It’s hard to say exactly when steel was first made. From time to time, it would be created when carbon diffused from the charcoal into the iron, strengthening it. But steel production was hard to control until a few hundred years ago, when the blast furnace was invented. Using bellows, steelworkers increased the temperatures of their coal fires to nearly three thousand degrees—hot enough to melt iron in large quantities. Today, blast furnaces are still the main method used to reduce steel. Current models are about a hundred feet tall, and can produce ten thousand tons of iron in a day. Instead of charcoal, they use coke, a processed form of coal. Coke and ore go in the top of the furnace, and molten iron comes out the bottom, infused with carbon; this iron can be easily processed into steel. The steel industry produces around two billion tons of it each year, in a $2.5-trillion market, while emitting more than three billion tons of CO2 annually, most of it from blast furnaces.
Fortunately, we’ve since learned that there’s more than one way to purify iron. Instead of using carbon to remove the oxygen from ore, creating CO2, we can use hydrogen, creating H2O—that is, water. Many companies are working on this approach; this summer, a Swedish venture used it to make steel at a pilot plant. If the technique were widely employed, it could cut the steel industry’s emissions by ninety per cent, and our global emissions by nearly six per cent.
Reducing steel with hydrogen has been done on a small scale in laboratories for years. Martin Pei, the chief technology officer of S.S.A.B., the steelmaking company, told me that there were no great scientific hurdles to scaling up the process. Instead, it’s mostly been a matter of optimizing the operating conditions: for instance, engineers needed to experiment with the machinery that heats the hydrogen before it’s pumped in. The real hurdle, Pei said, is the hydrogen supply. Pure hydrogen comes mostly from natural gas, typically methane—but getting hydrogen out of methane requires energy, and also creates carbon monoxide, which produces CO2 when burned. There is a green source of hydrogen: water. It’s possible to split water into hydrogen and oxygen, by running current through it, in a process called electrolysis. But electrolysis, in turn, is green only if the electrons involved also come from renewable energy.
Matthew Hutson
Tackling global warming requires major adjustments in every carbon-producing industry, not only energy generation and transportation. Still, switching to mass-use of hydrogen in key areas, from air fuels to steelmaking, carries some risks that I rarely see discussed. Hydrogen is a very volatile gas that can explode violently in contact with oxygen – this issue famously brought down the nascent dirigible business last century. It also needs to be refrigerated and compressed for transportation, increasing costs and potentially requiring upgraded infrastructure compared to natural gas.
Hydrogen-based steelmaking isn’t the only way forward. Other groups are exploring more experimental green-steel methods. An Austrian project called SuSteel—sustainable steel—uses hydrogen plasma, which is much hotter than hydrogen gas, to reduce and melt the iron ore; while it melts, they add carbon, combining ironmaking and steelmaking into one step. Boston Metal, an M.I.T. spinout company, is based on “molten oxide electrolysis”, in which electricity is run through melted iron oxide, producing steel and oxygen. ArcelorMittal is also testing “electrowinning”—a process in which it passes current through a solution containing particles of iron oxide, so that the iron collects on one electrode while oxygen collects at the other.
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