Creep Characteristics And Discrete Element Analysis Of Biomass Briquetting Process

The biomass briquette made by compressing the mixture of lignite and corn straw is an important way of efficient and clean utilization of coal,which not only reduces the emission of polluting gas during the combustion of lignite,realizes the efficient and clean utilization of lignite,but also promotes the resource utilization of straw.However,at present,the research on the creep characteristics and forming technology of biomass briquettes remains incomplete,with serious relaxation,poor forming quality and high energy consumption.Moreover,limited by the low efficiency of discrete element software simulation and difficult parameter calibration,the study of the creep characteristics of biomass briquettes is currently limited to macroscopic physical testing and finite element simulation.The microscopic mechanism of the creep process of biomass briquette is still unclear.In this thesis,lignite and corn straw particles were used as raw materials,and the effects of feeding amount,pressure,and holding time on the creep characteristics,relaxation ratio,and specific energy consumption of biomass briquette were analyzed by the pressure holding creep process.Under the optimized test conditions,a creep simulation model of biomass briquette was established using discrete element software PFC5.0 to solve the problems of low simulation efficiency and difficult parameter calibration in PFC software.,and then the microscopic mechanism of biomass briquette in the molding process is analyzed.The research methods and conclusions are as follows:(1)Through the single factor creep test,the test data of biomass briquette under different feeding amounts,pressure and pressure holding time are obtained,and the creep characteristic parameters are obtained by establishing the creep model.Combined with the creep characteristic parameters,the significance and influence law of each test factor on the relaxation ratio and specific energy consumption are analyzed,and the factors and levels that have significant influence on the relaxation ratio and specific energy consumption are screened out.(2)Through the response surface optimization test,the regression model between each test factor and index was established,and the significance order of each factor on relaxation ratio was obtained as follows: feeding volume > pressure > holding time;The significance order of the influence of each factor on energy consumption is as follows:pressure > feeding volume > holding time;By analyzing the influence law of the interaction of various factors,The optimal production process of biomass briquetting was determined as follows: feeding amount of 4.2 g,pressure of 15.17 k N,and holding time of15 s.Under this test condition,the relaxation ratio and specific energy consumption of biomass briquetting were the smallest.(3)By analyzing the composition structure and mechanical properties of biomass briquette,the Burgers model built into PFC software is used to describe the interaction between different particles.Aiming at the problem of low computational efficiency of PFC software,on the basis of previous optimization schemes,a model expansion method is proposed from the perspective of mesomechanics.Under the condition that the maximum error is not more than 1%,the computational efficiency of PFC software is improved by4,946 times,and the optimization effect of computational efficiency is very obvious,far exceeding other optimization schemes.(4)In terms of Burgers model parameter calibration,the control variable method is used to clarify the influence law of each meso parameter on the creep characteristic curve,further improve the debugging law of the traditional trial and error method,and speed up the parameter calibration by reducing the blindness of parameter debugging.In view of the disadvantage of trial and error method that depends on the debugging experience of the experimenter,a BP neural network calibration method is proposed.It is proved that the calibration method can calibrate the microscopic parameters without relying on the debugging experience of the experimenter,and the calibration accuracy is higher than 92%.Finally,based on the uniaxial creep test of corn stover,lignite and biomass briquet.Through parameter calibration,the parameters of the meso Burgers model among corn stover particles,between lignite particles,and between corn stover particles and lignite particles are obtained,and the virtual creep model of biomass briquette is established.(5)The creep behavior of biomass briquette is simulated by PFC software,which shows that the creep of biomass briquette is inseparable from the movement,rotation,arrangement and inlay of internal particle units.In the loading stage,the coordination number,force chain and velocity of the model are increasing,while the porosity is decreasing,and all indexes tend to be stable in the pressure retention creep stage.With the loading process,the main contact force of the biomass briquette is borne by the normal contact force rather than the tangential contact force,and the tangential contact force and the normal contact force both show obvious anisotropy characteristics,while the pressure retention creep can reduce the anisotropy characteristics of the contact force,so that the distribution of the contact force is more uniform.Finally,by analyzing the main contact force direction and the internal distribution direction of corn stover particles,it is found that the main reason for the cracking of biomass briquetting is that the distribution of corn stover particles in the main contact force direction is very small.