Different mechanisms of coke microtexture formation during coking coal carbonization

Abstract

Textural and microtextural evolution of five coking coals during carbonization were observed by optical microscopy (OM) and transmission electron microscopy (TEM) to determine the mechanism(s) of coke microtexture formation. Ultrathin sections, obtained by ultramicrotomy, from chars quenched at numerous close temperatures in the plastic stage, were examined by TEM in the 002 dark field mode. By coupling optical microscopy and Gieseler plastometry results, these observations led to two different mechanisms for coke microtexture formation. For MacClure coal which, in the present case, is the one passing through the best developed plastic stage, a mechanism similar to pitch carbonization was observed. Although they did not show the Brooks and Taylor mesophase structure, very small spherules up to 1.5 μm in size were found to occur, growing in number and size, and finally coalescing to give domains. These spherules were composed of practically parallel polyaromatic structural units (PSU). In the case of Norwich Park coal, with the highest coal rank and the least developed plastic stage, domains were obtained by a completely different process and were due to the improvement, during a practically solid state, of a pre-existing preferential planar orientation of the PSU.