Competitive reduction mechanisms and hydrogen utilization rate in the hydrogen-based reduction of carbon-containing iron ore composite pellets: Insights from gas evolution and microstructural characterization

Background Incorporating carbon-containing iron ore composite pellets mitigates locally low reduction rates from uneven H₂ distribution in hydrogen-based shaft furnaces. Methods The synergistic reduction of composite pellets with different C/O ratios in a H₂ atmosphere is studied through gas analysis, phase evolution, and microstructural characterization. Significant findings Composite pellet reduction accelerates markedly from the Fe₃O₄ → FeO stage versus carbon-free pellets. Catalyzed by iron oxides, CO generates via reverse water-gas shift below 500 °C. The steam-carbon reaction dominates carbon gasification above 745 °C. Higher temperatures enhance the H₂ utilization rate, up to a maximum of 27% higher than carbon-free pellets, but reduce the proportion of reduction contributed by H₂ while increasing that attributable to carbon. The apparent activation energy of the reduction reaction increases approximately linearly with the C/O ratio, ranging from 29.331 to 72.999 kJ/mol. C/O = 0.25 intensifies iron whisker sintering, whereas ratios ≥ 0.5 form hollow particle structures and substantially suppress sintering.