Assessment of Coal Structure

Coal structure can be subdivided into two categories (1) the physical strucmre and (2) the chemical structure. Coal contains extractable smaller molecules (Section 10.2) which are an integral part of coal and bear a physical and chemical relationship to the non-extractable material. In contrast to many earlier studies of coal structure, both are an essential part of any consideration of the structural types in coal and one cannot exist without the other. 

Therefore, in terms of physical structure, it is appropriate to note that coal is considered to be a two-component system, a mobile phase and a macromolecular netwoik (Given, 1984a,b  

Recent work has used the yield of the chloroform extract of coal as an indication of the extent of the mobile phase (Derbyshire et al., 1991). In untreated coals, only a portion of the mobile phase appears to be removable but after mild preheating there are sharp increases in the yield of the extract. It has also been noted (Brown and Waters, 1966a,b), perhaps to no one s surprise, that there is an increase in the yield of chloroform-extractable material with treatment temperature. This has been interpreted to mean that there is a gradation in the manner in which the smaller molecules are associated with the network. Thermal studies also tell us that the thermal chemistry of chemical bonds can also vary with temperature. 

caution is advised in the interpretation of the results of thermal investigations. There is a real need to recognize, or even differentiate between, the different chemical and physical events that can occur within the high-temperature domains, even though these events may differ from the preferred event. 

Indeed, the use of high temperatures in a mass spectrometric examination of the species that constitute the mobile phase might also be suspect (Yun et al., 1991 Winans et al., 1992), even though the data appear to reveal the existence of a thermally extractable bitumen-like fraction which is chemically distinct from the remaining coal components. 

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