Contrast test of liquid nitrogen freeze-thaw cycle cracking effect between anthracite and coking coal

In order to analyze the effects of coal ranks on the cracking effect of liquid nitrogen freeze-thaw cycle coal,a liquid nitrogen freeze-thaw cycle test was C arried out on high-rank anthracite and medium-rank coking coal. The nuclear magnetic resonance technique was used to test the variation of pore of coal samples after 3,6 and 9 times of liquid nitrogen freeze-thaw cycle. The effects of coal ranks and times of freeze-thaw on liquid nitrogen freeze-thaw cycle fracturing coal samples wer e investigated.The results show that the liquid nitrogen freeze-thaw cycle can promote the development of the internal pore structure of the coal sample and increase th e permeability of coal samples. After 3,6 and 9 times of liquid nitrogen freeze-thaw cycle,the average increase of T2 area of saturated meta anthracite is35.20%,5.97% and 4.20% ,and the average increase of the T2 map area of saturated coking coal is 44.72%,9.04% and 6.75% respectively.After thawing cycle ,the damage of coking Co al is obviously greater than that of anthracite. With the increase in the number of freezethaw cycles,the increase in T2 map area of saturated anthracite and coking coal has decreased significantly,and the effect of freeze-thaw damage on the improvement of coal pores has shown a decreasing trend.In the saturated state,the damage of coking coal after freeze-thaw cycles is significantly greater than that of anthracite.