International Comparison of Inertinite Reflectograms and Fluorograms

Inertinite is generally perceived to be a troublesome group of coal minerals, mainly because of the variable reactivity of its members in reference to most industrial processes. Although several Australian export coals contain large amounts of inertinite, their technological performance compares favourably with overseas coals containing less inertinite. Since the widely accepted Schapiro-Gray system of coke stability calculations allows only 1/3 of the inertinite to be fusible, many investigators of this problem suggested that the inertinite to be fusible, many investigators of this problem suggested that the inertinite of Permian Gondwana coals was more reactive than the inertinite of Carboniferous coals, on which the Schapiro-Gray system was based.

This notion was supported by the consistent failure of the Schapiro-Gray system to give correct estimates of coke stability indices in past ICCP ring analyses of Permian (and Cretaceous) coals. The failure was due to an underestimation of the fusibility of a considerable percentage of inertinite group macerals.

Having established close links between inertinite fusibility and both reflectance and fluorescence during previous NERDDP /ACARP Projects, we used these optical properties to investigate whether coals of different ages and origins contained

• different proportions of fusible inertinite, and / or 
• differed in the degree of inertinite fusibility.

The project consisted of a systematic survey of the reflectance and microgluorescence intensity distribution of inertinite in 75 Australian, Canadian, European and US bituminous coals. The samples were chosen to include high and low concentrations of inertinite macerals and a range in rank.

The study did not reveal any significant differences in the inertinite reflectance distribution between Carboniferous and Permian coals. It is concluded therefore that the proportion of fusible inertinite is similar in the coals studied by us. Likewise, no significant differences were found in the intensity of inertinite fluorescence in our samples.

Since good thermoplastic properties are indicated by high fluorescence intensities, whereas poor thermoplastic properties are matched by low fluorescence intensities, it is concluded that the degrees of inertinite fusibility are similar in our samples.

Considering that no significant differences were found in the reflectance cutoffs of fusible, partially fusible and infusible inertinite between coals of different origins and geological ages, it is suggested that the underestimation of inertinite reactivity by the Schapiro-Gray method might apply to Carboniferous coals as much as it does to Permian coals.

The aim of the project was to identify reasons for the perceived differences in inertinite fusibility in coals of different ages and origins. There are two extreme possibilities for such differences:

• either coals have similar levels of inertinite fusibility, but different proportions of infusible (with high reflectance) and fusible (with low reflectance) inertinite macerals apply to different coals; or 
• coals have similar proportions of high reflecting (=infusible) and low-reflecting (= fusible) inertinite, but different degrees of fusibility apply to coals of different ages and origins.

Work Program Description

The project consisted of a systematic survey of the reflectance and microgluorescence intensity distribution of inertinite in Australian, Canadian, European and US bituminous coals.

This photometric approach was adopted because inertinite reactivity is governed by its molecular structure which also determines the inertinite's optical properties.

Seventy-five samples were analysed by carrying out a random reflectance scan of inertinite based on a point count of 500 inertinite macerals per sample. A combined point-by-point assessment of the fluorescence intensity and random reflectance of inertinite was conducted in 8 Permian and 12 Carboniferous samples. In addition, each sample was subjected to a complete maceral analysis and mean random vitrinite reflectance determination.

The 500 reflectance measurements obtained from each of the analysed samples were statistically processed and illustrated in histogram form. Inertinite reflectograms derived in this manner from coals of different geological ages and origins were then compared in terms of reflectance range, mean, median and midpoint reflectance, as well as the difference between these parameters and mean vitrinite reflectance.

Fusibility categories were allocated on the basis of our previously established reflectance-fusibility correlation (Diessel, 1982, 1983; Diessel et al, 1986, 1987; Diessel and Wolff-Fischer, 1987). While the measured fluorescence intensities correlate with the degree of fusibility in coals of different ages and origins, the reflectograms show whether such coals have similar or different proportions of high-reflecting (= infusible) and low-reflecting (= fusible) inertinite macerals.

Major Findings and Conclusions

• The reflectance distributions were found to vary systematically with rank and palaeo-environment in Carboniferous and Permian coals. We expect the same to be true for coals of other ages and origins. 

• The study revealed no significant differences in the inertinite reflectance distribution between Carboniferous and Permian coals. It is concluded therefore that the proportions of fusible, partially fusible and infusible inertinite macerals do not vary significantly between coking coals of different ages and origins. 
• No significant differences were found in the reflectance cutoffs of fusible, partially fusible and infusible inertinite between coals of different origins and geological ages. Moreover, a coal-to-coke mass balance calculation of a Carboniferous coal and coke of German origin and an Australian Permian coal and coke used in the 1991 ICCP round robin has confirmed that in both coals only 50% of the inertinite remained unfused during carbonisation. It is suggested therefore that the underestimation of inertinite reactivity by the Schapiro-Gray method might apply to Carboniferous coals as much as it does to Permian coals.