Evolution characteristics of micropore and mesopore of different rank coal and cause of their formation

In order to investigate the evolution of the structure characteristics of micropore and mesopore of different rank coal and the cause of their formation, 7 different metamorphic coal samples collected from North China Permian coal basin were tested using low-pressure nitrogen and carbon dioxide adsorption techniques respectively. The change rule of pore size distribution(PSD), pore volume(PV)and specific surface area(SSA)of micropores(pore diameter less than 2 nm) and mesopores(pore diameter lies in 2~50 nm) with different metamorphic degrees of coal were analyzed using density function theory(DFT), Dubinin-Astakhov(DA)method, Dubinin-Radushkevich (DR) method, BET and BJH formula. Then the cause of the formation of micropores and mesopores was discussed. The results show that: PV and SSA of micropores are positively correlated with its vitrinite reflectance, the micropore with pore diameter below 2 nm is the dominant factor in coal adsorption; PSD curves of micropores are of bimodal distribution, and different coal samples have similar PSD curves, the ultramicropore has a fastest increasing amount in the micropores; PV and SSA of mesopore decrease with increase of coal rank, and its PSD show unimodal distribution. With the increase of metamorphism, BET and SSA of coal decrease firstly and then increase with “U” pattern; The formation of micropores in coal is mainly controlled by the microcrystalline parameters and the stacking structure of the aromatic layer, while the formation of mesopores is mainly controlled by change of coal side chains and the space of basic structure unite.