An experimental study on the evolution of aggregate structure in coals of different ranks by in situ X-ray diffractometry

Abstract

Coal samples of different ranks were selected based on geological ages and regions to investigate their thermal evolution in aggregate structure using in situ X-ray diffraction (XRD) spectroscopy. Samples were heated at a rate of 10 °C min⁻¹ in the range of 25–900 °C. The XRD profiles of the coal samples displayed an evident decrease in intensity of peak at 20° (λ-band), which suggests the decomposition of oxygen-containing functional groups in the coal matrix at higher temperatures. In contrast, the peak at 26° (so-called G-band) became sharper and shifted to higher angles, which suggests a more ordered crystallite structure of coal with increasing heating temperature. Note that the integrally diffracted intensity of these two bands weakened intensely above 400 °C, which suggests that the coal aggregate structure may be loosened due to the depolymerization of the carbonaceous matrix in coal. The primary phase of weight loss for the low-rank coals took place in the range of 300–500 °C, but the high-rank coals had an obvious second pyrolysis reaction above 500 °C. Moreover, this study shows that the heating temperature effect plays a key role in the evolution of the interlayer spacing of the crystalline structure (d₀₀₂) and height of the aromatic layers (L_c), whereas the diameter of the aromatic layers (L_a) is more dependent on coal ranks rather than heating temperature.