Study On Microstructure Change And Heat Conduction Law Of Coal Under Liquid Nitrogen Soaking

Liquid nitrogen soaking can cause certain damage to coal structure,and improve the permeability of coal.In addition to the characteristics of energy saving and environmental protection,the method of liquid nitrogen soaking to crack and increase coal permeability has high application value in pressure relief and pre-extraction of coal seam gas.Therefore,based on the previous research results,this paper conducts an in-depth study on the effect of liquid nitrogen cracking and the law of temperature propagation.In order to study the specific form and degree of influence of liquid nitrogen on coal cracking,this paper uses microscopic experimental methods to study and compare liquid nitrogen with scanning electron microscopy and CT scanning experiments under the condition of dry and saturated water of coal.The surface and internal structure of coal changed before and after cold soaking,and quantitative analysis of pore changes by nuclear magnetic resonance test;Because the ultra-low temperature nature of liquid nitrogen is the most likely to cause the structural damage of coal,this paper studies the internal heat transfer law of coal under the soaking of liquid nitrogen,and further derives the governing equation of coal freezing process under dry and saturated conditions,and uses the finite element analysis software to solve the governing equation.The numerical simulation results are compared with the experiment to verify the correctness of the simulation results.Research indicates:(1)The surface and internal micro-destruction forms of liquid nitrogen coal are different.For dry coal samples,liquid nitrogen erosion causes some coal particles near the surface of coal to fall off,and internal micro-cracks penetrate;for saturated coal samples After cold immersion by liquid nitrogen,the primary cracks on the surface appear widened and extended,and the internal microstructure changes mainly to produce secondary fractures on the basis of the primary fractures;(2)The cold soaking effect of liquid nitrogen also affects the pores of coal.With the extension of cold soaking time,the number of micro and small pores of dry and saturated coal samples gradually increases.it can be concluded that the change of coal temperature during liquid nitrogen cold soaking is inseparable from the damage caused to coal;(3)Under the action of liquid nitrogen,the temperature of the dry coal is slower than that of the saturated coal,and the total temperature history of the experienced temperature balance is longer.Using the established model,the temperature propagation law of coal during freezing process can be well analyzed.