Mechanism Of CO Generation From Low Metamorphic Coal And Early Warning Study On Spontaneous Combustion In Goaf

Article 52 of China’s Detailed Rules for Fire Prevention and Control in Coal Mines（2021 Edition）clearly stipulates that in mining coal seams that are prone to spontaneous combustion and spontaneous combustion,it is necessary to determine the gas and critical value for coal spontaneous combustion（CSC）.However,during the mining process of low metamorphic coal seams,they are often affected by the ambient temperature oxidation of goaf,leading to abnormal CO in the tailgate corner of working face during normal mining,posing a serious challenge to the early warning of CSC in mine.This article takes the brown coal mining face of Dananhu No.1 mine,Hami as the research object,and uses experimental research,theoretical analysis,numerical simulation,and on-site testing methods to systematically study the mechanism of CO generation in low metamorphic coal goaf and the early warning method of CSC.The main research work and achievements are as follows:（1）The CO production characteristics of low metamorphic coal during ambient temperature oxidation and the influence of ambient temperature oxidation on CO production during the heating oxidation process are obtained.The CO production characteristics of low metamorphic coal during ambient temperature oxidation under different particle sizes,O₂ concentrations,and multiple oxidation conditions are studied,and the CO production rules during heating oxidation process of coal samples treated with different ambient temperature oxidation time are analyzed.Research shows that:during ambient temperature oxidation,the CO production rate decreases first and then increases as the particle size increases,and only decreases with the decrease of O₂concentration when the O₂ concentration decreases to below 15%,and gradually decreases at the same time as the times of oxidation increases.The effect of coal’s ambient temperature oxidation on CO production during heating oxidation stage can be divided into two stages:the inhibitory effect before the critical temperature point and the promoting effect after that.（2）The CO generation mechanism of low metamorphic coal from ambient temperature oxidation to heating oxidation is clarified.The thermodynamic parameters of the CO generation chain reaction pathways of four initial chemical reaction molecules with different side chain lengths are studied.The influence of side chain lengths on CO production are analyzed,and the CO generation mechanism from ambient temperature oxidation to heating oxidation is revealed.Low metamorphic coal usually has a longer side chain,which results in a relatively low activation energy for CO production.After the long side chain initial chemical reaction molecules complete the primary chain reaction path to produce CO,they can also provide the initial chemical reaction molecular structure for the secondary side chain length CO production chain reaction path,so that the initial chemical reaction molecules with the long side chain structure can generate more CO,which is the reason why low metamorphic coal gobs can produce high concentration CO under ambient temperature oxidation conditions.The CO carbonyl precursor produced from ambient temperature to critical temperature point is mainly composed of primary aldehyde functional groups,and then turn out to be dominated by secondary aldehyde functional groups after critical temperature point.Ambient temperature oxidation consumes the primary aldehyde functional groups in coal,accumulating secondary aldehyde functional group precursors,resulting in significant influence of the duration of ambient temperature oxidation on the CO production during heating oxidation stage.（3）The CO production mechanism in low metamorphic coal goaf is revealed.The goaf can be divided into two types:ambient temperature oxidation goaf and heating oxidation goaf.Based on experimental research,the source term of CO production during ambient temperature oxidation is obtained.Based on theoretical analysis,a physical field control equation of goaf is established that introduces periodic weighting characteristics.A steady-state mathematical model of fluid solid chemical coupling in ambient temperature oxidation goaf and a transient mathematical model of heat fluid solid chemical coupling in heating oxidation goaf are constructed.The study on CO production mechanism in ambient temperature oxidation goaf and heating oxidation goaf is conducted using 1306 and 1305 goaf as research objects.The O₂ concentration in ambient temperature oxidation goaf is the main factor affecting CO production,making the dissipation zone the main contribution area of CO source in working face.The CO concentration output in tailgate corner is positively correlated with the volume of dissipation zone.The heating oxidation goaf has both ambient temperature oxidation and heating oxidation effects,and there is a mutual limit behavior between them.The enhancement of the heating oxidation ability of goaf will continuously narrow the range of ambient temperature oxidation,causing its CO production capacity to gradually decrease.Conversely,the enhancement of ambient temperature oxidation capacity in goaf will limit the range of dissipation zone and oxidation zone,which is not conducive to the occurrence and development of heating oxidation phenomenon.The mutual limiti behavior of the two oxidation processes results in a√shaped change in the CO concentration in tailgate corner with increasing temperature in low metamorphic coal goaf.（4）A method is proposed to determine the risk level of CSC in goaf based on the jointly monitoring speed of the working face,the monitoring of CO concentration in tailgate corner,and the trend auxiliary judgment index.By introducing a historical period sequence,the statistical velocity of the working face and the CO concentration in tailgate corner are transformed into the monitoring velocity of the working face and the monitoring concentration in tailgate corner.In the early warning process of low metamorphic CSC,the risk state of coal spontaneous combustion in goaf is first determined by comparing the monitoring speed of the working face with the minimum safe advance speed of critical coal spontaneous combustion.When the risk state of coal spontaneous combustion in the goaf is determined to be dangerous,the monitoring concentration and trend auxiliary judgment indicator are analyzed.After determining the approximate range of high temperature in the goaf based on the trend auxiliary judgment indicator value,the high temperature degree in goaf is accurately determined using the the monitoring concentration value.This method overcomes the problem of early warning false alarms caused by large dynamic fluctuations in instantaneous monitoring parameters for CSC.After adopting this warning method,the working face achieved safe mining and production.The thesis has 86 pictures,27 tables and 155 references.