Abstract: The test results of coal rock low-temperature nitrogen absorption experiment are significantly affected by the particle size of samples. At present, there are few systematic reports on the differential influence of sample particle size on the pore structure of coal with different metamorphic degrees. In this study, 7 coal samples with different metamorphic degrees (including lignite, gas coal, fat coal, lean coal, anthracite, and super-anthracite) were selected to carry out low-temperature nitrogen absorption experiments on the coal particles with four different particle sizes (40-50 meshes, 70-90 meshes, 160-200 meshes and >300 meshes); the impact of sample particle sizes on the pore structure of coal with different metamorphic degrees was explored. The results show that (1) the macerals of five pieces of bituminous coal and super-anthracite are dominated by desmocollinite, while those of two lignites (i.e., fusinitic coal and detritic coal) are mainly composed of fusinite and densinite, respectively; fusinite cell pores are mainly developed in fusinitic coal, while the lean coal and anthracite are dominated by gas pores. (2) The pore volume and specific surface area of total pore, macropore and mesopore of bituminous coal and super-anthracite are increased progressively with the decreasing of particle size, while the pore structure of detrital coal remains unchanged, and the macropore volume and specific surface area of fusinitic coal are gradually decreased. (3) As the degree of coal metamorphism is increased, the difference of pore volume and specific surface area between the minimum (>300 meshes) and maximum coal particle (40-50 meshes) is continuously increased. (4) In the progressive pulverization process, the absorption loop width of bituminous coal and super-anthracite is decreased gradually, while that of lignite remains basically unchanged. The study shows that lignite is dominated by open pores, and the proportion of closed pore volume can be increased sharply with the increasing degree of coalification; lean coal, anthracite and super-anthracite are dominated by closed pores. The original pore morphology in coal rock changed significantly during pulverization; progressive pulverization leads to not only the release of many isolated closed pores in bitumite and super-anthracite, which will be transformed into open pores with one end closed, but also significant damages to the cell pores in fusinitic coal. In addition, neglecting the effect of coal particle size on the low-temperature nitrogen adsorption test results may lead to an incorrect evaluation of the pore structure of coal reservoirs. Therefore, a reasonable sample particle size range is proposed for determining the pore structure in coal rock by means of low-temperature nitrogen adsorption method, ">300 meshes" is recommended as the optimal sample particle size range for determining total pores (i.e., closed and open pores) of coal rock, and "2-4 meshes" as the optimal particle size range for determining open pores in coal rock.

Key words: low-temperature nitrogen adsorption method, coal with different metamorphic degrees, pore structure, absorption loop, particle-size effect

摘要： 煤岩低温氮吸附实验的测试结果受测试样品颗粒粒径的影响显著，目前鲜有关于制样粒径对不同变质程度煤孔隙结构差异化影响的系统研究报道。选取7件不同变质程度的煤样（包括褐煤、气煤、肥煤、瘦煤、无烟煤和超无烟煤）开展了4种不同粒径（40~50目、70~90目、160~200目、>300目）煤颗粒的低温氮吸附实验，探讨了制样粒径对不同变质程度煤的孔隙结构的影响。结果表明：①5件烟煤—超无烟煤的显微组分均以基质镜质体为主，2件褐煤（丝质煤和碎屑煤）的显微组成分别以丝质体和密屑体为主；丝质煤主要发育丝质体胞腔孔，瘦煤和无烟煤的孔隙类型以气孔为主。②随着粒径变小，烟煤—超无烟煤的总孔、大孔、中孔的孔容和比表面积均逐级增大，而碎屑煤的孔隙结构基本不变，丝质煤的大孔孔容和比表面积均逐渐变小。③随着煤变质程度增加，最小煤颗粒（>300目）与最大煤颗粒（40~50目）的孔容差值占比和比表面积差值占比均持续升高。④在逐级粉碎过程中，烟煤—超无烟煤的吸附回线宽度逐渐变小，而褐煤的吸附回线形态基本不变。研究认为，褐煤孔隙类型以开放孔为主，封闭孔孔容的占比随着煤化程度增大急剧增加；瘦煤、无烟煤和超无烟煤孔隙类型以封闭孔为主。粉碎过程显著改造了煤岩的原始孔隙形貌，逐级粉碎会导致烟煤—超无烟煤的大量孤立封闭孔释放，改造形成一端封闭的开放孔，但也会显著破坏丝质煤的胞腔孔。忽略煤粒径对低温氮吸附测试结果的影响可能会造成煤储层孔隙结构的错误评价，提出了低温氮吸附法测定煤岩孔隙结构的合理制样粒径范围，建议">300目"为测定煤岩全孔隙（封闭孔和开放孔）的最佳制样粒径范围，"2~4目"为测定煤岩开放孔的最佳粒径范围。

关键词: 低温氮吸附法, 不同变质程度煤, 孔隙结构, 吸附回线, 粒度效应