RECENT PROGRESS IN COAL STRUCTURE RESEARCH

Coal is physically and chemically a heterogeneous rock which mainly consists of organic material. It is generally agreed that the organic coal material has been formed from plant debris. In a first, mainly biochemical stage, peat was laid down which in a later geochemical stage underwent coalification over a period of up to several hundred million years under the influence of pressure and temperature caused by overlying sediments. The organic sedimentary rock thus formed is composed of fossilized plant remains called macerals, and mineralized inclusions. The macerals are microscopically distinct areas which are differentiated into three major classes: vitrinite, exinite (or liptinite) and inertinite. Vitrinite occurs most frequently and is believed to be derived from woody plant material (mainly lignin), and the exinite from lipids and waxy plant substances. The origin of inertinite is possibly charcoal formed by prehistoric pyrolysis processes. As the result of its origin, coal is an almost non-volatile, insoluble, non-crystalline, extremely complex mixture of organic molecules varying considerably in size and structure. Detailed structural characterization has been found to be extremely difficult, so that coal structure research is still a challenging task and continues to be pursued intensively. In principle, two approaches can be used for determining the chemical structure. One attempts to degrade the coal macromolecules into representative fragments and to derive the original structure from the structures of such fragments. The alternative approach attempts to characterize coal directly by techniques which allow the non-destructive investigation of solid materials. Such techniques are, for instance, i.r. and solid-state n.m.r. spectroscopy, X-ray diffraction and small angle scattering, electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS, XANES, EXAFS), or Mossbauer spectroscopy. While for some of these techniques the application to coal is severely hindered owing to the lack of necessary transmittance or crystallinity, others have been developed recently in material science as important tools for the investigation of solids. In recent years further important insights into structural features of different coal varieties have been gained using new physical and chemical methods and known techniques that have been further developed. Some of these methods are reviewed and the results obtained from application to coals are discussed in the context of coal structure.