Pore structure characteristics of China sapropelic coal and their development influence factors

Sapropelic coal is similar to humic coal in organic matter content, and to shale in kerogen type. Research on the sapropelic coal pore structure can help to understand the differences between humic coal and shale in pore development. In this paper, the pore structures of 21 sapropelic coal samples from Shaanxi, Shanxi, Gansu and Shandong provinces were determined, coupled with proximate analysis, reflectance (R-o), maceral and fractal analysis, scanning electron microscopy (SEM), mercury porosimetry (MIP), and low-temperature nitrogen adsorption tests. The results indicate that Late Paleozoic and later age sapropelic coals are low rank with R-o values ranging from 0.31% to 0.82%, and Early Paleozoic sapropelic coals are high rank with R-o values ranging from 2.64% to 5.89%. The low-rank coals are mainly of Types IIa and IIb kerogen containing humic detritus. The high-rank coals are of Type I kerogen composed by bitumen matrix and minerals. According to the normalized pore size distribution (PSD) curves of MIP and low-temperature N-2 adsorption, three pore types (Type I-M, Type IIM, and Type IIIM) for seepage pores and three pore types (Type I-N, Type IIN, and Type IIIN) for adsorption pores are divided, respectively. For seepage pores, all the low-rank sapropelic coals have the Type I-M PSD, while the high-rank coals contain Type I-M, Type IIM and Type IIIM PSDs. From Type I-M via Type IIM, to Type IIIM, the content of mesopores and macropores increases; meanwhile, the fractal dimensions (D-t, derived from the thermodynamics model) also increase. For adsorption pores, from type I-N via type IIN to type IIIN, the BET specific surface area (SSA), density function theory (DFT) volume and Frenkele-Halseye-Hill (FHH) fractal dimension (D-f) successively increase, and the medium pore diameters successively decrease. It was found that the development of the saproplelic coal pore structure was controlled by the integrated effects of coal rank and composition. Generally, the seepage pores of sapropelic coals studied in this work are mainly primary or intergranular pores with weak correlation with the coal composition. However, the development of the adsorption pores has obvious correlations with organic matter content, ash yield rate (A(ad)) and moisture content (M-ad).