Construction of molecular structure model of Tunlan coal and its microscopic physicochemical mechanism

Physicochemical structure is essential for understanding physiochemical properties of coal reservoir and coalbed methane (CBM) occurrence. In this work, ultimate analysis, 13C nuclear magnetic resonance spectra (NMR), Fourier transform infrared spectroscopy (FTIR) and gas chromatograph/mass spectrometer (GC/MS) tests were adopt to investigate the coal molecular structure information. Then, by means of molecular simulation, stable three-dimensional (3D) molecular structure and physicochemical structure models of Tunlan coal were established to reveal the physicochemical response at a molecular level. Results show that compared with the single molecular structure, angle and torsion energies in nanopore structure play a more important role in maintaining a stable and stereoscopic structure. While the electrostatic energy in non-bond energy can simulate to form nanopore space in coal. Then, with the dissolution of low molecular weight compounds (LMWCs), the pore volume (PV) of micropores increases gradually, while the corresponding specific surface area (SSA) decreases first and then increases, suggesting that pore expansion and pore increase effects are dominant in the early and the later dissolution stages, respectively. Furthermore, observing physicochemical response, total pore structure distribution depends on whether vertical or horizontal changes of basic structure units in coal make a great contribution in this evolution stage. Actually, when the aromatic layer is mainly the increase of the stacking height Lc, it is easy to result in the decrease of total SSA and PV. In contrast, for mainly the increase of the diameter La, all pores in coal present an increase characteristic to induce the increase of total SSA and PV.