Research On Breaking Characteristics Of Hard Coal With High-pressure Water Presplitting And Cutting Picks

China is a country with large coal reserves and production in the world,and the coal industry is an important part of the national economic system.The development level of coal industry will directly affect the development of Chinese economic level.As an important strategic resource,coal plays a key role in basic industries such as heat and electricity.At present,the amount of coal mining in China is decreasing year by year,and is gradually developing towards thin coal seams,hard coal seams and other difficult coal seams.For the hard coal seam mining,the mechanized mining based on drum shearer is favored by coal producers.However,the increase of coal seam hardness will lead to the reduction of cutting efficiency,the increase of energy consumption,body vibration and the wear of drum,spiral blade and pick when the shearer is cutting coal seams.And will also lead to equipment shutdown and maintenance caused by mechanical failure,frequent replacement of pick and drum.At the same time,the increase of coal seam hardness will also lead to the increase of dust concentration under the mine.If the operating parameters of the shearer do not match,it will also reduce the lump coal rate,greatly increase the waste of raw coal,and reduce the profit space of coal production enterprises,which is not conducive to the development of China’s coal industry in the direction of green and efficient.Therefore,in view of the great difficulty in mining underground hard coal seams,from the point of view of changing the internal stress distribution of the coal seam,combining the existing methods,the high-pressure water injection method is selected to presplit and unload the coal seam,and the coal seam is cut by the pick.The process of fracture development and cutting in coal seam is a complex mechanical process.The strain curve and failure criterion of the elastic-plastic brittle model of coal wall are studied.Combined with the theory of energy dissipation,the mechanism of coal wall hydraulic fracturing is analyzed,and the mathematical model of critical hydraulic pressure is given.The crack propagation starts from the wing crack propagation.In the crack development model,the wing crack length development model is established,and the theoretical calculation formula is given.Finally,the mechanical model of pick cutting and the theoretical calculation model of instantaneous cutting resistance of pick are analyzed.By analyzing the structure of the shearer and the water presplitting technology,according to the similarity theory,the similarity ratio of the similarity model of the high-pressure water presplitting auxiliary cutting experimental platform is obtained,and the high-pressure water presplitting auxiliary coal and rock breaking experimental system is developed,including the cutting platform,the water injection presplitting system and the data acquisition system.The data acquisition system includes torque acquisition system and acoustic emission signal acquisition and analysis system.Three kinds of coal wall specimens with different coal cement ratios were poured in the laboratory,the samples were tested and calibrated,and the aggregate ratio scheme of f3-f5 hardness coal rock samples was determined.The relationship between coal and rock deterioration strength and presplitting water pressure and water injection time is established to provide test pieces and performance parameters for subsequent experiments and analysis.In order to study the influence of different water pressure and injection time on the temporal and spatial distribution of fractures and the effect of fracture development,the acoustic emission sensor is used to analyze the positioning error through the TDOA positioning algorithm,and the spatial distribution of fractures under 3MPa-10 MPa injection pressure is obtained.When the water injection pressure is close to and greater than the critical fracturing water pressure,the spatial distribution of cracks in the coal wall is better.With the increase of water injection pressure,the development time of cracks accelerates,and water stains appear on the coal wall surface,but the development range of cracks in the coal wall is small.Through the time-domain analysis of AE locating points of coal wall with 5MPa presplitting water pressure,the statistical values and spatial distribution of AE in different periods within 60 min are obtained.The spatial distribution results with time are basically the same as the statistical results of AE events.In order to study the specific energy consumption and lump coal rate of cutting under different cutting parameters and coal rock parameters,cutting experiments were carried out on presplitted and non presplitted coal walls.The experimental results show that compared with the non presplitted coal wall,the average cutting torque decreased by about 27.36%,the peak cutting torque decreased by about 26.24%,and the torque fluctuation decreased by about 43.75%.Compared with the cutting specific energy consumption and the blocking rate,the cutting specific energy consumption decreased by about 41.20% and the blocking rate increased by about 14.43%.The results verify the effectiveness of the presplitting method.The effects of cutting speed and traction speed on cutting specific energy consumption and lump coal rate are studied.With the increase of cutting speed,the cutting specific energy consumption and lump coal rate increase;With the increase of traction speed,the specific energy consumption of cutting decreases,and the lump coal increases first and then decreases.By comprehensively analyzing the influence of cutting speed and traction speed on them under different hardness coal rocks,it is concluded that the cutting parameters are cutting speed 40r/min and traction speed 3m/min when the cutting specific energy consumption is the lowest under different hardness coal rocks;When the lump coal rate is the highest,the cutting parameters are slightly different.When the hardness is f3 coal wall,the cutting parameters of the largest lump coal rate are 80r/min,2m/min;When the hardness is f4 coal wall,the cutting parameters of the largest coal rate are 60r/min,3m/min;When the hardness is f5 coal wall,the cutting parameters of the largest coal rate are 60r/min and 2m/min.In order to establish the best matching model of presplitting parameters,cutting parameters and coal rock parameters,a multi-objective optimization algorithm based on the combination of deep reinforcement learning and Pareto algorithm is designed.The objective functions of minimum specific energy consumption and maximum lump coal rate are established.Taking coal rock hardness,cutting speed,walking speed,water injection pressure and water injection time as decision variables,economic benefit and cost evaluation indexes are introduced as return values to locally optimize the global update principle of the algorithm.The relationship between the decision parameters in the target expectation is mined,and an algorithm performance evaluation system based on six evaluation indexes such as GD,DM,HV and IGD is established.It is proved that the algorithm proposed in this paper has the advantages of convergence,diversity and universality.The optimal matching relationship between high-pressure water presplitting and pick cutting parameters under any coal hardness is obtained,and a cost-effective optimization scheme is provided for different engineering needs.This paper has 70 figures,32 tables and 174 references.