Characterization of Ultramicropores and Analysis of Their Evolution in Tectonically Deformed Coals by Low-Pressure CO₂ Adsorption, XRD, and HRTEM Techniques

Porosity in coal is the main space and migration channel for coalbed methane, and more than 90% of the specific surface area of coal comes from ultramicropores (<1.1 nm), which are expected to be affected by metamorphic and deformation processes in tectonically deformed coal (TDC). It is important to know how ultramicropores occur and evolve for safe mining and coalbed methane development. In this work, we employ low-pressure CO 2 adsorption at 273 K (LPCO 2 ), X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM) techniques to investigate ultramicropores in one undeformed coal and seven TDCs from the Huaibei coalfield in Northern China. The results show that the pore volume (PV) and specific surface area (SSA) of ultramicropores ranged from 0.00839 to 0.02009 cm 3 /g and from 27.44 to 76.432 m 2 /g, respectively. As the deformation intensity increases, the PV and SSA of TDCs decrease in the brittle deformed stage and then increase in the ductile deformed stage. The results suggest that brittle deformation has a limited effect on the molecular structure of coal and only compacts the intermolecular space due to stress. However, ductile deformation can alter the molecular structure of coal to increase the PV and SSA of ultramicropores. f S and f L calculated by HRTEM images in different deformed tectonic coals have the same evolution law as do d 002 and L c obtained from XRD. The increase of L a and L c in ductile coal (73.4% and 47.6%) is much greater than that of brittle deformed coal (4.05% and 14.4%). SSA and d 002 are negatively correlated in the brittle deformed coal and are positively correlated in the ductile deformed coal. The rotation, folding, and recombination of the aromatic layer during the ductile deformation process by mechanochemical action is the main reason.