Effect of particle size on re-crushing characteristics of crushed coal during axial loading

The deformation and re-crushing characteristics of crushed coal and rock directly affect the porosity and permeability distribution of the mining gob and overburden strata movement, which significantly affects the methane enrichment zone and surface subsidence of mining gobs. The effect of particle size on re-crushing characteristics is investigated in this study based on an innovative compression–acoustic emission (AE) positioning method. The main results are as follows: 1) The AE counts and energy are associated closely with three deformation stages, i.e., initial compression, linear compression, and plastic compression. A larger particle deforms more easily and results in more AE counts and energy under the same compression; 2) crushed coal exhibits layered re-crushing characteristics, i.e., particle re-crushing first occurs in the middle layer, and as stress increases, re-crushing shifts gradually to the upper and lower layers; 3) larger particles re-crush more easily, and the re-crushing occurs more severely in the middle layer under the same compression stress; 4) screened results after final compression show and verify the characteristics of layered re-crushing. The lower layer exhibits the least reduction in porosity for each particle size. A larger particle exhibits a greater re-crushing rate under the same compression stress.