Comparative sensitivity of various analytical techniques to the low-temperature oxidation of coal

A number of analytical techniques were used to investigate the low-temperature oxidation of a high-volatile and a low-volatile bituminous coal. Two oxidation treatments were used: stockpiling at a size of < 3 mm out of doors, and treating in moist air at < 420 μm in a laboratory retort maintained at 50 °C, 65% r.h. 57Fe Mössbauer spectroscopy and computer-controlled scanning electron microscopy (CCSEM) were used to investigate the oxidation of included minerals; diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and electron spectroscopy for chemical analysis (ESCA) were used to examine maceral oxidation; and the alteration of several technological properties (Gieseler plasticity, free swelling index and coke reactivity) was also monitored. Gieseler fluidity was by far the most sensitive of these measurements to the early stages of oxidation, while the Mössbauer measurement of the oxidation of pyrite to sulphates or oxyhydroxides was the second most sensitive indicator of oxidation for the coals investigated. DRIFT and ESCA exhibited significant features derived from oxygen functional groups in macerals only after the oxidation was severe enough to have nearly destroyed the plasticity. However, band ratio techniques appear promising as a method of enhancing the sensitivity of DRIFT to oxidation.