Changes in the structure of coals of different rank due to oxidation - effects on pyrolysis behaviour

Relations between structural changes due to coalification and oxidation, and the modification of pyrolysis behaviour of coals are discussed. Five coals of different rank, ranging from high-volatile bituminous to semi-anthracite, were used. Fourier transform infrared spectroscopy (FTIR) was employed to obtain information about changes in functional groups. The pyrolysis behaviour of coals was evaluated by means of differential thermal analysis. The temperature of maximum volatile release could be used as a parameter indicative of coal rank. Oxidation gave rise to an increase in carbonyl and carboxyl functionalities at the expense of both aliphatic and aromatic hydrogen. The presence of these oxygenated groups in the structure of the coals gave rise to an increase in the percentage of volatile matter released in the low temperature range. The results obtained showed that the reactivity of aliphatic hydrogen plays a major role in pyrolysis behaviour in the intermediate temperature range. The relative proportion of volatile matter evolved at high temperatures may be related to the proportion of aromatic hydrogen. On the basis of the relative amount of volatiles released during the various stages of pyrolysis it is possible to construct a classification of coals that reflects rank and oxidation degree. (C) 1998 Elsevier Science B.V. All rights reserved.