Experimental study on multi-scale characterization and damage model of coal particle pores under circulating critical outburst pressure

In the process of coal seam mining, continuous high pressure gas circulation causes accumulating microscopic damage in coal, leading to a decrease in fracture toughness. When pore damage reaches a critical level, the coal body may fracture and become unstable under high pressure gas. Understanding this damage is essential for uncovering the mechanisms of coal and gas outbursts. This study presents experiments on circulating critical outburst pressure desorption using a high pressure desorption device, combined with low temperature liquid nitrogen adsorption tests, to analyze the pore parameters of different coal samples. The experimental results indicate that as the number of high pressure cycles increases, methane desorption generally increases. Bituminous coal exhibits the highest desorption, while lignite shows the lowest. Circulating high pressure significantly impacts the pore characteristics of coal samples. The pores in bituminous coal are well developed, with a liquid nitrogen adsorption growth rate of 50.01%, compared to only 16.49% for anthracite. The growth rate of micropore volume and specific surface area is the highest, indicating that these are the primary sites for gas storage. A comprehensive analysis of the circulating high pressure gas action, gas desorption characteristics, and pore evolution of coal particles led to the development of a quantitative model for pore damage. This model reveals the extent of damage in different coal types after circulating high pressure desorption, providing a theoretical basis for the migration characteristics of coalbed methane.