A Study on the Pore Structure and Fractal Characteristics of Briquettes with Different Compression Loads

In order to study the effects of different compression loads on the pore characteristics of coal, taking remolded coal as the research object, the mercury intrusion method was used to determine the pore structures of the briquettes under the compression loads of 50, 70, 90 and 110 MPa, and the Menger sponge model was used to conduct fractal research on the measured parameters. The results show that the compression load has a significant effect on the pore structure parameters of the briquettes. The hysteresis loop generated by the mercury-intrusion and mercury-extrusion curves of raw coal is small, and the pore connectivity is better. After different loads are applied for briquettes, the hysteresis loop becomes larger, and the pore connectivity becomes worse. From the process of the raw coal to the briquettes loaded at 50 and 70 MPa, the pore-specific surface area reduced from 5.069 m 2 /g to 1.259 m 2 /g, the total pore volume increased from 0.0553 cm 3 /g to 0.1877 cm 3 /g, and the average pore size increased from 43.6 nm to 596.3 nm. When the compression load reached 70 MPa, the specific surface area, total pore volume, and average pore diameter of briquettes remained basically stable with the change in the compression load. The minipores and visible pores and fissures of raw coal contribute 78% of the pore volume, and the micropores and minipores contribute 99% of the specific surface area. After being pressed into briquettes, the volume of mesopores and macropores increases, the volume of visible pores and fractures decreases and the volume of minipores changes little; additionally, the pore surface area contributed by mesopores and macropores increases, and the pore surface area contributed by micropores decreases, indicating that the effect of compression load on pores of 10–100 nm is not obvious, mainly concentrated in the 100–10,000 nm region. The fractal curve of briquettes is fitted into three sections, which are defined as low-pressure sections 1 and 2 and high-pressure section 3, and the fractal dimensions are D 1 , D 2 and D 3 respectively. The fractal dimension D 1 of briquettes with different compression loads is close to 2, D 2 is close to 3 and D 3 is greater than 3. The pore structures of briquettes have obvious fractal characteristics in the low-pressure sections 1 and 2 but do not conform to the fractal law in the high-pressure section. Furthermore, in the micropore stage of briquettes, the measured surface area and volume are both negative, indicating that the mercury intrusion method used to test the pore structure of the loaded briquette is more likely to cause the collapse of and damage to the pores in the micropore (<10 nm) stage.