Reaction Dynamic Characteristics Of Coal Dust And Gas Explosion And Disaster-causing Mechanism

When coal dust explosion accident occurs in the production,it often causes heavy casualties and property loss,and coal dust is involved in most of the gas explosion accidents due to the underground environment.In recent years,gas explosion accidents still occurred frequently.It has been the focus of research to prevent or mitigate the loss from coal dust gas explosion accidents and to reveal the propagation rule and the mechanism of coal dust gas explosion disaster.However,due to the large territory of China and the wide distribution of coal,coal and its deposit conditions vary greatly in each region,and the mineral contents variey from place to place,which leads to different physical and chemical properties of coal even in the same coal grade.Therefore,research scholars at home and abroad have not yet formed a unified understanding of the coal macromolecule structure of coal,and the mechanism of pyrolysis of coal and the combustion process of carbon particles has yet to be further studied,which poses a great challenge to the study of the dynamic characteristics of coal dust explosion reaction.Based on the analysis of the coal samples from lignite to anthracite,the microscopic physical and chemical structure of coal and the explosion characteristic parameters of coal dust were studied.The dynamic characteristics and mechanism of the explosion reaction of coal dust and gas were studied.In this paper,the relationship between the metamorphic grade of the coal dust particle and the related characteristic parameters was established by SEM scanning,Fourier Transform Infrared spectroscopy,laser co-focusing raman spectrum analysis,electron paramagnetic resonance analysis.The transparent coal dust dispersion system,the pipe coal dust explosion test system was designed and built.Combing with the 20 L spherical explosive ball test system,the explosion propagation law of coal dust andgas was studied systematically.The relationship between the pyrolysis of coal dust and the cracking reaction of the functional groups in coal macromolecular structure was revealed,and the dynamic characteristics of the chemical reaction were analyzed.The results of this study are of great significance to the study of the mechanism of coal dust and gas explosion.and the main work content and innovative achievements are as follows:（1）Carrying out the test of all types of coal,the detailed basic parameters and its microchemical structural parameter is obtained.With the increase of metamorphic grade,the specific surface area of coal dust particles increases gradually,and the specific surface area of anthracite can reach 3-4 times that of lignitic.There are many small particles attached to large particles on the surface of anthracite dust particle.The angles of these particles are relatively prominent,and the size of lignite particle with low metamorphic grade is well distributed.The results from peak-differentiating and imitating show that the fat chain length and the branched chain level of anthracite hit lowest,while those of bituminous coal and lignite with low-grade metamorphism are higher,and the aroma fa、I、DOC are better correlated with the degree of coal.The types of coal functional groups can vary according to different coal ranks.There are mainly such hydroxyl groups as hydrogen bonds,phenol hydroxyl,alcohol hydroxyl groups,as well as small amounts of free radicals in the coal dust of low metamorphic grade.And the hydroxyl groups of the middle and high metamorphism are phenolic hydroxyl and alcohol hydroxyl.When free radicals disappear,there remains little hydroxyl when it becomes anthracite.CH2/CH3 in the coal of low metamorphic grade is larger,with long fat chain and high branched chain.The ratio of CH2 to CH3 goes down as it gets down.The D1 and G peaks of the raman spectrum of coal can reflect the lattice defects of coal and the order of carbon structure.In the process of coal transforming to higher order,the peak of D1 is moving to the low wave area,and the distance between peak G and D is decreasing.And in the transition of coal dust particles from bituminous coal to anthracite,the value of AD1/AG increases gradually,but the law of change in low metamorphic lignite is not obvious.The size of the electron paramagnetic resonance g and the reflectivity of the lens were u-shaped,and the line width?H increased with the increase of the degree of metamorphic grade,and then gradually decreased.The relative concentration of free radicals increases with the increasing degree of coal,but when it increases to a certain extent（R0=2.0）,the concentration starts to decrease again.（2）The dispersion and explosive propagation characteristics of coal dust in different cavities is obtained.When the coal dust concentration and ignition energy are certain,the maximum pressure of the coal dust explosion is larger when the coal is low.And as the metamorphic grade increases,the correlation between the maximum pressure and metamorphic degree is not obvious,and the rising rate of maximum pressure in coal dust explosion and the explosion index show inverse u-shaped relationship with metamorphic grade.That is,when the（dp/dt）max and Kmax of bituminous coal stay between 1.25 and 1.8,vitrinite reflectance is relatively large,while in low grade lignite and high grade anthracite it is relatively low.The dispersion of the coal dust in the pipes is related to the pressure of the powder,the time and position of the spray powder,and for the transparent pipe dust dispersion system described in this paper,four dust nozzles arranged in the single section of pipe can achieve the optimum state of the dust dispersion in the pipe,with the first nozzle 125 mm apart from the blind plate,and the adjacent two nozzles 125mm apart.When the powder pressure is above 0.3 MPa,the powder pressure has little effect on the dispersive effect,and when the delay time of the system reaches around 20 ms,after 80 ms,the dust concentration in the pipe is basically dispersed.The spraying pressure of the pipe coal dust dispersion system is 0.35 MPa and the spraying time endures 200 ms.Because of the low volatility,the coal dust of high metamorphic grade is low in the early stage of explosion,but as the degree of coalification decreases and the reactivity of coal dust increases,it is shown that the propagation rate of explosive flame and the rate of pressure increases.From lignite to anthracite,the maximum explosion pressure of coal dust and the propagation speed of flame tend to decrease after the first increase.And the dust maximum explosion pressure of anthracite coal hit the lowest,bituminous coal the highest,followed by that of lignite.The optimum equivalence ratio concentration of the coal dust explosion is higher than the concentration of the stoichiometric ratio,and the equivalence ratio concentration under the condition of maximum explosion pressure and fastest flame propagation is about twice the concentration of the stoichiometric ratio.（3）The characteristics of gas-solid products after coal dust explosion and the mechanism of disaster through analyzing.In the explosion ball of 20L,the volatile content of the solid residue of the anthracite explosion is less than before the explosion,secondly,in the high metamorphic bituminous coal,the lower the metamorphic grade,the more its decrease of volatilization of the coal.And among them,the reduction of fat coal and gas coal is the greatest.The content of the fixed carbon content of the residual coal dust particles generally reduced by 40%,but there was no obvious correlation with the coalification degree of coal dust.The coal dust produces a lot of spherical particles after the explosion in explosion vessels of 20L,and several parts of the surface have ablative holes that lead to imcompletion.The damage of the surface is related to the metamorphic degree of coal dust,the surface structure of the particles after the coal explosion bears the biggest damage and there are maximum hole structures.The amount of spherical particulate matter produced is related to the metamorphic degree of the coal dust,among which the spheroidal particles formed after the explosion of anthracite dust reach the minimum,spherical particles formed after the coal explosion reach the maximum,in addition,a large number of holes in the surface exit.And there are also holes in the surface of residue from lignite explosion,but there are few spherical particles.The coal dust particles become plastomer after heating.Because the tar content in volatile component of bituminous coal is relatively larger,the volatile component from pyrolysis is accumulated in the coal dust particles,which causes the bulk volume to increase,and when the pressure builds up to a certain level,the accumulated gas carries tar from the surface of the coal,thus forming the structure of the hole after the explosion.The infrared spectrum of the dust particles before and after the explosion showed that several weaker shoulder peaks on the right side of the main peak before the explosion overlapped with a peak after the explosion,and the fat structure of 2800-3000cm^-1 was largely absent after the explosion,indicating that C-O、CH2-C=O and CH3-Ar were all involved in the pyrolysis and explosive reactions,and hydrocarbons was produced after the explosion from fat structure.After the explosion of coal dust in the pipeline,there are mainly N2,O2,CO2,CO,etc.Besides,there are also a small amount of CH4,ethane,ethylene,acetylene,etc.The concentration of N2 in the explosion was around 80%,the O2 concentration was generally less than 16%,and CO2 concentrations ranged from 3.89%to 8.63%.Coal dust particles,which are more coal-rich,consume less oxygen in the explosion than coal-dust particles that are less coal-rich.The concentration of CO in the environment after the explosion was 100 ppm,the highest8449 ppm,which was beyond the level that human body could bear.This result is consistent with CO poisoning as the main source of human casualties in the accident investigation.（4）The process of pyrolysis of coal dust and the chemical process of combustion of individual carbon particles through theoretical analysis.The volatile components in the process of coal pyrolysis mainly derive from the different functional groups of coal molecules,in which H2O,CO2,CO and H2 are derived from hydroxyl,carboxyl,ether and aromatic hydrocarbons,and C2H6 and CH4 are derived from the cracking of fat structures,and tar and other liquids are produced by various hydrocarbon compounds in coal.Different functional groups have different degrees of dissociation,and the thermal stability of the methoxide and carboxyl groups in the oxygen functional groups is relatively weak,and the thermal stability of the hydroxyl and carbonyl groups is greater.The lower the metamorphic grade,the lower the temperature of volatile component.The pyrolysis reaction of coal is divided into the first pyrolyzation and the secondary pyrolysis reaction and condensation polymerization.The first pyrolyzation mainly involves the fracture of the chemical bonds in the structure of coal macromolecular structure and the process of generating new free radicals;the secondary pyrolysis reaction occurs at higher temperature from the first pyrolyzation products,and the condensation reaction occurs mainly in the late stage of reaction.The combustion of carbon occurs in the lattice structure,where the O2 molecules enter the lattice surface or the lattice interface by diffusion and adsorption.And the carbon is broken down by heat or by the collision of other molecules,then the molecular structure of carbon is fragmented,and the small molecules formed then combine with oxygen to form the intermediate complex,which further react to produce CO2 and CO.