Modeling of Gas Desorption and Diffusion Kinetics in a Coal Seam Combined with a Free Gas Density Gradient Concept

Abstract

Gas diffusion in a coal seam plays an essential role in the efficient and effective exploitation of underground coalbed methane (CBM). Based on the concept of free gas density gradient-driven flow, in this work, we formulated and validated a theoretical model to quantify gas desorption and diffusion kinetics in a coal seam at different temperatures. Once the governing equations are formulated and non-dimensionalized, they are numerically solved with the finite difference method and then validated with the experimental measurements on gas desorption at different temperatures. The mechanism of temperature affecting gas diffusion kinetics was elaborately explored. Also, the advantages of the proposed free gas density gradient model (FGDGM) compared to the traditional unipore diffusion model (UDM) were well evaluated and discussed. It is found that the FGDGM simulations are basically consistent with the experimental data, but the UDM prediction deviates significantly from the experimental measurements at different temperatures. By introducing a new dimensionless temperature variable and performing sensitivity analysis, the dimensionless gas desorption is found to increase with temperature, while the incremental gas desorption varies across the entire temperature range. With the temperature increasing from 293 to 313 K, both the gas desorption increment and gas yield enhancement are found to be the largest. In addition, there exists a linear relationship between diffusivity and temperature because a high temperature increases the gas molecules’ activity and accelerates the pore expansion. It is clearly found that temperature has a considerable contribution to the gas diffusion kinetics in a coal seam, and appropriately boosting the coal temperature to enhance CBM recovery is feasible and valuable.

Experimental Study on Methane Diffusion Characteristics of Different Metamorphic Deformed Coals Based on the Counter Diffusion Method

“…According to the theory of gas molecular motion [57,58], temperature mainly impacts gas molecular diffusion through the root-mean-square speed and average free path of the gas molecules. Molecular motion theory in materials science states that the temperature of a gas is an indicator of the average kinetic energy of the molecules [32,59].…”

Characterization of Coal Particle Methane Desorption and Optimization of Desorption Model Based on Desorption Damage

“…A significant outburst of coal and gas accidents will be accompanied by energy release, which produces strong power in the period of underground mining. − Gas outburst produces a large amount of pulverized coal, and the desorption gas of pulverized coal will further facilitate the outburst. − Therefore, gas significantly influences the gestation and occurrence of gas outbursts. − As coal mining depths increase, the amount and intensity of gas emissions also increase, which will induce more gas calamities. − In addition to the high-intensity mining stage of deep coal seams, the coal has been strongly damaged many times during the coal-forming process, leading to the gas migration in the coal body and the circumstance of gas desorption–pulverization–desorption. , Consequently, studying the law of gas desorption in coal under damage conditions is crucial for understanding gas emission laws and gas flow mechanisms and predicting coal and gas outbursts. − …”

Coal Mine Fire Emergency Rescue Capability Assessment and Emergency Disposal Research

“…China is very rich in coal resources, and as an important source of energy for the country, it occupies a crucial position in the sustainable development of the national economy [1,2]. According to statistics, about 70% of mega-accidents and 50% of major accidents occur in the coal industry [3].…”