Occurrence of non-mineral inorganic elements in low-rank coal macerals as shown by electron microprobe element mapping techniques

Abstract

Significant proportions of inorganic elements (up to 1.5% Ca, up to 0.5% Al and up to 0.7% Fe) are consistently found in electron microprobe studies of such otherwise ‘clean’ coal macerals, especially vitrinite macerals, without any visible minerals or mineral inclusions in the macerals concerned. Detailed mapping of the concentration of these elements in visibly ‘clean’ macerals of several low-rank coals reveals that a majority of inorganic elements (Al, Ca and Fe) occur as non-mineral entities rather than discrete mineral particles, and are intimately distributed throughout the macerals; the distribution of the these elements is very similar to that of organic S, particularly in the collotelinite and fusinite of the same coal samples. The baseline concentrations seen in the inorganic element maps (Al, Ca, Fe and S) normally agree with the averaged concentrations derived from more general microprobe analysis of the same macerals in the coals, indicating that the elemental mapping technique provides a consistent basis and a powerful tool for evaluating the modes of inorganic element occurrence in coal macerals. The overall consistency in the baseline levels, instead of spike-like peaks expected from micron-sized mineral inclusions, suggests that these elements occur as an inherent part of the organic structure in the macerals concerned, possibly as a combination of exchangeable ions, carboxylates, chelates and other organometallic compounds; they may also be held by physical absorption and adsorption mechanisms. Better understanding the mode of occurrence of these non-mineral inorganic elements may provide important insights into a number of coal utilisation processes, such as catalyst reactions, slagging and fouling, and emission generation associated with combustion and carbonisation operations.