Effect of dynamic fragmentation on microscopic pore structure in coal: New insights into CH4 adsorption characteristics

Tectonic coal is a necessary condition for coal and gas outburst, and its pore structure is dramatically altered. The pore structure of tectonic coal has unique characteristics in terms of mechanics and gas flow, which plays a critical role in eliminating outburst risk. Therefore, it is important to study the effects of dynamic fragmentation on microscopic pore structure and CH4 adsorption characteristics of coal body. In this study, coal samples from Xintian (XT) and Pingdingshan No.6 (P6) coal mines were crushed and sieved into three particle size fractions of 0.5-1 mm, 0.2-0.25 mm, 0.074-0.1 mm, respectively, simulating dynamic fragmentation by reducing the particle size. A novel index that evaluates the average single-particle equivalent pore length was proposed. This index, L-p, provides a more comprehensive quantitative characterization of the effects of dynamic fragmentation on microscopic pore structure by considering the dynamic particle size reduction process. The L-p values of large-sized samples are 23.8 to 24.6 times that of medium-sized samples; the L-p values of medium-sized samples are 12.8 to 17.1 times that of small-sized samples. The experimental results show that dynamic fragmentation significantly alters microscopic pore structure, as demonstrated by the positive correlation between the L-p and particle size. With the decrease in particle size, the amount of CH4 molecules adsorbed in XT samples increases from 105.870 x 1019 p/g to 106.934 x 1019 p/g, increasing by 1.0 %; that in P6 samples increases from 39.660 x 1019 p/g to 48.037 x 1019 p/g, rising by 21.1 %. Dynamic fragmentation has a relatively insignificant effect on CH4 adsorption capacity. The total adsorption equilibrium time for XT samples decreases from 242 h to 32 h, and that for P6 samples decreases from 1146 h to 100 h as the particle size decreases. Dynamic fragmentation significantly reduces CH4 adsorption equilibrium time. The results of this study contribute to the understanding of microscopic pore structure under the effects of dynamic fragmentation and new insights into CH4 adsorption characteristics, which are of great guiding significance to the occurrence mechanism of outburst prediction and risk.