INFLUENCE OF HEATING-RATE VARIATION ON ANISOTROPY OF CARBONIZED VITRINITES

Six vitrinites of bituminous and anthracitic rank have been carbonized in nitrogen at three heating rates (1, 10 and 60 ° C/min) to temperatures between 20 and 950 ° C. The development of anisotropy, reflecting the growth and increasing ordering of the aromatic lamellae of the chars, has been assessed by reflected-light microscopy in two ways: 1. (a) qualitatively by observing the changes taking place in the morphology of the chars in plane-polarized light and under crossed polars, particularly the form of their mosaic structures 2. (b) quantitatively by measurements of oil bireflectance of the chars at 546 nm. Rising heating rate has its most pronounced effect in rapidly increasing the anisotropy of chars formed from vitrinites of medium-volatile bituminous rank. Fast heating rates have much less effect upon the anisotropy of chars from low-rank vitrinites and still less influence upon the anisotropy of chars formed from high-rank coking and anthracitic vitrinites, where the initial rank and the final carbonization temperature are much more important factors. The relation between 'softening', the development of mosaic structures and bireflectance is less well defined at fast heating rates than it is at more conventional heating rates of a few degrees/min and this aspect is discussed. Although bireflectance is not a highly specific parameter, its use in combination with measurements of mosaic size to assess thermal history or even the initial properties of coke-forming components seems to be a realistic possibility. The employment of fast heating rates to produce high levels of anisotropy in carbonized products for industrial processes, and the application of the data obtained in the current investigation to evaluate thermally metamorphosed coals in natural environments, are also briefly considered. © 1978.