## 5 Conclusions  
This paper focuses on the changes in unfrozen water content during the melting process of lignite coal after 150 min of liquid nitrogen freezing. The evolution of unfrozen water during the melting process is a key factor in controlling water migration and freezing. Figure 10: Change mechanism of unfrozen water content in the coal body during the thawing process. The difference in the distribution of unfrozen water directly affects the melting rate of coal seams. The study of the evolution law of unfrozen water during the melting process can further optimize the technical process of liquid nitrogen fracturing and determine the scientific and effective freezing time to achieve the best fracturing effect. The exploration of the intrinsic mechanism of the melting process in this paper will provide theoretical guidance for the engineering application of liquid nitrogen frozen permeability-enhancement technology. The main conclusions are as follows:  
1. The two-dimensional NMR T\({}_{1}\)-T\({}_{2}\) spectra reflected the unfrozen water space within the pore space. According to the T\({}_{1}\)-T\({}_{2}\) spectra at different temperatures, the unfrozen water content could be obtained in ascending order with increasing temperature. 2. In the melting process of frozen coal sample, the pore structure was composed of coal pore and ice pore. As the melting progresses, the small pores formed by the ice gradually disappeared, the number of small pores decreased and the number of intermediate pores increased. 3. The unfrozen water content was affected by temperature and pore size. The change of pore pressure with temperature directly affected the melting point of ice. The melting of coal samples started from the small pores until the temperature rose to a certain degree and the large pores began to melt. 4. Under the influence of intermolecular potential energy between water molecules and coal wall in the pore, the ice melting point decreased in small pore. Since the smaller the pore is, the larger the intermolecular potential energy is, a more significant effect occurs on the ice melting point reduction. This is the key reason why the ice in the small pores melted first during the melting process of frozen coal. In this paper, we only theoretically analyzed the effect of pore pressure on pore ice melting point and unfrozen water content under a single pore size condition. Due to the uneven pore size distribution of real coals, the control mechanism of pore size and distribution on unfrozen water content in coals with different degrees of metamorphism will be discussed in our future studies. 5. In this paper, it was found that the order of pore ice melting was small pores first and then medium-large pores, and the slow ice melting in medium-large pores will undoubtedly increase fracturing time and reduce the permeability-enhancement effect in the coal seam. At the same time, previous tests found that the freezing sequence was medium-large pores first and then small pores. Therefore, we believe that when applying low-temperature freeze-thaw fracturing technology, rapid freeze-thawing should be adopted to improve the freeze-thawing efficiency of medium-large pores, avoiding the waste of energy and time by freezing and thawing of small pores, so as to achieve the purpose of high-efficiency permeability enhancement and coalbed methane production increase. For example, rapid cyclic freezing and thawing or rapid hot and cold alternation can be adopted to fully reduce the consumption of liquid nitrogen, energy and time, as well as to promote the low-cost and high-efficiency application of this technology. Conceptualization, L.Q. ; methodology, T.L. and W.Y. ; formal analysis, X.Z. ; investigation, S.L. ; data curation, M.M. and J.L. ; writing--original draft preparation, T.L., X.Z. and L.Q. ; writing--review and editing, T.L. and L.Q. ; supervision, B.L. ; funding acquisition, T.L. and L.Q. All authors have read and agreed to the published version of the manuscript.