Effect Of Ultrasonic Excitation On Functional Groups And Pore Structures Of Coal

The key of gas prevention and control is to crack and increase the permeability of coal seam.Improving the permeability of coal seam to promote the flow of gas and improve the efficiency of gas drainage is an important method to prevent gas disaster in coal mines,and at the same time,it can effectively improve the utilization rate of gas.The traditional methods of coal seam cracking and permeability enhancement have achieved some results in practical application,but limitations still exist.Ultrasonic wave is a kind of sound wave with frequency higher than 20000Hz,having good directivity,strong reflection ability and is easy to obtain concentrated sound energy.It can be used for ranging,velocity measurement,cleaning,gravel,etc.Furthermore,it has numerous applications in the industry,medicine and military fields.Meanwhile,it also has a broad prospect in coal seam antireflection and fracturing.So far,the research on ultrasonic induced fracture is mainly focused on the theory at home and abroad,while the influence of ultrasonic wave on the pore structure and chemical properties of coal is seldom studied.In view of the above problems,this paper used the method of theoretical analysis and experiment to deeply study the change of pore structure and functional groups,the influence of ultrasonic on the gas adsorption characteristics,and the influence of ultrasonic frequency on the cracking effect of coal after ultrasonic excitation,providing theoretical support for the cracking of coal by ultrasonic excitation.The thermal effect of ultrasonic wave is the key factor of cracking coal.Therefore,the heat transfer law in the process of ultrasonic cracking was analyzed firstly in this paper.The infrared thermal imaging technology and thermocouple were used to monitor the surface and internal temperature of the coal,and the change rule of the internal and external temperature of the coal in the process of ultrasonic cracking was analyzed through the temperature change curve.Fourier infrared spectrum analysis was carried out.The influence of ultrasonic on the main functional groups of coal was analyzed by measuring the changes of infrared spectrum curve of coal before and after ultrasonic cracking.The results show that the content of oxygen-containing functional group,hydroxyl group,aliphatic hydrocarbon and aromatic hydrocarbon in the coal are reduced after ultrasonic cracking.Meanwhile,the change of the functional groups leads to the change of gas adsorption characteristics and the decrease of gas adsorption capacity of coal.By using low field NMR equipment,the influence of ultrasonic wave on micro pore structure of coal was analyzed,and the change of coal porosity and permeability was analyzed by T₂ relaxation curve.The results indicate that the total porosity,effective porosity and permeability of coal are improved after ultrasonic cracking,while the residual porosity is reduced.At the same time,the higher the ultrasonic frequency is,the more obvious the effect of fracture will be.Through PCAS analysis technology and wave velocity detection technology,the change law of the cracks on the surface and inside of the coal was studied,which proved that ultrasonic wave can effectively promote the development of the cracks on the surface and inside of the coal.The pores and fractures in the coal before and after ultrasonic cracking were reconstructed by three-dimensional reconstruction technology-3D-XRM,to observe the influence of the ultrasonic wave on the pore and fracture structures in the coal and make quantitative analysis.The results show that the number and volume of pores in the coal increased after ultrasonic cracking.It can be seen from the results of 3D reconstruction that the ultrasonic wave promoted the formation of new pores in the coal,and the cracks were further extended under the influence of it.All above show that ultrasonic wave can effectively crack the coal and reduce gas adsorption capacity of coal,thus promote gas flow in coal seam and improve gas extraction efficiency.There are 45 figures,16 tables and 100 references in this paper.