Vitrinite

To assess potential yield and maturity of source rocks and classify those according to their vitrinite reflectance . 

In 1963 a classification of coals by rank (differing from the ECE scheme) was pubUshed by the International Committee for Coal Petrology (Table 2) (9). This includes a classification of brown coal that correlates a number of important properties including the percent reflectance of vitrinite in the coal. This is a simpler version of that used in German practice, which further subdivides soft brown coals into foHaceous and earthy. Most brown coals belong to the latter group. 

Macerals. Coal parts derived from different plant parts, are referred to as macerals (13). The maceral names end in "-inite" as do the mineral forms of rocks. The most abundant (about 85%) maceral in U.S. coal is vitrinite, derived from the woody tissues of plants. Another maceral, called liptinite, is derived from the waxy parts of spores and poUen, or algal remains. The liptinite macerals fluoresce under blue light permitting a subdivision based on fluorescence. A third maceral, inertinite, is thought to be derived from oxidized material or fossilized charcoal remnants of early forest fires. 

The macerals in lower rank coals, eg, lignite and subbiturninous coal, are more complex and have been given a special classification. The term huminite has been appUed to the macerals derived from the humification of lignocellulosic tissues. Huminite is the precursor to the vitrinite observed in... 

The elemental composition of the three maceral groups varies. The vitrinite, which frequently is about 85% of the sample in the United States, is similar to the patent coal. The liptinites are richer in hydrogen, whereas the inertinites are relatively deficient in hydrogen and richer in carbon. The liptinites also contain more aliphatic materials the inertinites are richer in aromatics. The term inertinite refers to the relative chemical inertness of this material, making it especially undesirable for Hquefaction processes because it tends to accumulate in recycled feedstock streams. 

Vitrinite Reflectance. The amount of light reflected from a poHshed plane surface of a coal particle under specified illumination conditions increases with the aromaticity of the sample and the rank of the coal or maceral. Precise measurements of reflectance, usually expressed as a percentage, ate used as an indication of coal rank. 

Table 2. Vitrinite Reflectance Limits, Taken in Oil, and ASTM Coal Rank Classes ...

Seam correlations, measurements of rank and geologic history, interpretation of petroleum (qv) formation with coal deposits, prediction of coke properties, and detection of coal oxidation can be deterrnined from petrographic analysis. Constituents of seams can be observed over considerable distances, permitting the correlation of seam profiles in coal basins. Measurements of vitrinite reflectance within a seam permit mapping of variations in thermal and tectonic histories. Figure 2 indicates the relationship of vitrinite reflectance to maximum temperatures and effective heating time in the seam (11,15). 

Bonding in iVIa-cromoIecuIes. Conclusions regarding the chemical stmcture of the macromolecules within coal are generally based on experimental measurements and an understanding of stmctural organic chemistry (3,4,20,28). The description given herein refers to vitrinites. 

The molten part of a vitrinite is similar to the gross maceral, and a part of the maceral is converted to a form that can be melted after heating to 300—400°C. The molten material is unstable and forms a soHd product (coke) above 350°C at rates that increase with temperature. The decomposition of the Hquid phase is rapid for lower rank noncoking coals, and less rapid for prime coking coals. The material that melts resembles coal rather than tar and, depending on rank, only a slight or moderate amount is volatile. 

Given, P. H., Cronaucr, D. C., Spackman, W., Lovell, H. L., Davis, A., and Biswas, B., Dependence of coal liquefaction behavior on coal characteristics 1. vitrinite-rich samples. Fuel, 1975, 54, 34 39. 

Certain compositional differences between coals of differing origins can be inferred from available data. Differing anatomical distributions of cellulose, lignin and suberin, with implications for the origins of vitrinites, and differing distribution of phenolic substances in plants of different orders and families, have been referred to above. Some biochemical investigations of modern representatives of ancient plants have been made (e.g., refs. 14,... 

Teichmiiller (39) has indicated that coals formed in saline environments tend to be richer in hydrogen and nitrogen than freshwater coals. She also believes that certain fluorescent macerals may be relatively more abundant in coals formed in more saline conditions in accordance with this view, fluorinite and fluorescent vitrinite appear to be more abundant in coals from Illinois than in those in the Eastern province. 

Recent work has suggested that the coals of the Illinois Basin were never buried deeper than about 1500 m. (44), compared with an estimated 3000 m. or more for the coals of western Pennsylvania in the Eastern province. Presumably as a consequence, the coals of the Interior province tend to show low values of vitrinite reflectance and high values of moisture-holding capacity relative to coals of other areas of apparently similar rank (45). 

Further information on the trends of behavior of vitrinites with change of rank was provided by the examination with an optical microscope of residues from liquefaction experiments. This will be dealt with later. 

Figure 1. Conversions over short residence time for high-vitrinite coals of varying...

Earlier publications have documented the higher reactivities of vitrinite and liptinite group macerals and the lower reactivities of certain inertinite macerals in liquefaction (50,57,68). 

The reactive role of liptinite macerals in liquefaction has been partially documented (50,68). However, recent work has shown that unaltered sporinite often is encountered in the residues from both batch and continuous liquefaction runs. For example, sporinite was a common component in the residues of a high volatile A bituminous coal after hydrogen-transfer runs at 400° for 30 minutes (70). In spite of the relative unreactivity of the sporinite in this instance, the vitrinite clearly had reacted extensively because vitroplast was the predominant residue component. The dissolution rate of sporinite from some coals, even at 400°C, may be somewhat less than that of vitrinite. 

In contrast to sporinite, resinite from a Utah high volatile A bituminous coal reacted rapidly and more completely than the corresponding vitrinite. Table V shows the conversion levels achieved for a concentrate containing 75% resinite (mineral-free basis) reacted under relatively mild conditions. The results are curious. A fairly respectable level of conversion is achieved in 15 minutes at 350°C (under which conditions the associated vitrinite would presumably show little conversion), but longer times and a temperature of 370° have little further effect even raising the temperature to 400° does not show a major increase in conversion. 

The mean maximum reflectance of vitrinite (Ro max) is the mean of one hundred determinations. 

Figure 8 shows the effect of rank, as measured by the mean maximum reflectance of vitrinite, on the overall conversion. 

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