Study of Nanopores of Tectonically Deformed Coal Based on Liquid Nitrogen Adsorption at Low Temperatures

In light of the test results of liquid nitrogen adsorption at low temperatures in different types and degrees of deformation of gas coal, coking coal, and anthracite coal samples. The variations in the properties of the nanopores with coal rank and deformation are analyzed and the causes of these variations are explored. The results show that nanopores in coal can be classified into three types: mini-nanopore (3–10 nm), meso-nanopore (10–22 nm), and macro-nanopore (22–100 nm). As for coal with small amounts of deformation, the three types of nanopores’ volumes are mainly affected by the coal rank. All three types indicate a stepped decrease with an increase in coal rank in the beginning and then change regularly in their own way. In regard to mini-nanopores, except that coking coal is not significantly affected by relatively strong deformation and the anthracite coal is not excessively influenced by relatively strong deformation of brittle fracture. The influence of relatively strong ductile and shearing deformation on gas coal and anthracite coal is characterized by an increase in the different degrees of mini-nanopore volume and an enhanced peak height with synchronized changes occurring in the 3.2–4.5 nm pores. The influence of relatively strong deformation on meso-nanopores and macro-nanopores is exhibited by the increase in the different degrees of pore volume. A model of the nested chemical structure of coal is proposed, which includes units I and II and skeletons III and IV. In addition, the mini-nanopores, meso-nanopores, and macro-nanopores correspond with the gaps in unit II and skeletons III and IV.