Microstructural and microtextural evolution of metallurgical coke during reaction with CO₂ and H₂O

Abstract

The introduction of hydrogen into blast furnace is a promising solution to reduce the carbon intensity of ironmaking, however, it generates excess steam during the reduction of ferrous burden. Coke degradation behaviour in H2O is not fully understood and requires in-depth quantitative investigation to improve the fundamental knowledge of underlying mechanisms. To address these knowledge gaps, the gasification behaviour of partially gasified cokes was investigated in CO2 and H2O. The evolution of coke microstructure and microtexture was investigated using micro-computed tomography (CT) imaging and coke bireflectance analysis. New image processing algorithms were developed to locate coke reaction sites and quantify the radial mass loss. The thermogravimetric analysis (TGA) results showed that the average mass loss rate of coke in H2O was 3.8 times greater than CO2. Mass loss near the surface of the coke was greater during the reaction with H2O, while a more uniform radial mass loss was observed in CO2. Greater mass loss near the surface indicates that the balance between local reaction rate and pore diffusion tends more towards diffusion control in H2O. Higher carbon anisotropy and structural ordering of coke with higher Coke Strength after Reaction (CSR) were correlated with its lower gasification reactivity. Low bireflectance isotropic carbon forms were preferentially reacted at the early stages of gasification mass loss. CO2 showed a preferential reactivity with the isotropic inert-derived maceral components at the early stages of the reaction, while H2O reacted relatively non-selectivity with the isotropic inert-derived maceral components and the reactive maceral-derived components.