## 3 Experimental Results and Analysis  
### T\({}_{2}\) Spectrum Test Results  
The T\({}_{2}\) spectrum is the time constant describing the recovery process of the transverse component of the nuclear magnetization intensity and is therefore called the transverse relaxation time. The transverse relaxation process is caused by the exchange of energy within the nuclear spin system, so it is also called the spin-spin relaxation time. The NMR T\({}_{2}\) spectrum can approximate the content of hydrogen-containing material within the pores of the coal sample and its approximate distribution. Due to the complexity of the rock pore structure and the diversity of pore fluids and their fugitive states, the relaxation process is the result of multiple component contributions with different relaxation times. It can be expressed as a multi-exponential function: when the formation is saturated with water and the echo interval is relatively small, the T\({}_{2}\) spectrum corresponds well to the distribution of the rock pore size. Figure 2 shows the T\({}_{2}\) distribution curves of coal samples during melting at different temperatures. Usually, the T\({}_{2}\) distribution curve will show from one to three different peaks, which represent the relaxation times of hydrogen-containing substances in coal samples in different states. According to the special equation between the NMR transverse relaxation time and the sample pore size, it is calculated that the first peak represents the content of hydrogen-containing substances in micro small pores, while the second and third peaks correspond to the content of hydrogen-containing substances in medium and large pores. According to the existing research, the three peaks are classified. The characteristic values of the types are 2.5 ms, 100 ms, 10,000 ms and also can be the cut-off values. As shown in Figure 2a, when the temperature ranged from \(-\)196 \({}^{\circ}\)C to \(-\)20 \({}^{\circ}\)C, the area of the first peak increased, while the area of the second and third peaks did not change significantly. The main reason for this section was the continuous melting of ice in the tiny pores of coal samples. When the temperature ranged from \(-\)2 \({}^{\circ}\)C to 10 \({}^{\circ}\)C, the area of the second and third peaks increased significantly, and the ice in the large pores of this section began to melt continuously. In summary, during the melting process, the ice in the small pores melted first, and the temperature rose to a certain extent before the ice in the large pore melted. In addition, the unfrozen water in the pore space was mainly distributed in the pore space with a radius of 10 nm or smaller. Figure 1: Experimental equipment and process. As shown in Figure 2b, the first peak \"rightwing shift\" occurred in the T\({}_{2}\) spectrum of coal samples during the melting process. This is because during the freezing process, the water phase inside the mesopore and macropore becomes ice, and the ice compressions the remaining unfrozen water and fills the pores, which changes the size of the original pores and gradually forms ice pores. Therefore, in the negative temperature zone, the first peak not only represents the small pores in the coal sample but also includes the small ice pores in the large pores. With the increase of temperature, the ice in the large pore began to melt, the ice pore gradually disappeared and the area of the first peak gradually decreased and shifted to the right. The T\({}_{2}\) curve under four temperatures during the melting process of coal sample was integrated to obtain the curve of the change of peak area with temperature, based on which the change of unfrozen water content with temperature of coal sample was analyzed. As shown in Figure 3, the area enclosed by the T\({}_{2}\) curve and the horizontal axis represents the amount of pore water (unfrozen water). When the temperature of the frozen coal sample was between \(-196\)\({}^{\circ}\)C and \(-20\)\({}^{\circ}\)C, there was little change in the amount of unfrozen water.