Effect of Moisture on Methane Time-Varying Diffusion Kinetics of Low-Rank Coal

Moisture in low-rank coal (LRC) affects methane diffusion properties, which makes it difficult to predict risk of coal and gas outburst accurately. Therefore, four coal samples were prepared and analyzed in this paper. Isothermal adsorption experiment, desorption experiment of samples with different moisture content (0%~16.80%) and the time-varying diffusion model were conducted to study the influence of moisture on methane time-varying diffusion kinetics of LRC. Results show that methane adsorption capacity of LRC is stronger than the tested medium-rank coal sample. The VL (31.14 m3/t, 34.62 m3/t and 35.14 m3/t) of three LRC samples are larger than the VL (28.21 m3/t) of the tested medium-rank coal sample. With the moisture content increase, the desorption volume and desorption rate of LRC decrease obviously. The desorption volume fitting results by the time-varying diffusion model show that the diffusion rate decreases significantly as the moisture content increases. In addition, initial diffusion coefficient (D0) of samples has a negative linear relationship with the moisture content. Taking Puhe coal sample (lignite) as an example, as the moisture content increases from 0% to 16.80%, the D0 decreases from 7.27×10-8 cm2/s to 4.91×10-8 cm2/s. With the increase of moisture content, the time-varying diffusion coefficient (Dt) and its decay rate decrease obviously, indicating that moisture can significantly reduce the diffusion capacity of LRC. Furthermore, competitive adsorption between moisture and methane molecules is an important factor affecting the diffusion properties of LRC. Experimental results can provide a good theoretical basis for the prevention and control measures of hydraulic outburst in LRC mines