The tectonic stress–driving alteration and evolution of chemical structure for low- to medium-rank coals—by molecular simulation method

Abstract

Coal organic macromolecular structure can be significantly altered by tectonic stress, but such macromolecular models are seldom reported. Here, based on the ultimate analysis, FT-IR, XPS, and 13C NMR results, three molecular models of primary coal (P1) and TDCs (the brittle coal model B2 and ductile coal model D3) within periodic boundary conditions were constructed through the molecular mechanics (MM) and molecular dynamics (MD) calculation methods. By comparing the morphology characteristics of the above three models, the crystal parameters decrease with the increasing deformation intensity, which leads to a tighter configuration. Bridge bonds are easier to be stretched and even broken up within deformed coal, which can furtherly crack the coal molecules into several smaller segments. Oxygen functional groups and methyl can be lost by the deformation on coal, which leads to an increase of the disorder units within coal molecules. On the one hand, the ductile deformation increases the condensation degree of the aromatic layers through promoting the small disorder units (viz. CO) embed into the secondary structural defects. On the other hand, it can also improve the order degree and the stacking degree of aromatic layers. By comparative analysis, the evolution path of tectonically deformed coal molecules is furtherly proposed. The ultra micropore volumes of three models are several orders of magnitude larger than the pore volume within the range of 2–50 nm, indicating that ultra micropores are important storage spaces for excess methane resulting in coal and gas outburst.