Influence of Inertinite and Volatile Release Characteristics on Viscosity Development and Fusibility During Coking

This project builds on previous projects aimed at understanding the nature of interactions between inertinite and vitrinite. Prior research on Rangal coals (vitrinite reflectance 1.2%) found that mixtures of inertinite and vitrinite had lower viscosities and volatile release rates than expected from predictions based on weight-scaled additive behaviour of the component macerals. It was postulated that the inertinite might be adsorbing volatiles and; that this adsorption may aid viscosity development and fusibility.

This project extended prior research to include Moranbah coals to see if they behave the same way, and also introduced a new technique for characterising the nanoporosity of the coal, Small Angle X-ray Scattering (SAXS).

Vitrinite and inertinite concentrates were separated and crushed concentrates were blended and coked to study their viscosity development and volatile release rate using rheometry and thermogravimetric analysis (TGA). The vitrinite concentrates separated from the Moranbah and Rangal coals both had rank of approximately 1.3% (random vitrinite reflectance, R_r).

The interactions occurring between the vitrinite and inertinite were found to be minor, i.e., the measured viscosity and volatile release rates of the blends were close to those predicted using a weighted average of the behaviour of the components. This finding is in-line with results obtained in a previous project found no evidence of interactions for a high rank coal (R_r = 1.5%). Therefore, at this stage and given the small sample set, it is proposed that interactions between vitrinite and inertinite may be greater for lower ranked coals, or greater for lower ranked Rangal coals.

One of the Moranbah vitrinite concentrates studied contained a high proportion of collotelinite and was found to have a volatile release profile characterized by an initial suppression of volatile release followed by a sudden surge. This behaviour of suppression followed by a surge was enhanced when inertinite from the same coal was blended in. It is proposed that the inertinite could be retarding the release of volatiles by acting as a block. Similar volatile release behaviour was observed when inertinite from the other Moranbah coal was blended.

In contrast, when inertinite from the Rangal coal was added there was no suppression followed by a surge of volatiles released. Additionally, when the Moranbah inertinite was added to the Rangal vitrinite, no suppression followed by a surge was observed.

SAXS profiles of coals and semicokes showed that within the pore-size range probed here (<100 nm diameter), inertinite concentrates had a greater abundance of pores compared to the vitrinite concentrates and the Moranbah inertinites had a greater abundance of small pores < 10 nm diameter compared to the Rangal inertinite. Surface area measurements were found to be 4-5 m²/g for the Moranbah inertinites and 3.6 m²/g for the Rangal inertinite, whilst surface areas for the vitrinite concentrates were found to be <3 m²/g.

During coking up to the temperature of maximum fluidity, the scattering from pores of around 100 nm remained unaltered whilst the scattering from pores <50 nm reduced considerably, indicating that the larger scale structures in these coals are essentially conserved during coking. There was some recovery of pore numbers <5 nm up to 650°C, highest for the Rangal coal and less for the Moranbah coals. The semi-coked inertinite concentrates had slightly lower surface areas (by approximately 40%) compared to that of the fresh coal samples, with the Moranbah inertinites losing more of their surface area.

This work shows that the nanopore structure of the inertinites retained their surface properties on carbonisation. The porosity was expected to increase overall, given the magnitude of volatile matter loss. This was not observed within the size scale examined (pores < 100 nm), and there was a reduction in pore numbers at smaller sizes even though volatiles are lost. This suggests that the macromolecular structure of the inertinites may contract. Alternatively, volatiles may be adsorbing on the surface of the fine pores and coking, which would be expected to reduce the size of the pores.

The objective of this project was to determine whether pore properties of inertinites play a role in fusibility and fluidity development by allowing interaction with vitrinite via volatile adsorption. For the coals studied here, having a random vitrinite reflectance of 1.3%, no major influence of inertinite on fusibility/fluidity was detected, however, inertinite acting as a potential block for the release of volatiles was observed for Moranbah coals (suppressing volatile release).