Influencing mechanisms of microwave radiation duration on the efficiency of methane adsorption in coals
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摘要:目的
微波辐射时间是微波辅助高效提升煤层气采收率的关键因素，其对煤层甲烷吸附效率的影响机制尚不明确，需深入研究微波辐射时长条件对煤层甲烷吸附效率的具体影响，揭示其内在约束机制。
方法以黄陵矿区2号煤层为研究对象，用700 W功率微波对煤样辐射0~18 min，开展等温吸附、低温液氮吸附和傅里叶红外光谱实验，分析不同微波辐射时长下煤的甲烷吸附能力、微观孔隙结构和分子结构的变化；利用多元统计学分析辐射后煤体结构参数与甲烷吸附参数的相关关系。
结果和结论实验结果显示，随着微波辐射时长的增加，煤的甲烷吸附量呈现出先减小后增加的变化趋势。当微波辐射时长达到12 min时，甲烷吸附量降至最低，仅为14.94 cm3/g。微波辐射对煤分子结构影响显著，随着辐射时长增加，煤样中脂肪族结构(CH2/CH3)、含氧官能团、芳香结构及羟基吸收带的响应强度整体减弱，而芳香度(Har/Hal)则呈现升高的趋势，总体分子稳定性增强，表明微波辐射通过降低支链化程度和增加芳香结构缩合，改变了煤的甲烷吸附能力。随着微波辐射时长的增加，煤微观孔隙呈现出先扩孔、后增孔的变化特征，具体表现为介孔的比表面积(SSA)先减小后增大，而孔体积(PV)则先增大后减小。基于Pearson相关性矩阵及主成分分析方法，发现2~10 nm介孔比表面积是制约煤层甲烷吸附效率的关键参数。研究成果可为微波辅助煤层气高效开发提供辐射时长等理论参数，并为后续多参数耦合机制的深入研究奠定了基础，有助于推动微波辐射技术在煤层气增产工程中的应用。
Abstract:ObjectiveMicrowave radiation duration represents a critical factor in enhancing coalbed methane (CBM) recovery using microwave-aided methods. However, the influencing mechanism of microwave radiation duration on the efficiency of methane adsorption in coal seams remains unclear. This necessitates delving into the specific effects of microwave radiation duration on the adsorption efficiency and unveiling their intrinsic constraints.
MethodsThis study investigated the No.2 coal seam in the Huangling mining area. The coal samples were irradiated for 0‒18 min using 700 W microwaves. Using experiments and analyses including isothermal adsorption, low-temperature liquid N2 adsorption, and Fourier transform infrared spectroscopy (FTIR), this study analyzed the changes in the methane adsorption capacity, microscopic pore structure, and molecular structure of coals under varying microwave radiation durations. Furthermore, it examined the correlation between the structural parameters and methane adsorption parameters of coals post-radiation using multivariate statistical analysis.
Results and ConclusionsThe experimental results demonstrate that the methane adsorption capacity of coals initially decreased and then increased as the microwave radiation duration increased. The minimum methane adsorption capacity (only 14.94 cm3/g) was observed in the case of a microwave radiation duration of 12 min. Microwave radiation significantly affected the molecular structure of coals. With an increase in the microwave radiation duration, the response intensities of aliphatic structures (CH2/CH3), oxygen-containing functional groups, aromatic structures, and hydroxyl absorption bands in the coal samples decreased generally. In contrast, the aromaticity (Har/Hal) of the coal samples showed an increasing trend, and their overall molecular stability also increased. These findings indicate that microwave radiation altered the methane adsorption capacity of coals by reducing the degree of branching and promoting aromatic condensation. As the microwave radiation duration increased, the micropores in coals underwent pore expansion initially and then pore formation. Specifically, the specific surface area (SSA) of mesopores decreased first and then increased, while their pore volume (PV) showed an opposite trend. The Pearson correlation matrix and principal component analysis (PCA) revealed that the SSA of mesopores with sizes ranging from 2 nm to 10 nm is a key parameter that influences the efficiency of methane adsorption in coal seams. The results of this study can provide theoretical parameters such as optimal radiation duration for efficient CBM production using microwave-assisted methods while also laying a foundation for in-depth research on multi-parameter coupling mechanisms in the future. This study will promote the application of microwave radiation technology in CBM production growth projects.
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图 2 不同辐射时长的甲烷等温吸附曲线
Fig. 2 Methane isothermal adsorption curves under varying microwave radiation durations
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图 4 煤分子结构随微波辐射时长变化
Fig. 4 Variation of the molecular structure of coals with the microwave radiation duration
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图 5 低温氮吸附/脱附曲线及孔径分布曲线
Fig. 5 Low-temperature N2 adsorption and desorption curves and pore size distribution curves
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图 6 煤孔隙结构参数随微波辐射时长的变化特征
Fig. 6 Variations of pore structure parameters of coals with the microwave radiation duration
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图 7 微波辐射后煤结构参数与甲烷吸附参数相关性
注：VL，Langmuir体积；pL，Langmuir压力；S1，介孔的总比表面积；S2，2~10 nm孔的总比表面积；S3，10~50 nm孔的总比表面积；V1，介孔的总孔体积；V2，2~10 nm孔的总孔体积；V3，10~50 nm孔的总孔体积。
Fig. 7 Correlation between the texture parameters and methane adsorption parameters of coals after microwave radiation
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图 8 微波辐射促进煤层气排采生产模式及机理
(a) 地面井微波加热工程(据文献[33]，修改)；(b) 微波增产机理
Fig. 8 Pattern and mechanism of microwave radiation enhanced CBM production
表 1 煤样工业分析和元素分析测试数据
Table 1 Proximate and ultimate analyses of coal samples
样品名称 工业分析w/% 元素分析w/% Rmax/% Mad Aad Vdaf FCad Cdaf Hdaf Odaf Ndaf St,ad 黄陵煤样 1.02 9.40 32.21 57.37 72.70 4.82 20.91 1.33 0.24 0.71 注：St,ad：空气干燥基的总硫；Mad和Aad：空气干燥基的水分和灰分；Vdaf：干无灰基的挥发分；FCad：空气干燥基的固定碳；w：质量百分数，%。 
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