Two-Dimensional CFD Study of Carburization and Carbon Partitioning in an ENERGIRON ZR Shaft Furnace

This study develops a two-dimensional computational fluid dynamics model for the reactive zone of an industrial-scale shaft furnace operated under ENERGIRON ZR (zero-reforming) conditions, where carbon in direct reduced iron (DRI) is explicitly distinguished into combined carbon in Fe3C and free carbon in graphitic form. The model is validated against available plant data and then applied to investigate the effects of reducing gas temperature, gas composition, and gas feed rate on reduction, carburization, and carbon partitioning. The results show that in situ reforming, iron oxide reduction, and carburization are strongly coupled near the gas inlet. Increasing the reducing gas temperature from 1273 K to 1373 K raises the metallization degree from 0.9426 to 1.000 and the total carbon mass fraction from 0.01722 to 0.04938, while decreasing the combined carbon fraction from 97.6% to 86.3% because of enhanced Fe3C decomposition. The effect of CH4 content is temperature-dependent: at 1273 K and 1323 K, increasing CH4 from 15% to 25% decreases both metallization and total carbon because intensified endothermic reforming lowers the in-furnace thermal level, whereas at 1373 K the total carbon changes from 0.04849 to 0.04938 and then to 0.04179, reflecting a shift in the controlling factor from CH4 availability to thermal limitation. Increasing gas feed rate from 1400 Nm3/t-pellet to 1600 Nm3/t-pellet improves both reduction and beneficial carburization, with the total carbon mass fraction increasing from 0.02360 to 0.04047, while the combined carbon fraction decreases slightly from 93.9% to 92.5%. The predicted carbon partitioning results also show qualitative agreement with the limited industrial data, particularly the decreasing combined carbon fraction with increasing total carbon content in DRI.