Coal macerals reflectance

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. 

In the late 1950 s and early 1960 s the reflectance characterization of coal macerals was used with great success in carbonization studies. For example, it was shown by Spackman et al. (12) that the properties of the various coal macerals... 

Figure 1. Photomicrographs of coal macerals from the Elkhorn N0. 3 seam Eastern Kentucky - hvA bituminous rank. Reflected light in oil, diameter of field, 300 microns, crushed particle...

On the microscopic level, the lithotypes just described are seen to be composites made up of discrete entities called macerals. Optically differentiated by microscopic examination of coals under reflected light or by the study of thin sections of coal using transmitted light (Fig. 1), macerals are classified into three major groups vitrinite. 

On the other hand, semifusinite is intermediate between fusinite and vitrinite showing the well-defined structure of wood, but the cell cavities (round, oval, or elongated) vary in size and are smaller and sometimes less well-defined than those of fusinite. Semifusinite has the cell texture and general features of fusinite except that it is of lower reflectance. In fact, semifusinite has the largest range of reflectance of any of the various coal macerals going from the upper end of the pseudovi-trinite range to fusinite. Semifusinite is also the most abundant of the inertinite macerals. 

Davis, A. (1978) The Measurement of Reflectance of Coal Macerals Its Automation and Significance, EE. Tech. Rep. 2030-TR 10, Coal Research Section, U.S. Department of Energy and Pennsylvania State University. 

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. 

For American and European coking coals the behaviour of semi-fusinite is generally less important since only small quantities of this maceral are usually present. However, South African coal used in coke oven-blends contains as little as 40 per cent vitrinite and as much as 45 per cent reactive semi-fusinite (12). The partial reactivity of the semi-fusinite fraction during liquefaction of Australian coals has been reported by Guyot et al (13). They found that the low reflecting inertinite in two coals up to (a reflectance from 1.40 to 1.49) was reactive. This agrees with the results of Smith and Steyn (12) who consider that the semi-fusinite fraction in South African coals up to V- 5 (1.50 - 1.59) can be reactive to coking. 

Because llptlnltes comprise only a small fraction of most Australian bituminous coals, the M2J pyrograms for typical whole coals (Figure 2(E) and (F)) closely reflect the thermal behaviour of the aromatlc-rlch macerals. 

Coal reflectance (ASTM D-2798) is very useful because it indicates several important properties of coal, including determination of the maceral composition of coal, which, in turn, is helpful for the prediction of behavior in processing (Davis, 1978 Davis et al., 1991). Coal reflectance is determined by the relative degree to which a beam of polarized light is reflected from a polished coal surface that has been prepared according to a standard procedure (ASTM D-2797). Samples prepared by this practice are used for microscopical determination of the reflectance of the organic components in a polished specimen of coal (ASTM D-2798) as well as the volume percent of physical components of coal (ASTM D-2799). 

The refractive index of coal can be determined by comparing the reflectance in air with that in cedar oil. A standard test method (ASTM D-2798) covers the microscopic determination of both the mean maximum reflectance and the mean random reflectance measured in oil of polished surfaces of vitrinite and other macerals in coal ranging in rank from lignite to anthracite. This test method can be used to determine the reflectance of other macerals. For vitrinite (various coals), the refractive index usually falls within the range 1.68 (58% carbon coal) to 2.02 (96% carbon coal). 

Exinite microscopic coal constituent (maceral) or maceral group containing spores and cuticles. Appears dark gray in reflected light. See also Maceral. 

Vitrinite maceral and maceral group composing all or almost all of the villain and like material occurring in attrital coal as the component of reflectance intermediate between those of exinite and inertinite. 

The preliminary results of studies now underway suggest that the spectral peak (A max) of some macerals, is more sensitive to variation in coal rank than previously believed. In the high volatile bituminous coals studied, an increase of 3 V-types (0.45 -0.75% reflectance) resulted in about a 50 nm increase of A max. 

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