Mechanism of metallurgical coke gasification in CO2 and H2O using a reaction-diffusion model

• A reaction-diffusion model incorporating CT data was developed for coke reactivity. • Cokes reacted with H 2 O showed higher reactivity and effective diffusivity than CO 2 . • Higher Thiele modulus explains promoted surface reaction with H 2 O compared to CO 2 . • The model was used to predict the reaction mechanism of blast furnace feed size coke. Hydrogen-enriched blast furnace can lower CO 2 emissions in ironmaking, however, H 2 O generated during the process may react deleteriously with metallurgical coke, leading to accelerated structural degradation. A transient reaction-diffusion model was developed to investigate the gasification mechanism of cokes of varying strength, in either CO 2 or H 2 O reaction environments. The model was validated by experimental mass loss over time produced using TGA and over radius measured using micro-CT imaging techniques. This led to improved accuracy compared with previous coke gasification models in the literature. Coke mass loss rate in H 2 O was significantly higher than CO 2 , and the reaction rate was lower for cokes with higher strength, regardless of gas type. When coke was reacted with CO 2 , the mass loss occurred more uniformly with depth into lumps, while in H 2 O a greater mass loss was observed near the coke surface. The reaction rate constant in H 2 O was typically 3.5 times greater than CO 2 . The initial effective diffusivity of H 2 O was also higher than CO 2 (approximately 1.9 times). Compared to CO 2 , during the coke reaction with H 2 O, the local reactivity of carbon plays a more dominant role than gas diffusion. The developed model was used to predict the gasification behaviour of a 50 mm lump. Compared with CO 2 , greater mass loss at the surface of larger lumps in H 2 O led to shrinkage in size, while the core of the lumps remained less reacted.