The structural evolution characteristics for high volatile bituminous coal by in-situ heating in electronical microscope

Abstract

In order to visually investigate the aromatic layer arrangement and structural evolution characteristics, high volatile bituminous coal was selected and investigated by in-situ high-resolution transmission electron microscopy (HRTEM) and thermogravimetry coupled with mass spectrometry (TG-MS). HRTEM was applied to quantify the size and arrangement of aromatic layers (such as length, curvature, orientation, d002 and Lc) during in-situ heating from room temperature (RT) to 1000 ℃, thereby revealing aromatic structural evolution characteristics. TG-MS was employed to identify gaseous release characteristics from RT to 900 ℃·H2O, CO2, CH4, C3H6, benzene and H2 were monitored and measured in this study. Based on the HRTEM experimental results, it was found that with temperature increasing, the distributions of aromatic layers changed from disordered to well-aligned and then to disordered again. Among them, the naphthalene structure, the smallest aromatic layer structure identified in the HRTEM image formed by the condensation of two benzene rings, presents the most obvious variation. In addition, the relationship between aromatic structural evolution and gaseous generation was also discussed in this work. The results showed that the structural evolution of high volatile bituminous coal was complex and closely related to the gaseous generation characteristics. At the temperature of RT∼300 ℃, the decomposing of oxygen-containing functional groups caused an increase in the content of naphthalene, the layers became shorter but more disordered, d002 decreased significantly while Lc increased. Up to 400 ℃, the layers were extended and parallel, the d002 decreased and La increased, while Lc remained almost stable, demonstrating that the polymerization of smaller layers formed in the previous stage was the main reaction in this stage. 400–600 ℃ was the main pyrolysis stage with large amounts of volatile matter generated, new active sites were thus formed and reconnected, causing the growth of aromatics and improvement of the order degree. At 600 ℃∼700 ℃, the content of naphthalene increased again, which may possibly due to the decomposition of methylene-bridge bond connected between aromatic rings, accompanied by the release of CH4 and benzene. Then the layers arrangement reached the most order and the perfect crystallite structure began to be formed gradually up to 800 ℃, which is owing to the polycondensation between the aromatics accompanied with the release of H2. As the temperature rise to 1000 ℃, the value of Lc showed a significant increase and the d002 remained a slight decrease, indicating the improving of the stacking degree, however, the formation of new naphthalene through the aromatization of aliphatic rings caused the naphthalene content increase and La reduce significantly.