Estimating the Fusible Content of Individual Coal Grains and its Application in Coke Making

For coals of suitable rank the vitrinite, liptinite and some of the inertinite macerals are fusible during coking and the minerals and remaining inertinite macerals do not fuse. The amount of fusible inertinite present in a coal varies between coals extracted from differing coal basins and even from coal to coal from similar regions. Inertinite fusibility is related to reflectance, with low reflecting (fusible) inertinite being more reactive than high reflecting (infusible) inertinite. The size of infusible inertinite particles in the coke oven feed also impacts on coke strength as large inertinite present in coke oven feed impact adversely on coke quality.

This project demonstrated that CSIRO's Coal Grain Analysis (CGA) system, which provides whole coal reflectance fingerprints on individual particles, could be used to determine the amount of fusible and infusible inertinites in individual particles. To do so required that the reflectance cut off between fusible and infusible inertinite be established for each coal. This was achieved by using imaging methods on matching coal and coke surfaces obtained on two halves of individual particles, where one half had been coked, to determine the fusible reflectance thresholds between fusible and infusible inertinites. The CGA software was also upgraded so that it is now also able to extract the size dimensions for each individual structure within each individual particle.

The analyses of the matched coal and coke surfaces enabled the reflectance range between the end of the vitrinite and the beginning of the infusible inertinite reflectance value to be determined. This meant that when the individual particles present in the coke oven feed samples were analysed, this reflectance gap, rather than the absolute reflectance value was used to discriminate between the fusible and infusible inertinite structures in each particle. As commercial coal  products from a mine can come from different pits and seams and as the vitrinite is anisotropic, the random sectioning of individual particles results in vitrinite having a relatively broad spread of reflectance values in different particles.

However, within any individual particle, the inertinite always has a higher reflectance value than the vitrinite. The reflectance range for the fusible inertinite, which went from the end of the vitrinite reflectance range to the commencement of the infusible inertinite reflectance range, was consistent between particles. Hence it is proposed that rather than there being a single reflectance cut off between fusible and infusible inertinite for a coal there is a consistent difference between the inflection point at the top of the vitrinite reflectance distribution and the fusible / infusible inertinite reflectance boundary for each individual particle. For the 6 coals tested this difference was approximately 0.2%. This range was greatest for the lowest rank coals and decreased with increasing rank.

For each of the 6 coals the average reflectance cut off between fusible and infusible inertinite and the reflectance range for the fusible inertinite was determined. Also determined for each coal was the total amount of fusible and infusible inerts in each sample of coke oven feed and the amount of large (>1.5mm) inerts (infusible inertinite plus mineral) which were present in each coal. The two Rangal coals had a greater amount of large infusible inerts than their sister samples from the other coal measures. The additional information provided by this analysis technique could be used by coal producers to recommend milling strategies which optimise their coking performance. For example, the CGA results obtained suggest that the two coals from the Rangal Coal measures contained a greater proportion of large infusible inerts than did the coals of comparable ranks from other measures. If this is proven, this information may assist coal producers to develop specific milling strategies for these coals.