Comparative analysis of optical properties of coals by petrography and spectroscopy methods

Abstract

This paper reports determining the grayscale intensity of macerals in the coal lump polished sections oriented along and across the layering of three main groups of sub-bituminous, high volatile bituminous, low volatile bituminous coals. All measurements have been performed in white, red, yellow, green, blue light. The influence of all selected factors and most of their combinations on grayscale intensity is reliable; the most pronounced effects are established for the optical filter color and the maceral type. Comparative analysis of the grayscale intensity data array with the spectra of full reflectance in the visible region and the coefficient of diffuse reflection in the white light of fine powders of the same samples has been carried out. Green light yields a very dark image of vitrinite and liptinite with the grayscale intensity approaching zero. In two of the six samples, the reflectance level of inertinite in yellow light is significantly higher than the grayscale intensity of white light. This anomaly is associated with a hypsochromic shift in the inertinite maximum absorption. The approach used is an alternative method for quantifying the relative changes in reflectivity and position of the diffuse spectral minimum without applying a microspectrometer.