Adsorption behavior of water vapor in lignite pores:Experimental study and molecular dynamics simulation
Deeply understanding the adsorption of water on lignite is one of the theoretical foundations of lignite drying and upgrading tech-nology.The oxygen-containing functional groups and pore structure of lignite were analyzed using Fourier Transform Infrared Spectroscopy(FTIR),Scanning Electron Microscopy(SEM),and low-temperature nitrogen adsorption/desorption experiments.The adsorption behav-ior of water vapor in the pores of lignite was investigated using water vapor adsorption/desorption experiments and Molecular Dynamics(MD)simulations.Results show that the rich oxygen-containing functional groups and numerous micropores and mesopores(with the pore size mainly around 2 nm)in lignite samples provide adsorption sites and places for water vapor.The adsorption process of water vapor on the coal sample can be divided into three stages.In the first stage,with the relative vapor pressure(P/P0)＜0.21,water molecules directly adsorbe on oxygen-containing functional groups,and the adsorption speed is the highest.In the second stage(P/P0=0.21-＜0.71),water molecules interact with adsorbed water molecules,promoting the gradual growth of water clusters.In the third stage(P/P0≥0.71),water clusters fill the pores and the capillary condensation appeared,the adsorption speed in this stage is slightly higher than in the second stage.According to the fitting results of the Dent model on the water vapor adsorption isotherm,the adsorption type belonged to multi-stage adsorption,including primary adsorption(first stage)and secondary adsorption(second and third stages).The primary adsorption energy(-48.77 kJ/mol)is significantly greater than the liquefaction heat of water(EL=-43.99 kJ/mol),while the secondary adsorption energy(-42.28 kJ/mol)is only slightly less than EL,indicating that the water adsorbed on lignite is liquid.There is a significant desorp-tion hysteresis phenomenon during the water vapor desorption process,indicating that the water vapor adsorbes stably and is difficult to re-move.High pressure hysteresis occurr in the range of P/P0≈0.4-0.9,mainly caused by capillary condensation and the"ink bottle"effect.Low pressure hysteresis occurr in the range of P/P0＜0.4,caused by strong interactions between water molecules and oxygen-con-taining functional groups.The MD simulation results are consistent with the analysis of water vapor adsorption/desorption isotherm.Wa-ter molecules preferentially adsorb in the pore and form hydrogen bonds with oxygen-containing functional groups on the pore walls,with a diffusion coefficient(2.98×10-5 cm2/s)similar to that of liquid water.
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