Fractal study of adsorption-pores in pulverized coals with various metamorphism degrees using N2 adsorption, X-ray scattering and image analysis methods

Abstract

As a highly heterogeneous material, the porous structure of coal is usually described by fractal dimension, which generally can be obtained by gas adsorption, light scattering and image analysis methods. In this paper, the pore structure (pore volume and specific surface area) and adsorption performance (Langmuir volume and Langmuir pressure) of 12 different rank coal samples were investigated by liquid nitrogen adsorption experiment (LNA) and methane isotherm adsorption experiment. And the fractal characteristics of these coals were studied by three fractal dimensions D1, D2 and D3 acquired from LNA and small angle X-ray scattering (SAXS) and scanning electron microscopy (SEM), respectively. Results indicate that the adsorption pore volume and specific surface area show U-shaped evolutionary trends during the coalification process. The changes of pore volume and specific surface area are dominated by the mechanical compaction and dehydration effects before the second coalification jump (Ro,max = 1.3%), while they are then mainly controlled by concomitant metamorphic pores until the fourth coalification jump (Ro,max = 3.7%). Langmuir volume first increases and then reaches a steady value at the third coalification jump (Ro,max = 2.5%) with the increasing coal rank, however, Langmuir pressure monotonously decreases during the whole coalification process. D1 and D2 exhibit a dramatically increase from Ro,max = 0.6% to Ro,max = 2.0% and then a slowly rise when Ro,max increases from 2.0% to 3.5%, which is relevant to the changes of pore structure and macromolecule structure in coal. D3 increases at a low speed first and then increases at a high speed with the increasing coal rank, which reflects the surface morphology of pores in coal. Fractal dimension D1, D2 and D3 can be used to depict the methane adsorption properties of coal in varying metamorphism degrees. Generally, D2 is larger than D1, which is because N2 adsorption can probe more macropores but SAXS can detect more micropores. Additionally, D1, D2 and D3 are all negatively related to PL. Our investigations also confirm that SAXS may be more superior to character the adsorption capacity of coal according to its highest relativity. This work clarifies the intrinsic sense and difference for characterising the surface irregularity by various methods, and provides an understanding for the experimental means relating to adsorption pores.