A French research team from the National Center for Scientific Research (PROMES-CNRS) has demonstrated a method to produce pure sponge iron with zero carbon emissions by combining hydrogen and concentrated solar energy. This process replaces coal-fired blast furnaces—which account for nearly 70% of the 7% of global greenhouse gas emissions caused by steel production—with a solar-thermal reaction that emits only water vapor.
How does solar-thermal hydrogen reduction work?
The process uses concentrated solar energy as a direct heat source to reach temperatures between 800°C and 1,000°C. Hydrogen acts as the reductant to strip oxygen from iron ore (hematite, Fe₂O₃) through three sequential steps: Fe₂O₃ → Fe₃O₄ → FeO → Fe.
Stéphane Abanades, the lead researcher at PROMES-CNRS, stated the goal is to replace the combustion and use of coal with a process that produces only steam. Unlike traditional Direct Reduction of Iron (DRI) which often relies on renewable electricity to heat a furnace, this method uses solar thermal energy directly. Abanades noted that converting electricity to heat involves energy losses, making direct solar heat more efficient.
Why is “pure sponge iron” critical for green steel?
The output of this solar process is porous sponge iron. This material is high-purity metallic iron with voids where impurities were extracted. Because it is so pure, it melts more easily and creates stronger steel when processed in an Electric Arc Furnace (EAF).
EAFs are the key to decarbonization because they can be powered by renewable electricity. By feeding an EAF with solar-produced sponge iron instead of pig iron from a coal blast furnace, the entire production chain can move toward a zero-carbon footprint.
Overcoming the “stickiness” and visibility hurdles
The PROMES-CNRS team had to solve two primary engineering challenges to make the reactor viable:
- The Window Problem: The reactor is enclosed behind a glass window to maintain a pure hydrogen atmosphere. While previous solar work using methane or biomass caused carbon deposits to cloud the glass, Abanades confirmed that using only hydrogen produces only steam, leaving the window clean.
- The Adhesion Problem: To prevent molten metal from sticking to the reactor walls, the team utilized boron nitride (BN), a material standard in molten metal processing for its non-stick properties.
Direct Solar Heat vs. Green Electricity
Most industrial efforts to decarbonize DRI focus on using green electricity to heat furnaces. However, the PROMES-CNRS approach highlights a thermodynamic advantage. According to Abanades, using heat directly to supply the reaction enthalpy is more efficient than the multi-step process of converting electricity into thermal energy.
| Feature | Traditional Blast Furnace | Solar-Hydrogen Process |
|---|---|---|
| Reductant | Coal/Coke | Hydrogen (H₂) |
| Heat Source | Coal Combustion | Concentrated Solar Thermal |
| Primary Emission | Carbon Dioxide (CO₂) | Water Vapor (H₂O) |
What happens next for solar steelmaking?
While the proof-of-concept is successful, the researchers noted that “residence time”—the time the ore spends in the heat—is currently a geometry problem due to the lab-bench scale of the reactor. Scaling this to an industrial level will require moving from small-scale cavities to large-scale solar concentrator arrays.
Frequently Asked Questions
Can solar energy really get hot enough for steel?
Yes. Concentrating solar thermal systems can deliver process heat exceeding 1,000°C, which is sufficient for the reduction of iron oxide when paired with a reductant like hydrogen.
Why use hydrogen instead of just electricity?
Electricity provides heat, but it cannot remove the oxygen from the iron ore. A chemical reductant is needed. Hydrogen removes the oxygen as water vapor, whereas coal removes it as carbon dioxide.
Is this process ready for mass production?
The PROMES-CNRS team has demonstrated the process at a lab-bench scale. Industrial application will require solving geometry and residence time issues to handle larger volumes of ore.
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