Pore characteristics of soft medium rank coal in Huainan Mining Area based on low temperature liquid nitrogen immersion treatment

In order to study the influence of low temperature liquid nitrogen immersion treatment on the pore structure and fractal characteristics of middle grade coal in Huainan Mining Area, coal samples were treated with different liquid nitrogen immersion time, and the pore structure of coal was measured by mercury injection and liquid nitrogen adsorption method. The fractal theory was combined to analyze the development law and scale characteristics of coal pores under different immersion time from multiple angles. The results show that with the increase of liquid nitrogen immersion time, the total pore volume of coal increases from 198.089×10⁻³ cm³/g to 371.553×10⁻³ cm³/g, while the total specific surface area decreases from 4.984 m²/g to 4.496 m²/g, and the effect is significant. The adsorption pore decreases, the seepage pore increases, and the pore connectivity of the adsorption pore increases to form a larger level of pore, which gradually changes to the seepage pore. There was a negative linear correlation between D₁ (fractal dimension of seepage pore) and D₂ (fractal dimension of adsorption pore) and the immersion time of liquid nitrogen, and the influence of liquid nitrogen immersion on D₁ was more significant than that of D₂. D₁ and D₂ decreased gradually with the increase of liquid nitrogen immersion time, indicating that the structural complexity of internal pores decreased with the increase of liquid nitrogen immersion time, and the porosity and permeability of coal increased. D_c (comprehensive fractal dimension) decreases with the increase of average pore diameter and total pore volume of coal body, and increases with the increase of total specific surface area. With the increase of liquid nitrogen immersion time, the comprehensive fractal dimension of coal body decreases, the adsorption capacity of coal body is weakened, and the seepage capacity is enhanced, which is helpful to improve the coal-bed methane extraction effect of low permeability coal seam in Huainan Mining Area.