Research On Formation Mechanism Of High Pressure Pulverized Coal-Gas Two Phase Flow During Outburst And Its Disaster Characteristic

Coal and gas outburst is one kind of the most complex underground gas dynamic phenomena during the production of coal mines,which performs as a sudden ejection of coal,rock and gas into the mining space in a very short period,leading to the formation of coal-gas two phase flow.This process is usually accompanied by a strong coal pulverization phenomenon,and a positive feedback mechanism of pulverization – rapid gas desorption – further pulverization and further rapid desorption can take place,which could significantly promote the development of outburst in roadways.The study of the formation mechanism and motion feature of the outburst coal-gas flow,and the effect of coal pulverization in promote the development of outburst will be of great importance in enriching and perfecting the outburst mechanism,as well as guiding the outburst prevention in coal mines.This dissertation takes the rock mechanics,surface physical chemistry and twophase fluid dynamics theory as the guidance,combining the theoretical analysis,experimental testing and numerical simulation as the study methods,based on the quantitative characterization of samples’ basic parameters,pore structure and gas adsorption/desorption properties,aimed to obtain the effect of pulverization on the pore structure damage characteristics of coal samples and build the initial gas desorption kinetics model of the pulverized coal.Moreover,using numerical simulation and analog simulation experiments to study gas-bearing coal pulverization mechanism in the development of outburst and the motion properties of the coal-gas two phase flow in roadways,as well as the effect of gas-bearing coal pulverization and rapid gas desorption on the promotion of outburst development stage.The main research conclusions of the dissertation are list as below:1)Studies of the breakage and pulverization characteristics of the outburst coal using a DEM-CFD numerical simulation indicate that there exist two different collision patterns in the roadways during the development stage of outburst,namely the high frequency & low relative speed collision between particles and the low frequency & high relative speed collision between particles and roadway walls.Both two collision patterns could lead to the pulverization of coal,crushing products of the former one are primarily coal particles with large particle size whilst the products of the latter one are primarily coal powders with small size.With the enhancement of the outburst strengthen and the ejected coal mass,the interparticle collisions become more and more dominant and the coal pulverization phenomena also become much more obvious.The biggest pulverization rate determined by the analog simulation is up to 8.14%（outburst pressure: 0.5MPa）2)Pulverization process has no remarkable impacts on coals’ basic components however do modify the pore structures.Using the physisorption method to quantitative characterize the pore structures before/after pulverization and the results indicate that due to the introduction of additional pores,the specific surface area and pore volume of the coal both increase with coal pulverization degree.The specific surface area and pore volume values of <0.074 mm coal samples are several times to dozens of times higher than that of 1-3mm samples.Additionally,when the coals are crushed into a <0.074 mm particle size,some samples exhibit sudden increase of the surface area and pore volume,indicating that the pore structure of these coals have been completely ruined whilst also demonstrating that the matrix sizes of coals are closely related to coals’ metamorphic grade.3)The modification of coals’ pore structure after pulverization has significant influence on the gas adsorption/desorption properties.With the increasing of breakage degree,adsorption capacities and the cumulative desorption quantities within 130min(Q₁₃₀)show increasing trend（however the influence on WLH samples is slight）.Results from the initial gas desorption experiments indicate that comparing to Q₁₃₀ the rates of 0-5s gas desorption amount（Q₅）and 0-60 s gas desorption amount(Q₆₀)both increase as the particle size of the samples decrease.5 Q₁₃₀ Q value of <0.074 mm samples can reach 37.37% as well as the maximum 60 Q₁₃₀ Q can reach 66.67%.Initial gas desorption amount of other small sized samples is also very much remarkable and the initial gas rapid desorption of powder coals have significant effects on the ejection of coal mass when outburst is triggered as well as the movement of coal-gas two phase flow in roadways.4)Based on the initial（0-60s）gas desorption experiments,results demonstrate that for majority of coal samples the initial gas desorption amount Q_t has a highly linear correlation with the square root of desorption time（（?））.According to the experimental data,spherical diffusion model and Fick’s second law,an initial gas desorption model is established.After the verification of the model,results indicate that the model is of good reliability and practicability and can be used into the corresponding studies of the effects of powder coal desorption on outburst development.5)To study the structure,formation mechanism and motion properties of outburst coal-gas two phase flow,an independent development experimental instrument is established.Results indicate that due to the complicated gas-solid coupling effect between pulverized coal and gas flow,the flow patterns,shockwave overpressures,movement speed,ejected coal mass distribution and separation/pulverization properties of the outburst coal-gas flow are all controlled by the outburst pressure as well as the coals’ particle size composition.Generally,higher pressure and higher mass rate of <0.25 mm samples will lead to stronger outburst strength.Especially when the mass rate of <0.25 mm coal powder exceed 2/3,influenced by the higher pressure as well as the rapid desorbed gas,the coal-gas two phase flow will endure a self-acceleration effect.And after the secondary acceleration,powered by the hydrostatic pressure（>150kPa）the two-phase flow will keep moving in the roadway as a high speed（30m/s）plug flow.6)Using the weak adsorption gas Nitrogen as control group to study the effect of adsorbed gas in coal on coal and gas outburst,results indicate that the rapid gas desorption of the adsorbed gas has remarkable impact on the promotion of outburst.Even for the larger sized coal sample whose gas desorption rate is low,due to the influence of pulverization-promoted desorption in coal-gas two phase flow,it is still easy to find out that the outburst strengthen is enhanced（the relative strength of outburst is enhanced by 4.51-9.42%）.Moreover,the rapid gas desorption from powder coal samples can further induce the coal-gas two phase flow to form a self-acceleration flow and plug transport phenomena,which can significantly enhance the outburst strength and its destructive power.7)Referring to the energy conservation demaned by outbusrt,the effect of pulverization and rapid gas desorption of gas-containing coal on outburst development is investigated.Results indicate that the free gas and adsorbed gas in coal both attend the outburst process,however the rapid gas desorption from pulverized coal plays a decisive role in promoting the development stage of outburst.In an actual outburst accident,73.3%-89.95% of the total energy is supplied by the rapid gas desorption from coal powder.