An experimental study on pore complexity in single-particle coal and its impact on CH₄ diffusion kinetics

CH₄ diffusion kinetics in coal are critical for coal mine gas disaster control. Conventional qualitative analyses of coal gas diffusion, based on unit-mass pore parameters, neglect its fundamental origin within individual particles. In this study, a medium-volatile bituminous coal sample was gradually crushed by a jaw crusher and screened with a sample sieve into six particle size ranges: 0.3–0.5, 0.2–0.3, 0.125–0.2, 0.074–0.125, 0.045–0.075, and < 0.045 mm. The coal particle was modeled as homogeneous spheres and quantitatively characterized the full-scale pore structure (micropores: <2 nm, mesopores: 2–50 nm, macropores: 50–300 nm) using low-pressure N₂ (77 K) and CO₂ (273 K) adsorption. By integrating particle density and median size (D50), unit-mass pore parameters were converted into single-particle parameters. Results show that as the particle size decreased from 0.3 to 0.5 mm to <0.045 mm, the total pore volume within a single particle decreased exponentially by nearly four orders of magnitude. In addition, the initial CH4 desorption rate (V0–1) increased rapidly (5.5 times) as the particle size decreased, while the initial CH4 diffusion coefficient (D0) decreased linearly from 1.47 × 10−13 m2/s to 4.29 × 10−15 m2/s (34.3 times). The attenuation coefficient (β) increased exponentially from 8.97 × 10−5 to 1.401 × 10−3 s−1 (15.6 times). Analysis from the single-particle perspective reveals that smaller coal particles have simpler pores, accelerating initial CH₄ desorption but hastening decay. This contradicts unit-mass perspective suggesting finer coal has richer porosity, indicating that mass-averaging in traditional methods obscures the true impact of particle size on diffusion kinetics.