Study On Radon Exhalation Mechanism And Migration Law During Coal Spontaneous Combustion In Goaf

As the most important primary energy in China,coal provides a substantial energy guarantee for economic and social development.With the rapid growth of the coal industry,there are still many critical technical problems that have not been effectively solved.Among them,the precise detection of underground hidden fire sources has always been a complicated problem,severely restricting the efficient management of coal fire.Based on the different detection principles,domestic and overseas scholars have proposed various fire source detection methods.The surface-based radon detection method(SRDM)has the advantages of simple operation,low cost,and free from the limitation of detecting topography.It is expected to provide a feasible solution for accurately detecting hidden coal fire sources.For decades,scholars have conducted extensive studies on the radon exhalation characteristics and migration law from different perspectives.Although SRDM has made some progress in determining the fire source location on-site,the radon exhalation mechanism during coal spontaneous combustion has not been wholly studied,restricting the further development of SRDM.Simultaneously,the geological conditions of overlying strata in goaf are complex and changeable.The existing theories cannot fully explain the radon anomaly data under various geological conditions.The adaptability of SRDM needs further study.On the basis of theoretical research,the thesis first conducted an experimental study of the rule of radon exhalation from different coals at room temperature and its influencing factors,then small-scale coal oxidation heating experiments,numerical simulation experiments,and large-scale coal spontaneous combustion experiments were conducted to explore the radon exhalation mechanism during coal spontaneous combustion and the laws of radon migration under different overburden conditions.The main research contents and results are summarized as follows:(1)A radon exhalation model of broken coal at room temperature is established by referring to the radon emission model of single particle in porous media.The mechanism and influencing factors of radon exhalation from broken coal at room temperature are discussed.It is considered that radon exhalation from broken coal is mainly affected by radium content,mineral content,moisture content and pore structure.On this basis,combined with the characteristics of coal spontaneous combustion,the factors that may affect the radon exhalation during the process of oxidation temperature rise,such as moisture,pores,cracks,minerals,and combustion gas,are deeply discussed.(2)Seven coal samples with different degrees of metamorphism are taken as the research objects,including lignite,long-flame coal,weakly viscous coal,gas coal,coking coal,lean coal,and anthracite.Radon release intensity at room temperature and main physical parameters(radium nuclide content,emanation coefficient,moisture content,and pore structure)are measured.As the degree of coal metamorphism increases,the concentration of radon gas is rapidly decreasing.The gray correlation degree between radon exhalation intensity and physical property parameters is greater than 0.7,and the gray correlation degree is radium nuclide content,pore volume,moisture content and mineral content in descending order.(3)In the process of coal oxidation and heating,the variation of the radon exhalation rate of coal samples shows a noticeable trend of "single peak" or "double peak".The typical temperature of lignite is 100 °C,and the typical temperature of weakly caking coal,coke coal,and lean coal 1is 200 °C,the radon exhalation rate of lean coal 2 of the long flame coal changes in a "double peak" form,with the main peaks at 200 °C and 250 °C,respectively,and the secondary peak at 100 °C.(4)Combined with the results of isothermal drying experiment,low temperature nitrogen adsorption experiment,SEM experiment,room temperature treatment coal sample radon concentration measurement experiment and gas chromatographic analysis experiment,the mechanism of radon exhalation during coal spontaneous combustion is revealed.It is concluded that the dissolution of radon in water and the adsorption and closure of radon in capillary pores are the main existing ways of radon in coal.Water evaporation and coal pyrolysis are the main reasons for the increase in the radon exhalation rate.The difference between the number of dissolved radon atoms in the water and the number of closed radon atoms in the capillary leads to a "single peak" or "double peak" change in the radon exhalation rate curve.For coal with higher metamorphosis,its pyrolysis temperature is relatively high,so the typical temperature is relatively large.(5)On the basis of discussing the general differential equations of radon gas migration in uniform porous media,the two-dimensional partial differential equation of radon gas migration in the overburden strata in the goaf was simplified.Numerical simulation results of radon migration in the "two zones" overburden show that the radon concentration in the caved zone decreases linearly with the increase of migration distance,while the radon concentration in the fractured zone decreases rapidly in a logarithmic form.The greater the radon migration rate in the "two zones" overburden,the more obvious the abnormal phenomena of radon on the surface.When the radon gas in the goaf moves through the aquifer overlying rock to the surface,the abnormal surface radon concentration is weakened.Numerical simulation results of radon gas migration in the "three zones" overburden indicate that the lower radon migration efficiency in the continuous zone results in radon complete decay before it reaches the surface,and no radon anomaly occur on the surface.With the acceleration of the radon migration rate,the radon atoms in the goaf successfully passed through the "three zones" overburden area to the surface and formed radon anomaly on the surface.In addition,there is a small difference between the number of radon atoms from the goaf of multiple coal seams transported to the surface and the number of radon atoms on the surface of the goaf of a single coal seam,and both belong to the same level of radon concentration.(6)Based on the geological prototype of 32201 working face in Bulianta Mine,the radon migration experiment of coal spontaneous combustion in goaf under the condition of “two zones” overburden distribution was carried out.The experimental results show that in coal oxidation heating stage,water evaporation and coal pyrolysis lead to a slight increase in radon concentration in the coal spontaneous combustion simulation area.When the coal in the simulated area reaches the ignition point and burns,the surface radon concentration is 2.32 ?5.56 times of its background concentration.With the expansion of coal combustion range,surface radon concentration increased to 4.35 ? 10.42 times of its background concentration.When the aquifer is distributed in the overburden,the sureface radon concentration decreases to 2.53 ? 7.45 times of its background concentration.Based on the geological prototype of No.8 coal seam of Xiegou Mine,the radon migration experiment of coal spontaneous combustion in goaf under the condition of “three zones”overburden distribution was carried out.The experimental results show that when a small-scale coal combustion occurs,the continuous zone of the overburden with good integrity and low void ratio hinders the rapid migration of radon gas,resulting in no radon anomaly at the surface.With the expansion of coal combustion,the gas gathered in the fractured zone migrated to the surface under the action of concentration gradient and temperature gradient.The surface radon concentration rises to 2.00 ? 6.75 times of its background concentration.When a large area of coal combustion occurs in the goaf of No.13 coal seam,the radon concentration on the surface is 1.78 ? 4.90 times of background concentration,which coindents with the radon concentration on the surface of No.8 coal seam.