Thermodynamic analysis and life cycle assessment of an integrated coal-based supercritical water gasification and direct reduction iron system

The green transition of China’s steel industry is driving an urgent demand for direct reduction technology. However, the current mainstream gas-based shaft furnace process heavily relies on natural gas, creating a fundamental mismatch with China’s coal-dominated energy structure. To address this challenge, this study proposes an innovative coal-based integrated system for direct reduced iron production by coupling supercritical water gasification with a shaft furnace. The system employs a multi-stage heat exchange network and organic Rankine cycle technology to hierarchically recover waste heat from flue gas and top gas, thereby significantly reducing overall energy consumption. Exergy analysis reveals that the oxidation reactor, hydrothermal heat exchanger, and pressure swing adsorption hydrogen separation unit are the primary sources of exergy destruction, together accounting for approximately half of the total. Sensitivity analysis indicates that thermodynamic performance is optimized at a gasification temperature of 650 °C, coal concentration of 17.5%, and coal-water slurry preheating temperature of 200 °C, achieving energy and exergy efficiencies of 59.9% and 50.0%, respectively. By applying carbon capture and storage at the syngas purification stage and before top gas recycling, the global warming potential of the system drops sharply from 1600 kg CO₂ eq./t DRI to 814 kg CO₂ eq./t DRI under optimal conditions. Compared to conventional blast furnace ironmaking, the proposed system reduces the energy consumption per unit product, exergy loss per unit product, and global warming potential by 40.4%, 31.6%, and 37.5%, respectively. This work presents a viable pathway for steel production that supports both energy self-sufficiency and low-carbon transformation in coal-rich regions.