Factors Underpinning the Gasification Reactivity of Coke RMDC and IMDC with CO2

It is well known that the gasification reactivity of metallurgical coke is inversely correlated with its strength. The reactivity of coke is a key contributory factor to its integrity in the ironmaking blast furnace. Previous studies have generally indicated that under CSR test conditions, the coke inert maceral derived components (IMDC) show a higher rate of gasification than the reactive maceral derived components (RMDC); however, not all of the studies published in the literature concur.

In this project, a series of experiments was carried out to examine the factors that influence the CO₂ gasification reactivity of the individual IMDC and RMDC components of three Australian coal samples, including:

• Parent inertinite types;
• Degree of microtextural anisotropy;
• Accessibility of the IMDC;
• Ash chemistry.

One pair of head coals were matched in rank but differed in their ash chemistry and inertinite content, and the other pair of coals were matched in basicity index but differed in their rank. This enabled linkage of coke gasification kinetics to parent coal attributes.

Individual IMDC and RMDC components were formed by coking ~1.5g (per test) of inertinite group concentrate (IC) and vitrinite group concentrate (VC), respectively. Maceral group concentrates were collected using a hand-picking technique in an oxygen-free environment. Cokes were also formed from head coals and by using different proportions of IC and VC in the coking blend.

For each experiment, the kinetics of the reaction of the coke with CO₂ under CSR test conditions was measured using thermogravimetric analysis (TGA). Results were linked to coke microtextural attributes before and after reaction with CO₂ to 40% carbon conversion, parent inertinite types, and the specific surface area of the coke measured using BET analysis.

Coke lump and intrinsic gasification kinetics were compared for each case. The most influential factors on which the reaction rate was dependent were distinctly different between lump and powdered samples. The degree of isotropy of the carbon structure was the most important parameter controlling coke lump gasification kinetics with CO₂ at the 1100°C temperature, followed by the basicity index of head coal. Conversely, intrinsic reactivity was closely correlated with microporosity, which was measured via the BET specific surface area.

Importantly, the isotropy of the carbon structure was not found to be a critical parameter controlling the intrinsic gasification kinetics up to the 40% carbon conversion point. Our results suggest that in the lump form where gas diffusion limits the reaction rate, carbon structure and catalytic effect of coke minerals play the dominant role in coke gasification. We found that such behaviour leads to a selective reaction of gas with IMDC in coke lumps due to the isotropy of carbon and greater association with minerals.