Control of chemical structure on the characteristics of micropore structure in medium-rank coals

The formation and evolution of micropore was controlled by molecular structure to a certain extent. To study the influence of chemical structure on micropore structure, a well characterized sample set of coals covering a vitrinite reflectance (Ro,max) interval from 1.08% to 1.80% was selected from the Xishan coalfield, China. CO2 isothermal adsorption experiments at 273.15 K were used to characterize micropore structure in coals, and 13C nuclear magnetic resonance (13C NMR), FTIR and Raman spectrum were employed to obtain the chemical structural parameters of those coal samples. The relationship between structural parameters and micropore structure showed that micropore volume increased with increasing aromatic carbon (fa’), aromaticity (I) and the degree of condensation of aromatic ring (DOC), which demonstrated that micropore volume was mainly controlled by the aromatic part of the coal samples when Ro,max ranging from 1.21% to 1.80%. Moreover, the ZCD-2 sample (Ro,max = 1.21%) has the lowest micropore volume, which may be caused by the lower value of DOC or blocking of asphaltene in the second coalification jump. The micropore volume increases with increasing band position difference of D peak and G peak (G-D) and AD/AG, which suggests that the degree of order and more ordered rings (≥ 6 rings) mainly control the micropore formation. The findings help to understand the mechanism by which chemical structure controls the evolution of micropores, and further provide mechanistic insights of gas sorption and diffusion.