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Problems in pore property testing of lignite: Analysis and correction
International Journal of Coal Geology  (IF6.806),  Pub Date : 2021-07-25, DOI: 10.1016/j.coal.2021.103829
Fudong Xin, Hao Xu, Dazhen Tang, Ding Liu, Cao Can

Due to the low coalification degree of lignite, the colloid-like properties and loose characteristics lead to low energy utilization efficiency and seriously hinder the correct understanding of the original pore structure and its mechanical behavior. As a standard process for physical property test, removing the moisture in the pores by drying will cause the lignite to shrink sharply, destroying its original pore structure. Also, for test methods that need to be performed under high pressure, such as mercury intrusion porosimetry (MIP), the soft and loose texture of lignite will cause it to be significantly compressed, making the test results inaccurate. Aiming at the conventional lignite testing process problems, an optimized scheme suitable for lignite was proposed. By rapid freezing and sublimation at low temperatures, freeze-drying can effectively prevent the collapse of lignite. The solid-state of water protects the lignite's primary structure and shape and minimizes volume shrinkage. The average shrinkage rate of lignite sample after freeze-drying is only 9.72%, while that after hot air-drying is 34.8%. Therefore, dehydration by freeze-drying can be an effective method to protect the pore structure of lignite. Based on correcting the syneresis problem, the piecewise linear interpolation method is used to correct the compression effect of lignite. The corrected result excludes the increased pore volume due to the compression of the lignite skeleton. Also, it avoids the under-correction of the low-pressure part and overcorrection of the high-pressure part in the conventional correction method. Based on the corrected nitrogen adsorption and MIP results, the nuclear magnetic resonance (NMR) surface relaxivity of lignite is calculated. The average surface relaxivity value of the samples is 42.8 μm/s, which is significantly greater than that of medium and high-rank coals. The corrected NMR pore size distribution is closer to the actual pore size distribution of lignite. Finally, based on the analysis and correction of the problems in pore property testing of lignite, the improved workflow suitable for lignite is proposed, which provides some referential significance for the correct understanding of its actual physical property.