The steel industry, responsible for approximately 8% of global anthropogenic CO₂ emissions, may have found a practical interim solution for decarbonization while waiting for hydrogen-based technologies to mature. A new study published in Engineering demonstrates that upgraded biomass fuels—specifically torrefied biomass and biochar—can effectively replace up to 27%-30% of pulverized coal injection (PCI) in blast furnace operations without compromising combustion performance.

Researchers from Pusan National University, POSCO Technical Research Laboratories, and Dong-Eui University systematically evaluated four biocarbon samples: mildly torrefied biomass (MTB), hard torrefied biomass (HTB), mildly carbonized biomass (MCB), and hard carbonized biomass (HCB). Using thermogravimetric analysis (TGA), drop tube furnace (DTF), and laminar flow reactor (LFR) experiments, the team characterized how these materials behave under actual PCI conditions.

Raw biomass presents significant challenges for direct use in blast furnaces, including low grindability, high ash content, and low energy density. Through torrefaction (performed at 473-573K) and carbonization processes, these properties can be substantially improved. The study found that as biomass undergoes increasing thermal treatment, its combustion kinetics progressively resemble those of conventional PCI coal. Hard carbonized biomass (HCB), treated at 1073 K for one hour under nitrogen atmosphere, demonstrated the highest fixed carbon content at 68.73% and exhibited combustion behavior most similar to bituminous coal.

Co-firing tests revealed complex but promising dynamics. At low blending ratios of 3%, some biocarbon samples initially showed slightly reduced combustibility compared to pure PCI coal. However, as blending ratios increased to 5% and beyond, combustion performance generally improved, particularly for highly treated samples. HCB consistently outperformed PCI coal across all blending levels from 3% to 10%, with unburned carbon (UBC) decreasing steadily as the blending ratio increased. This improvement was attributed to enhanced particle fragmentation during combustion, which exposes fresh reactive surfaces and facilitates more complete burnout.

Radiocarbon dating analysis of collected unburned carbon confirmed that the vast majority originated from PCI coal rather than biocarbon, indicating that the biomass components were combusting efficiently. For MTB, over 99% of UBC came from coal at all tested ratios. Even at 10% blending, biocarbon-derived UBC remained below 2.1% for HTB and 0.4% for MCB.

The research established practical injection limits based on three criteria: combustion performance, heating value within ±5% of PCI coal, and total ash content below 10%. MTB and HCB exceeded the heating value threshold at approximately 27%-29% blending ratios, requiring adjusted fuel input to maintain thermal balance. HCB also faced constraints due to its elevated ash content of 14.89%, necessitating lower blending ratios to prevent operational issues.

From an economic perspective, the findings carry significant weight under expanding carbon trading systems. According to Intergovernmental Panel on Climate Change guidelines, CO₂ from biogenic carbon is considered carbon-neutral and excluded from net emission calculations. Substituting 10% of PCI carbon with biocarbon corresponds to an estimated reduction of approximately 0.09 tonnes of CO₂ per ton of PCI. At recent European Union emission trading system prices of approximately 78 EUR per tonne CO₂, this translates to roughly 7 USD economic benefit per ton of PCI through carbon credit value alone.

The study positions biocarbon injection as a technically feasible transitional strategy that complements rather than replaces future hydrogen-based ironmaking. While fully hydrogen-based reduction remains the long-term goal, current barriers including green hydrogen costs of 3-8 USD per kilogram and infrastructure limitations make immediate transition impractical. Given that most operating blast furnaces will remain in service for decades, particularly in regions where large-scale capital replacement is difficult, biocarbon co-firing offers immediate CO₂ reduction without requiring substantial furnace modifications.

The paper “Towards Carbon-Neutral Ironmaking: Stepwise Integration of Biocarbon in PCI with Combustion Behavior Characterization and Injection Limit Evaluation,” is authored by Min-Woo Kim, Min-Jong Ku, Jongho Kim, Gyoung-Min Kim, Chung-Hwan Jeon, Dae-Gyun Lee. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.12.004. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.