Abstract: Coal and gas outburst seams generally have the characteristics of high gas content and low permeability. In order to eliminate coal and gas outburst, coal samples from Dayun Coal Mine, Guizhou Province were studied and the method of carbon dioxide cracking was used to carry out low temperature nitrogen adsorption experiments to compare the microscopic pore changes of coal before and after cracking. The static volumetric method was used to test the coal samples with pore size between 3 nm and 200 nm before and after CO₂ cracking. The fractal theory was used to quantitatively describe the effect of CO₂ cracking on the inner surface of coal rock pores. The results show that the carbon dioxide cracking has a significant effect on the pore structure of the coal seam within 3 m, that the coal sample after the cracking has a significant reduction in the specific surface area and the diameter of the most pores, and an obvious increase in the average pore diameter and pore volume, that the effect of cracking on microporous damage is remarkable, and the number of micropores after splitting is reduced by an order of magnitude, with a maximum decrease of 83.93%, that the fractal dimension of raw coal is reduced from 2.705 93 to 2.553 78 after cracking, which indicates that the pore surface of Guizhou coal seam is complex and rough, and the use of carbon dioxide cracking can make the pore surface of coal seam smooth, and that specific surface area and fractal dimension have a positive correlation.

Key words: coal and gas outburst, carbon dioxide cracking, low temperature nitrogen adsorption, characteristic change of pore, fractal surface