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

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