Effects of preheating and oxidation on two bituminous coals assessed by synchronous UV fluorescence and FTIR spectroscopy

Modifications induced by the preheating process in the distribution of families of polyaromatic hydrocarbons (PAHs) in chloroform and pyridine extracts of two bituminous coals from Spain have been determined by synchronous UV fluorescence spectroscopy. Using PAH model compounds, three main synchronous fluorescence spectral regions can be distinguished according to the number of lineally condensed aromatic rings. The differences due to the different rank of the two parent coals (wet coals) are retained when they are subjected to oxidation and preheating prior to oxidation. Using chloroform and pyridine as extracting solvents, the lighter PAHs with two- or three-rings (mainly with one naphthalene skeleton) and the relatively larger and heavier PAHs with four- or more aromatic rings appear to be the compounds most affected by preheating of the highest rank coal. The differences between the parent and preheated coals in the composition of the extractable material are in good agreement with results from Fourier transform infrared structural studies, reflecting the relative higher aromaticity and degree of condensation of polyaromatic units, as well as the geochemical parameters of the entire organic matrix of these coals, such as the WC and O/C atomic ratios. The oxidation effects of the wet and preheated coals have also been studied using the same methodology by synchronous fluorescence and FTIR spectroscopic techniques. When the preheated coals were oxidized a different behavior was observed between wet and preheated coals. This was due to structural changes caused by the preheating at about 200 degrees C. The fluorescence data suggest that the highest rank coal (Mieres) after preheating had a similar oxidation response to Mieres wet coal, whereas the preheating-induced structural changes in Nalon coal provide fewer active sites to be oxidized.