Petrographic characterization of coals as a tool to detect spontaneous combustion potential

Textural features of 25 worldwide coals were studied after slow oxidation processing (0.5 °C min−1 from 20 to 250 °C in air) using oil immersion microscopy and image analysis techniques. The characterization of samples, before and after oxidation, showed important changes in vitrinite reflectance with high reactive coals, which also related to their intrinsic self-oxidation potential. The morphology of the coal particles was also altered after the oxidation, to produce at least six different morphotypes. Particles with 'homogeneous change of reflectance' and particles with 'oxidation rims' were predominant in the samples studied, which related to boundary reactive conditions (kinetic and diffusion control of the reaction respectively). These textural characteristics indicate how particles interacted with oxygen at low temperatures, which could be used to predict the most probable pathway during the early stages of oxidation which could then lead to a spontaneous combustion event. The magnitude of the reflectance change and the morphological characteristics of samples studied were also related to the reactivity properties, providing an additional source of information to identify coals prone to spontaneous combustion.