| Coalbed methane(CBM) resources is rich in our country, with the development of CBM exploitation technology, the exploitation of low rank CBM attracts more and more attention. However, the current studies of low rank CBM exploitation foucs on the technique and technology, lacking of researches on the control mechanism of low rank coal gas adsorption and desorption. Coal has complicated pore structure and chemical structure such as surface functional groups and these structures have great influence on gas adsorption and desorption performance. In this thesis, on the basis of physical experiments, microstructure characteristics of low rank coal were systematically analyzed from two aspects of pore structure and surface functional groups. The adsorption and desorption law of low rank coal was studied and key influencing factors on gas adsorption and desorption of low rank coal was discussed. On the basis of research results, the control mechanism of low rank coal adsorption and desroption was put forward, as well as the efficient extraction model.With the help of mercury intrusion and liquid nitrogen adsorption experiments, pore size distribution and pore types of low rank coal were studied. The results show that pore in low rank coal is more developed and there is a large amount of open pores. Mesopore and transitional pore occupy larger proportion which lead to a better connectivity between pores in low rank coal and pore types mainly present as ink-bottle and wedge shape. With the increase of coal rank, the proportion of micropores increase gradually and pore types in medium-high rank coal are mainly cylindrical and slit pores.Using fractal theory, volume fractal dimension and fractal characteristics of adsorption pore were calculated and the influence of coal composition on fractal dimensions were studied. The results show that the porosity and compressibility of low rank coal is large, which lead to a large volume fractal dimension, even larger than 3. Volume fractal dimension of medium-high rank coal is relatively small and the porosity increases with the decrease of volume fractal dimension. The fractal dimension in low pressure stage can be called surface fractal dimension, while the fractal dimension in high pressure stage can be called structure fractal dimension. Coal compositions cause the heterogeneity of pore surface and structure and will have an effect on fractal dimensions. Thus, fractal dimensions can be regard as a comprehensive reflection of the complexity of coal composition.By means of peak-differentiating and fitting of FTIR spectra, evolution law of surface functional groups in low rank coal was studied quantitatively and found that there is a large amount of hydroxyl and a certain amount of carboxyl in low rank coal. The types of hydroxyl are mainly hydrogen bond, phenolic hydroxyl group and alcoholic hydroxyl group, as well as a small amount of free hydroxyl. Oxygen-containing functional groups in low rank coal are mainly alkyl ether and alcohol, while in medium-high rank coal, they are mainly aryl ether and phenoxy ether. The content of C=O functional group is relatively high in low rank coal. During the process of metamorphism, part of –OH turned into C-O. As the coal rank increases, the C-O bond broken, and polycondensating into aromatic ring structure.The adsorption and desorption system was used to study the adsorption and desorption law of different rank coal samples and adsorption hysteresis coefficient was calculated. The results show that VL decreases first and than increases with the increase of coal rank. Adsorption capacity of brown coal is poor and VL of the rest low rank coal is larger than part medium rank coal(1.0<Ro,max<1.5) and smaller than high rank coal. The interaction between methane and low rank coal is relatively weak which lead to the slow increase of adsorption volume and it is hard to reach saturation adsorption.The theoretical desorption and recovery rate of low rank coal are apparently higher than that of medium-high rank coal. Adsorption hysteresis coefficient increases with the increase of coal rank.On the basis of experiment results, effects of microstructure characteristics on gas adsorption and desorption were studied. It is showed that gas adsorption capacity is controlled by pore size distribution and surface functional groups, while gas desorption ability is controlled by pore size distribution and pore type. Among all these factors, pore size distribution has a major effect on gas adsorption and gas desorption is mainly controlled by pore type.Finally, control mechanism of low rank coal gas adsorption and desorption, as well as efficient extraction mode for low rank coal CBM exploitation were put forward. |
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