Estimating the spontaneous combustion potential of coals using thermogravimetric analysis

In this paper, the reactive properties of coals associated with low-temperature oxidation were studied using thermogravimetric analysis (TGA). Coal samples were pulverized into a size fraction of <106 μm and analyzed using two non-isothermal thermogravimetric methods. The "adsorption of oxygen" test consisted of exposing coal samples to slow heating rates in air while measuring the weight increase produced by the oxygen adsorption. Two heating ramps were used (3 and 5 C min^-1), recording the net mass increases from 20 to 250 C. Results range from 0.0 to 4.4% weight gain, with the lowest heating rate giving the highest levels of adsorption. A second thermogravimetric test for spontaneous ignition potential (TG_spi) was developed, which focused on the linear segment of the weight derivative curve between 150 and 350 C at different ramp rates in air (3, 5, 7, 10, and 20 C min^-1). From these results, a relationship between the mass loss rate and temperature was obtained (TG_spi index), where high values were produced by highly reactive coals. The "oxygen adsorption" test related to the chemisorption step, which precedes "spontaneous combustion". The "TG_spi" test was more related to the combustion process, post-ignition. These results also agreed with the known behavior for the samples studied, indicating that these tests could be useful tools to identify coals prone to self-oxidation.