Coking behaviour of collotelinite and collodetrinite macerals: A case study of bituminous coals from the Bowen Basin (Queensland, Australia)

When predicting coke quality or coking behaviour, the more accurate models typically include vitrinite content in addition to rank. This work explores how the two most abundant vitrinite macerals (collotelinite and collodetrinite) behave during carbonisation. The insights gained highlight important differences, paving the way for more sophisticated coke quality prediction models and enhanced blending/processing strategies. Australian bituminous coals from Bowen Basin with random vitrinite reflectance (%R o ) ranging 0.81–1.18% were cut and polished into coal blocks having a 15 mm × 15 mm face running perpendicular to the bedding plane. Following petrographic analysis, these blocks were carbonised at 3 °C/min to 525 °C, set in resin, and repolished to examine coking behaviour of identified macerals. Vitrinite concentrates (VCs) were prepared alongside “whole coal” samples to measure viscosity using rheometry. Collotelinite underwent considerably more expansion than collodetrinite with the extent of collotelinite expansion greater for the lower reflectance bituminous coals; characterised by large pores and thin pore walls, while higher reflectance coals gave rise to smaller pores and thicker pore walls. Separated VCs were rich in collotelinite and generated significant fluidity, i.e. low viscosity. For whole coals, viscosities were higher due to the presence of inertinite, however, this work suggests that higher viscosities are also due to increased collodetrinite. The maceral picking process for one coal led to a situation where the VC and the whole coal had similar levels of vitrinite, however, the VC was richer in collotelinite and exhibited a significantly lower viscosity. This result suggests that fluidity development and expansion behaviour for these two vitrinite macerals is different, and aligns with visual observations. It is proposed that volatile release behaviour is different for collotelinite and collodetrinite due to the differences in microstructure. More specifically, volatiles are better retained in collotelinite which contributes to its greater fluidity development and expansion behaviour. The process of geochemical gelification that gives rise to collotelinite may enable it to have no, or very few, defects and therefore limited pathways for volatiles to leave. Instead, volatiles are trapped and contribute to both the liquid development and bubble expansion. With no inclusions and defects to destabilise bubble interfaces, significant expansion occurs. In contrast, whilst collodetrinite arises via a similar process, it is less than 10 μm in size and co-occurs with other macerals and minerals, held together physically owing to its detrital origin and/or its mechanical degradation within the palaeomires. Accordingly, there are defects and grain boundaries between components through which volatiles may diffuse out, preventing fluid development and expansion. Overall, the outcome of this study shows that the vitrinite size and relative ease of volatile release for different maceral types may play a crucial role in a coal's fluidity development, and consequently its coking behaviour.