Mechanisms Of Dust Generation Cut By Pick Used In Roadheader And Application Of Reducing Dust

Mine dust is one of the most serious hazards in coal mines,which leads to 15000new pneumoconiosis patients each year.Along with the wider use of roadheaders,the mechanization level and efficiency of tunneling are escalated,although the concentration of dust in heading working face is also increased dramatically.To control the high concentration dust at fully mechanized heading face,water spray,foam,de-dusting fan,wind curtain and other technologies have been researched and developed in recent years.However,the effect of dust control is still difficult to achieve the national standards.One of the main reasons is the weakness of basic research on coal dust generation and the lack of understanding of characteristics of dust generation.These cause the lack of pertinence in developing dust removal technologies and of scientific basis in formulating and implementing dust control measures.The inconsistency between dust reduction technologies and intensity of dust generation results in poor effect or redundant usage,and causing irrational technical economy.In order to solve the above problems,this dissertation adopts a comprehensive research method,combining theoretical analysis,laboratory experiments and numerical simulations,to systematically study the characteristics of coal dust generation cut by conical pick as well as application.The main results and conclusions can be obtained and summarized as follows:The discontinuous process of the cutting force of roadheader was analyzed to simplify the trough-peak of cutting force as a cutting unit.A dust generation experimental system,using a conical pick as the cutting tool,was designed,which is composed of pressure loading module,crack observation module and dust collection module.A sealed cover and a dust collection cover are adopted to prevent the outside air from disturbing the dust production process as well as the dust from escaping outward.By associating the process of coal breakage and dust generation with energy conversion,the energy conversion process was illustratiod.Energy is imported through the invasion of conical pick stored energy,stored inside the elastic deformation area,dissipated by the formation of crushing zone pulverized core,and released by the formation of new free surfaces and the stress relieving of the pulverized core.The cavity expansion model was further deduced,and the calculation formula of the radius of plastic zone and cutting force were given.The effects of physico-chemical properties of coal on the characteristics of dust generation were investigated.The experimental results show that the existence of water inside coal can suppress dust generation and reduce the cumulative proportion of respirable dust and PM2.5.Fixed carbon has significant positive correlations with the cumulative proportion and mass ratio of dust?MRD?,and the effects of volatile and ash on dust generation could be ignored.With increasing porosity of coal,the cumulative proportion of fine particle dust decrease,while the mass ratio of total dust generation increases.The fractal dimension,a macroscopic measure of the pore structure,has little effects on the characteristics of dust generation.Brittleness is a comprehensive property reflecting the mechanical properties of the coal body.Coal with higher brittleness would produce larger cumulative proportion of fine particles but lower mass ratio of dust generation.The influence of tip angle of conical pick on characteristics of dust generation was studied.The self-built dust generation experimental system was used to explore the relationship between tip angle and mass ratio of dust generation as well as the cumulative proportion of fine particles.A discrete element software,Particle Flow Code 3D(PFC^3D),was used to establish a conical pick cutting model with the flat joint contact based on the cavity expansion model.The relationships between tip angle and crack development as well as the work of pick were explored by numerical simulation.The process of coal breakage and dust generation was explained from the micro level of crack initiation and expansion.It was found that MRD and the cumulative proportion of respiratory dust and PM2.5 ascend with the increase of the tip angle.The proportion of tensile cracks is more than that of shear cracks.The range of hemispherical cracks?the pulverized core?and the total number of cracks formed at the initial stage of crack development rise with the increasing tip angle.The peak cutting force and the work done by pick also showed positive relation with tip angle,which further confirms the conclusion that a pick with larger tip would aggravate the generation of dust.The relationship between attack angle?the angle between the attack direction of pick and the contact plane of coal body?and the characteristics of dust generation was investigated.The contacting force of one pick during the cutting process were analyzed.The influence of the attack angle on the mass ratio of dust generation and the cumulative proportion of fine dust was studied experimentally.A finite element software,LS-DYNA,was used to simulate the change of cutting force,work done by a pick and energy of coal body.Results have shown that the larger attakc angle can reduce the cumulative proportion of PM2.5 and respirable dust,as well as the mass ratio of dust generation.The peak cutting force,work done by pick and peak intrinsic energy of coal increases with the decreasing attack angle.The energy accumulated before the coal broken is mainly used to form the crushing zone,which confirms that the larger attack angle can lower the degree of dust damage.The effect of cutting speed on the characteristics of dust generation was studied.The experimental results show that the cutting speed can barely change the mass ratio of dust generation as well as the cumulative proportion of fine particles.The process of coal breaking and dust generating is a physical and morphological change of the coal body with fracture and damage.The cutting speed is far less than the speed of stress waves and,thus,cannot affect the propagation of cracks.The quicker rotation of cutting head would increase the probability of secondary crushing and the number of picks in cutting coal,which would cause a increase of dust generation.The targeted dust reduction-control scheme was designed based on the characteristics of dust generation.Dust generation was reduced by using conical picks with small tip angle.The degree of dust generation was estimated based on the properties of the coal body,and various dust control schemes were designed.The application proved that the average concentration of dust were reduced by nearly 20%after using the conical pick with small tip angle in Dawan coal mine.The estimation of the mass ratio of dust generation based on the properties of coal bdy was consistent with the field dust concentration measurement results.Reducing the amount of dust-controling medium can hardly change the effect of dust control at Jiangzhuang coal mine but lower the operating cost and improve the working efficiency.The effect has been enhanced after increasing the amount of dust-controlling medium at Zouzhuang coal mine,which is propitious to reach the standard of dust control.The results of this dissertation provide theoretical support for correctly understanding the characteristics of dust generation cut by roadheader and interpret mechanisms of coal breakage and dust generation during a conical pick cutting.The proposed method of reducting-controlling dust based on the characteristics of dust generation is of great significance for amplifying the ideas of accurate dust control in tunneling working faces.The present dissertation has 99 figures,56 tables and 237 references.