Study of the Microstructure of Coal at Different Temperatures and Quantitative Fractal Characterization

In order to understand the influence of underground coalfireson coal fractures and pores, mercury intrusion porosimetry (MIP) andscanning electron microscopy (SEM) are combined to study the developmentof coal pore and fracture under high-temperature treatment and calculatethe fractal dimension to analyze the relationship between the developmentof coal pore and fracture and the fractal dimension. The results showthat the volume of pores and fractures of the coal sample (C200) treatedat 200 degrees C (0.1715 mL/g) is greater than that of the coal sample(C400) treated at 400 degrees C (0.1209 mL/g), and both are greaterthan the original coal sample (RC) (0.1135 mL/g). The volume increaseis mainly due to mesopores and macropores, and the proportions ofmesopores and macropores in C200 were 70.15 and 59.97% in C400. TheMIP fractal dimension shows a decreasing trend with the increase oftemperature, and the connectivity of coal samples improved with theincrease of temperature. The changes in volume and three-dimensionalfractal dimension of C200 and C400 showed the opposite trend and arerelated to the different stress of coal matrix at different temperatures.The experimental SEM images confirm that the connectivity of coalfractures and pores improves with the increase of temperature. Basedon the SEM experiment, the larger the fractal dimension, the morecomplex the surface is. The SEM surface fractal dimensions indicatethat the surface fractal dimension of C200 is the smallest and thatof C400 is the largest, which is consistent with the observationsmade by SEM. The combination of the two fractal dimensions is usedto characterize the self-similarity of coal using the fractal dimensiondifference. When the temperature increased to 200 degrees C, the unorderedexpansion of the coal sample resulted in the largest fractal dimensiondifference and the lowest self-similarity. When heated to 400 degrees C,the fractal dimension difference of the coal sample is the smallest,and the microstructure of coal shows a regular groove-like development.

Automated summary

Mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) were used to investigate the development of coal pores and fractures under high-temperature treatment. It was found that the volume of pores and fractures increased with temperature, and the connectivity of coal samples improved. SEM images confirmed the improvement in connectivity with temperature. Additionally, the fractal dimension was found to be related to the surface complexity of coal.