Coal macerals

Pieces of coal are mixtures of materials somewhat randomly distributed in differing amounts. The mineral matter can be readily distinguished from the organic, which is itself a mixture. Coal properties reflect the individual constituents and the relative proportions. By analogy to geologic formations, the macerals are the constituents that correspond to minerals that make up individual rocks. For coals, macerals, which tend to be consistent in their properties, represent particular classes of plant parts that have been transformed into coal (40). Most detailed chemical and physical studies of coal have been made on macerals or samples rich in a particular maceral, because maceral separation is time consuming. 

Changes In nuclear magnetic resonance measurements of an extensive suite of Australian coals on heating and exposure to pyridine are used to elucidate the molecular conformation of coal macerals Two types of fusible material are Identified In these coals One Is associated with llptlnltes of all ranks and Is typified by fusion commencing at temperatures below 475 K. The other Is associated with vltrlnltes and some Inertlnltes of bituminous coals only and Is characterized by a sharp onset of fusion at temperatures above 625 K. The temperature of onset of fusion Increases with rank for both types The effect of pyridine on the molecular stability of bituminous coals at ambient conditions Is strongly dependent on maceral composition at 86% C and on rank at higher carbon contents ... 

Nandi Q), in their investigation into the role of inert coal macerals in pulverised fuel... 

The contrast observed in electron microscopy depends largely on differences in atomic composition of the specimen. In this respect the coal macerals do not differ greatly from one another, and so very high contrast between them would not be expected and is not in fact obtained. Despite attempts to overcome this difficulty low contrast remains a serious limiting factor in observing structure. It is possible that staining techniques, as used in electron microscopy of biological specimens, may prove useful. 

The relative degree of denseness (i.e., opacity to electrons) of the macerals is exinite least dense, vitrinite intermediate, and inertinite most dense of the coal maceral groups. That is, the light/dark relations are similar to those observed in the usual thin sections examined in the light microscope. 

Aliphatic structures are still of major importance in the second group of resinites, those of the bituminous coals, but aromatic structures are present in significant amounts. The spectra of these resinites display the type of absorption pattern that has come to be associated with other coal macerals, particularly the sporinites and to a large extent the vitrinites. This pattern is established in the resinites of the high volatile bituminous coals. Furthermore, resinites of this group are reactive during carbonization and oxidation processes in which their behavior parallels that of similarly affected vitrinites of equivalent rank. 

The absorption curves given by coal macerals approached the horizontal (magnetic field strength) axis more slowly than a Gaussian distribution curve. Shape analysis (16) showed that over much of the curve, the form closely approximated a Lorentzian distribution curve, but both positive and negative deviations were found in the wings of the curves (that is, in various examples, the curves approached the axis either somewhat more or somewhat less rapidly... 

Possible Relationships of Materials with Coal Macerals of Higher Rank... 

Determinations of the density of various coal macerals have also been reported (Table 6.2), and although the variations are not great, the general order of density for macerals (having the same approximate carbon content) is... 

Binder phase continuous solid carbon matrix formed during the thermoplastic deformation of those coal macerals that become plastic during carbonization formed from the thermoplastic deformation of reactive (vitrinite and liptinite) and semi-inert (semi-fusinite) coal macerals of metallurgical bituminous coals (ASTM D-5061). 

C OAL is AN extremely complex, heterogeneous material that is difficult to characterize. It is a rock formed by geological processes and composed of a number of distinct organic substances called macerals and of lesser amounts of inorganic entities called minerals. Each coal maceral and mineral has a unique set of physical and chemical properties that contributes to the overall behavior of coal. Although much is known about the mineral properties of coal, surprisingly little is known about the properties of the individual macerals. 

This volume covers a wide range of fundamental topics in coal maceral science that varies from the biological origin of macerals to their chemical reactivity. Several chapters report novel applications of instrumental techniques for maceral characterization. These new approaches include solid l3C NMR, electron spin resonance, IR spectroscopy, fluorescence microscopy, and mass spectrometry. A recently developed method for maceral separation is also presented many of the new instrumental approaches have been applied to macerals separated by this new method. The contributions in this volume present a sampling of the new directions being taken in the study of coal macerals to further our knowledge of coal petrology and coal chemistry. 

All of the papers in this book deal with the chemistry and characterization of coal macerals and, as such, recognize the heterogeneous nature of coal. Coal is, in fact, a rock derived from a variety of plant materials which have undergone a variety of physical and chemical transformations. While a number of chemical studies of coal have found the concept of a single "coal molecule" useful, the papers in this book are aimed at characterizing in some way the various macromolecules that comprise the many kinds of coal macerals. 

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