Influence of coal particle composition and maceral associations on fluidity development during the coking process

Maceral composition is a key parameter used in the assessment of metallurgical coals and coke quality prediction. However, coal particles are typically a mix of different macerals and minerals. Fluidity development in coal particles depends on particle size and the extent of association between constituent maceral grains. As such, determining the composition of coal particles is crucial to understanding the drivers of thermoplastic fluidity. In this work, coal grain analysis (CGA) was used to determine the maceral grain compositional information of individual coal particles for two pairs of metallurgical coals demonstrating different thermoplastic behaviour, as determined by standard Gieseler fluidity and dilatation testing. Each pair was comprised of coals with comparable ranks and both similar (coals A and C) and dissimilar (coals B and D) maceral compositions. The experimental results showed that coals of finer vitrinite particle size and higher degrees of maceral grain association, i.e., finely dispersed macerals within coal particles, demonstrated lower dilatation and higher permeability. It was postulated that coals with close maceral associations are more prone to volatile gas escape during thermoplasticity, hindering bubble growth and thermoswelling while also increasing melt viscosity, leading to decreased Gieseler fluidity measurements. It was also observed that coals with different degrees of maceral grain associations demonstrate different thermoplastic behaviour, where the presence of pure forms of reactive macerals enhanced coal fluidity. Results suggest that some coals possess a higher “effective fluidity” than reported from standard Gieseler testing. Conversely, coals with limited maceral grain association and high compositions of vitrinite prolific particles benefit from enhanced bubble growth and coalescence. Apparent fluidities of such coals are more accurately represented by standard Gieseler testing.