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Coke reactivity with CO2 and H2O and impacts on coke microstructure and gas diffusion

Technical Market Support » Metallurgical Coal

Published: June 24Project Number: C34059

Get ReportAuthor: Ai Wang, Salman Khoshk Rish, David Jenkins, Subhasish Mitra, Arash Tahmasebi, Mark Knackstedt, Lydia Knuefing | The University of Newcastle, Australian National University

With the global shift to low-carbon ironmaking, partial substitution of coke and PCI with hydrogen in the blast furnace is one of the most promising blast furnace decarbonising solutions. In a hydrogen-enriched blast furnace, injected hydrogen reduces the ferrous burden while generating H₂O, decreasing fossil carbon input, and lowering CO₂ emissions. The altered reaction environment in the hydrogen blast furnace affects the reactivity and structural evolution of coke, thus influencing coke quality requirements.

The objective of this project was to examine the mechanism of coke gasification and degradation in CO₂ and H₂O. A combination of novel experimental tests, CT imaging and analysis, and innovative reaction-diffusion modelling methods were developed to investigate the reaction rate, microstructure and microtexture evolution, and the balance between local carbon reactivity and gas diffusivity during gasification of cokes of varying quality under CO₂ and H₂O.

Novel micro-CT image processing algorithms were developed to register pre- and post-reaction images to examine local microstructural changes, to link the change in grayscale values to mass loss, and to quantify the mass loss profiles across coke radius in 3D using an onion-skin analysis technique. In addition, a reaction-diffusion model was developed to determine the local carbon reaction and gas diffusivity rates. Modelling results were used to examine the balance between local carbon reaction and gas diffusivity as the dominant rate-controlling steps during coke reaction with CO₂ and H₂O.

The TGA gasification test results showed that the average mass loss rate in H₂O was nearly 4 times that of CO₂. During H₂O gasification, the reaction time to achieve a set conversion level was shorter than CO₂ for all samples tested. This was attributed to the higher reaction rate constant and diffusivity of H₂O. The average initial reaction rate constant in H₂O was nearly 3.5 times of CO₂, while the initial diffusivity of H₂O was nearly double that of CO₂. In general, higher CSR cokes showed a lower reactivity than lower CSR cokes, however, this difference was smaller under H₂O.

The micro-CT image analysis of unreacted and partially gasified cokes showed that compared with CO₂, reaction with H₂O led to a greater mass loss near the surface of the coke. Although the initial coke quality influenced the time required to reach a certain conversion and the temporal profiles of mass loss, it had a limited influence on radial mass loss profiles.

The effective diffusivity of CO₂ and H₂O increased with the progression of gasification due to the generation of new porosity. An opposite trend was observed for coke reactivity, which decreased with mass loss. However, the both diffusivity and reactivity of H₂O remained greater than CO₂ and the difference in gas diffusivity increased at higher conversions.

A dimensionless parameter, Thiele modulus, was used to evaluate the roles of local carbon reactivity and gas diffusion in coke gasification. Results show that there is a balance between reaction and diffusion as the rate-controlling step. The Thiele modulus values for H₂O gasification were on average 1.75 times greater than CO₂, suggesting that the internal gas diffusion plays a somewhat lesser role and the local reactivity with carbon plays a more dominant role during H₂O gasification.

The gasification behaviour was simulated of a 50 mm BF coke feed lump using the developed model and found that for these large lumps, the overall mass loss rate in H₂O was nearly 2.5 times that of CO₂. Model results show that the impact of gas type on radial mass loss profiles was greater than the initial coke quality. Larger lumps also showed greater surface mass loss during H₂O gasification, which led to the shrinkage of coke at above 40% mass loss due to complete carbon conversion of carbon at the outer surface of coke. Results suggest that reaction with H₂O would eat away the surface of a 50 mm lump, much more than reaction with CO₂.

Coke microtexture analysis showed a higher bireflectance value for high CSR cokes, indicating a higher carbon anisotropy and structural ordering which is understood to show a lower gasification reactivity. The anisotropy of coke lumps increased with the progression of gasification, suggesting that low bireflectance isotropic carbon forms were preferentially reacted at the early stages of gasification mass loss.

New knowledge on the mechanism of reactivity and degradation of metallurgical coke CO₂ and H₂O was achieved in this research. The project allowed an opportunity to develop unique experimental and modelling tools to evaluate coke reactivity.

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