| Methane gas is a disaster, but also a clean energy. In order to achieve coal mine safe production and the efficient use of clean energy, the study for adsorption and desorption characteristics is particularly important. In order to ensure the safety of production, measurement of gas parameter determination is gradually in high demand, but the existing conventional measurement fails to meet the demand of frequent and rapid measurement in gas parameter determination for coal as the period for conventional measurement is long and relevant process is complex. Therefore, we’d support the rapid determination of gas parameter in coal, which has close relationship with the dynamic characteristics of coal’s adsorption and desorption. In order to further reveal the characteristics of gas’ adsorption and desorption in coal seam, this paper studies main controlling factors of coal’s adsorption ability for gas, its isothermal adsorption model, the desorption and hysteresis mechanism of gas in coal, its evaluation index, diffusion model of coal particle based on coal centre concentration and the timevariant characteristic of diffusion coefficient, etc. under the guidance of adsorption theory, interface chemistry, hydromechanics and other theoretical methods, by the ways of experimental study, theoretical analysis and numerical simulation. On such basis, rapid determination of gas parameters in coal was proposed and instrument as well as application development was conducted. Main conclusions of this paper are as follows:(1) The porosities determined by the mercury intrusion experiment and concentration experiment are similar, and they’d both firstly increase and then decrease along with the increasing of metamorphism degree. Hysteresis degree for coal samples in 2 #, 4 # and 6 are low, which illustrates that pores in the three coal samples are mainly cylindrical and v-shaped holes; hysteresis degree for coal samples in 1 #, 3 # and 5 # are relatively large, especially the coal sample in 1 #, which shows that the three coal samples contain a lot of the ink-bottle-shaped pores. It is easy to learn from the loop line of isothermal adsorption that large quantities of sharp wedge-shaped pores with all sides open exist in 1 ~ 5 # coal samples. By employing the pore-size ranges measured by mercury intrusion experiment and liquid nitrogen adsorption experiment, we can combine the results together. When D≤33 nm, the results of DFT model for liquid nitrogen adsorption experiment can be adopted and the results of mercury intrusion experiment can be employed when D > 33 nm. Except for coal sample from 6 #, pore volume and specific surface area of the rest samples firstly decrease and then increase along with the rising metamorphism degree, which conforms to the general law of metamorphism degree’s influence on the pore development of coal.(2) Fractal dimension for six coal samples were respectively analyzed by employing mercury intrusion experiment and liquid nitrogen adsorption experiment. Considering that mercury intrusion experiment would do harm to the coal in the section of high pressure and micropore filling would occur in the low-pressure section of liquid nitrogen adsorption experiment, this paper uses experimental data from the lowpressure section of mercury intrusion experiment and high-pressure section of liquid nitrogen adsorption experiment. The fractal dimension obtained from mercury intrusion experiment would firstly decrease and then increase along with the higher degree of metamorphism degree and fractal dimension obtained from liquid nitrogen adsorption experiment also shows the same trend expect for coal sample from 3 #. It shows the fact that along with the increase of metamorphism degree, pore complexity and surface roughness of the six groups of coal samples firstly decrease and then increase.(3) Through the isothermal adsorption by experiment under different conditions, various factors’ influence on the adsorption was obtained. Through the molecular structure of coal, metamorphism degree would influence coal’s adsorption domain, adsorption sites and adsorption potential to methane molecules, which would lead to the situation that coal’s adsorption ability to methane molecules would firstly decrease and then increase. Pore structure would influence coal’s adsorption domain and surface curvature to methane molecules, which would result in the situation that coal’s adsorption capacity to methane molecules would increase along with the increase of its adsorption domain and surface curvature. Fractal dimension actually reflects the roughness of coal surface, and the experiment proves that the adsorption ability of coal would gradually be strengthened along with the increase of fractal dimension. Water would decrease coal’s adsorption capacity by occupying adsorption and blocking the pore throat, while temperature and particle would change coal’s adsorption capacity by changing methane molecules’ work function and increase its diffusion path. It is proved by experiment that the increase of both moisture and temperature all reduce the adsorption capacity of coal, while the increase of particle ship improves the Langmuir pressure instead of changing coal’s adsorption capacity.(4) Through the relationship between BET specific surface area and fractal dimension as well as coal’s adsorption capacity, the two ways of monolayer adsorption and micropore filling for the adsorption forms of methane molecules are determined. The modes of action that can influence coal’s adsorption to gas are as follow: the adsorption domain on the surface of coal, adsorption sites in certain area, the size of adsorption potential between the surface of coal and methane molecules, the surface curvature of micropore in coal, coal impurity and environmental temperature. The DALangmuir isothermal adsorption equation for the adsorption of gas in coal was established based on the Dubinin-Astakhov equation of micropore filling and Langmuir equation of monolayer adsorption. Through this equation, the isothermal adsorption curves for six groups of coal samples are fitted. Moreover, it is also compared with the fitting rule of Langmuir equation, and it is found that the degree of fitting for DALangmuir is higher.(5) Through isothermal adsorption and desorption experiments under different conditions, different factors that influence desorption hysteresis can be obtained. Through increasing the diffusion paths and diffusion resistance, the increased particle improves the methane’s hysteresis degree in coal; although the increase of pressure balance result in the increase of hysteresis amount, it fails to improve the hysteresis ratio; the increase of temperature causes the decrease of both hysteresis amount and hysteresis ratio. The increase of water decreases hysteresis amount and increases hysteresis ratio; along with the increase of metamorphism degree, both the hysteresis amount and hysteresis ratio firstly decrease and then increase, while the increased range of hysteresis ratio is lower than that of hysteresis amount. Both the pore volume and specific surface of area mercury intrusion method shows positive correlation with hysteresis ratio. Both the fractal dimension mercury of intrusion method and fractal dimension of liquid nitrogen adsorption method also shows positive correlation with hysteresis amount, and shows certain positive correlation with hysteresis ratio, which is not so obvious. It is shown that the more complex the pore structure in coal and the rougher its surface, the greater its hysteresis amount and hysteresis ratio and the bigger the effect degree of desorption hysteresis for gas in coal.(6) The main causes for the hysteresis in desorption of gas in coal are pore throat, distorted adsorption and being blocked by impurity. Because of the overlaid adsorption potential field in pore throat, its adsorption capacity for adsorbate particle is stronger and then the circulation of methane particle would be blocked, which would cause the hysteresis. At the same time, after the coal’s adsorption to methane, expansion and deformation of the matrix would occur and it can lead to the shorter diameter, which would cause the retention of methane molecules that could run away. In addition, in the process of gas desorption and diffusion, impurities in the pore would flow out with the air, they’d block the smaller pore throat in the process and they’d block the flowing of air, which cause the hysteresis of gas would desorption in coal. The hysteresis of gas desorption in coal would result in low measured content of gas and adsorption constant. From the perspective of safety production in coal mine, the development of coalbed methane as well as geological storage of greenhouse gas, this paper puts forward the index for the evaluation of hysteresis happened in gas desorption of coal--limit amount of hysteresis in gas, so as to interpret the hysteresis capacity of gas in coal. Moreover, the isothermal adsorption and curves for adsorption mentioned above was calculated and summarized.(7) Starting from the shortage of the diffusion model for homogeneous coal particle, the change rules of coal centre concentration and pore temperature were put forward. Supposing the gas pressure is P, and the desorption process is the continuous process of adsorption equilibrium whose pressure is from 0 to P. Therefore, we can obtain the corresponding coal center concentration according to the residual gas content of coal. Gas desorption, an endothermic process, reduces the temperature of methane molecules and pore surface in the pore, and then changes diffusion coefficient of methane molecules within the pore. On such basis, the diffusion model for homogeneous coal particle was based on the changes of coal centre concentration and pore temperature. By using the above model, the software COMSOL was employed to make numerical simulation for the diffusion rule of coal samples, whose particle size is 1 ~ 3 mm, gas pressure is 2 MPa, and temperature is 30 ℃. Finally, the distribution law of gas concentration within the coal particle in different point-in-time, and rule of concentration change and desorption curve for the particles in coal was obtained, and the simulation result was verified by using desorption data in the laboratory.(8) By using the one-to-one correspondence for desorption curve and gas parameter, the rapid determination method for gas parameters in the coal was put forward. After conducting desorption experiment for coal samples with particles of different partical sizes, plus water and environmental temperature after adsorption equilibrium under the same condition, we determine through experimental rule that the dry raw coal sample of 1~3mm as the standard coal samples for experiment of rapid determination and put forward the correction formula of temperature in the experimental results. Based on the above-mentioned methods, rapid tester for the gas parameters in coal was developed. As for the instrument, the design of intrinsic safety type was employed, three types of test time was set up as follow: 30, 45 and 60 minutes, and three models for using the Ann model design, set up three kinds of test time, 30, 45, 60 minutes, and two kinds of matched pattern were determined as follow: the method of the including loss volume and direct method. Take the 92 nd mining area, II1 coal seam, Jiulishan Mine as example, corresponding database was designed and temperature correction was made. In eight dill holes constructed in the 92 nd mining area, the gas parameters were determined by respectively using conventional methods and rapid tester. It is shown by the results of the measurement that the measuring error is within 25%. Therefore, the test results obtained from the rapid tester for gas parameters in coal is basically reliable. |
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