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Steam reforming or steam methane reforming (SMR) is a method for producing synthesis gas (hydrogen and carbon monoxide) by reaction of hydrocarbons 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 or methanol. The reaction is represented by this equilibrium:^[1]

$CH{\phantom{A}}_4+H{\phantom{A}}_{2}O\stackrel{-\rightharpoonup }{\leftharpoondown -}CO+3H{\phantom{A}}_2$

The reaction is strongly endothermic (ΔH_SR = 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]

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

Pre-reforming

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.

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^[11] 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 is put into contact with a nickel catalyst.^[11] 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 (Raschig rings). Additionally, these shapes have a low pressure drop which is advantageous for this application.^[12]

Steam reforming of natural gas is 65–75% efficient.^[13]

The United States produces 9–10 million tons of hydrogen per year, mostly with steam reforming of natural gas.^[14] The worldwide ammonia production, using hydrogen derived from steam reforming, was 144 million tonnes in 2018.^[15] Energy consumption has been reduced from 100 GJ/tonne of ammonia in 1920 to 27 GJ by 2019.^[16]

Globally, almost 50% of hydrogen is produced via steam reforming.^[10] It is currently the least expensive method for hydrogen production available in terms of its capital cost.^[17]

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.^[17] Despite this, implementation of this technology remains problematic, costly, and increases the price of the produced hydrogen significantly.^[17][18]

Autothermal reforming (ATR) uses oxygen, carbon dioxide, or steam to react 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 the outlet pressure can be as high as 100 bar.^[19]

In addition to reactions [1] – [3], ATR introduces the following reaction:^[20]

$\left[4\right]{CH}_4+0.5O_2\rightleftharpoons CO+2H_2\Delta H_R=-24.5kJ/mol$

The main difference between SMR and ATR is that SMR uses air as the combustion heat source to generate steam, whereas ATR uses purified oxygen. The advantage of ATR is that the H₂:CO ratio can be varied, which can be useful for producing speciality 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).^[21]

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.^[22]

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.

• 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