Gasification Kinetics of Coke Lumps under Simulated Conventional and H2-Rich Blast Furnace Processes

The introduction of hydrogen-rich fuels into blast furnace (BF) is identified as a promising approach to reducing the carbon footprint of ironmaking. In this process, injected hydrogen promotes the indirect reduction of the ferrous burden generating excess H2O in the process. Partial replacement of CO/CO₂ with H₂/H₂O is expected to alter the gasification rate and coke degradation mechanism as steam is known as a potent gasifying agent.

The overall objective of this project was to improve the fundamental understanding of the rate and mechanism of coke gasification under the hydrogenenriched BF environment, benchmarked against conventional reaction conditions. This was achieved by conducting a series of tests to obtain gasification kinetics of coke lumps under a controlled CO₂/CO/H₂/H₂O/N₂ atmosphere in the temperature range of 900-1200 °C, simulating the region above the cohesive zone of BF, where coke gasification is dominant.

A novel experimental test facility was developed to investigate the gasification reactivity of coke lumps under simulated conventional and H₂-rich BF environments. In addition, a modified random pore model (RPM) was developed to study the impact of reaction conditions, initial coke quality, and process parameters such as temperature on the mechanism of coke reactivity and degradation. The developed model took both pore diffusion and interfacial reaction for determining the reaction mechanism; hence, it was an improvement on the previous models reported in the literature.

Three pilot oven cokes were examined in this project, which were generated from coal blends, the blend components of which were sourced from different Australian coal measures. The ash chemistry of the coal blends was controlled at a consistent level, therefore, the variations observed in the gasification reactivity of the cokes were related to the differences in their microstructural and carbon structural properties.

The experimental and modelling work was conducted in two stages.

• The gasification behaviour of the coke samples was investigated in single gas reaction conditions, i.e., in CO₂ and H₂O to isolate the impact of the two gases and to develop a base kinetics model.
• The reactivity of cokes was investigated under simulated conventional and H₂-rich BF reaction conditions by controlling CO₂/CO/H₂/H₂O/N₂ concentrations in the reaction gas mixture. In this stage, the combined impacts of CO₂ and H₂O on the coke reactivity were analysed and a more complex kinetics model was developed.

Results showed that the reaction rate of coke with H₂O was up to 3.7 times faster than that of CO₂. Moreover, the effective diffusion coefficient of H₂O was up to 6 times greater than CO₂. The combination of a higher reaction rate and diffusion coefficient of H₂O significantly reduced the time required to achieve the complete conversion of coke lumps. This effect was exacerbated at elevated temperatures, in particular at above 1100 °C. The reaction temperature was also found to greatly influence the rate-controlling mechanism during gasification, where a shift from chemical reaction-controlled to diffusion-controlled reaction occurred at above 1100 °C.

Moreover, the concentration of H₂O in the reaction gas and the initial coke CSR were found to greatly influence the mechanism of reactivity and degradation during gasification under conventional and H₂-rich BF conditions. Higher H₂O concentrations promoted reaction on the outer surface of coke lumps, while the core remained less consumed. Conversely, gasification under conventional BF conditions took place more volumetrically throughout the porous structure of coke. Using this observation, we postulate that the presence of excess H₂O in an H₂-rich BF shifts the mechanism of coke gasification from chemical-controlled to diffusion-controlled, where due to the significantly higher reaction rate, the diffusion of reactant to the interior pore structure of coke becomes the rate-limiting factor. The consequence is that under H₂-rich BF reaction conditions, coke degrades more at the outer layers while maintaining a stronger core.

Results showed that the surface-promoted reaction in the presence of excess steam was more pronounced in higher CSR cokes, suggesting that with increasing the initial coke quality, gas diffusion becomes the rate-limiting factor. In contrast, lower CSR cokes were found to react and degrade more volumetrically. These observations suggest that a higher CSR coke may be required for an H₂-rich BF operation to minimise the structural degradation and that a high CSR coke may have a performance advantage under H₂-rich BF conditions.

New knowledge on the mechanism of reactivity and degradation of metallurgical coke under conventional and H₂-rich BF reaction conditions has been generated. The project allowed an opportunity to develop unique experimental and modelling tools to evaluate coke reactivity under different blast furnace operation conditions.