Study On The Heat-Flow Coupling Evolution Laws And Low-Oxygen Characteristics In Shallow Goaf

With the shift of coal mining strategy towards the west,the importance of the northwest mining area in China’s energy security system has become increasingly prominent.Due to its unique geological conditions,the Shanxi-Shaanxi-Inner Mongolia region is prone to the occurrence of surface-goaf-workface“three zone”connectivity,forming gas leakage channels.A large amount of surface gas enters the mining coal-rock mass through surface cracks,and the oxidized gas and low-concentration oxygen flow across zones to the low-pressure goaf of the coal seam,and finally surge into the workface,resulting in the“three-zone”multi-gas inhalation-flow-oxidation heating effect.This leads to frequent“three zone”heat-flow disasters（coal-rock self-ignition,low-oxygen in the workface,etc.）,seriously affecting coal mine safety production.In response to this problem,this thesis starts with theoretical analysis,combines field engineering parameters and experimental research,and establishes a numerical model of heat-flow coupling evolution in shallow goafs.The research results are as follows:（1）Based on the theory of traditional zonal collapse characteristics in goaf,the process of fragmentation-compaction of the overlying coal-rock mass in shallow coal seam goaf,the law of development of surface cracks and the characteristics of porous media in the goaf were summarized,revealing the mechanism of the formation of“three zone”connected channels and multi-gas inhalation-overflow.Based on the basic theories of fluid mechanics,heat and mass transfer,etc.,a mathematical model of multi-gas“three zone”seepage and multi-field coupling in shallow coal seam is established,laying a theoretical foundation for the simulation study of heat-flow coupling evolution law in shallow goaf in the following sections.（2）Taking the S1204 working face and its main 2^-2 coal seam in the Ningtiaota mining area as the engineering background,the on-site data of the working face are investigated and analyzed.Based on the coal samples collected on-site,coal program heating experiments are carried out to analyze the trends of oxygen consumption rate,derived gas generation rate,and heat release rate during coal heating and oxidation,and the experimental results are fitted and analyzed with the Arrhenius equation.Combined with the on-site investigation results of the previous section,basic parameters are provided for the numerical simulation in the following section.（3）Based on the multi-field coupling mathematical model of multi-gas“three zone”overflow,combined with field data and experimental parameters of coal oxidation,a simulation study of the heat-flow coupling evolution law in shallow goaf with air leakage was carried out.By analyzing the evolution law of flow field,oxygen component field and temperature field in shallow goaf,the heat-flow coupling evolution law and low-oxygen characteristics of shallow goaf under“three zone”connectivity were explored.（4）Based on the numerical model established in the previous section,this study further discusses the impact of multiple variables in coal seam mining and the presence of overlying coal seams on the heat-fluid coupling evolution and low-oxygen characteristics of the goaf under“three zone”connected.The results show that when there is an overlying coal seam in the goaf,the phenomenon of oxygen enrichment in the upper part disappears,and the oxygen concentration near the return airway is generally low,which exacerbates the risk of low-oxygen in the working face.In addition,the presence of overlying coal seams causes air leakage from the ground to carry heat into the goaf,accelerating the oxidation process of residual coal.During the coal seam mining,the intensity of air leakage in the overlying rock,ventilation resistance and air volume in the working face,thickness of residual coal layer and residual coal distribution factor in the goaf are all positively correlated with the low-oxygen gas outburst and temperature rise rate in the goaf.An increase in these parameters will lead to an increase in the risk of low-oxygen and spontaneous combustion of residual coal in the goaf.The relevant research results can provide ideas for the prevention and control of heat-flow disasters in the exploitation of shallow coal seams in the western region.