Effect of particle size on gas energy release for tectonic coal during outburst process

Based on the particle size evolution of tectonic coal in formation process, the differences between tectonic coal and intact coal in pore structure and initial gas desorption characteristics were studied. Then, the influences of particle size reduction on pore structure and initial gas desorption characteristics were investigated; and the initial gas energy release model was derived, which was used to calculate the gas expansion energy of tectonic coals with different particle sizes. This study concludes that the particle size of tectonic coal will be reduced under the pulverization effect during the formation process, and the particle sizes of these types of tectonic coal reach millimeter level or even smaller scale under pulverized state. The pore volume and specific surface area of minipores (pore size: 10–100 nm), mesopores (pore size: 100–1000 nm) and macropores (pore size: 1000–10000 nm) in tectonic coal are larger than those in intact coal by over 10 times. Tectonic coal's average gas desorption rate in the first 10 s reaches up to 11.47 times of that of intact coal. Within the 5 particle size greater than 0.074 mm, the pulverization obviously transforms the minipores and mesopores in coals, but its effect on micropores (pore size: 0.9–10 nm) is limited. When the particles are pulverized to blow 0.074 mm, the micropores are obviously transformed. The initial gas energy release model for coal samples with different particle sizes is derived and verified through the experimental data. As calculated using the initial gas energy release model, the gas expansion energy will be reduced sharply with the particle size reduction when the particle size is smaller than 1 mm. Therefore, the particle size of tectonic coal decides its initial gas desorption capacity, which is of critical significance to the gas energy release in the outburst process. This study will be of great scientific significance for exploring the outburst mechanism and prevention.