Study On The Thermal And Deformation Effects In The Process Of Methane Adsorption/Desorption In Coal

CBM(Coalbed methane)is a kind of efficient and clean energy,whose main component is methane.The development of CBM has strategic value to the alleviation of the tense situation of oil and gas resources,the reduction the degree of mine disasters and decrease greenhouse gas emissions of China.It also has the important significance to enhance the ability of independent energy security and adjust and optimize the energy structure of China.Comprehensive factors of the meso structures with differet maceral and mineral composition,temperature and adsorption pressure.In addition,the thermal-deformation effect exists during methane adsorption on coal.Namely,the releases heat lead to the temperature rising of coal during methane adsorption,and the swelling and extrusion deformations between inner structures of coal occurred as well.Taking the “Study on the thermal and deformation effects in the process of methane adsorption / desorption in coal” as the subject,large amount of scientific theoretical exploration and experimental research were conducted in this paper.The significant non-uniform potential wells theoretical model of methane adsorption in coal was put forward,and the meso mechanism of thermal-deformation effects during methane adsorption was also profoundly studed.The main contents are as follows:1)The methane adsorption characteristics of coal with non-uniform potential wells and the variation of adsorption heat are studied under the co-action of adsorption pressure and temperature with the high temperature test system,.Research shows that: the coal sample with non-uniform potential wells have different methane adsorption characteristics with the ideal Langmuir model.In methane adsorption process,methane molecules tend to be adsorbed by the deep adsorption potential wells,and the relationships between depth of potential wells and the coverage rate obeys the logistic curve(S type)with the parameters of temperature and adsorption pressure.Under constant pressure of adsorption,the adsorption heat in higher temperature stage is larger than that in the lower temperature stage.At constant temperature,the adsorption heat in the lower pressure stage is larger than that in the higher pressure stage.2)The kinetics numerical model of methane adsorption in coal is established based on Monte Carlo method,the methane adsorption processes of two kinds of coal models with non-uniform adsorption potential well are calculated and analyzed.Researches show that the non-uniform distribution characteristics of potential well of coal sample models have effects on the variation of adsorption heat and the sensitivity of adsorption capacity to temperature and adsorption pressure.The isotherm adsorption equation of non-uniform well is derived.From the physical experiment,it is verified that isotherm adsorption equation of non-uniform well can provide more accurate desorption on processes of methane adsorption and description in coal than the ideal Langmuir equation.3)Through the algorithm of coal sample surface layer extraction,the CT scanning gray image of coal sample surface can obtained,which forms the good correspondence with the SEM image.Based on the image processing SEM-EDS test,the quantitative assessment of the pore structure characteristics and components accounted of different types of meso structures in coal.It is shown that the pore structure characteristics and components accounted of different types of meso structures in coal are closely related to the distribution and the filling degree of clay mineral,which is,The meso structures with clay minerals non-compactly filled have more developed of pores.4)With the independently developed device of infrared thermal imaging of methane adsorption / desorption in coal,the infrared thermal imaging characteristics of methane adsorption / desorption in coal samples are studied under different pressures.The results indicate that different regions of coal has different adsorption capacity,so that the regions of remarkable methane adsorption have greater temperature rising/decrease than any other regions during methane adsorption/desorption.The same non uniform characteristics are shown in the processes of the temperature rising of methane adsorption and the temperature decrease of methane desorption,and the higher adsorption pressure the greater temperature variation.5)A method combining scanning electron microscopy and energy dispersive spectrometry(SEM-EDS)with infrared thermal imaging is used to observe meso structures,methane distribution,and evolution characteristics in coal.The results indicate that the different meso structures of coal have different methane adsorption characteristics,leading to a non-uniformity of the methane distribution and evolution.In other words,compared with the cavity pores and meso fractures in telocollinite,which are compactly filled with clay minerals,the cell cavity pores and meso fractures in telinite,which are non-compactly filled with clay minerals,have more inter-gravel pores and broken coal structures at different scales,and can store great amounts of methane owing to a larger surface area and greater number of adsorption sites for methane molecules.Therefore,the methane content in coal increases sharply in these meso structures with increasing adsorption pressure,resulting in a centralization of the adsorption locations and increasing inhomogeneity of methane storage at different locations.In addition,owing to the shallower potential wells for methane adsorption,the coverage rate of the adsorption sites in these meso structures increases more slowly during an increase in pressure,leading to a decreased aggregation of methane storage in coal.6)To investigate the effects of pore pressure of methane adsorption on porosity and swelling deformation of coal,a micro-CT system named ?CT225kVFCB is adopted and coal sample is molded to cylinder which diameter is 5mm.The conclusions presented that the coal porosity decreased and the volume swelling deformation are caused by methane adsorption.The change of the volume swelling deformation and porosity are non-homogeneity in meso-scale coal sample,and the porosity in the adsorption state is effectively estimated by determination of coal bulk volume swelling deformation.In the process of gas adsorption,the decreasing of pore volume and the expanding of bulk volume are resulted from coal skeleton volume swelling.Both of these have effects on the porosity of coal,and it is inclined to squeeze original porosity of coal to obtain spatial of expansion.7)With a method combining of SEM-EDS and micro CT scan,meso structural deformations of coal during methane adsorption and desorption were observed.It is found that there are two kinds of deformation phenomena,which are the swelling deformation and extrusion deformation of the coal structure during methane adsorption / desorption.The mechanism meso of deformation of coal during methane adsorption / desorption has been revealed,namely: the swelling deformation and extrusion deformation of the coal structure during methane adsorption / desorption are result from the the interaction of expansion stress between different meso structures in coal.During the methane adsorption,the structures with higher density are prone to swelling deformation,and lead to the extrusion deformation of the lower structures.Due to incoordination of deformation and recovery capacities between the inner structures,the residual deformations in coal mainly exists in the regions with strong density inhomogeneity.