Technical Market Support » Metallurgical Coal
Coking coal quality is based on a number of indices which characterise the quality of the coke product. Many standard tests exist to compare the parent coals; the resulting evaluations, however, do not always reflect accurately the real behaviour of the coals during the coking process. Using standard coal maceral qualities, the coking performance of some coal measures can exceed expected results, suggesting that they are undervalued. One reason for this discrepancy is thought to lie in the variable nature of the macerals that comprise the coals. It is known that for coals of suitable rank some of the inertinite macerals, as well as vitrinite and liptinite macerals, are fusible during coking (fusibles) and the remaining inertinite macerals and minerals do not fuse (infusibles). The challenge has been to discern accurately the attributes which determine the fusibility of inertinites, the extent of their fusibility, and the role played by the size of the fusible and infusible structures. Classifying this transitional material chemically and identifying its differences from vitrinite and semi‐fusinite proper will provide an understanding of how this material should be classified and how it will behave in coking.
The aim of this project was to apply a more fundamental approach to the prediction of coke strength and coherence from coal properties, hypothesising that:
- The fusibility of inertinite is linked to the chemistry and reflectance of the inertinite.
- Differences in reflectance between the different inertinite structures, and also the differences seen between vitrinite and inertinite structures, are due to the varying chemistry of these maceral structures.
Attenuated total reflection Fourier‐transform infrared spectroscopy (ATR‐FTIR) was used to quantify the aliphatic and aromatic content of vitrinite, fusible and infusible inertinite maceral structures with a range of reflectance values. From this analysis we hoped to determine a relationship between reflectance and fusibility for the different macerals.
Initially, the relationship between aliphatic content (from FTIR), maceral type and reflectance for single phase coal grains was explored. 'Infusible' inertinite in these grains was shown to contain significantly lower proportions of aliphatic content when compared to vitrinite. This was expected and confirmed previous work that inertinite contains less aliphatic functional groups than vitrinite due to the greater degree of aromaticity. Interestingly, 'fusible' inertinite appeared to contain similar proportions of aliphatic content to that of the vitrinite grains examined. An initial hypothesis therefore is that this transitional inertinite material may have the chemical structure, and therefore the coking behaviour, of slightly higher reflecting vitrinite during the coking process, rather than that typically expected of inertinite.
Transects across mixed phase grains were then investigated where FTIR spectra were collected from locations containing varying maceral content. It was observed that increasing proportions of 'fusible' material (<50% overall content measured at the FTIR collection point) appeared to result in only slight decreases in aliphatic content, reinforcing the hypothesis that fusible inertinite may have a similar chemical structure and therefore coking attributes to higher reflecting vitrinite rather than infusible inertinite.
Overall, it was observed that aliphatic content decreases with increasing reflectance in mixed phase coal grains, and whilst fusible inertinite rich grains display lower aliphatic content that vitrinite, their chemical functionality appears to be much more akin to that of vitrinite than the higher reflecting infusible inertinite. This similarity in chemical structure could be a contributing factor as to why some coals with high proportions of fusible inerts, such as the Rangal coal measure (samples 1 and 6), often exhibit different coking attributes (higher CSR and density matched by rank) than initial vitrinite and inertinite maceral proportions would suggest. Further research is required in order to link chemical functionality and other previously reported coking properties, such as reduced swelling, of Rangal coals.
Fusible material proportion >50% appeared to result in a dramatic 'cliff‐like' drop off in aliphatic functionality observed for many of the samples. One reason for this could be that as the proportion of infusible inertinite content increases, the nearby fusible material associated with it is likely to be at the upper end of the fusible inertinite range, indicating that the material is indeed 'transitional' and not a discrete structure; this suggests that further distinctions within the fusible inertinite classification could be made in the future.