Written by Ryan McGuine //
The modern age has been characterized by the skyrocketing use of a number of materials, including steel. Remarkable for its strength as well as its durability, steel is the key metal of industrialization — in 2014 steel production was almost 20 times larger than that of aluminum, copper, zinc, and lead combined. As countries build out the infrastructure needed for the energy transition and urbanization, global steel demand is poised to grow by over one-third by 2050.
The main constituent of steel is iron, which makes up about 98% of the metal (the other 2% being carbon), and is extracted from the earth in the form of iron ore. Most of the world’s steel is produced in one of two methods. Virgin steel is produced by using coke and hot air to melt iron ore in blast furnaces to produce “pig iron,” then combining the molten pig iron, nearly-pure oxygen, and scrap steel in basic oxygen furnaces. Alternatively, steel can also be produced by passing an electric arc through recycled steel or “direct-reduced iron.” It takes a huge amount of energy to produce steel by either method, but using an electric arc furnace generates half or less carbon dioxide per ton of steel.
Iron was used widely throughout antiquity – it began to displace weapons and tools made of softer bronze some 4,000 years ago, kicking off an era during which iron was so ubiquitous that historians named it for the metal. The ancient Chinese made wide use of cast iron in agricultural and military tools as early as the 8th century BC, while iron had moved from rarity to commodity by about 500 BC across the Near East. Around 400 BC, Indian metalworkers invented a smelting technique which bonded carbon to the iron, producing something similar to modern steel. This new metal was even harder and nearly as ductile as iron, and the recipe spread widely. The Syrians used steel to produce the almost-mythical Damascus sword, the Spaniards pumped out interchangeable swords for the Roman army, and the Japanese masterfully produced katanas that would be passed down within families for generations.
Even though steel was made and used throughout antiquity, it took centuries to truly take off. The first step along this track was taken by British pot producer Abraham Darby I, who began using coke instead of charcoal for heat around 1710. This enabled larger iron charges and higher profits, but the steel produced had too much carbon, making the resulting metal too brittle. There were numerous methods of reducing the carbon content after production, but they were mostly small-scale and labor-intensive until the 1850s, when Henry Bessemer pioneered a process of using oxygen to remove carbon from molten steel by forcing air through it. This caused a chemical reaction that caused the carbon to combine with oxygen and leave the mixture. However, the Bessemer Process could do nothing about phosphorus impurities, so only low-phosphorus iron could be used as an input. In the 1870s, Sidney Gilchrist Thomas worked out the last major piece in the puzzle when he discovered that phosphorus could be eliminated from the final product by lining furnaces with limestone.
Cheap, wildly-available, and uniform steel proved to be the key to unlocking widespread industrialization. During the 19th century, steel production exploded to enable railroad expansion, skyscraper construction, and plow manufacturing. Since the Industrial Revolution, a wide range of both singular and incremental improvements have been made. These advances, paired with steel’s unique combination of cost-effectiveness and beneficial material properties, mean that steel has remained one of the most produced and most traded commodities in the world over the last two centuries. Today’s built environment would be nearly impossible without it – steel is the backbone of everything from homes to schools, and from bridges to vehicles, not to mention a wide array of clean energy technologies including solar panels, wind turbines, and dams.
Unfortunately, steel comes with myriad environmental consequences. For example, the process of extracting iron from the earth both requires a massive amount of energy, and produces a significant amount of local pollution. For example, open pit mining involves heavy machinery scraping and driving over the ground, generating lots of particulate matter; rocks containing metal ore are often high in sulfides, and exposing them can acidify water and soil near mines; processing rare earth elements uses hazardous chemicals, creating both solid waste and wastewater; and brine mining can damage water quality and reduce water levels. Further, mining is very energy intensive, accounting for about 4-7% of global carbon dioxide emissions (not all of that is associated with iron ore extraction, but iron is one of the most heavily-mined materials) and expanding mining operations can negatively impact biodiversity.
Additionally, steel production accounts for over 7% of total global carbon emissions. Similar to cement production, carbon emissions are produced by generating heat to facilitate chemical reactions, and from the chemical reactions themselves (roughly 88% and 12% respectively for steel). While technologies exist for low-carbon electricity generation and transportation, heavy industrial sectors face unique challenges – assets are expensive and long-lived, and there are few alternatives to fossil fuels for producing high-temperature heat. One way of reducing the carbon emissions from primary steel production is using hydrogen as an energy source. Many heating applications within steelmaking can be electrified, and where possible, that is the most direct way of reducing emissions. However, hydrogen will likely play a role because of its relatively high gravimetric energy density. Carbon capture will also be needed in many cases. Most steelmaking facilities have not yet reached the end of their operational life, and adding carbon capture technology allows companies to continue paying them off while reducing further emissions.
Similar to other industrial sectors, the modern steel industry has high capital costs and low profit margins, and many of its applications are critical to public safety. As a result, producers tend to be very risk-averse and slow-moving when it comes to changes. Today it is free to pollute and expensive to be green, which creates incentives that lock in high-pollution outcomes, but environmental policies can help reverse that situation. For example, America and Europe struck an important trade agreement last fall. Further, policies to decarbonize the electricity grid will also benefit large users like steel because electrification is a key pillar in economy-wide decarbonization. But in the end, improvements in both energy efficiency and material efficiency are needed. These measures are not sexy, but they require no advanced technology, and have the potential to contribute up to 40% of the emissions reductions needed for net-zero targets.