Chemical structural transformations of different coal components at the similar coal rank by HRTEM in situ heating

The structural transformations in three coal components, barkinite, vitrinite and resinite, on heat-treatment from 200 to 700 °C were investigated by high-resolution transmission electron microscopy (HRTEM) technique. The orientation in chemical structure was examined by HRTEM image. The orientation in aromatic layers of three coal components increased gradually on heating from 200 °C to 700 °C. Vitrinite followed barkinite's transition some 50 °C later in temperatures of chemical structural change than barkinite. The transformations of structural parameters derived from TEM images of three coal components were different, including the interlayer spacing (d), the layer size (La), the number of aromatic layers per stack (n), and the aromatic layer stacking height (Lc). At least three temperature stages (200-400, 400-600, and 600-700 °C) were shown. Barkinite has similar change trends of d, Lc, and n with resinite however the La values were similar to vitrinite at 200-700 °C. Both barkinite and resinite had significant decreased d values at 200-400 °C and gradual decreased from 400 °C to 600 °C. The values of Lc and n for barkinite and resinite exhibit no obvious changes at 200-400 °C and increased significantly >450 °C. Vitrinite decrease significantly in d from 450 to 600 °C (4.32-3.62 Å) and increase greatly in Lc and d up to 450 °C (12.84-20.29 Å). Vitrinite and barkinite have two temperature ranges for increasing significantly in La from 400 °C to 700 °C. When the temperature is >600 °C, these coal components had much larger Lc, n, and La values with temperature increasing and decreased gradually in d values. The variation of structural parameters when heating was explained by the alteration of chemical structures in coal on pyrolysis and plastic behavior.