| The coal spontaneous combustion is one of the major disasters affecting the mine safety production in the goaf,which not only produces toxic gases,but also induces a series of secondary disasters such as gas or coal dust explosion.With the increase in the depth and intensity of coal mining in China,the prevention and control of spontaneous combustion disasters are becoming more and more complex.Especially in the fully mechanized caving goaf,affected by the amount of residual coal and air leakage,the risk of spontaneous combustion is strong.Therefore,it is of great significance to master the law of oxidation temperature rise of residual coal in goaf and construct an efficient early warning method for coal spontaneous combustion disaster prevention.Coal spontaneous combustion in goaf is the final result of the composite reaction of residual coal and oxygen,and the composite reaction rate of coal and oxygen has a nonlinear relationship with temperature and oxygen concentration.In this paper,theoretical analysis,experimental test,numerical simulation and field application are used to study the nonlinear relationship between coal-oxygen composite reaction rate and oxygen concentration,the characteristics of staged oxygen reduction and oxidation of coal,the evolution law of residual coal spontaneous combustion in fully mechanized caving goaf under push mining conditions and the dynamic early warning method of coal spontaneous combustion.The main results are as follows:(1)Combined with theoretical analysis and temperature programmed test,the change rule of coal oxidation reaction order at low temperature was analyzed.It was found that the reaction order of experimental coal samples decreased first and then increased with coal temperature.With the increase of temperature,the reaction series gradually approached 1.The functional relationship between the reaction order and the coal temperature was obtained by piecewise fitting,thus clarifying the nonlinear relationship between the coal oxidation reaction rate and the oxygen concentration.The nonlinear mathematical models of oxygen consumption rate and gas product formation rate are derived,and the variation of coal oxidation characteristic parameters obtained by nonlinear and linear models with temperature is compared.The results show that the calculation results of nonlinear model are lower than those of linear model in high temperature stage,and the difference is mainly due to the nonlinear model considering the actual attenuation characteristics of oxygen concentration in coal sample tank in the direction of air flow.(2)Through the calculation results of the nonlinear model in the iconic gas,combined with the coal-oxygen composite reaction order and the gas yield change rate,the stage characteristics of the coal spontaneous combustion process were analyzed.The coal spontaneous combustion process was divided into five stages:autothermal,critical,pyrolysis,cracking and fission.Taking into account the characteristic temperature and the attenuation characteristics of the environmental oxygen concentration of the residual coal in the goaf during the mining process of the working face,the staged oxygen reduction temperature programmed experiment was carried out,and the coal oxygen reaction rate and kinetic parameters under different oxygen reduction environments were analyzed by the nonlinear oxidation reaction rate model.It was obtained that when the coal was oxidized to a certain temperature under the condition of high oxygen concentration,the coal oxidation reaction rate had a certain improvement compared with the test results of low oxygen level during the oxygen reduction and continuous heating process,and the improvement degree was positively correlated with the oxygen reduction temperature.The relationship of oxygen consumption rate and gas production rate with oxygen concentration and temperature was analyzed,and three oxygen concentrations which had significant influence on coal oxidation rate were obtained,which were 5%,9%and 17%,respectively.The oxygen concentration had little effect on the apparent activation energy and the mutation characteristics of coal spontaneous combustion process.(3)Based on the nonlinear relationship between the coal-oxygen composite reaction rate and the oxygen concentration,the four-dimensional nonlinear oxidation numerical simulation model of the residual coal in the goaf was constructed considering the factors such as the mining speed of the fully mechanized caving face,the spatial distribution characteristics of the residual coal,pores in the goaf,and the particle size influence function.The simulation calculation was carried out combined with the key parameters determined by the experimental test,and the accuracy of the model was verified by the natural ignition period and the field gas monitoring results determined by the natural ignition experiment.The simulation results show that the temperature rise of residual coal in goaf has obvious"partition" characteristics,that is,the temperature of residual coal in "two lanes" position is high,the temperature in the middle of goaf is low,and there is a nonlinear relationship between the maximum temperature of goaf and the advancing speed.Oxygen concentration is one of the key factors affecting the temperature rise of residual coal in goaf.When the oxygen concentration decreases to less than 5%,the temperature rise rate and CO yield of residual coal decrease significantly.(4)Based on the stage characteristics of residual coal oxidation and the change law of gaseous products under different oxygen environments,three single gas indicators and five commonly used coal spontaneous combustion early warning composite indicators were established.A multi-indicator and stage-by-stage early warning system with R2,C2H4,and C2H6 as main indicators and CO,R1,R3,R4,and R6 as auxiliary indicators was optimized and constructed,and the quantitative relationship between multi-indicator and coal temperature at different stages was determined.Considering the change of predicted temperature and oxygen concentration of residual coal in goaf,and combining with the nonlinear characteristics of coal-oxygen composite reaction,the prediction model of spontaneous combustion period of residual coal in goaf is constructed,which can predict the time required for coal temperature to reach different warning levels under different temperatures and oxygen environments.(5)Based on the classification and early warning method of coal spontaneous combustion in goaf,the prediction model of residual coal temperature and spontaneous combustion period,the dynamic early warning system of coal spontaneous combustion was developed considering the monitoring and early warning requirements of mine spontaneous combustion disaster,and the field industrial test was carried out in the fully mechanized caving face of a mine in Shaanxi.This method and system can preliminarily realize the dynamic perception and early warning of coal spontaneous combustion risk in goaf,improve the processing efficiency of coal spontaneous combustion information and the stability of early warning results,and avoid the distortion of results caused by single index.The field application effect is good.The research results have certain practical significance for strengthening the prevention and control level of spontaneous combustion disasters in coal mine goaf. |
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