| With the continuous exploitation and utilization of coal resources,the mining depth will be further extended,the ground stress and gas pressure will continue to increase,t the frequency and intensity of outburst will increase significantly.It will cause increasingly serious economic losses and personal injuries.Gas outburst is a very complex dynamic phenomenon.The mechanism of gas adsorption,desorption,seepage and diffusion in coal has not been clearly studied,and the outburst mechanism is still in the hypothesis stage.Therefore,it is necessary to determine a more practical and reasonable prediction indicator.A large number of mine actual situation and laboratory simulation experiments on gas outburst proved that there would be temperature changes before coal and gas outburst.Therefore,through experimental research,adsorption theory,surface chemistry theory and quantum chemical calculation and other methods,the process of gas adsorption-desorption and its temperature variation and energy conversion and other issues were comprehensively and systematically explored in this paper.It will provide a reference for the improvement of coal and gas outburst mechanism.The main research conclusions include:（1）There were six kinds of particle sizes lean coal samples to be collected and prepared,such as columnar,24 mesh,48 mesh,816 mesh,1630 mesh and 3060 mesh.The existing adsorption and desorption experimental equipment in the laboratory was designed and processed.An thermal insulation device and a temperature measuring device were added to it.In order to eliminate the influence of gas compression and expansion on coal temperature,the experimental tank was calibrated with granite sample,and the calibration formula and fitting function were given.Aerogel felt with low thermal conductivity was selected as insulation material,and its insulation performance was tested to meet the experimental requirements.The equipment can accurately measure the temperature variation in the process of gas adsorption and desorption experiment.（2）The temperature variation of coal in the process of adsorption and desorption under different pressure and particle size was studied.The experimental results showed that:1)in the process of carbon dioxide adsorption and desorption has obvious temperature variation.The temperature increases in the gas injection process and decreases in the exhaust process.It shows that adsorption is exothermic process and desorption is endothermic process.2)under the same gas injection pressure,the smaller the particle size of coal sample is,the greater gas adsorption capacity is when adsorption is equilibrium,and the larger the maximum temperature change is;For the same particle size coal samples,the greater the gas injection pressure is,the greater the equilibrium pressure is when adsorption is equilibrium.The greater the adsorption capacity is,the greater the temperature rise range is,the greater the change rate is.3)under the same equilibrium pressure,the smaller the coal particle size is,the larger the maximum temperature change is.Under same size coal,the larger the adsorption equilibrium pressure is,the larger the temperature decrease and the higher the change rate is.4)the temperature variation in the desorption process increases with the increase of the pressure and content of gas inside the coal.From this rule,the variation of temperature can be used to characterize the gas accumulation in coal and predict gas outburst.（3）By using the theory of gas adsorption model,the adsorption mechanism of gas in the coal surface is analyzed,and combined with the thermodynamics formula,surface chemistry theory,the law of conservation of energy and other knowledge,the law of energy variation during gas adsorption and desorption process was analyzed,the results showed that:1)BET model is more suitable for describing the adsorption of carbon dioxide by coal,thus it is concluded that multi-layer adsorption occurs when coal absorbs carbon dioxide.2)in general,the smaller the particle size of the coal sample is and the greater the equilibrium pressure is,the greater the surface free energy and heat change value of the coal in the process of adsorption and desorption is.With the increase of the equilibrium pressure,the increase rate of the surface free energy reduction value of coal samples with different particle sizes gradually slows down.It indicates that there is a non-uniform adsorption potential field on the coal surface.3)by comparing the differences between the heat data of columnar and granular coal samples in the process of adsorption and desorption,it can be concluded that:in the process of adsorption,the reduced surface free energy is transformed into the adsorption heat and deformation energy,and the adsorption heat and deformation energy jointly lead to the increase of coal temperature;in the process of desorption,the gas desorption and the coal shrinkage deformation together lead to the decrease of coal temperature.（4）By using quantum chemistry software,the adsorption models of CO2 and CH4 molecules on the surface of coal were built,and the adsorption energy was calculated.The results showed that:1)the more stable adsorption configuration between carbon dioxide molecule and coal molecule is that the projection of carbon atom of carbon dioxide molecule coincides with the central carbon atom of coal molecule,and the projection of carbon dioxide molecule structure coincides with a C-C bond of coal molecule structure.2)the more stable adsorption configuration between CH4 molecule and coal molecule is that the projection of C-H bond of CH4 molecule coincides with the C-C bond of coal molecule,and CH4 molecule is adsorbed on the surface of coal molecule in the form of positive triangle cone.3)the adsorption potential well values of the stable adsorption configurations of CO2 and CH4 molecules are-10.739 kJ/mol and-4.350 kJ/mol respectively,both of which are physical adsorption.The adsorption configurations of CO2 molecules are more stable,which proves that the adsorption performance of CO2 on the surface of coal is better than that of CH4 under the same adsorption conditions. |
