Abstract: In this paper, the small-angle X-ray scattering (SAXS) is used to study the structure and distribution characteristics of
nanopores in the coals of a natural evolution series. Our results show that, with the increase of coal rank, the pore surface fractal changed
in multiple stages: Ro < 0.89%, the exinite began to liquefy gradually, developing a lot of pores, and the fractal dimension increased
continuously; 0.9%<Ro<1.5%, the pores were filled with volatile oil, and the crude oil asphalt underwent aromatization, both lead to the
micropore surface smooth and the fractal dimension decreased; 1.5%<Ro<3.5%, the vitrinite pyrolysis generated a large number of
nanopores, and the fractal dimension increased again; then it gradually graphitized, and the surface fractal decreased again. The
nano-sized pores in coal are mainly concentrated in the range of 50-100 nm. The volume percentages of the fine mesopores (2-10 nm),
the mesopores (10-25 nm), and coarse mesopores (25-50 nm) are 0.21%-3.12%, 5.06%-11.28%, and 21.06%-26.36%, respectively.
Large pores (50-100 nm) account for the largest percentage of volume, up to 64.63%-68.36%. With the increase of coal ranks, the most
measurable pore size of coal sample decreased continuously from 80 to 10 nm, and its decreasing speed increased gradually; the volume
percentage of mesopore and fine mesopore increased continuously, showing a logarithm and linear relationship with maturity
respectively, while the percentage of rough mesopores and macropores decreased, showing a logarithm relationship with maturity. The
most measurable pore diameter changed obviously. In the low-coal bituminous coal stage, as the degree of coalification increased, the
most measurable pore diameter decreased slightly (the pore diameter at the peak ranges from 75 to 71 nm). In the middle-high coal
bituminous coal stage, with the increase of the degree of coalification, the most measurable pore diameters showed a more obvious
downward trend (the pore diameter at the peak ranges from 78 to 53 nm). In the anthracite stage, the pore size decreased rapidly (the
pore size at the peak ranged from 72 to 9 nm)

Key words: coal, SAXS, nanopores, pore size distribution