Molecular model construction of Chifeng lignite and analysis of adsorption mechanism of O2 at low temperature

Abstract

Oxygen adsorption by coal is the key step to trigger coal spontaneous combustion. In this manuscript, the results of elemental analysis, 13C NMR and XPS analysis were combined with molecular mechanics (MM) and molecular dynamics (MD) methods to establish a macromolecular model of Chifeng lignite and its unit cell structure. The mechanism of oxygen adsorption by coal was studied by means of Grand Canonical Monte Carlo (GCMC) and Density Functional Theory (DFT) at temperatures of 273 K, 288k and 303k and at pressure range of 0 ∼20 MPa. The results show that a single coal molecular cluster reaches saturation after adsorbing 22 oxygen molecules, and the van der Waals force and electrostatic interaction play a major role in the adsorption process. With the increase of oxygen adsorption capacity, physical adsorption began to change to chemical adsorption. Moreover, the adsorption strength of oxygen in coal molecules is affected by the adsorption physisorption site, location and direction. Oxygen tends to be adsorbed on each functional group structure of coal in a parallel direction, and the adsorption capacity of each functional group structure for oxygen is as follows: aromatic ring (-3.13∼-3.32 kcal/mol) > methyl (-2.677∼-2.463 kcal/mol)> hydroxyl (-2.378∼2.092 kcal/mol)> ether bond (-2.375∼-1.955 kcal/mol)> carbonyl (-2.27 9∼-1.910 kcal/mol) > carboxyl (-1.536∼-2.238 kcal/mol). The research results can be used to predict the oxygen adsorption capacity of different coal types, and provide a reference for preventing coal spontaneous combustion.