Investigation on the size and fractal dimension of nano-pore in coals by synchrotron small angle X-ray scattering

Abstract

Coal is a heterogeneous porous medium with complex pore structures. These pore structures directly influence the adsorption, desorption and migration of gas in coal reservoirs. The characteristics of pore structure play an important role in various aspects of coal utilization including extraction of methane from coal seams, CO2 sequestration in coal and water purification by activated carbon. In this paper, the synchrotron small angle X-ray scattering (SAXS) was applied to test micropore structure (1–100 nm) in six coal samples with different ranks. In the experiment, the particle size of coal powder was 60–80 mesh. The recorded SAXS two dimensional images were processed and quantitatively converted into one dimensional scattering data using FIT2D software. Based on the corrected SAXS data, the pore size distribution and specific surface area were quantitatively estimated. The Porod scattering curves, Guinier scattering curves, pore size distribution, specific surface area, and fractal characteristics of six coal samples were explored and analyzed. The results indicate that the SAXS data of six coal samples show the positive deviation of Porod theory. This observation proves that there are additional density fluctuations in the coals. Thus, the original SAXS data essentially needs to be corrected to achieve the true scattering results. The polydisperse distribution of micropores is observed based on the Guinier curves of six coal samples. Moreover, the pore size distribution of tested coals was analyzed based on the maximum entropy theory and the “double peaks” feature was observed. Both the average pore diameter and the most probable pore diameter of the tested samples decrease with the increase of coal ranks, which is consistent with the results proved by the methods of mercury injection and low temperature nitrogen adsorption. The fractal dimension of micropores in the tested coal samples was also discussed. According to the double logarithmic curves which shows the realationship between scattering intensity I and the scattering vector q , through different range of q value, method of tangent double logarithm is adopted to get the corresponding q value range of fractal dimension of coal pore. The surface fractal dimensions of micropores were calculated and presented as the scattering vector q stating at the relative lower value. However, the pore fractal dimension related to the distribution of micropores was presented as the scattering vector q rising to the relative higher value. It was found that the surface fractal dimension of micropores experience decreasing firstly, then increasing and finally drop again with the increase of coal ranks. The pore fractal dimension of micropores decreases at beginning and then increases with the rise of coal ranks. It is also found that the minimum value of pore fractal dimension appears as the R o,max equals to 2.07% based on the current test results. SAXS is a powerful tool to study the micro pore structure of coal, however it is necessary to point out that the small angle scattering data processing methods, different processing methods or theories have advantages and limitations. Therefore, in the follow-up study, increasing the sample number and variety of different metamorphic degree of coal, the pore size, the ratio surface area with the metamorphic degree of variation is more abundant and exact.