Photoacoustic FT-IR study of weathered stockpiled coking coals

The process of weathering (natural oxidation) of two medium volatile bituminous coking coals stored at the INCAR open stockyard for several months has been studied by means of Gieseler plastometry, the free-swelling index (FSI) test, and by photoacoustic Fourier transform infrared spectroscopy (PA-FT-IR), the latter technique allowing only the surface of the sample exposed to the atmosphere to be analyzed. The proximate and ultimate analysis of the coals did not reveal any significant changes in their chemical composition. The Gieseler maximum fluidity (MF), however, was found to decrease with weathering time; using PA-FT-IR, a reduction in the aliphatic hydrogen content and a shortening of the alkyl chains was also detected. In addition, changes in the proportion of the oxygen-containing structures were observed, although the evolution of this kind of groups with weathering time was dependent on the raw coal. (C) 2002 Elsevier Science B.V. All rights reserved.