Computational fluid dynamics simulation of effect of methane volume fraction on shaft furnace reduction, carburizing and cooling process

To address the low-carbon transition needs of the iron and steel industry, hydrogen-based shaft furnace process is considered a potential solution for low-carbon ironmaking. Based on the ENERGIRON process, a two-dimensional numerical simulation model of a hydrogen-based shaft furnace was established using computational fluid dynamics (CFD), comprehensively considering gas-solid flow, heat and mass transfer, and key reactions, including iron oxide reduction, methane reforming, and carburization. The model is validated against plant data. The effects of CH₄ volume fraction in the reducing gas and cooling gas, as well as cooling gas flow rate, on the reduction, carburization, and cooling performance of the shaft furnace were systematically investigated. Simulation results show that increasing the reducing gas CH₄ volume fraction from 2% to 20% intensifies methane reforming and carburization reactions, increasing the total endothermic reaction heat by 54.3%. Although this raises the direct reduced iron(DRI)carbon mass fraction from 0.004 to 0.020, the metallization degree decreases from 0.949 to 0.823. By increasing the temperature and flow rate to raise the sensible heat of the reducing gas by 19.5%, the heat loss caused by the higher CH₄ volume fraction can be effectively compensated, increasing the DRI metallization degree to 0.946 and achieving synergy between high metallization and high carbon content. When natural gas is used as the cooling gas, it exhibits superior carburization and cooling performance, while coke oven gas is more favorable for maintaining a high metallization degree. Increasing the cooling gas flow rate continuously decreases both the DRI metallization degree and gas utilization degree. Considering only the reduction and carburization effects of DRI, the suitable cooling gas flow rate is 10 312.5 m³/h for natural gas and 20 625 m³/h for coke oven gas. If DRI needs to be cooled to 313 K, the minimum required flow rates are 30 937.5 m³/h for natural gas and 51 562.5 m³/h for coke oven gas. These findings provide a quantitative theoretical basis for selecting and optimizing gas composition and flow parameters in hydrogen-based shaft furnace operations, enabling the customization of DRI with different metallization degrees and carbon contents as required.