Changes in optical properties of coals during air oxidation at moderate temperature

Organic components in thermally altered coal waste: Preliminary petrographic and geochemical investigations

“…Organic matter oxidation also leads to an increase in carboxylic groups and to changes in the proportions of different bitumen fractions (Martinez and Escobar, 1995). During oxidation, vitrinite particles develop brighter 'oxidation rims' (Chandra, 1962;Stach et al, 1982;Calemma et al, 1995;Ndaji and Thomas, 1995). However, this is not always the case as Ingram and Rimstidt (1984) have reported darker rims.…”

“…9C). These are formed at temperatures of 200°C (Calemma et al, 1995;Ndaji and Thomas, 1995) or caused by oxygen diffusion (Calemma et al, 1995). In a number of cases, the oxidation rims are zoned (Fig.…”

“…However, this is not always the case as Ingram and Rimstidt (1984) have reported darker rims. During vitrinite oxidation, irregular cracks have also been shown to develop (Stach et al, 1982;Calemma et al, 1995;Ndaji and Thomas, 1995). These changes occur mainly in the vitrinite macerals; inertinite particles tend to develop microfractures and pores (Ingram and Rimstidt, 1984).…”

Review of experimental methods to determine spontaneous combustion susceptibility of coal – Indian context

“…the formation of rims along grain boundaries and micro-fractures, the formation of micro-pores and micro-fissures, increased relief, coke formation and variation in reflectance) using different petrographic technique Crelling et al, 1979;Gray et al, 1976;Marchioni, 1983). The changes observed in the petrographic texture of oxidised coal may be due to the reaction temperature and the chemical changes of coal, which can be determined by studying the change in reflectance and the presence of an oxidation rim (Avila, 2012;Benedict & Berry, 1964;Calemma et al, 1995;Gray et al, 1976;Marchioni, 1983;Nandi, Ciavaglia, & Montgomery, 1977 ;Pearson & Creaney, 1981;Ribeiro, Suárez-Ruiz, Ward, & Flores, 2016). Artificial oxidation due to heating develops brightness of vitrinite particle and at the boundaries of particles (oxidation rim) whereas natural weathering processes result in dull rims (Avila, 2012;Calemma et al, 1995;D.…”

“…Similarly, microscopy petrographic studies are widely used due to its simplicity, low cost and wide availability. A geological examination may be used to predict the propensity of a coal to spontaneously heat by performing a study of the maceral compositions and rank of the samples (Avila, 2012;Calemma, Del Piero, Rausa, & Girardi, 1995;D. Chandra, 1958D.…”