Effect of annealing temperatures and pressure on the evolution of structure and optical texture of coke in the blast furnace

Current coke quality evaluation relies predominantly on the Nippon Steel Corporation test method. However, its simplified experimental conditions cannot fully reproduce the complex environment of the blast furnace (BF), thereby limiting the reliability of coke quality assessment. In this paper, two metallurgical cokes (Coke A (CA) and Coke B (CB)) with different blending ratios were selected for comparison, of which CA had a higher proportion of high-quality coking coal. The effects of annealing temperatures and pressure on the degradation behaviour and evolution of pore structure, crystalline structure, and optical texture of cokes were investigated using a self-developed Si–Mo furnace, optical microscopy, and X-ray diffraction. The results indicate that coke degradation exhibits various characteristics in different BF zones. In the lump zone (1000 °C–1100 °C), coke undergoes limited weight loss and particle-size reduction while maintaining relatively low gasification reactivity. The cohesive zone (1200 °C–1300 °C) represents a critical transition stage for coke degradation, during which gasification rate accelerates markedly. In the dripping zone (>1300 °C), coke degradation continues to intensify. Furthermore, the inherent structural differences between the two cokes lead to distinct degradation behaviours. Compared with CB, CA has a lower porosity, smaller equivalent pore diameter, and a higher proportion of high-order anisotropic optical textures. These features are expected to restrict CO 2 diffusion and reduce the effective carbon–CO 2 reaction area, thereby limiting carbon consumption. Additionally, at 1600 °C, cokes exhibit greater weight loss and particle-size degradation under the pressurised condition than under the corresponding non-pressurised condition. Based on these findings, it is proposed to consider preferentially distributing CB near the furnace walls and to concentrate CA in the central region to balance gas permeability and supporting capacity of the burden. The study provides valuable insights for evaluating coke quality and optimising coke utilisation in low-carbon ironmaking.