Study on wetting and dust reduction characteristics of in-situ pulsating water injection of large-scale coal

In order to study the influence of combined physical and chemical effects on the wetting and dust reduction of coal seam water injection, a large-scale coal sample true triaxial coal seam pulsation water injection test system is built independently, and on the basis of a comprehensive analysis of the wetting performance of eight single surfactants, orthogonal compounding of sodium dodecyl sulphate （SDS） and diethanolamine coconut oleate （CDEA） is carried out by adding acid and inorganic salt additives. Based on the acoustic emission system monitoring, wetting speed test, wetting angle test and coal sample impact dust production experiments, the wetting performance of the compounded solution, the expansion of coal body pores and cracks under different pulsation frequency, water injection parameter and water composition, the damage evolution characteristics, the distribution of water injection volume and water content in the area, and the characteristics of the impact crushing and dust production are investigated. The results show that under the combined effect of surfactant-acid and inorganic salt additives, the average settling time of coal dust is only 48 s, and the average contact angle is reduced by more than 28° compared with that of pure water. Based on the number of acoustic emission ringing and the distribution law of positioning points, it can be seen that the pulsation water injection method can greatly increase the degree of development of coal body pore cracks, low-frequency pulsation fracturing speed is slow, but it can make the coal body produce a uniform and rich fracture network, and the high-frequency pulsation can make the coal samples produce larger fatigue damage faster, but it is easy to form unidirectional large fissures. Compared with the traditional constant pressure water injection method, under the synergistic injection efficiency of pulsation + additives, the water injection volume of the coal body reaches up to 180.5037 mL, which is 33.41% higher, the average water content reaches up to 17.74%, which is 4.95% higher, and the percentage of dust below 75 μm in the crushing of the coal body decreases by up to 78.13%. The change of water content in each block of the coal body shows a high correlation with the acoustic emission energy and the size of the number of ringing, and the water flow expanded along the fissure network within the coal body, mainly concentrating in the area with stronger acoustic emission energy and more ringing.