Enhancing Direct Reduced Iron (DRI) for Use in Electric Steelmaking(2)
Enhancing DRI's value-in-use
DRI Preheating
Advantages of charging DRI hot coming out of the shaft reactor have been well documented, namely productivity increase and decrease in power use. Few DRI plants in the world have the ideal setup with an EAF downstream of the DRI plant, where the DRI coming out of the shaft furnace can be charged hot into the EAF. However, a significant number of steelmaking plants that use DRI receive it at ambient temperature, thus reducing its potential value-inuse. Preheating the DRI presents a way for such operations to increase this value-in-use of DRI before it is charged into the furnace. Air Products’ DRI preheating process is shown in Figure 2. In this embodiment of the process, DRI is preheated using oxy-fuel burners on a conveyor belt before charging into the furnace. The end section of the transport conveyor is envisioned to be converted to a refractory-walled tunnel housing the oxy-fuel burners. Oxy-fuel combustion pertains to combustion of fuel in presence of pure oxygen as the oxidizer. In contrary to air-fuel combustion, in oxy-fuel combustion nitrogen is not present to take away the heat of combustion through the flue gases. Thus, with oxy-fuel combustion, more heat is available to the product, increasing the efficiency and achievable temperature. Oxy-fuel combustion is effectively used in EAFs to supplement the electric energy for melting steel, as well as in the glass industry for melting glass.
Various suggested parameters used in the process are described in Table 2. Additional CO2 impact of combustion is estimated to be insignificant at 0.02 MT/MT of DRI. Calculations suggest that addition of this preheating process for an EAF mill charging cold DRI can lead to $2.3/liq. ton savings for the mill after considering improved fixed cost utilization and operating costs.
Currently this process has been evaluated for feasibility and applicability of oxy-fuel burners for preheating DRI. For example, Figure 3 shows results from single pellet preheating experiments conducted at Air Products combustion labs. Significant preheating temperatures >800 ˚C (Figure 3b) can be achieved at single pellet level with insignificant re-oxidation of the pellets (Figure 3c). Currently further evaluation is underway with multi-layered pellet stacks simulating the charge load to be preheated on a conveyor belt. Multiple layers do present some challenges with heat transfer from the burners but can be overcome using increased momentum on the burners, distribution of the pellets, and effective recirculation of product gases of combustion. Next steps involve scaling up the experiments to prototype scale and a field trial at an EAF mill.
