Experimental study on the effects of ultrasonic excitation on pore structure of water-bearing coal

Coal seam water injection is one of the main coal mine dust control measures. To study the effects of ultrasonic excitation on the pore structure of water-bearing coal in depth, ultrasonic excitation experiments were conducted with varied variables including ultrasonic excitation time, ultrasonic power, and fluid pressure on coal. The effects of sodium dodecyl sulfate (SDS) treatment and ultrasonic excitation were also compared. The coal samples treated under different conditions are characterized by low-temperature nitrogen adsorption and scanning electron microscopy (SEM) respectively for the quantitative analysis of pore volume and specific surface area and qualitative analysis of surface pore and fracture structure. The results show that ultrasonic excitation can significantly increase the pore volume and specific surface area of coal and damages the inner wall surfaces of the pore to different extents, making the pore inner surfaces rougher. However, the effect of ultrasonic excitation becomes weaker after the pore volume increases to a certain extent, resulting in more uniform pore size and volume distributions. Ultrasonic excitation time affects the pores with smaller size more significantly, and there is a time threshold effect. The volume and specific surface area of large pores and fractures show limited responses (or increases) to as the excitation time increased. The energy is mainly used for the fracturing and expanding the pores with smaller size. The effect of ultrasonic excitation on pore expansion is more obvious under the fluid pressures lower than 4 MPa. High fluid pressures weaken the fracturing and expanding effect of ultrasonic excitation on the pore structure of water-bearing coal. The ultrasonic power threshold effect is also observed. Once exceeding a certain value, further increasing the ultrasonic power does not dramatically affect the pore structure of water-containing coal. Ultrasonic excitation can enlarge the primary pores and fracture and generate secondary pores and fracture in coal. In contrast, the 0.85% SDS treatment is only able to clean the primary small pores, and no new pore structures are formed.