Reactivity and structural evolution of metallurgical coke in hydrogen enriched Blast furnace conditions

<p dir="ltr">The reduction of CO2 emissions from blast furnace (BF) operations is critical to meet decarbonisation targets in the steelmaking sector. Introducing hydrogen gas into the BF displacing pulverised coal or coke is a promising solution to decrease the carbon usage of BF ironmaking because it generates H2O instead of CO2 from the reduction of the ferrous burden. However, replacing pulverised coal and coke with hydrogen can increase the concentration of H2O and change the thermal and chemical conditions in the furnace. These changes affect the gasification reaction rate and mechanism, as well as the structural evolution and degradation of coke.</p><p dir="ltr">This research aimed to evaluate the impact of using H2 in BFs on coke reactivity, reaction mechanisms, structural evolution, and degradation, which is essential for understanding coke quality requirements in low-carbon ironmaking processes. Using thermogravimetric analysis, kinetic modelling, micro-CT imaging, compression testing, fractography analysis, and Pearson microtexture analysis, the study provides a comprehensive assessment of coke performance under H₂-rich conditions. </p><p dir="ltr">According to thermogravimetric and kinetic modelling results presented in this thesis, the gasification of coke with H2O is significantly faster, up to 4.7 times, and has an effective diffusion coefficient up to six times greater than with CO2. CO2 gasification produces a more uniform reaction throughout the coke lump, with the chemical reaction being the primary mechanism. In contrast, H2O gasification, diffusion dominates due to its higher reaction rate with carbon, leading to increased carbon conversion at the surface of the coke lumps and a gradient of reaction, with the core remaining less reacted. </p><p dir="ltr">When considering the gaseous compositions of conventional blast furnace operating environments, and modelled gaseous compositions in a blast furnace with hydrogen injection (H2-rich environment), differing behaviour is also yielded. In an H₂-rich environment, coke reactivity is up to 1.5 times higher compared to conventional conditions. This increased reactivity is due to the higher reaction rate of H₂O with carbon. Therefore, using less reactive coke in H₂-rich conditions can slow the reaction rate and reduce damage to low CRI coke above the tuyere level, emphasising the need for low-reactivity coke to mitigate damage in these highly reactive environments. Additionally, in conventional conditions, chemical reactions are evenly distributed throughout the coke lump. In contrast, in H₂-rich environments at high temperatures (e.g., 1473 K), diffusion predominates, leading to higher carbon conversion near the surface and a less-reacted core. This effect is more pronounced in low CRI coke, which has reduced porosity and slower gas diffusion.</p><p dir="ltr">Micro-CT image analysis supports the kinetic modelling results for both conventional and H2-rich conditions. In H2-rich environments, the difference in carbon conversion between the coke surface and centre is more significant than in conventional conditions. Carbon conversion at the surface is about 2.6 times higher than at the centre in H2-rich environments, compared to roughly 1.9 times in conventional conditions.</p><p dir="ltr">Tensile strength testing and fractography analysis show that reaction mechanisms affect coke degradation differently in conventional and H2-rich environments. Conventional conditions lead to uniform weakening and middle breakage, resulting in varied fragment sizes that can impair furnace performance. In contrast, H2-rich conditions cause edge breakage due to concentrated surface reactions, producing smaller fragments. Low CRI coke maintains higher tensile strength and a stronger core in H2-rich environments, while high CRI coke weakens uniformly across its structure, regardless of the reaction conditions. This highlights the advantage of low CRI coke in preserving core strength and maintaining furnace permeability under high reactivity conditions.</p><p dir="ltr">Furthermore, high CRI coke shows greater bireflectance under conventional conditions than in H2-rich environments. In contrast, low CRI coke, which has a more anisotropic carbon structure, displays similar bireflectance values in both environments. This difference arises because high CRI coke reacts more selectively with isotropic carbon structures in conventional conditions, while low CRI coke is less selective towards either isotropic or anisotropic carbon structures in both environments.</p><p dir="ltr">To conclude, high-quality coke (low CRI) maintains better structural integrity in H2-rich environments, which is essential for efficient BF operations. In contrast, low-quality coke (high CRI) degrades more easily, potentially impacting furnace performance and stability. Thus, the study highlights that careful coke selection is necessary when transitioning to H2-rich environments to ensure optimal performance and avoid excessive coke degradation. High-quality coke is one of the key factors enabling the successful injection of H2 into BFs.</p>