Influence of hydrogen and carbon monoxide on reduction behavior of iron oxide at high temperature: Effect on reduction gas concentrations

The purposes of this study are to reduce Fe2O3 using hydrogen (H-2) and carbon monoxide (CO) gases at a high temperature zone (500 degrees C-900 degrees C) by focusing on the influence of reduction gas concentrations. Reduction behavior of hematite (Fe2O3) at high temperature was examined using temperature programmed reduction (TPR) under non-isothermal conditions with the presence of 10% H-2/N-2, 20% H-2/N-2, 10% CO/N-2, 20% CO/N-2 and 40% CO/N-2. The TPRCO results indicated that the first and second reduction peaks of Fe2O3 at a temperature below 660 degrees C appeared rapidly when compared to TPRH2. However, TPRH2 exhibited a better reduction in which Fe2O3 entirely reduced to Fe at temperature 800 degrees C (20% H-2) without any remaining of wustite (FeO) whereas a temperature above 900 degrees C is needed for a complete reductionin 10% H-2/N-2, 10% and 20% CO/N-2. Furthermore, the reduction of hematite could be improved when increasing CO and H-2 concentrations since reduction profiles were shifted to a lower temperature. Thermodynamic calculation has shown that enthalpy change of reaction (Delta Hr) for all phases transformation in CO atmosphere is significantly lower than in H-2. This disclosed that CO is the best reductant as it is a more exothermic, more spontaneous reaction and able to initiate the reduction at a much lower temperature than H-2 atmosphere. Nevertheless, the reduction of hematite using CO completed at a temperature slightly higher compared to H-2. It is due to the presence of an additional carburization reaction which is a phase transformation of wustite to iron carbide (FeO -> Fe3C). Carburization started at the end of the second stage reduction at 600 degrees C and 630 degrees C under 20% and 40% CO, respectively. Therefore, reduction by CO encouraged the formation of carbide, slower the reduction and completed at high temperature. XRD analysis disclosed the formation of austenite during the final stage of a reduction under further exposure with high CO concentration. Overall, less energy consumption needed during the first and second stages of reduction by CO, the formation of iron carbide and austenite were enhanced with the presence of higher CO concentration. Meanwhile, H-2 has stimulated the formation of puremetallic iron(Fe), completed the reduction faster, considered as the strongest reducing agent than CO and these are effective at a higher temperature. Proposed iron phase transformation under different reducing agent concentrations are as followed: (a) 10% H-2, 20% H-2 and 10% C; Fe2O3 -> Fe3O4 -> FeO -> Fe, (b) 20% CO; Fe2O3 -> Fe3O4 -> FeO -> Fe3C -> Fe and(c) 40% CO; Fe2O3 -> Fe3O4 -> FeO -> Fe3C -> Fe -> F,C (austenite). (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Automated summary

This study investigated the reduction behavior of hematite at high temperatures using different gas concentrations, revealing that CO is a more efficient reductant than H2 as it can initiate reduction at lower temperatures. However, H2 is faster in completing the reduction process at higher temperatures due to its stronger reducing ability.