| With the implementation of West Development Strategy,coal industry of China was promoted to the west and north area.It is imperative to develop new clean coal technology aiming at the west and north area.At present,the west and north area of China has abundant coal resources and most of them are lignite and sub-bituminous coal and other low rank coals.However,it is difficult to separate the low rank coals by the current wet methods,such as dense media separation and flotation.Especially,many technology problems will arise if using the wet methods,such as the blocking of slime-water treatment system and high moisture of separating production.Above the current conditions,this paper researches the dry air separation theory and process technology using the self-developed variable-diameter pulsed airflow separator,by means of <6mm steam fine coals from Sinkiang and Inner Mongolia as the research objects.The methods of mineral processing theory,fluid mechanics,numerical analysis and numerical simulation were used to research the separation process of <6mm fine coals.Such research contents were accomplished,including dynamics mechanism analyzing of variable-diameter pulsed airflow separating process of fine coal,parameters optimization of structure and operation of the separator,flow field distribution features of pulsing airflow and numerical simulation of separating process.The main research content of this paper includes six parts:(a)separation mechanism of the separator using variable-diameter tube and pulsed airflow;(b)collaboration and matching mechanism of structure and operation parameters in the separating process of fine coal;(c)influence of material property difference on separation and classification process;(d)numerical simulation of separation process based on gas-solid two phase coupling;(e)continuous separation test of fine coal in variable-diameter pulsed airflow separator.The main research contents and important conclusions are listed as follows:(1)The transmission mechanism of pulsating air flow waveform in variable-diameter pulsed airflow separator was analyzed based on the wave theory.Wave equation of pulsating air flow was established in the conditions of linear damping.And the wave equation was solved by analytic expressions.As a result,fluctuating velocity in the separator conditions was compounded by travelling wave along the flow direction and travelling wave reverse flow direction.And the amplitude fluctuating velocity in linear damping conditions was mainly decided by damping coefficient and scale factor in the variable-diameter reducer structure.(2)Based on the computational fluid dynamics(CFD)method,the influence of pulsation rate,fluctuating velocity amplitude and solid volume concentration on drag coefficient in pulsing airflow field was discussed using the theory of cell model.The results show that,in the pulsating flow field,the drag coefficients is bigger than in the field of static and steady state.And,as the fluctuating velocity amplitude increased,the drag coefficients increased linearly,and the pulsation rate had little effect on drag coefficients.On this basis,the modified formulas in pulsing airflow field was presented to improve the standard drag coefficient curve by adding the terms standing for the volumetric concentration and fluctuating velocity amplitude.(3)By the dynamic analysis of particle motion in pulsating airflow field,the dynamic differential equation of particle in the pulsating flow was set up.Combined with the modified formulas of drag coefficient in pulsing airflow field,and using numerical analysis method,the force situation of fine coal particle in the variable-diameter pulsed airflow separator was analyzed.The results show that,the effects of cumulative and periodic oscillation has certain positive role to strengthen particles by density separation trend.Further more,the variable-diameter tube can strengthen separation of particles by density.(4)The structure of separator was optimized by study the influence of gas distributor structure,feeding position,straight tube height and the variable-diameter reducer structure,on the separation performance of fine coal.The results show that,the bigger the discharge gate of the gas distributor,the lower the separation density,and meanwhile,ecart probable moyen reaches minimum value when the discharge gate is 40 mm occupying 16% of the bottom surface,under the certain condition of operating parameters.The closer the distance between feeding position and the discharge gate on the bottom,the higher the separation density,and the lower the ecart probable moyen.When the distance between feeding position and the discharge gate on the bottom exceeds a certain value,separation density gets to change slowly.And more,it is better to feed on the position of the variable-diameter reducer than of the straight tube.The higher the straight tube,the higher the separation density,and the lower the ecart probable moyen.When the height exceeds 0.75 m,separation density and ecart probable moyen get to change slowly.The bigger the angle of variable-diameter reducer,the lower the separation density,and ecart probable moyen get to change slowly.(5)The operation parameters of the variable-diameter pulsed airflow separator was optimized by study the influence of primary air flow,pulsating air flow and pulse period,on the separation performance of fine coal.The results show that,the bigger the primary air flow,the higher the separation density,and the higher the ecart probable moyen.The bigger the pulsating air flow,the higher the separation density.When the volume of the primary air flow is <250m3/h,the pulsating air flow has less influence of ecart probable moyen,and when the volume of the primary air flow is >250m3/h,the bigger the pulsating air flow,the higher the ecart probable moyen.The effect of pulsating air flow on separation performance was complex,when the volume of the primary air flow is low,the pulse period has less influence of separation density,and when the volume of the primary air flow is high,the longer the pulse period,the higher the separation density.Meanwhile,when the pulse period gets long,ecart probable moyen becomes high.(6)Based on the response surface method(RSM),the influences of primary air flow,pulse period,pulsating air flow and angle of reducer on the separation performance of fine coal were analyzed by analysis of variance.The results show that,the effect of primary air flow on the separation density was extremely significant,pulse period and the angle of variable-diameter reducer was significant,and pulsating air flow was not significant.Meanwhile,ecart probable moyen has been greatly affected by the interaction among many factors.The effects of primary air flow,pulse period,interaction between primary air flow and the angle of reducer,interaction between primary air flow and primary air flow on ecart probable moyen were extremely significant,pulsating air flow was significant,and the angle of reducer was not significant.On this basis,mathematical correlations of separation density and ecart probable moyen and various factors was established.(7)The best matching combination of structure and operation parameters was provided using parameter optimization methods on the condition when the separation density was fixed.As a result,when the given separation density was low,it is much more suitable for the separation process using a little angle reducer,and when the given separation density was high,it is much more suitable for the separation process using a big angle reducer.(8)The influence of material property difference on separation and classification process was studied,through separation and classification experiments using 6~3mm fine coals of different size consists and density consists.The results show that,washability of coal samples has little effect on separation results of fine coal,and has great effect on classification results.While,size consist has great effect on separation results of fine coal,and has little effect on classification results.In addition,the bigger proportion of coarse fraction,the lower separation density,and the bigger proportion of high density fraction,the lower partition size.And,partition coefficients of one certain density fraction should be equal to the weighted sum of partition coefficients of every size fraction in this density fraction,partition coefficients of one certain size fraction should be equal to the weighted sum of partition coefficients of every density fraction in this size fraction.(9)The separation and classification process of fine coal using variable-diameter pulsed airflow separator was comprehensive assessment.The results show that,along with the change of primary air flow,separation density and partition size have positive correlation with primary air flow.And density ecart probable moyen has positive correlation with primary air flow,and size ecart probable moyen has negative correlation with primary air flow.(10)The distribution of the flow field in the variable-diameter pulsed airflow separator was simulated by CFD method by accounting for the influence from the volume of particles and the forces that result from the collision of particles.The results show that,before the particles were feeding,pressure drop in the separator shows a character of periodic variation with the change of pulsating air flow,and the change was caused majorly by the pressure loss along the separator tube.After the particles were feeding,the flow field in the separator becomes asymmetrical,the value of pressure drop increase rapidly.Moreover,the closer to the feeding position,the lower the air flow speed is.Moreover,the greater handling capacity of the pulsing air flow separator,the more significantly change of pressure drop,and this change was caused mainly by the pressure loss on account of the inputting dispersed phase.(11)The pulsing airflow separation process of fine coal was simulated by comparing DDPM+DEM and DPM.The results show that,DDPM+DEM model can simulate the separation results by the low and high handling capacity situation simultaneously and accurately.While DPM was just suitable for the low handling capacity situation.(12)Combining with the results of numerical simulation in variable-diameter pulsed airflow separation of fine coal,the approximate mathematical model of separation density and air volume and handling capacity was established.(13)Separation efficiency of 6~3mm,3~1mm and 1~0mm fine coal was evaluated using the laboratory continuous separation system.The results show that,at the same air flow condition,separation density and ecart probable moyen increased with the handling capacity increasing.When the primary airflow was 270 m3/h and the handling capacity was 200kg/h,organic efficiency of air flow separation of 6~1mm fine coal was 91.02%. |
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