72 / 2023-06-04 10:19:18

Effects of degassing temperature and particle size on pore structure of moderately metamorphic coals

moderately metamorphic coals; adsorption characteristics; granularity; degassing temperature

The gas adsorption capacity of coal seam is closely related to the specific surface area of coal, and the larger the specific surface area, the stronger the adsorption capacity. However, when the low-temperature nitrogen adsorption test is carried out on the coal, there are no corresponding standards to clearly give the best experimental particle size and degassing temperature, and when different coal sample particle sizes and degassing temperatures are used for testing, the specific surface area and pore structure of coal are different, which affects the determination of the adsorption capacity of coal. In order to clarify the effects of degassing temperature and particle size on the pore structure of coal, this paper took coal samples with medium metamorphic degree as the research object, selected the fat coal from Pingdingshan No. 8 Mine and the coking coal from Pingdingshan No. 11 Mine, and carried out relevant research with the help of thermogravimetry-mass spectrometry system, low-temperature liquid nitrogen adsorption instrument and scanning electron microscopy. The main conclusions are as follows:

(1) The thermal weight loss curves of coal samples with different degrees of metamorphism varied in three stages, and the experiment mainly studied the change of physical structure of coal body with temperature in the second stage. The experimental temperature range of physical change of both fat and coking coal was 110°C-300°C. When the pore structure experiment of coal is carried out by low-temperature nitrogen method, the pore structure parameters of hard coal of fat and coking coal change slightly more than soft coal with the increase of degassing temperature, and the degassing temperature has little effect on the pore structure of fat and coking coal. However, when the temperature reaches 250 °C, some pyrolysis condensate appears, indicating that the degassing temperature cannot be higher than 250 °C. The scanning electron microscopy showed that when the degassing temperature of fat and coking coal was 150 °C, the impurities on the surface of the coal were significantly reduced, and when the degassing temperature increased to 250 °C, the coal matrix appeared expansion deformation and the pore structure was distorted.

(2) With the decrease of particle size, the pore structure parameters of hard coal of fat and coking coal changed significantly. When the particle size reaches 200 mesh, the maximum adsorption capacity, specific surface area of BET and total pore volume of coal samples increase sharply, and the average pore size only changes slightly. The influence of particle size on pore area distribution was mainly concentrated in pores < 2nm and 10-100nm, and the influence on pore volume was mainly concentrated in the ranges of 10-100nm and >100nm. The scanning electron microscopy tests showed that the exposure of the pore structure on the surface of the coal became larger with the decrease of particle size, and the influence of external crushing on the exposure of the pore structure of the coal sample was much greater than the impact on the damage of the pore structure of the coal body, and the effect of particle size on the surface morphology of soft coal of fat and coking coal was small.

(3) The optimal degassing temperature for low-temperature liquid nitrogen experiments of fat and coking coal is 150°C, and the optimal particle size is <200 mesh.