Measurement of reflectance of coal macerals: its automation and significance

Abstract

One of the prime objectives of The Penn$ylvania State University's DOE program is the characterization of 1,300 samples of U.S. coals. Reflectance determination plays a major role in meeting this objective, because it is used as an accurate rank index both to select coals for testing and to investigate property-behavior interrelationships using the Penn State/DOE Coal Data Base. The reflectances of coal macerals are relate•d to their refractive and absor•ptive indices by the Beer equation; the refractive index of the medium and the wavelength of • 1 ight need to be specified. Determinations usually are made in immersion oil"at 546 nm. The properties of vitrinite make it the most suitable maceral for these measurements, but the variety of vitrinite selected may depend upon the intended application. Vitrinite reflectance is considered to be dependent upon the degree of aromatization of its structural units. Although some of the earlier investigators believed that the reflectnnr.e of coals increased in a discontinuous, stepped manner as their rank increased, the bulk of assembled data suggests that the change is continuous. However, some recent results indicate that there may be more than one coalification track for coals with different geological histories, resulting in displacement from the general trend. Several techniques have been employed to determine the optical properties of coal constituents. The usual comparative method involves the use of a microscope cope photometer to compare the intensity of light reflected by particles \~ithin a-polished pellet of coal td that of a glass or synthetic mineral standard. Because coal 1s anisotropic it is common to measure either maximum reflectance in polarized light or r~ndom reflectance in non-polarized light. Various equipment modifications and accessories have been used to improve the ease of measuring maximum reflectance. i i Reflectograms produced by fully automated refJectance microscopy (ARM or Rapid Scan) can be interpreted to obtain measurements of vitrinite maximum reflectance and maceral composition. Earlier attempts had been limited severely by the technology available, but current work at The Pennsylvania State University has the benefit of a modern minicomputer facility. The very rapid data acquisition_ rate achievable with a minicomputer makes it an excellent tool to couple with a microscope photometer. The difference in reflectance between the organic components of coal and pyrite enables pyrite chordsize distributions to be measured by summing tonsecutive readings on pyrite during continuous scanning. The system gives reproducible results, and, because it is capable of simultaneously quantifying rank, petrography, and pyrite size and amount, it holds promise as a means of coal quality control. Further work, aimed at gaining resolution from the ~se of a pulsed step scanning stage,is in progress. Vitrinite reflectance provides a rapid, accurate measurement of coal rank _requiring only a small amount of material. Consequently several systems of cla•ssifyin...

The timing of coalification in relation to structural events in the Grande Cache area, Alberta, Canada

“…2). for ASTM rank classes as published by Davis (1978): For the interpretation of regional reflectance changes, averaged values were used for those locations where reflectances had been determined on upper, middle, and lower portions of the same scam.…”

Petrology, palynology, and depositional environments of coals of the Upper Carboniferous Barachois Group (Westphalian A and C), southwestern Newfoundland

“…American Society for Testing and Materials ranks were assigned to the samples according to the scheme in Davis (1978) by measurement of maximum vitrinite reflectances. Petrographic composition was determined by maceral analyses using a slightly modified Stopes-Heerlen system for classification of bituminous coals (Bustin et al 1989).…”

Maturity studies in the Carboniferous Dobroudja coal basin (northeastern Bulgaria)—coalification, clay diagenesis and thermal modelling

“…Previous coalification studies in the basin (Nikolov,197 4) documented high volatile to low volatile bituminous ranks (ASTM rank classes as published by Davis, 1978) or "flam" to lean bituminous ranks (Bulgarian State Standard 15193-80, as published by Siskov et al, 1985). Fig.…”