Exploring consecutive cycles of iron powder combustion for sustainable thermal energy

• The study demonstrated ten consecutive cycles of iron powder combustion and hydrogen-based reduction, proving its potential as a recyclable energy carrier. • The recycled iron powder maintained stable combustion performance, comparable to virgin iron powder, ensuring consistent energy output. • The reduction process effectively restored iron powder, allowing for significant material recovery and reuse. • The porous structure of recycled iron powder enhanced ignition stability, while its particle size remained stable over multiple cycles. • Emissions, including nanoparticle formation and NO emissions, remained low throughout the cycles, making iron powder a cleaner alternative to conventional fuels. • Challenges such as mass loss during handling and limited round-trip efficiency highlight the need for further optimization to improve energy recovery and system scalability. To advance the realization of iron powder as a clean energy storage and carrier, ten full combustion-reduction cycles of iron powder have been successfully demonstrated. Combustion was conducted using a lab-scale semi-practical metal powder burner, the "Metal Cyclonic Combustor (MC 2 )", while the reduction of iron oxide back to iron powder was achieved using hydrogen in a lab-scale semi-practical fluidized-bed reactor. The reduction process demonstrates high conversion, resulting in 87±0.8 wt% of iron content in the recycled iron powder. The recycled iron powder achieves a combustion efficiency of 85±0.9% compared to the theoretical maximum heat release of iron to hematite, which is comparable to the 86% efficiency of virgin iron powder. It also maintains stable combustion characteristics over ten cycles. Despite slightly lower gas temperatures due to residual iron oxides in the recycled powder, the combustion process remains stable, with consistent iron flames and uniform gas temperatures. The stability in combustion performance is attributed to the preserved particle size of recycled iron powder and its porosity, which facilitates ignition. Furthermore, emissions, including nanoparticle and NO formation, remain constant throughout the cycles, with NO emissions below 2.5 mg/MJ, which is significantly lower than those of other combustion fuels. These promising results highlight the feasibility of iron powder as a clean and safe energy storage and carrier medium, offering a lower-risk alternative to conventional fuels.