Detection of weathering in stockpiled coals by Fourier transform infrared spectroscopy

The process of natural oxidation of two low-rank coals exposed to the atmosphere for 11 months has been studied by Fourier transform infrared (FT-IR) spectroscopy. The study was carried out on samples taken at different time intervals and additionally from zones where signs of high oxidation and self-ignition were detected. The aliphatic hydrogen (3000-2800 cm(-1)) and oxygen-containing structures (1800-1500 cm(-1)) regions of the spectra were examined by curve-fitting analysis and a series of structural parameters based on ratios of integrated absorbance areas of curve-fitted bands were established. The aliphatic hydrogen content of samples tended to decrease with increasing time of storage and carboxyl groups only increased slightly under conditions of low pile activity. When oxidation and self-ignition processes took place, the structural changes were more significant. Aliphatic structures decreased drastically and net production of oxygen-containing structures was observed. Aliphatic hydrogen content evaluated from integrated absorbance measurements of normalized spectra and the CO/aliphatic hydrogen ratio seemed to be very sensitive in detecting signs of weathering even at very low levels of activity.