Effects of coalification on nano-micron scale pore development: From bituminous to semi-anthracite

Abstract

Surface atom functionalization and the variation of aliphatic/aromatic structures within coal macromolecules are key factors to determine the pore structures within coalification. The characteristics of nano-micron scale pore structures were analyzed with the aid of coupled fluid intrusion and molecular probe technologies. Results indicate that the cleavage of long-chain aliphatic structures enhances the size of the pore throat and reduces the specific surface area and pore volume of the mesopores (2–50 nm). Macropores (>50 nm) were slightly impacted by coalification and their development was primarily controlled by diagenesis. Micropores (<2 nm) overall increase with the enhancement of aromatization and condensation polymerization; however, in the stage of low volatile bituminous, the blocking of minerals or low boiling hydrocarbon solids will temporarily reduce micropores. Meanwhile, similar surface structures and functional groups lead to similar characteristics of the pore size distribution for mesopores and micropores. Furthermore, the peak value of microporous pore volume migrates to the smaller pore size, and pores <0.4 nm in size generated from clearances among the aromatic layers gradually become dominant with coalification. The macropores fractal dimension (Df1) is not only related to coalification. However, the heterogeneity and surface roughness of micropores (Dm) and mesopores (Dv2) are controlled by coalification. Dv1 of mesopores is affected by diagenesis. The pore walls are contributed by aliphatic structures before the third coalification jump. Meanwhile, micropores among aliphatic structures developed relatively independently and small-scale pore networks frequently exist within them. Accompanied by the cleavage of aliphatic structures, the aliphatic structures around the pore walls will be gradually replaced by the aromatic structure. Micropores of relative independence are decreased, and large-scale pore networks are gradually formed. The pore morphology developed to be more complex and strongly heterogeneous. In addition, high-density oxygen/nitrogen/sulfur functionalization will increase the micropores.