Effect of the lump size on methane desorption from anthracite

• When lump size exceeds the coal matrix size, pore-fracture structural characteristics of anthracite are invariant.
• When coal size drops below the coal matrix size, the average pore size increase with decreasing grain size.
• With increasing lump size, gas diffusivity decrease and then change little when lump size exceeds the coal matrix size.

Anthracite fracture and pore structure of four different sized coal samples were measured using a light microscope and mercury porosimetry, respectively. The desorption kinetics of methane, at three different pressures, were also studied on six size fractions of coal samples using a volumetric experimental apparatus. The effect of lump size on methane desorption from anthracite combined with its pore structure was further discussed. The results showed that (1) for the lump coals, three pressure regimes in the process of mercury injection are indicated by distinct values of the fractal dimension; in order of increasing pressure, these correspond to cleat penetration, pore penetration, and coal matrix compressibility; (2) When the lump coal size drops below the coal matrix size, the pore volume and average pore size increase with decreasing grain size. When the lump size exceeds the coal matrix size, pore-fracture structural characteristics of anthracite, such as cleat aperture, cleat porosity, the matrix average pore size, and matrix porosity are basically invariant; (3) with increasing lump size, the desorption time constant changes according to positive exponential rules, eventually obtaining a stable value after the lump size fraction of 70–75 mm; the ultimate desorption ratio, initial desorption rate, attenuation index of the desorption rate and gas diffusivity decrease significantly and then change little when the lump size exceeds the coal matrix size.