The influence of moisture content on methane adsorption in lignite: A molecular simulation study

As a high-efficiency clean energy and chemical raw material, coal-bed methane (CBM) has received wide attention in recent decades. At the present stage, new strategy to achieve a breakthrough in the CBM exploration and development is regarded as the primary goal for China's lignite region. Usually, lignite contains a high moisture content in our country. Therefore, it is of great practical significance to understand the effect of moisture content on methane adsorption in lignite. Herein, molecular simulations of Monte Carlo and molecular dynamics methods are employed to elucidate this effect on methane adsorption with three moisture contents of 0%, 1.6% and 3.2%, and three temperatures of 298 K, 313 K and 373 K and the pressure range of 0-10 MPa are considered in the work. The lignite substrate model is composed of an aromatic skeleton amorphous structure with 88 atoms, including carbon, hydrogen, oxygen, nitrogen and sulfur. The adsorption amount, adsorption heat, and atomic radial distribution function along N, O, and S atoms are investigated in turns. The results show that the adsorption amount of methane decreases with the increase of moisture content in lignite, because the water molecules would be superior to methane to occupy the adsorption sites in lignite at the initial stage. The adsorption amount of methane decreases with the increase of temperature due to the molecules thermaldynamic property. The adsorption heat of methane decreases with the increase of moisture content in lignite. In addition, the N and S atoms exhibit the stronger interactions with methane than the O atom in lignite. This molecular simulation results are not only helpful to understand the adsorption mechanism of methane in lignite, but also provide theoretical prediction and scientific evidence for the exploration of CBM.