Multi-physics Coupling Mechanism Of Co-existence Hazards For Coal Spontaneous Combustion And Gas

Spontaneous combustion tendency and gas adsorption of coal, the two basic characteristics of coal, are the fundamental cause resulting in gas and spontaneous combustion disaster. Recently, with the increase of mining depth, coal seam gas content and gas emission amount increase, geothermal gradient also increase rapidly, the symbiosis of gas and coal spontaneous combustion disaster in cross-scale fractures field is more common, the risk of disaster increases, the safety situation of coal production is grim extremely. The symbiotic disaster of gas and coal spontaneous combustion has become a universal mode resulting in serious accidents of coal mine. Not only the efficiency of disaster treatment, but also the safety should be considered during the treatment of symbiotic disaster. Taking the fundamental approach for gas disaster treatment—gas extraction as an example, we should consider the efficiency of gas drainage should be considered, and we must also ensure that it will not lead to secondary disasters such as coal spontaneous combustion and gas explosion during gas extraction.However, the symbiotic disaster of gas and coal spontaneous combustion is the coupling result from multi-scale,-time and-physics processes. Up to now, it is still lack of the systematic theory study on the interaction relationships between the coal-rock fractures field, the diffusion-seepage field of compositional gas, the chemical field of coal-oxygen reactions, and the transport field of thermal energy, leading to the result that it is not very clear for us to understand the disaster occurrence, development and evolution mechanism of gas and coal spontaneous combustion in coal-rock fractures field. The theory is obviously behind the practice in the prevention of disasters. Therefore, disaster-causing mechanism of multi-field coupling processes needs urgently to be studied for the symbiotic environment of gas and coal spontaneous combustion in the cross–scale fractures field(coal seam and gob loose porous media, etc).(1) The hydrological-mechanical coupled mechanism of the coal seam gas and air mixtureConsidering the coal matrix shrinkage/expansion process induced by gas desorption-diffusion-seepage in coal seam matrix, gas-air mixture flow in fractures and coal gas desorption/adsorption in the gas extraction process, the hydrological-mechanical coupled dual-porosity medium model was established to evaluate the gas extraction quality. The results revealed the hydrological–mechanical coupled mechanism and controlling parameters of low quality in the gas extraction process, and a new in-situ control technology of gas-air mixing, which uses fine particles to seal leakage fractures field to improve gas concentration, was proposed, providing a scientific basis for efficient and safe gas extraction in coal seam.(2) The hydrological-mechanical-thermal-chemical coupled mechanism of coal spontaneous combustionThe multi-physics coupling processes of the coal geomechanical deformation, compositional gas seepage-transmission and energy transport were linked together during spontaneous combustion of underground coal seams. A HMTC coupled model was established to determine quantitatively the time and locations of spontaneous combustion. Furthermore, the self-heating susceptibilities associated with extrinsic and intrinsic factors, incorporating coal permeability, pressure difference, oxygen-consumption rate, and reaction heat of coal oxidation, are gained using the model. The results indicate that the self-heating rate and gas velocity are positively correlated with above factors showing ‘‘S-type’’ upward trends, whereas the oxygen concentration has an ‘‘S-type’’ downward trend.(3) Multi-field coupled mechanism hazards for the co-existence area of coal spontaneous combustion and gas in coal seamsThe coal deformation(gas-desorption induced coal shrinking and self-heating induced coal swelling), methane diffusion from matrix, compositional gas flow in fractures, coal-oxygen reaction kinetics, and energy transport process were further coupled. The methane-rich coal self-heating evolution model was established, and it was applied to quantify the self-heating susceptibilities associated with methane desorption diffusion time, coal permeability, differential pressure of leakage, coal-oxidation reaction heat and coal-oxidation rate. The results show the compositional gas flow in fractures is related not only with coal permeability, but also with self-acceleration effect induced by gas thermal expansion and gas desorption rate; The desorption gas in coal matrix infiltration into the fractures can not only dilute oxygen concentration and inert combustible coal, but also hinder outside air leakage into porous coal media; The larger differential pressure of leakage, coal permeability and coal-oxidation reaction heat correspond to the higher coal medium seepage velocity, and the coal-oxidation reaction is more severe, whereas the self-heating rate of coal is not completely determined by the coal-oxidation rate, but is closely related with activation energy and pre-exponential factor. The smaller activation energy is or the larger pre-exponential factor is, the more severe oxidation reaction of coal is.(4) Multi-field coupled mechanism hazards for the co-existence area of coal spontaneous combustion and gas in gobsTaking the time-space evolution of gob permeability induced by continuous advancing of working-face into account, coal spontaneous combustion and gas emission flow show the strong “history effect” with time. A multi-field coupled model for the co-existence area of gas and coal spontaneous combustion in the active gob with high-gas emission and easy-spontaneous combustion was established. Subsequently, the impact of factors(such as working face length, ventilation quantity, wind resistance, working-face advance rate, coal-oxidation reaction heat and coal-oxidation rate) on the flow fields of gas and coal spontaneous combustion were investigated. The results reveal the disaster occurrence, development and evolution mechanism of gas and coal spontaneous combustion in coal-rock fractures field, enhancing the theoretical level of awareness for the synergistic prevention of gas and coal spontaneous combustion disasters in gobs.(5) Software development of multi-field coupling simulation and its application associted with co-existence hazards of coal spontaneous combustion and gasBased on the potential hazards such as gas explosion and coal spontaneous combustion in the complex symbiotic region of gas and coal spontaneous combustion, the concept of regional safety is firstly proposed. The mathematical expression of safety degree is defined according to the hazard criteria of gas and spontaneous combustion. Based on the independent development platform of Comsol Multiphysics and Matlab(CMM) and the established coupling model of gas and spontaneous combustion, an integrated evalution program is developed. The program is successfully applied to gas extraction cases in the coal seam and gob, the dynamic safety evolution of coal seam and gob in the gas-drained processes was respectively obtained. It provided a theoretical method and tool for collaborative prevention of gas and coal spontaneous combustion.