DIFFERENT MECHANISMS OF COKE MICROTEXTURE FORMATION DURING COKING COAL CARBONIZATION

Textural and microtextural evolution of five coking coals during carbo nization were observed by optical microscopy (OM) and transmission ele ctron microscopy (TEM) to determine the mechanism(s) of coke microtext ure formation. Ultrathin sections, obtained by ultramicrotomy, from ch ars quenched at numerous close temperatures in the plastic stage, were examined by TEM in the 002 dark field mode. By coupling optical micro scopy and Gieseler plastometry results, these observations led to two different mechanisms for coke microtexture formation. For MacClure coa l which, in the present case, is the one passing through the best deve loped plastic stage, a mechanism similar to pitch carbonization was ob served. Although they did not show the Brooks and Taylor mesophase str ucture, very small spherules up to 1.5 mum 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 str uctural units (PSU). In the case of Norwich Park coal, with the highes t coal rank and the least developed plastic stage, domains were obtain ed by a completely different process and were due to the improvement, during a practically solid state, of a pre-existing preferential plana r orientation of the PSU.