Molecular Simulations of the Adsorption Behavior of NH3 and O2 in Xiaolongtan Lignite

Abstract

Massive quantities of NH₃ generated after blasting in underground coal mines are believed to enhance the adsorption of O₂ by coal and accelerate the rate of coal’s spontaneous combustion oxidation. The adsorption behavior of coal for NH₃ and O₂ after blasting in underground coal mines provides critical insights into the mechanisms of coal’s spontaneous combustion. This research was conducted on Xiaolongtan lignite, examining the adsorption characteristic of NH₃/O₂ binary gas mixtures and single-component NH₃ and O₂ on lignite at 278.15–323.15 K and 0–500 kPa using Grand Canonical Monte Carlo simulations. Additionally, the kinetic properties of the lignite/NH₃ and lignite/O₂ systems were analyzed at 278.15–323.15 K using molecular dynamics simulations. The results revealed that the adsorption isotherms conformed well to the Langmuir equation. Under the specified conditions, the adsorption amount of lignite decreased with increasing temperature, and the gas adsorption amount followed the order: NH₃ > O₂. The adsorption selectivity coefficient of NH₃ and O₂ was largely unaffected by the molar ratio but decreased as the temperature increased. The integral area of the relative concentration curve confirmed that the adsorption of NH₃ and O₂ onto lignite decreased with increasing temperature, maintaining the order NH₃ > O₂. Under identical temporal conditions, the mean square displacement and diffusion coefficients of the gases increased with temperature, with O₂ exhibiting a higher diffusion coefficient than NH₃. Furthermore, the interaction energy of the lignite/NH₃ and lignite/O₂ systems decreased as the temperature increased, with NH₃ exhibiting the strongest interactions with lignite at the same temperature.