Experimental study on micro-structure of coal by ultrasonic treatment

This paper is devoted to reveal the microscopic mechanism of ultrasonic vibrating to improve coalbed methane extraction rate. The effects of ultrasonic treatment on the micro-structure of coal is investigated using scanning electron microscopy, surface area and aperture analyzer, and nuclear magnetic resonance analyzer. The results show that the vibration of ultrasonic can clean the micro-particles on the surface of the water-containing coal and in crack channel, open the sealing pores in the coal, and partially break the loose coal, thus creating new cracks in coal. After the ultrasonic treatment, the total pore volume, specific surface area, average pore diameter, porosity, and T2 spectrum peak of coal all increase. The connectivity of coal pore fracture also increases, which is beneficial to the desorption, diffusion, and seepage of coalbed methane. Furthermore, the N2 adsorption ability of coal increases after ultrasonic treatment because there is adsorption hysteresis in the process of adsorption and desorption, which results in the formation of a large type C hysteresis loop in the isothermal adsorption desorption curve. The porous and fissure structures of coal are changed by ultrasonic cavitation, mechanical vibration, and thermal effect. This study provides a reference for improving coalbed methane extraction rate using ultrasonic treatment.