Multiscale Fractal Characterization of Pore Structure for Coal in Different Rank Using Scanning Electron Microscopy and Mercury Intrusion Porosimetry

Abstract

Multiscale fractal analysis of the pore system for coal is necessary to obtain more inner information. The techniques of Scanning Electron Microscopy (SEM) and Mercury Intrusion Porosimetry (MIP) are combined to characterize the pore structure of natural coal. A total of eight coal samples, of a different rank and coalification degree, are prepared for experiments. Methods of SEM image processing, piecewise curve-fitting and correction of intrusion data are adopted to obtain more useful results. According to the pore size range of the MIP probe, pores in coal are classified as seepage pore (pore size ≥ 1000 nm), transition pore (pore size ≥ 50 nm and <1000 nm) and mesopore (pore size < 50 nm). Variations of multi-scale fractal dimensions are studied from the perspective of coalification degree or coal rank. Fractal dimension from SEM data (D1) and fractal dimensions of seepage pore, transition pore and mesopore (D2, D′2 and D″2) from MIP data are calculated by fitting curves, and consequently correlations of those with volatile matter (Vdaf), pore volume and pore size are analyzed and discussed. The U-shape relationships between fractal dimensions (D1, D2 and D′2) and Vdaf are observed. Macropores are presented as the isolated clusters embedding in the network of smaller pores, and the difference of the order of magnitude of the pores’ size affects the connectivity between pores. Both the pore size and volume have a direct influence on multiscale fractal dimensions. Overall, multiscale fractal analysis is beneficial to explore the structure of natural coal.