Investigation of the oxidation rate in reducing coking propensity of individual maceral grains residing naturally in lump coal using FTIR microscopy

The mechanism of coal oxidation has been studied by various researchers due to its importance towards coal utilisation and commercial implication. Changes in the coal technological properties are normally acquired to assess its behaviour during pyrolysis either as an individual or as a component in a coal blend. The various instrumental techniques and methods applied to assess coal oxidation degree can be broadly classified into optical microscopy methods and non-optical methods.

Petrological studies using optical microscope on oxidised coal reveals that discolouration of coal macerals can be observed with dark oxidation rims recorded at the vitrinite periphery. With respect to non-optical methods, rheological tests, such as Gieseler Plastometer and dilatometer, are used extensively as these tests are sensitive to the change in early stages of coal oxidation. However, as traditional measures of coal plasticity were deemed inadequate for predicting coke quality, the result of these tests might underestimate the quality of coke produced from oxidised coal samples which, in turn, lower their potential value.

FTIR microscopy (or μFTIR), a non-destructive analysis technique, can offer a unique ability to study coal oxidation as it can identify the structural change of individual maceral grains instead of the bulk coal as conventional FTIR, which requires coal to be ground to fine powder. μFTIR couples midinfrared spectroscopy with visible light microscopy to provide visualization and identification of functional group abundances in samples across 2D regions. Between the two μFTIR components, the microscope helps to determine the distribution of maceral grains in coal sample based on their reflectance level. This result will then be used as a guidance for FTIR analysis to be conducted precisely on individual macerals. As a result, this feature has the potential to allow coal oxidation studies to be conducted in situ on individual maceral, thus, offering the collection of oxidation information on coal maceral level.

This project aimed to take advantage of the μFTIR technique on investigating the structural change of macerals oxidation during storage and transport. This was achieved by utilising the FTIR microscope (or μFTIR) unit at the University of Newcastle. The apparatus combined the Bruker Tensor 37 (FTIR) with the Hyperion 2000 (microscopy) that allows the acquisition of good quality spectra even from small maceral grains thanks to its high spatial resolution (2.7 μm).