Thermodynamic calculation of metallic Fe yield and CO2 emissions in gas-based shaft furnace direct reduction process

Under the "dual-carbon" goals background, the Chinese iron and steel industry urgently needs to undergo a low-carbon transformation. Currently, promoting the substitution of coke oven gas (COG) for natural gas to develop shaft furnaces is a crucial initiative for implementing sustainable development. Through thermodynamic calculations, the iron yield and CO₂ emissions of the direct reduction process in the shaft furnace were systematically analyzed. A thermodynamic calculation based on mass balance and heat balance was established to clarify the effects of key parameters [total iron content (TFe) of iron ore, metallization degree (MD), and generation temperature of metallic Fe (T_MFe)] on the process performance. The thermodynamic calculation results show that, (1) Heat balance dominates the total carbon consumption. (2) Enhancing TFe is one of the most effective strategies to improve the yield of metallic Fe (Y_MFe) and reduce CO₂ emissions. Taking 100 mol of COG as the calculation basis, under the conditions of MD = 98% and T_MFe = 900 °C, increasing TFe from 45 to 70%, raises Y_MFe from 1.38 to 2.24 kg, while CO₂ emissions per ton of metallic Fe (t-MFe) decrease from 1200.04 to 740.08 kg. Furthermore, MD and T_MFe exert significant regulatory effects on both Y_MFe and CO₂ emissions. (3) Under the conditions of TFe = 66%, T_MFe = 900 °C, and MD = 94%, the consumption of COG for direct reduction is 838.38 m³/t-DRI. For a coke oven with a production capacity of 1.2 million tons, with gas production of 53,736 × 10⁴ m³/a, the generated COG can support the shaft furnace to achieve an annual Y_MFe of 48.77 × 10⁴ t and CO₂ emissions of 41.64 × 10⁴ t. When the top gas is recycled as fuel of the heating furnace, the heat consumption of the heating furnace can be supplemented by no more than 20% of the top gas. The remaining 80% of the top gas can be recycled after the CO₂ capture, which can significantly reduce the production costs, carbon consumption, and CO₂ emissions. The present work may provide the theoretical guidance for choosing optimal parameters of shaft furnace process in China.