Reflectance of vitrinite

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. 

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. 

Figure B. Clarite with microspores in a high volatile bituminous coal (37% V. M.). Above untreated, below after a-irradiation. The reflectance of vitrinite and exinite increased to the degree of reflectance of a medium volatile coal (with 26% V. M.). Polished section, oil immersion, X 500 (from Brennstoff-Chemie 46, 13 (1965))...

Let us take another example. At the U.S. Gulf Coast, the Pleistocene and the Upper Tertiary (Pliocene and Miocene) reach a thickness of more than 6000 meters. Nevertheless, the coaliflcation has only reached the stage of high volatile bituminous at this depth. This observation resulted from our studies of samples from a deep borehole located in Terrebonne Parish, La. By reflectance measurements on humic inclusions in claystones of the uppermost Middle Miocene encountered at a depth of 5440 meters, we obtained a mean value (under oil) of 0.95% reflectance, which is equivalent to the reflectance of vitrinites (out of deep boreholes) with 35-40% volatile matter (daf). On the other hand, we found low volatile bituminous coals (14-16% volatile matter, daf) at a depth of 5100 meters in a borehole in the Lower... 

In this paper, we demonstrate how mean maximum reflectance of vitrinite in oil (hereafter referred to as R ) can be used in place of conventional chemical-rank parameters (volatile matter and fixed carbon) to estimate the relative yields of carbonization products, specific properties of gas produced by carbonization, and chemical properties of coal such as calorific value and free swelling index (FS1). Further, we illustrate that measured R can be used to detect coal oxidation, to categorize coals for certain combustion uses, and to help classify coals by rank. 

This investigation shows that the average reflectance of vitrinite in coal (Ro) can be used to estimate carbonization product yields, by-product gas properties, chemical properties, oxidation effects, and combustion behavior. Moreover, R along with calorific value and volatile matter content might be employed to classify accurately and consistently coals of all ranks. 

ASTM D-2798. Standard Test Method for Microscopical Determination of the Reflectance of Vitrinite in a Polished Specimen of Coal. 

The mean maximum reflectance of vitrinite was determined in oil (ca. 1.517 index of refraction) and in 546-nm wavelength green light. A photomultiplier photometer was used to measure vitrinite reflectance. Two glass standards were used to calibrate the equipment. A Leitz MPV2 microscope photometer was used. The reflectance values were determined as designated in ASTM D2798-85 (2 5). Twenty-five vitrinite reflectance values were determined for the Rasa coal. 

Figure 4. Correlation of reflectance of vitrinite and carbon content of coals (6)...

Because of the complexity of the coalification processes, different measures are used to define different levels of rank (Table I) high moisture, low heating value, and nonagglomerating character of the coal define the rank (group) within the lignite and subbituminous classes and volatile matter (or fixed carbon) define the various groups of rank in the bituminous and anthracite classes. In addition to these properties, the reflectance of vitrinite, carbon content of the coal (dry, mineral matter free), and some other properties change proportionately as rank increases (Table I). 

Hutton A. C. and Cook A. C. (1980) Influence of alginite on the reflectance of vitrinite from Joadja, NSW, and some other coals and oil shales containing alginite. Fuel 59, 711-714. 

In a single coal, vitrinite, which is usually the commonest maceral, has a higher reflectance than the associated liptinite, but a lower reflectance than inertinite. There is, therefore, a correlation between carbon content and reflectance, and this is used to precisely determine rank. The mean maximum reflectance of vitrinite in oil (Romax) as the level of organic maturity, or rank, of a coal sample. 

The maceral content defines the coal type sapropelic, with >50% liptinite, or humic, more abundant, usually presenting a banded structure. On the other hand, based on the different optical properties of macerals, the reflectance of vitrinite is an essential characteristic used in coal identification and related to rank. A good analysis of maceral content provides knowledge about the chemical composition of a coal, their behavior in different conversion processes, and can also be used as a parameter of coal rank (see section on Petrographic analysis). 

Reflectivity studies using vitrinite, an organic component, were initially carried out on coal to determine its rank, or thermal maturity. These studies were then applied to hydrocarbon generation, as hydrocarbons such as oil and gas are generated over time by the action of heat on fossil organic material. The reflectivity of vitrinite in the hydrocarbon source rocks reveals maturity and the likelihood of the presence of oil and gas in the sediments. 

As described earlier, the optical reflectance of vitrinite depends on its degree of aromatization and, hence, the loss of volatile matter. This natural process can be substituted by artificial heating without oxidant (pyrolysis) in a laboratory furnace. Because of evolving anisotropic reflectance character during such coking processes, the maximum reflectance of vitrinite is reported to be used for temperature tracing [3], but Bunt et al. [8] also demonstrated the applicability of the mean reflectance. 

Usually the thermal decay starts at temperatures around 300 °C where the first increase of reflectance of vitrinite occurs. Between 400 and 900 °C, a nearly linear increase up to i max > 8.0% can be observed, whereas at higher temperatures, as soon as all volatile matter is released, the reflectance stabilizes at a constant final value even decreasing slightly. 

Another problem remains, which is the wide scatter of different parameters within one rank or—seen from the other perspective—the variety of ranks for one volatile matter, heating value, or reflectance of vitrinite, which may be also contradicting each other in tendency. 

The overall ASTM D-388 [13] classification scheme is presented in Table 3.4 and values for the mean random reflectance of vitrinite and bed moisture are also added [3], There are four main classes of coal distinguished with gradual... 

Method of Determining Microscopically the Reflectance of Vitrinite, International Standards Organization, Geneva, Switzerland, www.iso.org. 

The reflectance of vitrinite (a commonly used rank indicator for high-rank coals) is measured by comparing the intensity of a beam of light incident on a polished surface of vitrinite with the light reflected directly back from that surface. 

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