Study On Methane Migration And Accumulation In Spontaneous Coal Combustion Environment Of A Coal Mined-Out Area

The coal mined-out area is a porous space where the spontaneous coal combustion and the methane migration take place.With the increasing depth of the coal mining and the increasing mining intensity,the spontaneous coal combustion disaster and the methane disaster are seriously threatening the safe mining.The coal combustion-triggered methane explosions in the coal mine gobs cause severe casualties,damage of devices and loss of resource.Therefore,it is important to study the disaster formation process of the coal combustion-induced methane explosion and the disaster prevention measures to ensure the mining safety.Although a lot of numerical simulation research,physical simulation research and engineering research are conducted,the disaster formation process is still not clear enough,resulting in the lack of the basis for the disaster prevention measures.The difficulty of the research on the coal combustion-triggered methane explosion is the complex interaction between the spontaneous coal combustion and the methane migration.The effect of the spontaneous coal combustion on the methane migration is difficult to analyze and capture in the therotical research and practical research.To systematically study the disaster formation mechanism,the combination of the theory analysis,the experimental simulation and the numerical simulation is applied to investigate the methane migration and accumulation in the spontaneous coal combustion environment of the coal mine gob.In this research,the coal combustion-induced buoyancy effect is achieved in the experient and the gas flow model.The main research content and the main conclusions are listed below,1.Based on the fluid dynamics,the gas flow model for the spontaneous coal combustion environment in the coal mine gob was built after the analysis on the airflow change caused by the high temperature of the coal combustion.According to the mass conservation law,the air flowing into the coal combustion area speeds up while the outflow velocity reduces.Based on the momentum conservation law,the upward airflow is formed by the buoyancy force in the coal combustion area.According to the species transport equation,the airflow change,the temperature change and the porosity in the coal combustion area cause the local species accumulation.The gas flow model coupled the buoyancy effect was built for the coal mine gob based on the fluid dynamics.In the gas flow model,the effect of the coal combustion temperature on the gas density is reflected in the mass conservation law,the momentum conservation law and the energy conservation law,respectively.Meantime,the effect of the mechanical dispersion on the gas movement is considered in the porous medium.2.The experimental platform was designed and built to conduct the research on the methane migration in the spontaneous coal combustion environment of the coal mine gob,and the methane accumulation was found at the coal combustion location.The experimental platform consists of the coal mine gob,the air-inlet roadway,the mining face,the gas releasing chamber,the ventilation device and the electrical heating modules.In this experimental platform,the thermal properties of sand is close to the real environment in the coal mine gob.The homogenious methane is released from the coal mine gob bottom.At different ventilation velocities,the methane migration is studied under different spontaneous coal combustion conditions.According to the experimental results,without the spontaneous coal combustion in the coal mine gob,the methane concentration shows the monotonous increase trend in both the strike direction and the dip direction.With the spontaneous coal combustion on the air-inlet side in the coal mine gob,the methane concentration distribution is in the shape of ‘?' around the coal combustion area due to the methane accumulation at the coal combustion location.With the spontaneous coal combustion on the air-return side in the coal mine gob,the methane concentration distribution is in the shape of ‘n' upside down around the coal combustion area due to the methane accumulation at the coal combustion location.The increase of the ventilation velocity decreases the methane concentration in the coal mine gob and the accumulated methane concentration at the coal combustion location.3.The numerical simulation results are validated by the experimental results,and the porous chimney effect at the spontaneous coal combustion location is proposed to reveal the disaster formation mechanism of the spontaneous coal combustion-triggered methane explosion in the coal mine gob.Besides,the revealed disaster formation mechanism is used to study the disaster prevention effect of the ventilation enhancement.The numerical simulation results show the obvious methane accumulation at the coal combustion location in the coal mine gob,which is in accordance with the experimental results.Without the buoyancy effect,the numerical simulation result can not obtain the experimental methane accumulation at the coal combustion location.The gas flow model coupled buoyancy effect is sensitive to the temperature.The porous chimney effect can cause the methane accumulation at the coal combustion location of 400 K.With the increase of the temperature,the accumulated methane concentration gradually grows to the maximum.After the analysis,the disaster formation mechanism can be explained by the porous chimney effect as follows,(1)the spontaneous coal combustion decreases the gas density to produce the buoyancy effect;(2)the buoyancy effect develops into the porous chimney effect in the porous coal mine gob,leading to the upward airflow and the negative pressure in the coal combustion area;(3)the negative pressure causes the dense methane to flow into the coal combustion area and the buoyancy force pushes the methane to flow upward.The methane accumulation at the coal combustion location under the influence of the air leakage;and(4)the accumulated methane is ignited by the coal combustion to develop into the methane explosion disaster in the coal mine gob.The disaster prevention effect of the ventilation enhancement is different due to different spontaneous coal combustion conditions in the coal mine gob.With the spontaneous coal combustion on the air-inlet side in the coal mine gob,the ventilation enhancement can decrease the methane concentration below the explosive methane limits to prevent the disaster.With the spontaneous coal combustion on the air-return side in the coal mine gob,the ventilation enhancement cannot decrease the accumulated methane concentration below the explosive methane limits at the early stage(400 K)of the spontaneous coal combustion,but decrease the methane concentration into the explosive methane limits to increase the risk of the methane explosion at the middle stage(400 K)and late stage(900 K)of the spontaneous coal combustion.This dissertation includes 118 figures,10 tables and 173 references.