Particle size effect on pore structure characteristics of lignite determined via low-temperature nitrogen adsorption

There is currently no uniform particle size of lignite for low-temperature nitrogen adsorption. Further, it is still unclear whether particle size has an effect on lignite pore structure. In this study, low-temperature nitrogen adsorption tests were used to study the pore structure of fresh lignite samples, using eight different particle size ranges from the Hailar Basin, northeastern China. The results show that lignite mainly contains macropores, and the proportion of macropore volume to total pore volume is 63.67-75.67%. The specific surface area and volume of macropores and mesopores decrease with decreasing particle size, and the reduction of macropores is more significant than that of mesopores. Optical and scanning electron microscopy observations showed that the predominant pore types in the lignite are plant tissue pores, intergranular pores, and microfractures, which occur in open pores. These differences in pore structures over the varied sizes are related to mechanical crushing. Specifically, decreases in coal particle size lead to a greater degree of collapse and destruction to open pores during progressive pulverization. Herein, the results show that the particle size of lignite is important to accurately determine its pore structure. To facilitate the comparison of the experimental results of different samples in future works, a mesh size of 1-2 is recommended as the optimal size range of lignite for low-temperature nitrogen adsorption studies.