Research On Spontaneous Combustion Law And Prevention Technology Of Overlying Goaf In Shallow Buried Short Distance Coal Seams Under Mining

The mining areas in western China have abundant coal resources.However,during the mining process,there is a potential safety hazard of spontaneous combustion in the goaf at any time,which is attributed to the characterized of shallow burial,short distance,and susceptibility to spontaneous combustion in coal seams.The spontaneous combustion of goaf not only affects the safe and efficient mining of coal resources,but also seriously threatens the life safety of underground miners.Especially during the mining of shallow and short distance coal seams,the upper goaf is prone to form through cracks that are connected to the surface,which are the main channels for fresh air to leak into the goaf,leading to the formation of a good oxygen supply environment in the goaf.At the same time,surface water and groundwater flow into the goaf through these cracks,causing the residual coal in the upper goaf to be soaked in water for a long time,which will lead to changes in the spontaneous combustion characteristics of coal.Given the above issues,the geological and mining conditions of shallow buried coal seams not only increase the difficulty of natural fire prevention and control,but also make it more difficult to determine the fire area in the upper goaf of coal seams.Therefore,studying the spontaneous combustion law of the shallow goaf overlying close coal seams is of great significance for the prediction and prevention of goaf fires with the same geological characteristics.The research methods adopted in this article include theoretical analysis,on-site observation,laboratory experiments,numerical simulation,and on-site application.These methods are used to conduct a systematic study on the distribution and prevention technology of dangerous areas in goaf with characteristics such as shallow burial depth and close coal seam distance.In this article,the development law of surface fissures connecting the upper goaf during the lower coal seam mining process was analyzed,and the mechanism of the change in the spontaneous combustion characteristics of residual coal in goaf is revealed,which occurs under the influence of surface fissure air leakage.The numerical simulation research on goaf based on dynamic crack development laws was conducted,and the prediction of the risk area of spontaneous combustion in the upper goaf was achieved.In addition,the integrated comprehensive fire prevention and extinguishing technology system for goaf has been established.The main research results are as follows:(1)The development law of surface cracks under mining of shallow and short distance coal seamBased on the topography and geological mining conditions of Sujiagou coal mine,the formation mechanism of surface fractures was theoretically analyzed.The types of fractures as the main air leakage channels in the goaf were confirmed.The range of crack occurrence areas connecting the surface and goaf is divided.The dynamic evolution law of surface cracks with advancing working face was obtained.(2)Macroscopic characterization and microscopic mechanism analysis of the impact of crack air leakage on the spontaneous combustion characteristics of coal soaked in goafThe programmed heating experimental system was adopted to measure the gas production and oxygen consumption rate of two types of coal samples during the heating process,one of which was treated with water immersion and air drying,and the other was untreated.Determine the characteristic temperatures of two types coal samples based on the variation law of apparent activation energy.Furthermore,the change rule of the relative content of coal molecular functional groups was analyzed based on in-situ FTIR,and the mechanism of “water soaking-air drying” process changing coal spontaneous combustion characteristics was revealed by combining coal elementary reaction model.By comparing the experimental results,the indicator gas for predicting coal spontaneous combustion in Sujiagou Coal Mine is determined,and a risk assessment system for predicting goaf spontaneous combustion is constructed,of which composed gas generation amount and characteristic temperature.(3)The distribution pattern of oxygen field,airflow field,and spontaneous combustion danger zone in goaf under crack air leakage conditionsThree dimensional geometric models and multi field coupling mathematical model of goaf are established,including seepage,diffusion and oxidation reaction of multi-component gas.Based on the above models,the distribution pattern of oxygen and airflow fields under the dynamic development of fractures in the overlying goaf was obtained.The risk area of spontaneous combustion in the upper goaf has been determined on basis of standard for classification of spontaneous combustion hazard zones.(4)Establishment and application of integrated fire prevention and extinguishing technology system for goaf overlying close coal seamsPropose an integrated comprehensive fire prevention and extinguishing technology system for goaf with "monitoring,plugging,and inerting" as the main body.Based on the experimental results of coal spontaneous combustion characteristics,a monitoring system that uses iconic gases and characteristic temperatures as indicators for predicting spontaneous combustion was establish.Based on the distribution characteristics of oxidation temperature-rising zone in goaf,the key sealing areas for surface cracks were determined.Implementing joint sealing technology for the ground and working face to achieve air leakage control in the overlying goaf.Furthermore,the minimum nitrogen injection required for fire prevention and extinguishing in goaf was obtained according with the actual conditions of Sujiagou Coal Mine and the predicted range of spontaneous combustion danger zone.The nitrogen injection plans and safety technical measures have also been introduced in succession.