Thermodynamic assessment of an iron-based circular energy economy for carbon-free power supply

As the urgency for decarbonization of economies around the world is becoming more pressing, green energy carriers synthesized with renewable energy are emerging as tradable commodities for connecting regions with abundant renewable energy to those with high energy demand. Among the various options, metals – especially iron – have been identified by the scientific community as promising green fuels due to their high volumetric energy densities. However, there persists a gap in comprehensive thermodynamic analyses despite the growing interest. This study provides a rigorous thermodynamic assessment of an iron-based circular energy economy for carbon-free power supply. The circular system encompasses the storage of renewable energy through the thermochemical reduction of iron oxide powder to metallic iron powder, intermediate storage, energy release in iron-fired power plants via thermochemical oxidation of the iron powder, and long-distance inter-regional transport. Each sub-process of the iron-based energy cycle is described and evaluated using comprehensive thermodynamic models, addressing technical implications and thermodynamic limitations. Two technological options for the hydrogen direct reduction of iron oxides – namely, the shaft furnace and the flash reactor hydrogen direct reduction – are compared. The thermodynamic assessments reveal that the flash reactor is superior to the shaft furnace concept, primarily due to the elimination of additional process steps for particle size adjustments. Moreover, the study underscores the feasibility of iron-fired power plants as a means to retrofit and decarbonize existing coal-fired power plants. The analysis shows that iron-fired power plants attain higher efficiency levels than coal-fired power plants, even under non-ideal conditions. Regarding transport, industrial practices and regulations for handling iron and its oxides are well established globally, providing further confidence in the feasibility of the approach. The findings indicate that integrating an iron-based circular energy economy with the repurposing of existing infrastructure presents a compelling option. This approach effectively addresses the temporal and spatial mismatch between energy demand and supply serving as a critical enabler for renewable energy transport and long-term storage, which is essential for a successful energy transition.