Effect on liquid nitrogen impregnation of pore damage characteristics of coal at different temperatures

In order to study the fracture expansion law and the pore damage characteristics of different prefabricated temperature coal that has been immersed by liquid nitrogen,microscopic observation,ultrasonic wave velocity test and nuclear magnetic resonance (NMR) technique are used respectively to study the fracture expansion,internal microfracture development,internal pore development,and pore size distribution of coal before and after being immersed by liquid nitrogen. The experimental results show that the increasing prefabricated temperature leads to the increase of thermal stress and characteristic fracture area. In addition,there is a strong correlation between the thermal stress and the increase ratio of coal surface fissure area. The higher the prefabricated temperature is,the bigger the porosity increase ratio of the coal after immersion gets. During immersion,there are two stages in the development of coal pore fractures development. In the first stage,the conversion number of micro pores to the medium or large pores is greater than the number of new ones. In other words,the number of micro pores falls while the number of medium or large pores rises. As the prefabricated temperature increases,it moves to the second stage,which the number of newborn micro pores is greater than that the conversion to the medium or large pores. In other words,the number of all pores rises. This paper studies the relationship between the fracture expansion law and the internal damage of different prefabricated temperature coal that has been immersed by liquid nitrogen. As a consequence,it finds the positive correlation among the characteristic fracture increase ratio,sonic wave velocity change rate and porosity change rate. The above experimental results show that the prefabricated temperature of coal has a significant effect on the fracture development and pore damage characteristics of coal which has been immersed by liquid nitrogen. Furthermore,there is a positive correlation between the fracture expansion and the internal damage.