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

• The H 2 utilization rate is enhanced during the synergistic reduction. • Higher temperature boosts carbon reduction but reduces H 2 reduction. • The synergistic reduction outpaces the reduction by H 2 alone. • The reverse water-gas shift reaction begins at as low as 500 °C in a H 2 atmosphere. Incorporating carbon-containing iron ore composite pellets mitigates locally low reduction rates from uneven H 2 distribution in hydrogen-based shaft furnaces. The synergistic reduction of composite pellets with different C/O ratios in a H 2 atmosphere is studied through gas analysis, phase evolution, and microstructural characterization. Composite pellet reduction accelerates markedly from the Fe 3 O 4 → 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 H 2 utilization rate, up to a maximum of 27% higher than carbon-free pellets, but reduce the proportion of reduction contributed by H 2 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.