Comparative study of a vitrinite-rich and an inertinite-rich Witbank coal (South Africa) using pyrolysis-gas chromatography

Ofentse M Moroeng,Nicola J Wagner,Vimbai Mhuka,Mathew M Nindi,R James Roberts

doi:10.1007/s40789-019-00274-3

Abstract

This study aims to compare iso-rank vitrinite-rich and inertinite-rich coal samples to understand the impact of coal-forming processes on pyrolysis chemistry. A medium rank C bituminous coal was density-fractionated to create a vitrinite-rich and an inertinite-rich sub-sample. The vitrinite-rich sample has 83 vol% total vitrinite (mineral-matter-free basis), whereas the inertinite-rich counterpart has 66 vol% total inertinite. The vitrinite-rich sample is dominated by collotelinite and collodetrinite. Fusinite, semifusinite, and inertodetrinite are the main macerals of the inertinite-rich sample. Molecular chemistry was assessed using a pyrolysis gas chromatograph (py-GC) equipped with a thermal desorption unit coupled to a time of flight mass spectrometer (MS) (py-GC/MS) and solid-state nuclear magnetic resonance (13C CP-MAS SS NMR). The pyrolysis products of the coal samples are generally similar, comprised of low and high molecular weight alkanes, alkylbenzenes, alkylphenols, and alkyl-subtituted polycyclic aromatic hydrocarbons, although the vitrinite-rich sample is chemically more diverse. The lack of diversity exhibited by the inertinite-rich sample upon pyrolysis may be interpreted to suggest that major components were heated in their geologic history. Based on the 13C CP-MAS SS NMR analysis, the inertinite-rich sample has a greater fraction of phenolics, reflected in the py-GC/MS results as substituted and unsubstituted derivatives. The greater abundance of phenolics for the inertinite-rich sample may suggest a fire-related origin for the dominant macerals of this sample. The C2-alkylbenzene isomers (p-xylene and o-xylene) were detected in the pyrolysis products for the vitrinite-rich and inertinite-rich samples, though more abundant in the former. The presence of these in both samples likely reflects common source vegetation for the dominant vitrinite and inertinite macerals.

Highlights

The organic chemistry of coal is a complex product of the various progenitors for the individual macerals
The main purpose of the present study is to examine differences and/or similarities in the organic molecular chemistry of iso-rank bituminous coals, a vitrinite-rich and an inertinite-rich sample, using a pyrolysis gas chromatograph equipped with a thermal desorption unit (TDU) coupled to a time of flight (TOF) mass spectrometer (MS) in order to understand the effect of source vegetation and origin pathways on coal pyrolysis products
This suggests that the major form of sulphur in the parent coal is present as sulphide minerals, which reported to the 1.8 sink fraction, with organic sulphur comprising a minor proportion

Summary

Introduction

The organic chemistry of coal is a complex product of the various progenitors for the individual macerals. The origin of inertinite macerals is variable for different macerals belonging to this group (van Krevelen 1993; ICCP 2001), derived through multiple origin pathways (Hower et al 2011a, 2013; O’Keefe et al 2013; Moroeng et al 2018a, b, c). There is some general consensus that some macerals of this group, such as semifusinite and fusinite, formed through the charring of woody plant matter (Austen et al 1966; Scott 1989, 2002, 2010; Diessel 1992, 2010; ICCP 2001; Glasspool 2003a, b; Hower et al 2013; O’Keefe et al 2013; Moroeng et al 2018a, b, c). Inertodetrinite represents reworked plant matter that would have produced primary inertinite macerals, and secretinite reflects extreme oxidation or charring of resin and humic gels (ICCP 2001; O’Keefe et al 2013)

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