* [[Timeline of hydrogen technologies]]

== References ==
{{Reflist}}

{{environmental technology}}

{{DEFAULTSORT:Fossil Fuel Reforming}}
[[Category:Hydrogen production]]
[[Category:Chemical processes]]
[[Category:Fuel gas]]
[[Category:Catalysis]]
[[Category:Industrial gases]]