Experimental study on gas desorption characteristics of hydrous coal by multistage pulsed ultrasonic excitation

In order to further clarify the influence of multistage pulse ultrasound on the characteristics of gas desorption in water-bearing coal, the ultrasonic stimulation test system of gas-containing coal is used to analyze the pore structure and gas desorption of coal under the condition of continuous ultrasonic stimulation of coal with different moisture contents and multistage pulse stimulation of water-saturated coal, and the mechanism of multistage pulse stimulation of water-bearing coal is revealed. The results show that the average pore diameter, specific surface area and total pore volume of coal increase with the increase of ultrasonic power and water saturation degree. When ultrasonic power is 1000 W, compared with dry coal, the increase of specific surface area and total pore volume of coal increases from 4.369% and 3.504% of 25% moisture content to 7.699% and 8.992% of 100% moisture content, respectively. There is a Langmuir-type relationship between coal gas desorption and time, and coal gas desorption amount and desorption rate are positively correlated with ultrasonic power and water saturation degree of coal. Compared with dry coal, the gas desorption capacity and desorption rate increase linearly with the increase of water saturation degree of coal, and ultrasonic stimulation has the best effect on the pore reconstruction and gas desorption of water saturated coal. With the increase of ultrasonic multistage pulse times, the specific surface area, total pore volume, gas desorption capacity and desorption rate of water-saturated coal increase. Compared with the 1000 W ultrasonic excitation, the increase of specific surface area and total pore volume of coal is a positive power function of the number of multistage pulses. The increase of gas desorption amount and desorption rate of coal increases from 2.745% and 4.598% of the first multistage pulse to 27.222% and 11.106% of the third multistage pulse, respectively, which is in a positive linear relationship with the number of multistage pulses. The combined effect of vibration and cavitation generated by multi-stage pulse ultrasonic stimulation of water-bearing coal causes fatigue damage to the coal matrix, strengthens the pore transformation of coal, and increases the molecular kinetic energy of gas, thus promoting gas desorption in coal. On site, the gas extraction in coal seam can be enhanced through the development of multi-stage pulse ultrasonic emitter combined with hydraulic technology.