89 / 2021-09-27 10:48:49

Study on the Evolution of Pore Structure of Anthracite Coal under LN2 Freeze-Thaw Cycles

Freeze-thaw cycle,Cryogenic cracking,pore structure,anthracite coal

    In order to analyze the evolution characteristics of the internal pore structure of anthracite by liquid nitrogen (LN₂) freeze-thaw cycles, the nuclear magnetic resonance (NMR) equipment was used to test the water-saturated anthracite samples which were frozen-thawed by LN₂ for 0~9 times.The pore size distribution, development degree and variation characteristics of different pores in anthracite coal samples subjected to LN₂ freezing were studied by analyzing the changes of parameters related to T₂ spectra of coal samples. The results show that:①LN₂ freeze-thaw cycles are beneficial to the development of internal cracks in anthracite obviously, and it has the greatest promoting effect on macropores. The crack development rate of coal samples decreased with the increase of the number of freeze-thaw cycles. The crack development rate was the fastest in the first freeze-thaw cycle, and the crack development did not increase significantly after the fourth freeze-thaw cycle. ②After four freeze-thaw cycles, the absolute growth rate of T₂ spectra area of the sample reaches to 35.54%, which effectively strengthened the development degree of coal fissure, and improved the gas extraction conditions of coal body, and effectively enhanced the gas extraction effect. ③The pore structure of anthracite is under the influences of thermal stress and frost heave force. Therefore, it can be determined that four times of LN₂ freeze-thaw cycles has the best effect in the LN₂ injection to increase coal permeability and recover coalbed methane(CBM). The results of this study provide theoretical support for the field application of coal freezing cracking and anti-reflection promotion pumping.