Abstract
Due to the complexity of the internal structure of natural coal and its characteristic of multicomponent, the depth of its methane adsorption potential well is nonuniform, which makes it difficult to accurately evaluate the adsorption capacity of coal. Besides, in order to find out the factors affecting the depth distribution of potential wells in natural coal, this paper calculated the depth and number of potential wells during methane adsorption in coal according to the Langmuir adsorption kinetics process. Coal samples with different metamorphic degrees were tested and analyzed by infrared spectroscopy diffraction technology. The relationship between the structural parameters of functional groups in coal samples with different metamorphic degrees and the distribution of different depths of adsorption potential wells in coal samples was studied. Some main conclusions are as follows: The number of adsorption potential wells at different depths in natural coal with different metamorphic degrees has multipeak distribution characteristics. With the increase of the metamorphic degree of coal sample, the structures such as aliphatic branched chain structure and oxygen-containing functional groups in coal structure break, fall off, and deoxygenate. The relative content of aliphatic hydrocarbons is significantly reduced and condensed into aromatic hydrocarbons and aromatic ring structures. The different types and quantities of functional groups on the surface of coal samples lead to different forces between coal molecules and methane gas molecules, thus affecting the distribution of different depths of adsorption potential wells in coal samples.
Highlights
An adsorption potential well refers to the position on the surface of coal where the local potential energy is relatively low and methane can be adsorbed, whose depth is related to the mutual attraction between coal and methane [1,2,3]
The different types and quantities of functional groups on the surface of coal samples lead to different forces between coal samples and methane gas molecules, which further affects the distribution of different depths of adsorption potential wells in coal samples
This is consistent with the variation law of the distribution ratio of adsorption potential wells in the range of 0-5 kJ/mol and 5-10 kJ/mol with the metamorphism of coal sample, indicating that the length of the fat side chain of coal sample structure and the structural parameter ‘C’ of oxygen-containing functional groups play a key role in the distribution ratio of low and medium depths of adsorption potential wells
Summary
An adsorption potential well refers to the position on the surface of coal where the local potential energy is relatively low and methane can be adsorbed, whose depth is related to the mutual attraction between coal and methane [1,2,3]. Many scholars [8,9,10,11] have measured and calculated the potential well depth or the value of adsorption heat when the coal body is adsorbing methane gas by using physical experiments and molecular simulation methods. Nie and Duan’s calculation [12] shows that the potential well depth on the coal surface is basically consistent with the measured data of adsorption heat. According to the existing research, we can speculate that the potential well depth on the surface of coal has obvious nonuniformity, and there are both deep and shallow depths of potential wells that can adsorb methane on the coal surface
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