Adsorption pore structure and its fractal characteristics of coals by N2 adsorption/desorption and FESEM image analyses

Abstract

The adsorption pore structure and its fractal characteristics of coals (0.69–2.79% Ro,m) were investigated by N2 adsorption/desorption and field emission scanning electron microscopy. First, organic matter (OM) pores and interparticle pores (InterP) with various shapes are distinguished in different rank coals. The OM pores present irregular shapes in low-/middle-rank bituminous coals, and circular, oval and slit shapes in high-rank bituminous coal and anthracite with width from 50 nm to 2 μm. In contrast, the InterP pores show polygonal/elongated shapes and exist between mineral grains or mineral and OM with width >100 nm. Second, the coals with H1 type hysteresis loop of N2 adsorption/desorption isotherms are favorable for gas adsorption due to large pore surface area, whereas coals with H2 and H3 types are conducive to gas desorption and diffusion because of open pores with good connectivity. Moreover, the fractal dimension of pore shape was obtained from FESEM images and ranges from 1.068 and 1.30, indicating the adsorption pore shape is relatively regular. The surface and volume fractal dimensions of micropores and mesopores were calculated from the N2 adsorption data on the basis of surface fractal model and FHH model, respectively. The fractal dimension of micropore and mesopore surface respectively ranges from 2.18 to 2.69 and 1.78 to 2.41, whereas that of micropore and mesopore volume respectively ranges from 2.18 to 2.67 and 2.29 to 2.79. The pore structure and coal composition during different coalification stages have specific effects on the fractal characteristics of pore shape, surface and volume.