The Microstructure of Pore Space in Coals of Different Rank

Scanning electron microscopy (SEM), small angle neutron scattering (SANS) and small angle X-ray scattering (SAXS) were used to analyse pore space for a series of Bowen Basin coals ranked in the range VR=0.7% toVR =3.1%. The linear pore size range assessed in the small angle scattering (SAS) experiments was 1.3 nm to 200nm. Using fractal analysis of the SAS data we showed that for the lower rank coals (VR less than 1%) the pore-coal interface is rough in the entire pore size range. For coals with VR larger than 1%, the interface becomes smooth on scales larger than 100 nm, but remains rough at smaller scales. The degree of roughness is decreasing with increasing coal rank and for anthracites the interface becomes smooth in the entire scale range.These findings are quantified by the calculated values of the specific area of the internal pore-coal interface. Owing to the fractal character of the pore space for sizes above 10 nm, the specific surface area was obtained by an appropriate scaling procedure and depends on the size of measuring yardstick. For the particular case of coals used in this study, when measured with the 5 A (0.5 nm, atomic size) yardstick the specific area decreases from about 200 m2/g for low rank coals to 3 m2/g for anthracites. The corresponding sorption capacity for methane at saturation is calculated to decrease from about 40 mg/g to 0.5 mg/g of coal. For a 60 Å (large molecule size) yardstick these limits are 20 m2/g and 3 m2/g, respectively.Apart from the fractal micro-architecture at scales above 100 Å, an additional microstructural feature of the characteristic smallest dimension of 20Å was observed. This feature develops in coals as their maturity increases. As a consequence, for mature coals the specific internal surface area may be significantly larger than that calculated solely from fractal scaling.SEM observations were performed on fragments of samples previously analysed by SAS. Samples were cleaved in the direction perpendicular to the bedding plane and internal surfaces of relatively large pores (of the order of 1µm across) opening to the exposed surface were observed at magnifications from x500 to x500000. For low rank coals, structural features down to the size of 5 nm were observed. The wide size range of the observed structural detail is consistent with the notion of fractal microstructure of coals, as determined using small angle scattering techniques.