Dynamic Simulation Based on a Simplified Model of 1/3 Coking Coal Molecule and Its Formation Characteristics of Hydration Films

Abstract

To comprehensively elaborate the formation characteristics of hydration films on 1/3 coking coal molecule, this paper reports the construction of a realistic simplified model for calculations of electrostatic potentials on the coal molecular surface to foresee the major immersion locations. On this basis, interactions at the interface of coal molecules with different numbers of water molecules and their effects on each functional group of coal molecules were investigated. Using the scanning electron microscopy experiment, changes in the coal matrix before and after water leaching were compared and analyzed by fractal dimension calculations. Hydration characteristics of coal were described from a combined macroscopic and microscopic perspective. The results showed that both positive and negative electrostatic potential of coal molecules occurred near the O-containing functional groups. The hydroxyl group’s electrostatic potential (−OH) rose, resulting in higher electrostatic potential in coal–water molecules and providing many immersion sites. Deficiency in water molecules led to the complete immersion of water molecules. The interface of coal molecules could not be covered entirely, which led to the low number of active sites and Z values. The interface of coal–water molecules did not affect the average bond lengths of water molecules but decreased the bond angle by 3–4°. The influence ofwater molecules on the −OH groups of coal molecules was the most prominent when water molecules were incorporated into the coal molecules. Water damage for the coal matrix is more pronounced than in the raw coal itself. In view of above research, the formation characteristics of the hydration film from a microscopic point of view explained that the initial hydration of coal molecules was owing to H-bonds. From a macroscopic perspective, it was mainly due to structure changes for the coal matrix. This provides valuable references for field experiments in hydraulic fracturing and perforation.