Experimental Study on Temperature Response of Different Ranks of Coal to Liquid Nitrogen Soaking

Abstract

Liquid nitrogen freeze–thaw has been used in oil, shale gas and coalbed methane exploitation as an efficient fracturing technology. This paper aimed to study the effect of different coal ranks and liquid nitrogen soaking times on the temperature distribution of coal samples, and to explore the temperature evolution mechanism of different coal ranks during liquid nitrogen soaking. For these objectives, the temperature change process, thermophysical parameters and infrared spectrum of different coal ranks under liquid nitrogen soaking were tested using, respectively, (a) liquid nitrogen soaking temperature measurement, (b) laser thermal instrument and (c) Fourier transform infrared spectrometer. The results showed that the temperature curves of coal samples under liquid nitrogen soaking were divided into an accelerated cooling stage I, a decelerated cooling stage II, and a maintained low-temperature stage III. As the number of liquid nitrogen soaking increased, the time required to reach low-temperature Stage III gradually shortened. During the rise in coal sample temperature, it increased with time in accordance with a logarithmic function. The order of absolute values of maximum heating/cooling speed was lignite > bituminite > anthracite. The higher coal rank is, more oxygen-containing functional groups were removed by coalification. The less content of oxygen-containing functional groups led to closer molecular structure, which resulted in smaller thermal conductivity and ultimately caused slower temperature transfer. The study results are of important guides to understand further the action process and mechanism of liquid nitrogen soaking on coal.