Research On The Mechanism Of Multiple Water Molecules Being Adsorbed On Low-rank Coal

As the national economy develops, the reserves of high-rank coal is decreasing gradually, making it increasingly important to exploit and utilize low-rank coal, i.e. lignite. The poor energy quality and low temperature of the waste heat during lignite utilization can be used to make up for the cost of coal drying, significantly improving cycle efficiency and thermal economy of the power plant. High water content, one of the marked characteristics of lignite, deeply influences the utilization of lignite, increasing the transportation cost and the fixed investment of the power plant yet decreasing the combustion efficiency. Therefore it is imperative to study the interaction of lignite and water.Theoretical calculation of quantum chemistry is applied in the study on the interaction of lignite surface molecular model and multiple water molecules. Lignite surface model is established in GaussianView. Optimization of the equilibrium configuration of lignite molecular model and multiple water molecules is conducted at the level of B3LYP-D3 in ORCA. The interaction energy and surface electrostatic potential distribution of the equilibrium configuration is calculated in Gaussian-09 W. Combined with Multiwfn and VMD, reduced density gradient function(RDG) is used to graphically demonstrate the strength, type and location of the weak interaction between lignite surface molecular model and multiple water molecules.Theoretical analysis of surface electrostatic potential of lignite molecular model and the equilibrium configuration indicates that extreme points of surface electrostatic potential mostly concentrate at the surface area of oxygen functional groups and water molecules. The adsorption of water molecules does not affect surface electrostatic potential distribution of lignite molecule greatly but more adsorption sites with strong chemical activity are thereby available. Calculation results of coal-water equilibrium configuration, adsorption sites and water molecular cluster structure indicates physisorption between lignite surface molecular model and multiple water molecules. Most of the interaction between lignite molecular model and water molecule is hydrogen bond, with the rest being van der Waals weak interaction. As more water molecules get adsorbed onto lignite molecule surface, it is increasingly possible to detect water dimmers, trimmers and cluster structures in the equilibrium configuration. Cluster increases along with water molecules and the interaction energy of the equilibrium configuration gets stronger, making the structure more stable. The value of function RGD on every spatial spot of the configuration is calculated. Hydrogen bond, electrostatic potential, van der Waals action even steric effect are demonstrated graphically and the corresponding results are consistent with the calculation results above, conforming to what is currently understood and studied on the binding force of coal structure and molecular simulation of coal-water interaction.