Study On The Influence Of Secondary Oxidation Characteristics Of Coal In Composite Gobs On The Working Environment Of Longwall Faces

Underground mining is the most common type of coal mining in the world.The working face for workers is a limited working space.The safety of personnel working space environment depends on mine ventilation.When the mine air flows through the working face,a small amount of air passes through the residual coal gap in the goaf to form air leakage in the goaf.The mine air in the working face is separated from the dirty air in the goaf through the hydraulic support to form a parallel network.The working space of the coal face and the goaf space of the dirty air are mutually coupled and integrated environmental space.Especially in the mining of close coal seams,under the dual action of the primary oxidation of the residual coal in the mined out area and the secondary oxidation of the oxidized residual coal in the overlying goaf,it directly affects the distribution of the temperature field and gas concentration field in the goaf,and then affects the working environment of the working face.Therefore,it is urgent to carry out experimental research on the temperature rise of coal secondary oxidation,explore the temperature rise characteristics of coal secondary oxidation gas production,establish a mathematical model of the impact of residual coal oxidation in the composite goaf on the working environment of the working face,including the residual coal in the extracted coal seam and the oxidized residual coal in the overlying goaf,and reveal the impact of the secondary oxidation of oxidized residual coal in the overlying goaf on the working environment of the working face.In this paper,the research method of combining laboratory test,theoretical analysis,engineering measurement is adopted to carry out the constant temperature difference guidance method experiment of coal samples with different preoxidation degrees,and the laws of macro temperature rise and gas production are obtained;Test and analyze the microstructure change of each coal sample,explore the influence mechanism of coal secondary oxidation;The mathematical model for calculation the working environment temperature and gas concentration of the working face is established,and the model is tested and verified;Taking an engineering example,carry out numerical simulation prediction under different assumptions,analyze the impact of residual coal oxidation on environmental parameters such as O₂concentration,CO concentration and temperature at the return air corner and the rear of hydraulic support,and put forward special preventive measures for residual coal oxidation.The changes of industrial components,pore structure and surface active functional groups of different coal samples with the deepening of preoxidation were studied by coal quality industrial analyzer,specific surface area and pore size distribution tester and Fourier transform infrared spectrometer.The results show that with the deepening of oxidation degree,the water content of pre oxidized coal sample decreases,the ash content increases,and the volatile content first increases and then decreases compared with the raw coal sample;The specific surface area increased first and then decreased-OH functional groups showed a single downward trend,while-CH₂-,-CH₃ and-C=O functional groups increased first and then decreased.The coal spontaneous combustion simulation device is used to carry out the constant temperature difference guidance method experiment of coal samples,and the variation law of adiabatic oxidation time of coal samples with different preoxidation degrees and the generation law of CO and CO₂ gas products are analyzed.The results show that the adiabatic oxidation time of 6 h and 24 h preoxidation at 50℃is shorter,the generation time of gas products is earlier and the amount of gas products is larger than that of raw coal;The adiabatic oxidation time of 6 h and 24 h preoxidation at 120℃and 200℃is longer than that of raw coal,the generation time of gas products is later and the generation amount is smaller.the mathematical model for calculating the working environment temperature and gas concentration of the working face is established.Using the finite element method and the grid stretching technology,the simulation calculation of multi physical coupling processes such as gas flow,heat transfer,mass transfer and reaction in the goaf and working environment during the dynamic advancement of the working face is realized.Introducing the source terms of the expression of oxygen consumption rate and gas generation rate of engineering example coal samples,the O₂ concentration,CO concentration and temperature at the return air side in the goaf are calculated by using the mathematical model,and the simulation results are compared with the field engineering experimental results.The simulation results are in good agreement with the engineering experimental results,which verifies the correctness and reliability of the established model.The developed mathematical model for predicting the working environment of working face is employed to analyze the temperature and concentrations of O₂ and CO in the return air corner of SIIN₂6-9 working face in the Liujia Coal Mine of Pingzhuang Energy Co.,Ltd.The working environment with the cumulative advance distance of 100 m is simulated under three assumed conditions.The results show that the O₂ concentration in the return air corner is reduced from 20%to 19%due to the secondary oxidation of oxidized residual coal in the overlying SIIN₂6-7 goaf,The decrease of oxygen concentration will reduce workers’labor efficiency;The concentration of CO gas has increased from 20 ppm to 60 ppm,which has far exceeded the 24 ppm specified in the occupational exposure limit for hazardous factors in the workplace.Working for a long time in this environment will endanger the health of workers.It is revealed that the secondary oxidation of oxidized residual coal in the overlying goaf is an important factor to aggravate the hypoxia and co overrun in the working environment.