Comparison of elemental composition of macerals determined by electron microprobe to whole-coal ultimate analysis data Colin R. Ward ⁎, Zhongsheng Li, Lila W. Gurba School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney NSW 2052, Australia ABSTRACT ARTICLE INFO Article history: Received 28 February 2008 Received in revised form 9 May 2008 Accepted 9 May 2008 Available online 18 May 2008 Keywords: Coal analysis Coal petrology Maceral chemistry Electron microprobe Australia The elemental composition of the individual macerals in a suite of Australian coals has been determined in polished sections using light-element electron microprobe techniques. The analyses of the individual macerals in each coal were combined with data on maceral abundance to produce an inferred chemical composition for the organic matter of the respective whole-coal samples, and this was compared, for each sample, to the respective whole-coal ultimate analysis data, corrected to a dry, ash-free (daf) basis. Except for slightly lower values in some lower-rank coals, the inferred percentages of whole-coal C estimated from the microprobe data were found to be very close to the respective whole-coal C percentages as determined by conventional ultimate analysis. The proportion of O in the coals indicated by the microprobe study, however, appears to be as much as 2% higher than that derived from the ultimate analysis data, especially in the lower- rank coal samples. The difference it may represent errors in calculating the O percentages in ultimate analysis, errors in the microprobe analysis due to difficulties in calibration or measurement, or increased proportions of O in the coals due to factors such as take-up with storage of the polished sections. The percentages of whole-coal N calculated from the microprobe data are up to 0.5% (absolute) below the proportion of N determined directly by whole-coal ultimate analysis. This may reflect the inherent difficulty of dealing with a light element at low concentrations by the microprobe technique, or it may indicate that some of the N occurs in the coals in mineral form. The percentages of whole-coal (organic) S calculated from the microprobe study are close to the percentages of organic S determined for each sample by more conventional techniques. With the exception of (organic) O, which may be affected by other factors, and also possibly of N, the electron microprobe technique appears from the study to provide results that are consistent with ultimate analysis over a wide rank range. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Ultimate analysis is a well-established procedure for providing fundamental data on the proportions of C, H, O, N and S in individual coals, which are then used for classification and utilisation purposes. Such data are usually generated by analysis of “whole-coal” samples, embracing moisture and mineral matter as well as the organic constituents (Given and Yarzab, 1978), and corrections need to be made to the results if the composition of the organic matter alone needs to be evaluated. The composition of the organic matter obtained from ultimate analysis data, corrected to a moist, mineral matter free (mmmf), dry, mineral matter free (dmmf) or dry, ash-free (daf) basis, is used in coal classification and as an indicator of coal rank (e.g. ASTM, 1997). However, notwithstanding the possibility of errors associated with the different corrections involved, the composition of the organic matter determined in this way inherently represents an aggregation of the composition of the different maceral components. Variations in chemical composition from ultimate analysis data therefore really reflect variations in the coal type (i.e. the mixture of macerals present), as well as the rank of the coals concerned. Although specific information on the chemical composition of the individual macerals within whole-coal materials is not often obtained, such information is more fundamental than whole-coal data in the understanding of coal composition, and in the response of different coals to processes such as rank advance (cf. Ward et al., 2005; 2007) and igneous intrusion (Walker et al., 2007). It may also be significant in evaluating the chemical mechanisms associated with coal utilisa- tion, since it is the individual macerals that react, independently or with each other, during combustion, gasification, and coking (Walker and Mastalerz, 2004). It is inherently difficult, however, to isolate cleanly the individual macerals in a coal for separate chemical analysis, without contamina- tion by minerals or other organic components. Density gradient centrifugation (DGC) of finely powdered coals has been used to obtain maceral concentrates for more detailed analysis of particular coal components, allowing a number of useful comparisons and other evaluations to be made among different maceral groups (e.g. Dyrkacz International Journal of Coal Geology 75 (2008) 157–165 ⁎ Corresponding author. Tel.: +61 2 9385 8718; fax: +61 2 93851558. E-mail address: C.Ward@unsw.edu.au (C.R. Ward). 0166-5162/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2008.05.010 Contents lists available at ScienceDirect International Journal of Coal Geology journal homepage: www.elsevier.com/locate/ijcoalgeo