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

High moisture content affecting coal bed methane (CBM) occurrence and transport is a typical feature of low-rank coal (LRC). The mechanism of the influence of moisture on methane diffusion properties of LRC is critical to improve CBM production and prevent coal and gas outburst. In this paper, a new method was designed to prepare coal samples with different moisture contents (0–16.80%). The desorption experiment at different moisture contents was conducted to study the time-varying diffusion properties of selected LRC and medium-rank coal samples. Results show that with the moisture content increase, the desorption volume and the initial desorption rate of LRC decrease obviously. Based on the negative linear relationship between the initial diffusion coefficient ( D 0 ) and moisture content ( M ), a mathematical model between them is established, which can be described as D 0 = λM + D dry , where λ and D dry are the material constant and D 0 of the dry coal sample, respectively. Moreover, with the increase in moisture content, the time-varying diffusion coefficient ( D t ) and its decay rate decrease obviously. It indicates that moisture can significantly reduce the diffusion capacity of LRC, and the competitive adsorption between moisture and methane molecules is an important factor affecting the diffusion properties of LRC. It is worth noting that there is a critical value of moisture content, beyond which increasing moisture content has no significant effect on reducing desorption and diffusion capacity. Results can provide a theoretical basis for taking hydraulic technology to prevent outburst in LRC mining. Article Highlights Moisture can significantly reduce methane desorption capacity of low-rank coal. Effect of moisture on the time-varying diffusion properties of methane in low-rank coal was studied. Mathematical model of methane initial diffusion coefficient and moisture was established.