THE DIFFERENCE OF THE MOLECULAR STRUCTURES BETWEEN DEFORMED SOFT AND HARD COAL AND THEIR INFLUENCES ON THE CHARACTERISTICS OF ADSORPTION METHANE

This paper aims to study the influence of molecular structural difference between deformed soft and hard coal on methane adsorption and obtains the quantitative indexes that reflect the above influence degree. In this paper, XRD and FTIR experiments were adopted to determine the molecular structure of typical deformed soft and hard coals with different ranks. The molecules structural parameters of deformed soft and hard coal were compared and analyzed. Based on the above structural difference characteristic, four corresponding physical models of macromolecular structure are established. A simulation for deformed soft and hard coals was established to test out the behavior difference of adsorption and its mechanism; by using Materials Studio (MS) software, same scenario is also applied on the other four physical models listed above. Then, the isothermal adsorption methane experiments of four kinds coal samples were carried out to verify the above simulation results. The results show that the molecular structure of deformed soft and hard coals has significant differences, i.e. comparing with hard coal, deformed soft coal has smaller interlayer spacing d002; greater lateral sizes La, stacking heights Lc and the crystal nuclei sizes La/Lc. Deformed soft coal had a more orderly structure crystal nuclei, bigger crystal volume, the larger basic structure unit (BSU) and more flattened. Deformed soft coal has less oxygen-containing functional groups, lower fatty hydrocarbon content and more C=C. These findings verified that dynamic metamorphism promotes aromatization and large molecular polycondensation of the coal, thus improving the aromatic degrees of the crystal nucleus. Due to the increasing of the lateral sizes being beneficial and the stacking heights being not conducive to the adsorption of the coal, the La/Lc was proposed as an indicator of the coal adsorption methane ability. The isothermal adsorption experiments show that the adsorption capacity of deformed soft coal with the same rank is greater than that of hard coal, which is in good consistency with the above research results. Also, the above experimental results provide a theoretical basis for the determination of coalbed methane content and the estimation of CBM.