Posted on 03 May 2024
Steelmaking, the fiery process that undergirds modern life, comes with a huge cost to the climate. Greenhouse gases gush from the burning fossil fuels that drive 1600°C blast furnaces and melt raw iron ore. Purifying the molten ore by mixing it with refined coal, or coke, releases a second, bigger surge of carbon dioxide. A third stream comes when the resulting pig iron is turned into steel by cooking it a bit further—baking off most of the remaining carbon—and alloying it with additives such as chromium or titanium.
In the end, the emitted greenhouse gases weigh roughly twice as much as the steel itself. Nearly 2 billion tons of steel is produced worldwide each year, accounting for about 7% of human greenhouse gas emissions, more than Russia or the entire European Union. The U.S. Department of Energy (DOE) is now hoping to change that. In March, DOE announced $1.5 billion in grants for low-carbon ironmaking, and last month, the agency’s Advanced Research Projects Agency – Energy (ARPA-E) announced another $28 million in grants for more cutting-edge, speculative approaches.
In iron ore, positively charged iron is bound to negatively charged oxygen, forming iron oxide, or rust. To purify the iron for steelmaking, that oxygen needs to be stripped away. In a traditional blast furnace, carbon monoxide from burning coke removes the oxygen, binding to it to form carbon dioxide. “The fundamental chemistry of this industry hasn’t changed in 2000 years,” says Christina Chang, a chemist and former DOE official who helped conceive the ARPA-E initiative. She is now a partner with Lowercarbon Capital, which invests in a low-carbon iron startup called Electra. “Today we may be able to reinvent this industry.”
One idea is to swap the coke for hydrogen. DOE is spending nearly $1 billion on two commercial-scale iron smelters, one in Mississippi and another in Ohio, that can bathe heated pellets of iron ore in hydrogen. It strips away the oxygen, forming water as a byproduct instead of carbon dioxide, and at lower temperatures of about 1100°C.
But this approach has limitations. Today the technology only works with ore that has a high iron content. And although DOE is also pushing to make so-called “green” hydrogen with renewable electricity, the gas is costly and made mostly using fossil fuels.
Some startups funded by ARPA-E, like Electra, which received nearly $2.9 million, are pursuing a more fundamental shift: using electricity to power the whole process. These electricity-driven approaches “are acknowledged as being the end game,” says Mike Walsh, a longtime iron and steel industry analyst and co-host of the podcast The Green Steel Challenge.
Electra’s factory on the outskirts of Boulder, Colorado, is located in an office building that once housed a nut butter manufacturer. At temperatures no warmer than a cup of coffee, vats of acid dissolve iron ore. Iron ions are attracted to electrodes that can be powered by renewable electricity. Sheets of pure iron emerge at the end of the process.
“The loudest thing down there is the pump we have pushing the fluid through the electrical cells,” says Trevor Braun, a chemist at Electra who oversees development of the process. “It’s pretty quiet.”
Electra expects to open a new pilot plant in 2025. The company says its technology can accommodate low-quality ore left behind by mines, and because it operates at low temperatures and can be turned on and off quickly, it can use fluctuating renewable energy. Braun estimates that emissions from Electra’s process currently only equal one-third of the weight of the iron—and nearly all come from mining and processing iron ore before it reaches their plant.
At a pilot plant, Boston Metal, which previously received money from ARPA-E, also relies on electricity to separate and purify iron. But the company uses the heat generated by the electricity itself, rather than acid, to liquefy the ore.
Similar electrolytic techniques have been used for decades to extract aluminum from bauxite. Until recently, they weren’t attractive for iron because blast furnaces are relatively cheap and effective, says Adam Rauwerdink, senior vice president of business development at Boston Metal. But the growing demand for low-carbon steel, the spread of renewable energy, and advanced electrodes able to withstand molten iron have changed that picture. “Ten or 20 years ago it would have been impossible to try to do what we are doing,” Rauwerdink says. The company’s test furnace can make several hundred kilograms of iron per day, and it hopes to build a commercial plant by 2026 that could produce several thousand tons.
On the still more speculative end are efforts to build a 21st century iron furnace. At DOE’s Argonne National Laboratory, scientists are using microwaves to heat hydrogen to 600°C, until it forms a plasma. At these temperatures, the ionized hydrogen can strip oxygen even from solid ore. “It’s going to take quite a while to scale,” says John Kopasz, a chemist leading the research. So far, it has worked in a laboratory, producing just a few grams of iron at a time.
At the University of Utah, meanwhile, metallurgic engineer Zak Fang and his colleagues received $3.5 million from ARPA-E to refine a way to use hydrogen to purify an iron oxide powder, cutting energy-intensive steps normally used to turn iron ore into furnace-ready pellets. The powder could then be shaped into steel products ranging from machine gears to rebar through a process called “sintering” in which the powder is baked much like a clay pot, at temperatures far lower than those needed to melt it.
For the moment, however, these innovations are dwarfed by the magnitude of the industry. The United States and Europe, centers for clean steel research, make less than 15% of the world’s iron and steel. About half is made in China, where conventional coal-powered blast furnaces are the standard. New blast furnaces are being built at a breakneck pace in developing countries such as India.
Dabo Guan, an environmental scientist at Tsinghua University, says simpler measures to decarbonize iron and steelmaking could have a more immediate impact: recycling more used steel in electric furnaces, and capturing carbon dioxide as it rises from smokestacks at iron smelters.
Although the high-tech approaches might seem attractive, “what is not promising is the time it takes” for them to mature, Guan says. With the planet rushing toward greenhouse gas levels that will push temperatures beyond a 2°C increase, he says, “we are in a very alarming phase right now.”
Source:Science | AAAS
