Steam Reforming

From natural gas to hydrogen

Nearly all of the hydrogen the chemical industry uses is not dug up or electrolyzed — it is pried out of natural gas. Steam reforming (more precisely, steam methane reforming, SMR) is the reaction that does it. Feed methane and water vapor over a nickel catalyst hot enough to glow, and the methane is torn apart, releasing its four hydrogen atoms and leaving the carbon bound to oxygen as carbon monoxide:

CH₄ + H₂O ⇌ CO + 3H₂   ΔH°₂₉₈ = +206 kJ/mol

That product — a mixture of carbon monoxide and hydrogen — is called synthesis gas, or syngas. Reforming alone hands you three molecules of H₂ per methane. A second reaction, the water-gas shift, then reacts the carbon monoxide with more steam to extract a fourth hydrogen and turn the CO into easily removed CO₂:

CO + H₂O ⇌ CO₂ + H₂   ΔH°₂₉₈ = −41 kJ/mol

Add the two together and the overall stoichiometry is strikingly clean — one methane and two waters make one CO₂ and four hydrogens:

CH₄ + 2H₂O ⇌ CO₂ + 4H₂   ΔH°₂₉₈ = +165 kJ/mol

Why it is endothermic and runs so hot

Methane is a deeply stable molecule. Each of its four C-H bonds carries a bond-dissociation energy of about 414 kJ/mol, and breaking them is the expensive part of reforming. The new bonds formed — the triple bond in CO and the H-H bonds in hydrogen — give some of that energy back, but not enough: the reforming step still absorbs roughly 206 kJ for every mole of methane converted. Reforming is, in short, a way of storing furnace heat as chemical energy in hydrogen.

Because the reaction is endothermic, its equilibrium constant rises with temperature (the van 't Hoff relationship, d ln K / dT = ΔH/RT²). At 500°C the equilibrium barely favors products; by 850-950°C, methane conversion can exceed 90%. The reaction also produces more gas molecules than it consumes (2 in, 4 out), so by Le Chatelier's principle it is favored by low pressure. Yet real plants run at 15-40 bar — a thermodynamic sacrifice made for two practical reasons: downstream ammonia and methanol synthesis want high-pressure gas anyway, and it is cheaper to compress the methane feed than to compress the larger volume of hydrogen-rich product gas later. The penalty in conversion is simply paid back with extra temperature and steam.

The two reactions that make hydrogen

Property	Reforming	Water-gas shift
Reaction	CH₄ + H₂O ⇌ CO + 3H₂	CO + H₂O ⇌ CO₂ + H₂
ΔH° (kJ/mol)	+206 (endothermic)	−41 (exothermic)
Favored by temperature	High (≥800°C)	Low (200-400°C)
Effect of pressure	Hindered (more moles)	None (equal moles)
Typical catalyst	Ni/Al₂O₃	Fe₃O₄/Cr₂O₃ then Cu/ZnO
Purpose	Crack methane to syngas	Convert CO to extra H₂ + CO₂

What happens on the nickel surface

The reforming reaction is heterogeneous catalysis: it happens on the surface of nickel crystallites dispersed across a porous alumina or magnesia-alumina support. Methane first chemisorbs and loses hydrogen atoms one at a time in a sequence of dehydrogenation steps (CH₄ → CH₃* → CH₂* → CH* → C*), with the first C-H cleavage usually being the rate-determining step. Steam dissociates on neighboring sites into adsorbed O* and OH*. The surface carbon and oxygen then combine to release CO, while the stripped hydrogen atoms recombine and desorb as H₂. Nickel is used not because it is the best catalyst — platinum, rhodium, and ruthenium are all more active per atom — but because it is hundreds of times cheaper and good enough when you can pack tonnes of it into a furnace.

Two things ruin nickel. The first is sulfur: even 0.1 ppm of H₂S in the feed will chemisorb onto nickel and block the active sites, so the natural gas is first hydrodesulfurized over a cobalt-molybdenum catalyst and scrubbed across a zinc-oxide guard bed (H₂S + ZnO → ZnS + H₂O) until sulfur falls below ~0.1 ppm. The second is coking — carbon laid down on the metal either by methane cracking (CH₄ → C + 2H₂) or by CO disproportionation, the Boudouard reaction (2CO → C + CO₂). Coke buries the catalyst and can crack the support. The defense is to run with excess steam: a steam-to-carbon ratio of about 2.5 to 3.5 keeps surface carbon gasified (C + H₂O → CO + H₂) faster than it can accumulate.

Inside a steam reformer

A commercial reformer is a fired furnace the size of a building. Desulfurized natural gas, mixed with steam, flows down through dozens to hundreds of vertical alloy tubes — heat-resistant HK-40 or HP-modified steel about 10-15 cm wide and 10-13 m long, each packed with nickel catalyst pellets. Rows of burners fire along the tubes, keeping the tube walls near 850-950°C and supplying the 206 kJ/mol the reaction demands. Because the reaction is so heat-hungry, the design challenge is heat transfer into the catalyst, not the chemistry itself; reformer tubes are among the most thermally stressed pressure parts in any chemical plant.

The hot syngas leaving the tubes then passes through two shift reactors in series. A high-temperature shift over iron-oxide/chromia at ~350°C does the bulk of the CO conversion quickly; a low-temperature shift over copper-zinc oxide at ~200°C then pushes the exothermic equilibrium further, leaving CO below ~0.3%. Finally the gas is cooled and the hydrogen separated — modern plants use pressure-swing adsorption (PSA), where the CO₂, residual CO, methane, and water are adsorbed on molecular sieves and hydrogen passes through at 99.9%+ purity. The adsorbed off-gas is burned back in the reformer furnace, recycling its fuel value.

Steam reforming versus the alternatives

Reforming is the cheapest large-scale route to hydrogen wherever natural gas is cheap, but it is not the only one. Partial oxidation and autothermal reforming burn part of the feed to supply heat internally rather than through furnace tubes; coal gasification runs in regions with cheap coal; and water electrolysis splits H₂O directly with electricity, producing zero-carbon "green" hydrogen if the power is renewable.

Routes to hydrogen, compared

Route	Feedstock	Heat source	CO₂ per kg H₂	Notes
Steam reforming (SMR)	Natural gas	External furnace	~9-12 kg	Cheapest at scale; "grey" H₂
Autothermal reforming (ATR)	Natural gas + O₂	Internal combustion	~8-10 kg	Compact; easier CO₂ capture
Partial oxidation (POX)	Heavy oil / gas	Internal, no catalyst	~10-13 kg	Tolerates dirty feed
Coal gasification	Coal	Internal combustion	~18-20 kg	Common in China; high carbon
Water electrolysis	Water + electricity	Electric	0 (if renewable)	"Green" H₂; far costlier today

The numbers explain why steam reforming still dominates: with cheap gas, SMR hydrogen costs on the order of $1-2/kg, while electrolytic hydrogen is typically $3-6/kg. The catch is carbon. SMR emits roughly 9-12 kg of CO₂ per kg of hydrogen — about 830 million tonnes of CO₂ a year worldwide. Hydrogen made this way is called grey hydrogen. Bolting carbon capture and storage onto the plant (capturing 60-90% of the CO₂, easiest from the concentrated stream after the shift reactors) yields blue hydrogen, and electrolysis from renewables yields zero-carbon green hydrogen.

Why it matters

Steam reforming is the quiet keystone of the modern chemical economy. Its largest single customer is the Haber-Bosch process, which combines reformed hydrogen with nitrogen to make ammonia — the source of the synthetic fertilizer that feeds roughly half the planet. Reforming hydrogen also drives methanol synthesis, the hydrocracking and hydrotreating that desulfurize gasoline and diesel in every oil refinery, and increasingly the hydrogen for fuel cells. The CO-rich syngas, before the shift step, is itself the feedstock for Fischer-Tropsch synthesis of liquid fuels and for hydroformylation. When people talk about decarbonizing "hard-to-abate" industry, a huge fraction of the challenge is simply finding a cleaner way to do what steam reforming has done cheaply for a century.