Effect of particle size on difference of gas desorption and diffusion between soft coal and hard coal.

Engineering practice shows that the gas diffusion rules during desorption of hard and soft coal particles are significantly different. This is the key reason leading to the distinction of coal and gas outburst hazard between soft coal and hard coal. The difference of gas diffusion can be used to predict and prevent outburst disaster. Particle size is one of the key factors that cause the difference. Influence rules and mechanisms of particle size have not been found yet. Based on gas transport theory in multi-porous medium, D-values of gas diffusion velocity between soft coal and hard coal with different particle sizes were studied using physical simulation method. The study shows that the D-value declines with particle size decreases when the particle is between basic size and limited size. If particle size is less than the basic size, then the gas diffusion velocity and gas diffusion parameters are no difference between the different coals. If the particle size is equal to or more than the limited size, then the D-value will reach its maximum value, to which is almost stable. The ratios of soft coal to hard coal on gas diffusion kinetic parameters appear to have similar variation rules. According to the Fick's second law, the variation rule of gas diffusion kinetic parameters is a direct reason that results in the change rules of gas diffusion velocity. Different characteristics of pore structure between soft coal and hard coal were investigated using the mercury injection. Also, the influence mechanism of particle size on the different gas diffusion between coals was analyzed. The research results show that the volumes of macropore and mesopore are larger in soft coal, therefore they increase with the particle size decrease. That is, particle size decrease reduces the gap of pore structure between soft coal and hard coal. It reduces the difference of gas diffusion path channels, which is a natural reason. The research results provide a theory base for choosing particle size during determination of gas initial diffusion velocity index (Δp), gas desorption index of drill cuttings (K1, Δh2), and coal seam gas content. [ABSTRACT FROM AUTHOR]