The world cannot meet climate goals without decarbonising steel-manufacturing, which adds 7.2% of total greenhouse gas emissions per year, with the fastest-growing rate of any sector.
The main carbon culprits in steel are coal and coke, used for energy and as chemical inputs in the manufacturing process. Growing regulations on industry sustainability are setting companies on a race to adopt cleaner alternatives, including the EU’s carbon border adjustment mechanisms and a possible ‘green steel club’ between the bloc and the US.
Recently, the Natural Resources Research Institute (NRRI) at UMD received a $2.9 million dollar grant from the U-S Department of Energy (DOE) to develop a biobased replacement for fossil materials in steel-making – a renewable, plant-based material designed to replace at least some of the coke used to make steel.
The material in question is known as a biocarbon, which is any product made through burning biomass in a very low-oxygen environment.
What is the material and how can plants decarbonise steel?
Why steel-making uses fossil materials
The NRRI is developing its biobased material for the electric arc furnace method. About one-quarter of the world’s steel is produced using this method. Invented in France in 1907, it is one of three main routes for steelmaking alongside blast furnaces and open hearth processes.
Metal scraps rather than iron ore are the starting material for the blast furnace process. Scrap metal is collected into a 60 tonne capacity basket, which is itself lowered into an electric arc furnace, where electrically charged metal rods heat the material. When the scrap metal melts, it runs in liquid form into another ladle furnace. Here, the finishing touches are applied to the metal, making it a sellable product.
Today, the electric arc furnace method relies almost wholly on fossil inputs to induce chemical reactions in the furnace as the steel is melting. Fossil coke, a hard, porous material made from heating coal without air to very high temperatures, is injected into the electric arc furnace.
Coke is used because it is rich in carbon, a vital ingredient for enhancing the formation of a foamy material called slag, which accumulates in a floating layer on top of melting steel. The properties of slag are very important to the steel making process. The layer absorbs impurities from the melted metal below it, and perfecting it is crucial to ensuring a sellable end product. The slag should also be thick enough to cover the electric arcs during the melting process since this prevents heat damage to the furnace roof and keeps the heat in the melting metal beneath.
Carbon from the coke enhances the chemical properties of the floating slag, improving foaming and resilience to making it more stable. This improves the overall efficiency of the process.
Can biomass replace coke?
The push to replace some fossil steel inputs with biobased ones has been gaining ground for years, with researchers proposing various biobased materials like bio-oil or biochar, a charcoal-like substance that comes from biomass heated without oxygen.
It is now widely recognised that biomass can be used as a replacement for coke as properly processed biomass has similar physicochemical properties to coal or coke. Some even have less ash content and impurities than fossil coke, which benefits the steel making process.
Various kinds of plant matter can be used as feedstock: pyrolysis charcoal and even palm shell char have been demonstrated in experiments as effective replacements, with one study showing the latter had a better interaction with electric arc furnace slag compared to conventional coke.
The suitability of biocarbon for the steel making process all depends on how the biomass is processed and what kind of biomass feedstock is used since variations impact on the structural properties of the biocarbon and how they affect the steel melting process.
Blends of various types of biomass, the ratio of biomass to conventional coke, and the size of the biomass pieces (whether they are used in pellet or particle form) can also have an impact on the properties of the final steel product.
Full substitution for coke using biobased materials is not possible due to the huge biomass volumes that would be required. Depending on the application and type of manufacturing process, there can also be some loss of function with 100% biocarbon. However, by blending it in mixes with fossil coke, biomass could significantly cut the use of coke by the industry.
NRRI aims for 2026 demo plant
The NRRI’s biocarbon is the product of biomass heated in low oxygen kilns, then pressed into briquettes or pellets. Making the burnt biomass into uniform sizes reduces carbon losses from dust formation while making them stronger for transport. NRRI has chosen locally sourced woody biomass as its feedstock, at least some of which will be from organic waste streams rather than logged wood.
However, one of the major challenges of using organic waste as feedstock steelmaking inputs is related to its quality, such as a higher ash content.
“Our goal will be to tweak the formulation and, by 2026, demonstrate this process at a tonnage scale in an actual steel mill,” explained Brett Spigarelli, the project’s principal investigator.
The NRRI also plans to develop a way to capture any gases released by the reacting of coke and slag, recirculating it back into the system as an energy source. This could displace some of the fossil energy used to power the process, usually natural gas.
Finnish, British, and US companies invest in biocarbon for steel-making
The NRRI is not the only entity working towards biobased fuel and chemical replacements for steelmaking inputs.
British Steel is also supporting a University of Lincoln study into biochar as a carbon source for electric furnace steel production. It plans to use willow as a source of feedstock, sourced from sustainable farms located on UK peatland.
British Steel claims that sourcing willow feedstock from sustainable farms on native peatland could save this valuable habitat. Regenerative farming could prevent the habitat from decaying, releasing large carbon leaks in the process.
In November 2023, US wood processing tech company Bruks Siwertell secured a contract to design a new biocarbon pellet plant for Steel Dynamics, a large US steel producer, for its electric arc furnace mills.
The University of Nottingham, miner Rio Tinto, and the sustainable tech company Metso Outotec have been developing another biobased steel manufacturing input to replace coal, this time for making metallic iron another steel making method called the blast furnace method. The blast furnace method uses iron ore instead of scrap metal inputs.
Their biobased material is branded Biolron, which uses lignocellulosic biomass – tough, hard-to-use plant matter that is normally cast off as agricultural waste such as wheat straw, sugar cane bagasse, canola sticks, or barley straw. Rio Tinto have stated that they will not use wood biomass for their biocarbon that is sourced from old growth or High Conservation Value forests.
Testing for the Biolron process finished in 2022 and its carbon reducing potential was confirmed in an independent review by Hatch.
Finally, Sweden’s Envigas is one of Europe’s first companies to invest in large-scale and high quality biocarbon for the steel industry.
Biobased coke may be an emerging application in most of the world but in the Brazilian steel industry, it is relatively common already. 10% of its steel is produced using charcoal rather than coal coke to reduce iron ore. Normally, the cost of using wood or charcoal compared to coke is more expensive but in a country with large forest cover and scarce coal, it can reduce costs.
How sustainable is it?
In a life cycle assessment conducted on a laboratory reactor in Italy, researchers found using hydrochar injection improved the sustainability profile of fossil-based EAF process in the areas of greenhouse gas emission, terrestrial acidification, freshwater eutrophication, and water consumption impact categories.
The two cokes were roughly equal in their impacts on ozone formation, fine particulate matter formation, freshwater ecotoxicity, marine ecotoxicity, human carcinogenic toxicity, and land use were not significantly different, showing how difficult achieving a more sustainable steel industry will be.
One potential issue is that, depending on the material, more biocarbon needs to be burnt to achieve the same chemical effects as pure coke. Some research conducted on partially switching to biochar from coke can result in higher carbon emissions for this reason.
However, carbon is not the only pollutant emitted by coke in steel making. Another toxic, planet-warming gas associated with the industry is sulphur dioxide. The same research above showed that sulphur emissions decrease when the switch is made to biochar blends. A review paper noted that the charcoal replacement of coke could result in a 94% lower sulphur load for a metallurgical operation.
On the other hand, biobased steel-making fuel faces the same sustainability hurdles as biofuels. Whether it brings net environmental benefits compared to pure coke depends on how the biomass is sourced and which environmental metrics you’re measuring. With certain feedstocks, the carbon released from land-use change could impact overall net benefits. Land competition with agriculture and biomass feedstock competition with other biobased sectors also need to be considered.
Greening steel will always be relative: this is an energy and material intensive process. However, replacing coke with biobased options can reduce various environmental impacts from the industry, carbon emissions among them. Investigations into using biobased blends of coke will only grow as legislative stimulus favours sustainable steel production and demand for more sustainable steel will grow.
