## 4 An Improved HDRSF Process with Waste Heat Recovery  
Under typical conditions, to ensure sufficient reduction reactions in the shaft furnace and achieve the required metallization rate of DRI, the amount of reducing gas introduced into the furnace often exceeds the theoretical demand for the reduction process. This leads to a significant amount of unreacted H\({}_{2}\) and CO in the top gas. For economic reasons, it is essential to separate and recover the unreacted reducing gas from the top gas. However, as the top gas is typically above 300 \({}^{\circ}\)C, it must be cooled and dehydrated before purification. This approach considers practical factors such as the technical feasibility and cost-effectiveness of gas separation and purification processes [45; 46]. In addition to the required endothermic nature of chemical reactions and irreversible heat losses that cannot be recovered, the residual heat carried by the top gas exiting the reduction section and the cooling gas leaving the cooling section can be recovered.  
In order to achieve the energy efficiency and consumption reduction of the HDRSF process, an improved HDRSF production process with waste heat recovery is proposed. Inthe proposed approach, the waste heat of top gas and discharged high-temperature cooling gas is recycled in the HDRSF process, and the surplus reducing gas in the top gas is also purified for recycling. The flow of the improved HDRSF process with waste heat recovery is shown in Figure 4. For the goal of energy efficiency and consumption reduction, the following two main enhancements are implemented:  
1. The top gas first enters the heat exchanger to preheat the reducing gas before entering the heating furnace so as to recover and utilize the waste heat of the top gas and reduce the energy consumption during the heating process of the reducing gas in the electric heating furnace.
2. The discharged high-temperature cooling gas is used for preheating the iron ore entering the furnace, thereby realizing recovery of its waste heat and increasing the heat of the iron ore entering the furnace. According to Equation (18), it is evident that increasing the heat content of iron ore entering the furnace can reduce the external energy input required by the shaft furnace, accordingly decreasing the amount of reducing gas needed.  
The improved HDRSF process with waste heat recovery operates as follows: External supplementary reducing gas is primarily heated by transferring heat with the top gas. It is then combined with the reducing gas separated from the top gas after undergoing heat transfer, cooling, dehydration, purification, and pressurization processes. Subsequently, the mixed gas is heated to the required process temperature in an electric heating furnace before entering the shaft furnace for iron ore reduction, completing the gas circulation in the reduction section of the shaft furnace. In the cooling section, cooling gas enters from the bottom of the shaft furnace and is discharged from the upper part. The high-temperature cooling gas discharged from the shaft furnace first enters a waste heat recovery device to preheat the iron ore before it enters the furnace. After cooling, purification, and pressurization, the cooling gas returns to the furnace to complete the circulation process. A gas-water heat exchanger is utilized to cool the top gas before dehydration and entry into the furnace, ensuring continuous process operation. The cooling gas after preheating the iron ore is also cooled again by a gas-water heat exchanger. After exiting the gas-water  
Figure 4: The flow of the improved HDRSF process with waste heat recovery.  
heat exchanger, the cooling water is cooled in a cooling tower and then returns to the heat exchanger to complete the circulation.