Devolatilization behaviour of naturally weathered and laboratory oxidized bituminous coals

The influence of natural weathering and low temperature laboratory oxidation on the devolatilization behaviour of bituminous coals was investigated. The study was conducted on six sets of coals, each comprising a fresh and an oxidized (crop) coal from the same seam. One of the fresh coals was oxidized in a laboratory at 200 °C for 2 and 72 h to elucidate the differences between natural weathering and low temperature laboratory oxidation and their effect on the devolatilization behaviour of the coal. The compositional analyses revealed that the volatile matter of the oxidized coals were higher than those of the corresponding fresh coals. However, the volatiles release rates, as observed in a thermogravimetric analyser, were slower and the volatiles were released over a wider temperature range for the oxidized coals than for the fresh coals. The volatiles released during flash pyrolysis of the crop coals, as determined by gas chromatography-mass spectrometry, were leaner in aliphatic and aromatic hydrocarbons and other combustible gases and richer in oxygenated species than those from their fresh companions. The differences in the nature and the quality of the volatile matter between the naturally weathered, laboratory oxidized and fresh coals indicate possible differences in the ignition behaviour of these categories of coals.