What is hydrogen gas used for? Refineries ammonia, metal annealing, generator cooling, food and the ways it is delivered

Hydrogen is used for what it does to other substances and for how it moves heat. Chemically it is the universal reducing and hydrogenating agent: it strips sulphur from diesel, turns nitrogen into ammonia, turns oxides on steel back into metal, and turns liquid oil into solid fat. Physically it is the lightest gas there is, with a thermal conductivity seven times that of air, which is why the largest electrical machines on earth are filled with it. The chemical uses consume hydrogen by the tonne from on-site reformers; the physical and small-scale uses are what a merchant gas company delivers in cylinders.

Oil refineries are the largest hydrogen users. Hydrotreating passes crude fractions over a catalyst with hydrogen at 30 to 100 bar to convert sulphur to hydrogen sulphide, which is then removed; India's move to BS-VI fuel at 10 ppm sulphur raised refinery hydrogen demand. Hydrocracking uses hydrogen at higher pressure to break heavy residues into diesel and kerosene. Refineries make most of this hydrogen themselves by steam methane reforming and recover more from catalytic reformer off-gas.

Ammonia synthesis consumes about 178 kg of hydrogen per tonne of ammonia, from the stoichiometry N₂ + 3H₂ → 2NH₃ (ammonia is 17.8 % hydrogen by mass), and nearly all of it goes to urea and other fertilisers. Methanol synthesis from CO and hydrogen is the third large consumer. Together these three take the overwhelming share of world hydrogen, over 90 % by most accounts, and none of it travels in cylinders. They matter to a cylinder buyer only as the reason hydrogen is a mature, well-understood industrial gas with established safety codes.

This is the largest merchant use. Bright annealing of stainless steel strip, tube and wire runs in pure hydrogen or in nitrogen-hydrogen atmospheres at 1,050 to 1,150 °C; the hydrogen reduces the chromium oxide film that would otherwise form, so the steel emerges bright and needs no pickling. Copper and brass are annealed in a few per cent hydrogen in nitrogen to reduce cuprous oxide. Sintering of powder-metal parts, brazing of stainless and copper assemblies in continuous belt furnaces, and reduction of tungsten and molybdenum oxides to metal powder all use hydrogen atmospheres.

The hydrogen fraction is chosen for the oxide being reduced, and the dew point is as important as the purity: a wet hydrogen atmosphere will oxidise chromium even though the hydrogen is nominally reducing. Dissociated ammonia, at 75 % hydrogen and 25 % nitrogen, is the traditional cheap atmosphere, and nitrogen-hydrogen blends from a PSA nitrogen plant with cylinder hydrogen are the modern replacement where ammonia storage is unwanted. Float glass is a special case: the molten tin bath on which the glass ribbon floats would oxidise in air, so the bath enclosure is kept under nitrogen with a few per cent hydrogen to scavenge oxygen.

Large steam and gas turbine generators are filled with hydrogen instead of air. Two properties make the case. Thermal conductivity: about 0.18 W/m·K for hydrogen against 0.026 for air at room temperature, so the windings and core run cooler for the same gas flow, which lets the machine be rated higher in the same frame. Density: hydrogen at one fourteenth the density of air cuts the windage loss of a rotor spinning at 3,000 rpm by a similar factor, which is worth a measurable fraction of a per cent of station output. Machines above roughly 100 MW are hydrogen-cooled at 3 to 5 bar, with oil-film shaft seals, a purity monitor and a scheduled purge sequence: air is displaced by carbon dioxide before hydrogen is admitted and the reverse on shutdown, because the 4 to 75 % flammable range in air must never be crossed inside the casing. Purity is held at 98 % or better by bleeding and topping up, and that top-up is cylinder or bundle hydrogen.

Hydrogenation of vegetable oil over a nickel catalyst at 150 to 200 °C and a few bar converts liquid oil to the semi-solid fat sold in India as vanaspati, and to the shortenings and margarines used by bakeries. Fully hydrogenated fat blended with liquid oil is replacing partially hydrogenated fat as food regulators limit trans-fat content, but the hydrogen requirement remains. Fine chemical and pharmaceutical plants hydrogenate intermediates in batch autoclaves on hydrogen from cylinders or bundles. Hydrogen peroxide is made by the anthraquinone process, which is a hydrogenation followed by an oxidation.

Electronics and optical-fibre plants use hydrogen in epitaxy, in the oxy-hydrogen flame that fuses silica preforms, and as a carrier gas. Glass works beyond float lines use oxy-hydrogen torches for fire-polishing and quartz working because the flame is hot, around 2,800 °C with oxygen, and leaves no carbon. Laboratories use hydrogen as the fuel for flame ionisation detectors in gas chromatography and as GC carrier gas, and weather services fill sounding balloons with it where helium is unavailable or costly.

Hydrogen stores less than the ideal-gas sum in a cylinder because its compressibility factor is above 1 at room temperature: about 1.09 at 150 bar, so a 46.7 litre cylinder filled to 150 bar at 15 °C holds about 6.4 m³ rather than the 7.0 m³ a nitrogen or oxygen cylinder of the same size holds. The arithmetic for every gas is on gas cylinder sizes and contents in India. Hydrogen's heating value is the highest of any fuel per kilogram, 120 MJ/kg lower heating value, but only about 10.8 MJ per cubic metre at 0 °C, one third of methane, which is why cylinder hydrogen is never bought as a fuel.

Hydrogen burns across 4 to 75 % in air, ignites from about 0.02 mJ, the same minimum ignition energy as acetylene, and burns with a flame that is nearly invisible in daylight. It is also 14 times lighter than air, so a leak rises and disperses fast outdoors and collects under roofs indoors. Those four facts drive every rule on hydrogen safety and handling: store and use cylinders outdoors or under a ventilated roof, keep ignition sources and gas group IIC electrical equipment in mind, detect leaks with a detector or soap solution rather than by looking for a flame, and treat the hydrogen safety data sheet as the reference. Supply forms and specifications are on the KIGL hydrogen page.