School of BEES - Chemistry of Coal Macerals

RESEARCH BY PROFESSOR COLIN WARD, Dr ZHONGSHENG LI and DR LILA GURBA

Because it is derived from compacted plant debris, coal is inherently a heterogeneous material. The organic fraction of coal consists of a diverse range of intimately admixed microscopic components referred to as macerals. It is the individual macerals in the coal that react, independently or with each other, during rank advance and in different utilisation processes.

Although much useful information is gained from traditional whole-coal analysis, sample-specific knowledge of maceral chemistry can provide important insights into the response of different coals to particular utilisation processes, allowing reactions affecting the individual organic constituents to be studied separately. Such an understanding allows the coals in question to be matched more precisely to particular utilisation requirements.

The recent development of special techniques for light-element analysis using the electron microprobe provides an opportunity for directly determining the elemental composition of the individual macerals in coal polished sections, by analysing areas in situ only a few microns in size. Such an approach avoids the need to isolate the individual macerals from each other for bulk analysis, and allows the composition of the organic matter to be determined without corrections for mineral matter.

This study has used the electron microprobe to measure variations in the elemental composition of the macerals in a series of coals ranging from sub-bituminous to anthracite in rank. The chemistry of the macerals has been related to other chemical and petrographic indices, such as vitrinite reflectance, as a basis for evaluating more fully the role of coal petrology in assessing the performance of individual coals for different technological applications.

ELECTRON MICROPROBE ANALYSIS

The electron microprobe can be used to determine the quantitative elemental composition of a very small volume (effectively a spot a few microns wide) within a larger mass of material, such as the individual macerals in a polished section of a coal sample. The technique is based on bombarding the spot to be analysed with an electron beam under vacuum conditions. The impact of the electron beam generates X-rays from the target material, with a spectrum that depends on the elements present and their respective concentrations. The emitted X-ray spectrum is analysed by a series of wavelength-dispersive spectrometers, using analysing crystals appropriate to the wavelengths in question.

Images of coal polished sections under the electron microprobe. Labels show the maceral type (TC = telocollinite, DSC = desmocollinite, SP = sporinite), along with the carbon and sulphur contents at each individual point analysed

SOME FINDINGS

Data on maceral chemistry, determined by electron microprobe, can be used to assess changes in the individual macerals in coals with rank advance, and thus to delineate the coalification tracks of the individual organic components.

The elemental composition of particular macerals, such as vitrinite, provides in some cases a better indicator of coal rank than the other petrographic index, vitrinite reflectance. Reflectance in some coals may be anomalously low (suppressed) due to the influence of marine or other depositional conditions.

Studies using the microprobe also show that organic sulphur and organic nitrogen are more abundant in the vitrinite macerals of coals than in the inertinite components of the same coal samples. This has implications for coal behaviour and environmental impact in different utilisation processes.

Microprobe studies also show the occurrence of inorganic elements, such as Al, Ca and Fe, in the macerals, especially the vitrinite, of lower-rank coals. These usually disappear as the chemical structure of the macerals changes with rank advance.

RECENT PUBLICATIONS

Ward, C.R., Li, Z.S. and Gurba, L.W., 2005. Variations in coal maceral chemistry with rank advance in the German Creek and Moranbah Coal Measures of the Bowen Basin, Australia, using electron microprobe techniques. International Journal of Coal Geology 63, 117-129.

Gurba, L.W., and Ward, C.R., 2000. Elemental composition of coal macerals in relation to vitrinite reflectance, Gunnedah Basin, Australia, as determined by electron microprobe analysis. International Journal of Coal Geology, 44, 127-147.

Ward, C.R. and Gurba, L.W., 1998. Occurrence and distribution of organic sulphur in macerals of Australian coals using electron microprobe techniques. Organic Geochemistry, 28(11), 635-647.