The influence of impregnation by hydrocarbons on coal structure during itsthermal evolution

The present work analyses the changes in the chemical structure of a perhyd rous coal during its thermal evolution at different temperatures in an open -medium pyrolysis system. The results obtained were compared with those des cribed for non-perhydrous coals in order to establish the effect of the sub stances assimilated by the coal structure (hydrocarbon/oil-like substances) on the thermal evolution of the coal. The transformation ratio at each sta ge of thermal treatment was determined and the chemical-structural characte risation of the resultant products was performed. Changes in textural and m icrotextural properties associated with structural modifications during the evolution were also tested. The results obtained show that this perhydrous coal develops a specific evolution pathway different from that followed by non-perhydrous coals with a normal H/C ratio. The substances assimilated b y the perhydrous coal cannot be easily and totally released from its struct ure so that they can be only partially removed after thermal treatment. Thu s, the treatment debilitates the interactions between the substances and th e coal matrix in addition to weakening and cracking the matrix during the t hermal process. The increase in temperature also leads to the conversion of some of the heavy assimilated substances into lighter compounds. However, the assimilated substances are present even at high temperatures of the the rmal process, providing hydrogen which has the effect of stabilising the ra dicals originated during the pyrolysis and improving the fluidity propertie s in the reacting medium. (C) 2001 Elsevier Science B.V. All rights reserve d.