To study the effect of uniaxial compression on coal nanostructure during uniaxial compression, in situ synchrotron radiation small angle X-ray scattering experiments were carried out on four coals with different ranks under uniaxial compression. According to the scattering data during the uniaxial compression process, the fractal characteristics and the variation feature of fractal dimension with stress were obtained. Four coals with different ranks all possess two fractal characteristics: pore fractal occur in the smaller pore range (7–17 nm) in the high q value range, and surface fractal occur in the larger pore range (17–52 nm) in the low q value range. For two low rank coals, with increasing stress, the pore fractal dimension DP decreased and the surface fractal dimension DS increased, respectively; the variation trends of DP and DS were obvious. This indicates that with increasing stress, the heterogeneity and complexity of the pores decrease, the surface roughness of the pores increases, and stress has a significant effect on the nanopore structure. The smaller pores are more susceptible to stress, and the influence range of stress on low rank coals is larger than that on high rank coals. The change rate of fractal dimension (RD) has a poor relationship with compressibility during uniaxial loading and is related to coal rank. The RD per unit stress for high rank coals is larger than that for low rank coals. Nanostructure response to uniaxial compressive stress is more significant in low rank coals than in high rank coals. Compared with low rank coals, high rank coals have strong aromatization and molecular structure, and the nanostructures are less susceptible to failure under uniaxial stress.
Gas adsorption and desorption capacities and ad-/desorption hysteresis in coal are important for carbon capture and storage (CCS) and coalbed methane (CBM) development. To investigate the impact of fractal features on gas adsorption and desorption capacities and ad-/desorption hysteresis in coals, five coal samples were collected and carried out methane (CH4) and CO2 isothermal ad-/desorption experiments. Small angle X-ray scattering (SAXS) was applied to characterize the fractal features of the coal pore structure. The results show that five coal samples show surface fractal features, represented by surface fractal dimension (Ds). The adsorption and desorption capacities of CO2 are stronger than those of CH4. In the adsorption stage, Ds and Langmuir adsorption volume (VL-ad) show a positive relationship for CH4 and CO2, due to the van der Waals force and available adsorption sites. In the desorption stage, Ds and Langmuir desorption volume (VL-de) show a positive relationship for CH4 and CO2, because most adsorbed gas molecules can desorb and diffuse out of the pores when gas pressure decreases. No obvious correlation was found between Ds and Langmuir adsorption pressure (PL-ad) as well as between Ds and Langmuir desorption pressure (PL-de) for CO2 and CH4. An improved hysteresis index (IHI) was adopted to characterize the degree of gas ad-/desorption hysteresis. The IHI values of CO2 vary from 12.2 to 35.2%, and those of CH4 vary from 8.9 to 50.3%. The curves of Ds vs. IHI for CO2 and CH4 are like an irreversible “V” shape, which yields to be further studied. This work further extends SAXS application in exploring the impact of coal pore structure on gas adsorption related phenomena, which is beneficial for CCS technology and CBM development.
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