<p>The full-oxygen blast furnace (OBF) has the potential to mitigate CO₂ emissions but suffers from the “overheating” issue in the lower furnace. This paper presents a systematic study of the hearth gas injection for a 380-m³ industrial OBF to overcome this problem by using a 3D multi-fluid BF process model. The effects of four key variables, <i>i.e.</i>, hearth gas injection rate, hearth gas injection temperature, CO₂ content in the hearth injected gas, and allocation ratio between hearth injection and shaft injection, have been quantified under fixed hot metal (HM) temperature. The results show that (1) the fuel rate first decreases to a minimum and then dramatically increases with increasing hearth gas injection rate. This is because under low hearth gas injection rates, increased gas injection rate enhances the indirect reduction of iron ore in the lumpy zone. However, excessive gas injection rate leads to enhanced heat dissipation through top gas and furnace wall and increased carbon consumption by chemical reactions. (2) While increasing hearth gas injection temperature reduces the fuel rate, excessively high gas injection temperature lowers the upper furnace temperature, thereby suppressing indirect reduction in the lumpy zone and increasing the direct reduction degree. (3) Increased CO₂ content in hearth injected gas substantially raises the fuel rate due to the thermal and carbon consumption during CO₂ decomposition in raceways. Although injecting CO₂ into the hearth can reduce the theoretical flame temperature, excessive CO₂ content may cause “overheating” in the upper furnace. (4) The hearth allocation ratio has limited influences on fuel rate but significantly affects the in-furnace states. A higher hearth allocation ratio simultaneously addresses the “overcooling top” and “overheating bottom” issues in OBF. In contrast, a higher shaft allocation ratio only mitigates “overcooling top” issue while lowering the CZ position. These findings should be helpful to the industrial OBF operation.</p>