Thermodynamic insights and equilibrium phase diagrams of carbon deposition and carburization on iron surfaces in C–H–O system
摘要
The direct reduction process can reduce carbon emissions by over 50% compared to traditional blast furnace ironmaking. Carbon deposition and carburization are critical for ensuring process stability and economic viability. Thermodynamic phase diagrams were developed to intuitively represent carbon deposition and carburization preferences in CH4–CO–H2 ternary atmospheres. High carbon potential and low oxygen potential significantly enhance carbon deposition and carburization. Increasing temperature from 500 to 1000 °C shifts the dominant reactions from CO-based to CH4-based, increasing maximum carbon deposition from 0.55 to 0.80 mol and carburization from 0.25 to 0.80 mol per mole of reducing gas. Increasing pressure suppresses CH4-based reactions while promoting CO-based reactions, reducing maximum carbon deposition from 0.8 to ~ 0.7 mol and increasing maximum carburization from 0.80 to 0.85 mol per mole of reducing gas. Equilibrium phase diagrams for various carbides were also developed, revealing preferences for Fe3C2, Fe7C3, Fe5C2, and Fe3C as the Fe/C ratio increases. Higher temperatures and CH4 concentrations favor the formation of carbides with higher carbon content. Carburization preferences under typical Energiron ZR and Midrex atmospheres were highlighted, and the higher carbon content in direct reduction iron produced by the Energiron ZR process was thermodynamically confirmed.