“Hydrogen can pretty much do everything, but that doesn’t mean it should,” says Russell McKenna at ETH Zurich in Switzerland.

In a recent study, McKenna and his colleagues analysed the CO2 that would have to be emitted to produce and transport low-carbon hydrogen in 2000 planned projects worldwide, comparing this with the CO2 emissions this hydrogen could displace. They found that hydrogen could have the biggest positive climate impact in the steel, biofuels and ammonia industries.

Using hydrogen for road transport, power generation and domestic heating, on the other hand, wouldn’t reduce emissions as much.

Steel

In a blast furnace, coke made from coal not only provides heat to melt iron oxide ore, but also carbon for a reaction that strips the oxygen away from that ore. So it isn’t enough to just heat the metal with renewable electricity instead. You need something to stand in for carbon in the reaction, which hydrogen can do, emitting water rather than CO2.

“The technology we have today that’ll work to make iron at full industrial scale out of iron ore without making CO2, that technology is hydrogen,” says David Dye at Imperial College London. “Anything else requires you to invent a lot of future technologies.”

Stegra, a green steel start-up, is building a facility in northern Sweden that plans to produce steel with an electric arc furnace and green hydrogen produced from river water on site by late 2026, becoming the first carbon-free steel plant. There are also projects under way elsewhere in Europe, Asia and North America.

But cheap renewable electricity has to be available to make the green hydrogen and power the arc furnaces. ArcelorMittal, a multinational steel-making corporation, turned down €1.3 billion in subsidies this year to convert two steel plants in Germany to hydrogen because it said electricity prices were too high.

Ammonia

In order to grow, plants need nitrogen in the form of nitrates, but soil holds a limited quantity of them. In the early 20th century, however, chemists Fritz Haber and Carl Bosch developed a process to make the nitrogen that is abundant in the air react with hydrogen to produce ammonia, which can be converted into a variety of fertilisers.

This enabled a revolution in agriculture and a boom in global population, and hydrogen today is largely consumed for ammonia production, as well as oil refining. About 70 per cent of all ammonia goes towards fertiliser, while the rest helps manufacture plastics, explosives and other chemicals.

“We can’t electrify that… because it’s a chemical reaction that needs this input,” says McKenna. “So we need the hydrogen, but it needs to be decarbonised hydrogen.”

Countries like Saudi Arabia have begun building factories to produce hundreds of thousands of tonnes of green ammonia with solar and wind energy, mostly for export. Meanwhile, start-ups have been developing small modular plants that produce green hydrogen and ammonia on site at farms in the US. But for now, all these approaches rely for on government investment or tax credits.

Alternative fuels

Ammonia can also be burned in an engine. While cars and many lorries can run efficiently on electricity, long-distance transport like heavy lorries, ships and planes may struggle to carry and recharge batteries. Hydrogen is likely to be crucial to manufacturing low-carbon fuels for this sector.

McKenna and his team’s study found that producing hydrotreated vegetable oil was one of the most impactful uses of hydrogen. This involves treating used cooking oil with hydrogen to break down the fats into hydrocarbons that can be burned.

Both ammonia and hydrotreated vegetable oil are being considered as replacements for heavy fuel oil in shipping, which accounts for 3 per cent of global emissions. Aviation, with its similar carbon footprint, could switch to ammonia as well.

But hydrogen could also be used to manufacture synthetic aviation fuel, which could go into any plane today, as it is almost identical to kerosene fuel, only produced without oil.

In the longer term, researchers at institutions like Cranfield University in the UK are designing aircraft with ultra-strong tanks to hold compressed hydrogen. While hydrogen or ammonia produce nitrogen oxide pollution when burned, they can instead combine with oxygen in a fuel cell to produce electricity and water. Fuel cell aircraft are the ultimate goal, says Phil Longhurst at Cranfield University.

“Hydrogen is the cleanest, most zero-carbon fuel we can get,” he says, “so that’s kind of the Holy Grail.”