Steam Methane Reforming: Process Stages, Conditions and CO2

Steam methane reforming (SMR) is the industrial process that makes hydrogen by reacting methane, the main component of natural gas, with high-temperature steam over a nickel-based catalyst. The reaction yields carbon monoxide and hydrogen; a second “water-gas shift” step converts the carbon monoxide into more hydrogen and carbon dioxide, and pressure swing adsorption purifies the product. It is the dominant hydrogen route in the United States and worldwide.

Short answer

SMR reacts methane with steam, CH₄ + H₂O → CO + 3H₂, at roughly 700–1,000 °C and 3–25 bar in the US Department of Energy’s description. The reaction is strongly endothermic, so heat is supplied from outside the catalyst tubes. A shift reactor then converts CO to extra H₂ and CO₂, and pressure swing adsorption removes CO₂ and other impurities. The CO₂ has to be captured or released, which is why “grey” and “blue” hydrogen are labels for the same chemistry with different carbon handling.

Definition and scope

The US Department of Energy’s hydrogen and fuel cells program (Office of Energy Efficiency and Renewable Energy) groups SMR under “natural gas reforming” and states that 95% of the hydrogen produced in the United States is made by natural gas reforming in large central plants. A Department of Energy Office of Science technology status document gives a different figure with a different scope: high-temperature catalytic SMR accounted for approximately 75% of the hydrogen produced globally in 2019. The two numbers do not conflict. One is a national share of production by route family, the other is a global share attributed to SMR specifically, for a different year.

“Steam methane reforming” names one reaction (methane plus steam) but, in practice, an entire plant. The National Energy Technology Laboratory (NETL) reference design for a large SMR hydrogen plant treats the reformer as one block in a chain: feed desulfurization, a prereformer, the primary reformer, heat recovery, water-gas shift, purification and compression. The sections below follow that chain because most of the engineering lives in the stages around the reformer, not in the reformer reaction itself.

The process is a chemical-engineering topic. This page explains how and why it is built the way it is; it does not give operating instructions. For the discipline around it see What Is Chemical Engineering?, and for how such plants are modelled see Aspen Plus: Chemical Process Simulation Guide.

How the process works, stage by stage

The two reactions

The primary reforming reaction is CH₄ + H₂O ⇌ CO + 3H₂, with a reaction enthalpy of +205.8 kJ/mol in the NETL report, “strongly endothermic”. Equilibrium conversion is favoured by high steam-to-carbon ratio, low pressure and high temperature. The water-gas shift reaction, CO + H₂O ⇌ CO₂ + H₂, is mildly exothermic (−41.2 kJ/mol) and recovers additional hydrogen from the carbon monoxide. The DOE page lists a small amount of CO₂ already appearing in the reformer step.

The reference plant, in order

The NETL report (DOE/NETL-2022/3241, 12 April 2022) models a 200 million standard cubic feet per day plant (483,000 kg of hydrogen per day) delivering 99.90 vol% hydrogen at 6.48 MPa (925 psig), pipeline-ready. Its stages, and what each one protects against, are:

Stage	What happens	Why the stage exists
Sulfur guard bed	Zinc oxide bed removes sulfur compounds from the preheated natural gas feed (feed preheated to 370 °C; 98% of methanethiol removed, 0.1 ppm or less in the outlet).	“Even a small amount of sulfur present can poison the reformer catalysts.”
Prereformer	Adiabatic fixed bed (500 °C feed) that reforms ethane, propane and butane (C2+) so that less than 500 ppm C2+ leaves.	Reduces carbon deposition downstream.
Primary reformer	Nickel-based catalyst inside high-alloy steel tubes, fired from outside with recycled PSA off-gas plus supplemental natural gas.	Supplies the heat for the endothermic reforming reaction.
Heat recovery	Convective section recovers heat from the hot syngas and generates superheated steam.	About 53% of the superheated steam generated is used for reforming; the remainder is exported.
Water-gas shift	Catalytic shift of CO with steam to CO2 and H2 (a high-temperature shift train in the report’s capture-oriented design).	Converts the CO into additional H2; steam to dry-gas ratio is held at 0.25 at the shift exit to prevent carbon deposition.
Pressure swing adsorption (PSA)	Adsorbent beds remove CO2 and other impurities, leaving essentially pure hydrogen (design H2 recovery 85%).	Sets product purity. The hydrogen-rich off-gas is sent back to fire the reformer.
Compression	Product hydrogen compressed to pipeline pressure.	Delivers the product at 6.48 MPa.

The data table below the article compiles the numeric design values from these sources, together with the scope each one applies to, so the figures can be compared side by side.

Why the reformer conditions look the way they do

Two temperature ranges circulate in introductions to SMR, and they measure different things. The DOE description gives steam at 700–1,000 °C and 3–25 bar with a catalyst. The NETL design notes that tube metallurgy usually limits the reaction to 760–925 °C (1,400–1,700 °F); in the reference plant the raw syngas leaves the primary reformer at 870 °C and 2.8 MPa (411 psia), with more than 77% of the methane in the reformer feed converted, at a molar steam-to-carbon ratio of 2.56 at the reactor inlet. The textbook range is a general window; the 760–925 °C range is what the tube alloy permits in a modern design. Note also that 2.8 MPa is 28 bar, slightly above the top of the general 3–25 bar range, which illustrates why a single “typical conditions” line should be read as an approximation.

The trade-off behind the steam-to-carbon figure is carbon deposition. Excess steam pushes the equilibrium toward hydrogen and protects the catalyst from coking, but every extra mole of steam must be generated and heated. That is why the plant is built around heat recovery and why the feed-side stages (sulfur guard and prereformer) exist at all: they protect the catalyst so the reformer can run hard.

Related routes and how they differ

Partial oxidation

The DOE page describes partial oxidation of methane as CH₄ + ½O₂ → CO + 2H₂ + heat. It is exothermic and faster than steam reforming, allows a smaller reactor, but yields less hydrogen per unit of fuel. The NETL report evaluates an autothermal reforming (ATR) case alongside SMR (Case 3 against Cases 1 and 2); ATR combines partial oxidation and steam reforming so that the heat for reforming comes from the oxidation reaction rather than from fired tubes.

Thermocatalytic decomposition

The Office of Science technology status document notes that thermocatalytic decomposition of methane yields 2 H₂ per CH₄, against SMR yielding twice as much hydrogen because of the water-vapor conversion. Decomposition avoids producing CO₂ directly but is a different route with its own carbon product.

SMR with and without capture

The NETL report compares Case 1 (SMR, no capture) with Case 2 (SMR with capture). Capture cases recover more than 90% of the carbon using the water-gas shift plus a solvent separation (the report names MDEA and Shell Cansolv). Without capture, SMR has the highest cold gas efficiency in the comparison, 76.7%. Adding capture lowers effective thermal efficiency by about 7 percentage points, and in the report’s modelled SMR case capture raises the levelized cost of hydrogen by 54%. These are model results for one reference design on the report’s own basis, not forecasts for any plant.

Where it matters for labs and research operations

Hydrogen purity and the gas that reaches the bench

PSA output is “essentially pure hydrogen” in the DOE description, and 99.90 vol% in the NETL reference plant. For applications with strict specifications, that is not automatically enough. The Office of Science document notes that fuel-cell anodes need high-purity hydrogen and that fuel-quality standards limit CO, sulfur compounds, ammonia and halogenated compounds, limits that “can be challenging for H₂ produced by SMR”. Whether a gas stream meets a given specification is a question for the supplier’s certificate and the relevant standard, not for the process description alone. Labs that purchase hydrogen cylinders should treat these as separate questions from how the hydrogen was made; see Compressed Gas Cylinder Safety for handling and Gas Chromatography: Columns, Carrier Gases and Detectors for a common bench use of hydrogen.

Emissions and life-cycle accounting

The Office of Science document cites average emissions of 10 kg CO₂ per kg H₂ for the majority of global hydrogen produced via SMR. The NETL report assesses life cycle global warming potential following ISO 14040/14044 and reports, for its capture cases, 4.6 lb CO₂e per lb H₂ (Case 2) and 5.7 (Case 3), against 20 lb for a coal case without capture. Anyone comparing hydrogen “carbon intensity” figures should check the system boundary first: a plant-gate number and a life-cycle number are not interchangeable. For how inventory data underpins such comparisons, see Life Cycle Inventory: LCA Data Sources & Methods.

Research funding context

The Office of Science document is a product of the Basic Energy Sciences program, the program that also funds research on hydrogen production science; see DOE BES: What Basic Energy Sciences Funds.

Common misunderstandings

“Steam reforming is a single reaction”

The reforming reaction produces CO and H₂. The water-gas shift is a separate step with its own catalyst and temperature window, and the pure hydrogen comes from the purification step, not from the reformer.

“The reformer is heated by the reaction”

It is the opposite: the reaction absorbs heat (+205.8 kJ/mol in the NETL report), so the tubes are fired externally. Partial oxidation is the exothermic route, and the DOE page lists it separately.

“SMR is 95% of hydrogen” or “SMR is 75% of hydrogen”

Both appear in credible sources and both are correct for their stated scope: 95% for US production by natural gas reforming in large central plants (DOE), approximately 75% for global production in 2019 by SMR (Office of Science). Quote the scope with the number.

“Blue hydrogen is a different process”

In the NETL comparison, SMR with capture is the same reforming chain with CO₂ recovery added (more than 90% carbon recovery), and the cost of that recovery shows up as lower efficiency.

“Any small amount of impurity in the feed is harmless”

The report singles out sulfur: a small amount can poison the reformer catalyst, which is why a dedicated guard bed with a 0.1 ppm or lower outlet target is the first stage.

Frequently asked questions

What temperature does steam methane reforming run at?

The Department of Energy describes steam at 700–1,000 °C and 3–25 bar. In the NETL reference design, tube metallurgy usually limits the reaction to 760–925 °C, and raw syngas leaves the primary reformer at 870 °C.

Why is steam methane reforming endothermic?

Breaking methane and water into CO and H₂ absorbs energy (+205.8 kJ/mol in the NETL report), so heat is supplied by firing outside the catalyst tubes, using recycled PSA off-gas plus supplemental natural gas.

What is the water-gas shift step for?

It converts CO and steam into CO₂ and more H₂ (−41.2 kJ/mol), raising hydrogen yield before purification.

Why does steam-to-carbon ratio matter?

Higher steam-to-carbon favours reforming equilibrium and helps avoid carbon deposition; the reference reactor inlet ratio is 2.56. More steam costs more heat, which is the design trade-off.

How much CO₂ does SMR produce?

The Office of Science document cites average emissions of 10 kg CO₂ per kg H₂ for SMR-produced hydrogen. Capture cases in the NETL report recover more than 90% of the carbon, at an efficiency penalty of about 7 percentage points.

Sources

• US Department of Energy, Office of Energy Efficiency and Renewable Energy, “Hydrogen Production: Natural Gas Reforming”, energy.gov (read 2026-11-12).
• National Energy Technology Laboratory, “Comparison of Commercial, State-of-the-Art, Fossil-Based Hydrogen Production Technologies”, DOE/NETL-2022/3241, 12 April 2022, netl.doe.gov PDF (read 2026-11-12).
• US DOE Office of Science, Basic Energy Sciences, “Foundational Science for Carbon-Neutral Hydrogen Technologies” technology status document, OSTI 1809223 (read 2026-11-12).