An improved CFD study of direct reduction shaft furnace for low-carbon ironmaking

The direct reduction shaft furnace (DR-SF) process is a key alternative technology for ironmaking decarbonisation, however, the in-furnace phenomena remain unclear. In this work, a two-dimensional axisymmetric steady-state SF model based on the two-fluid model (TFM) framework is developed and improved by considering reversible homogeneous reactions. It is validated against industrial plant data. Then, the simulation captures the key smelting behaviours, including the gas flow pattern, temperature field, and species distribution, as well as iron oxides reduction and carburisation processes. The cases with and without reversible homogeneous reactions are compared, indicating that neglecting reversible reactions may be misleading in predicting carbon content, with deviations exceeding 15%. Additionally, the effect of the flow rate at the cooling gas outlet (FR-CGO) on the SF internal state and DRI quality is studied and quantified. The simulation results show that (1) the FR-CGO increase suppresses the space available for its uniform upward flow, by 4.08% for every 1% increase in the proportion of FR-CGO to the flow rate at the cooling gas inlet (FR-CGI); (2) when 96% of cooling gas flows out from the cooling gas outlet, the gas flow pattern will be significantly altered, leading to mixing of the cooling gas and reducing gas, which is undesired and unfavourable for the cooling gas cycle system; and (3) the metallisation rate and carbon content of DRI can be regulated by adjusting the FR-CGO, with increasing by 0.94% and 0.21%, respectively, as the proportion of FR-CGO to FR-CGI rises by 1% from 86% to 95%. This study reports an improved cost-effective tool capable of studying the complex in-reactor behaviours and provide SF process optimisation, advancing low-carbon ironmaking.