The ratio of carbon dioxide produced, relative to steel produced, is surprising. The bigger surprise is how to improve that ratio:
A new manufacturing technique could drastically reduce the footprint of one of our dirtiest materials.
Steel production accounts for around seven per cent of humanity’s greenhouse-gas emissions. There are two reasons for this startling fact. First, steel is made using metallurgic methods that our Iron Age forebears would find familiar; second, it is part of seemingly everything, including buildings, bridges, fridges, planes, trains, and automobiles. According to some estimates, global demand for steel will nearly double by 2050. Green steel, therefore, is urgently needed if we’re to confront climate change.
To understand steel, you need to think at the level of high-school chemistry—even the chemistry you learned on the first day will suffice. Basically, steel is iron, with a little carbon added in to increase strength: tiny carbon atoms nestle between the larger iron ones, making the steel denser and more ductile. In a sense, iron isn’t so hard to find—it makes up five per cent of the earth’s crust, by weight—but metals in rock are mixed with other elements. You must get them out, in pure form, before you can build that sword or Eiffel Tower. In this respect, iron presents a particular challenge: iron atoms bind tightly with oxygen atoms, like complementary pieces in a jigsaw puzzle. Two irons and three oxygens make ferric oxide, or Fe2O3—a complete picture that’s hard to pull apart. Ferric oxide forms easily—so easily that, in the presence of water, naked iron will stick to oxygen in the air, creating rust.
For most of human history, therefore, the problem of iron extraction was unsolvable. Five thousand years ago, the ancient Egyptians made beads out of iron—but they got their metal from meteorites, in which it had already been split from oxygen by some unknown extraterrestrial process. Another thousand years would elapse before making usable iron became possible, through a process called reduction. Sometime around 2000 B.C.E., it was discovered, possibly by accident, that iron-heavy rock, or ore, became malleable when it was heated over charcoal fires. Today, we can explain why this happens: at high enough temperatures, iron atoms loosen their grip on oxygen atoms. The oxygen binds to the carbon in the charcoal, forming CO2, which flies off into the air. What’s left behind is purified, or “reduced,” iron. The process of reduction allowed the Iron Age to begin.
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. That’s a big step toward saving the world…
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