Diffusion and Separation Mechanism of Multi-component Materials in Gas-solid Separation Fluidized Bed

Author:Lv Bo

Supervisor:luo zhen fu


Degree Year:2019





Coal is an important primary energy source and a basic guarantee for national economic construction in China.However,coal has caused a series of environmental problems in the process of development and utilization,which not only hindered the development of the coal industry,but also violated the original intention of China’s national environmental protection policy.Therefore,the research and application of clean coal technology is inevitable in line with China’s national conditions and promoting the sustainable and stable development of coal resources.Combined with the regional differences of coal resources and water resources in China,taking into account the coal industry development strategy of the northwest,a series of dry coal preparation technologies,especially dry fluidized separation technology,are gradually being developed and promoted.At present,some basic theoretical researches on dry fluidized separation technology still have some shortcomings,especially the complex multi-scale multi-component system in gas-solid fluidized bed and the diffusion behavior of various components have not been deepened.These problems are extremely unfavorable for improving the separation precision of fluidized bed and promoting the industrialization of dry fluidized separation technology.In view of the above problems,the basic research on the diffusion behavior of multi-component materials in gas-solid fluidized systems and the synergy between them was conducted in this study.Based on a variety of mixture models,i.e.,the PGM model,the Tanaka formula,and the modes of operation shifts,a binary fluidized bed suitable for dry fluidized separation is constructed by combining experimental research.Moreover,the diffusion law of binary medium particles in the fludized bed is explored: the mixing of fine coal and magnetite is dominated by lateral mixing and supplemented by axial mixing.The rising of bubbles plays an important role in promoting axial mixing.Both the fracturing and scattering of air bubbles and the undulating characteristics of the bed are the key for the mixing process of binary medium particles.Bubbles play a crucial role in the mixing of the components of binary particles.Meanwhile,the lateral diffusion coefficient of fine coal is obtained by Coulomb diffusion equation.The effective lateral diffusion coefficient increases exponentially with the gas velocity,but has little relationship with the height of the bed.The agglomeration mechanism is used to explain the moisture transport process between the heavy medium particles themselves(or the coal and heavy medium particles).Based on the capacitance tomography technique,the transfer law of external moisture in the fluidized bed is analyzed.Moisture is mainly introduced into the bed through the entrainment of the separated coal.It exists mainly in the form of free moisture and external moisture on the coal surface.In conclusion,both free moisture and external moisture enter the fluidized bed in the form of agglomerates in the fluidized bed.The difference between these two cases is that the agglomerates formed by free moisture are mainly based on magnetite,and tend to move to the lower side wall of the bed under the action of their own gravity and bed bubbles,while those formed by external moisture are in the form of magnetite adhering to the coal surface,and due to the difference in density,the agglomerates formed by low-density coal are mainly concentrated in the upper part of the bed,and the agglomerates formed by high-density coal mainly falls to the bottom of the bed.Finally,the moisture is transported in the fluidized bed as the agglomerates move.The force of coal particles in the fluidized bed is analyzed,and the stratification process of coal particles in the fluidized bed is further studied: since the low-density coal reached the final position in a short time after entering the bed.The continuation of the sorting time has less effect on the low-density coal.The sedimentation coefficient of high-density coal can be divided into three categories,i.e.,the rapid reduction zone at low gas velocity(u<10.81cm/s),the dynamic equilibrium zone at middle gas velocity(10.81<u<12.78cm/s),and the fluctuation zone under high-speed conditions(u>12.78cm/s).The formation mechanism of coal mismatch phenomenon in the process of Stratification is expounded,and it is pointed out that the bubble plays a big role in the mismatch behavior of coal.The response mechanism of fluidized bed fluidization characteristics during the process of coal floating and sedimentation is revealed.The fluid dynamics of the flow field near the scraper in the dynamic gas-solid separation fluidized bed is introduced,and the related influencing factors are discussed.Meanwhile,the direction and size of particle collision pressure in different regions under the action of scraper are analyzed,which provides a basis for predicting the trajectory of particles.In addition,the sub-regional sampling method is used to reveal the migration distribution of coal particles in the fluidized bed: the migration distribution of coal particles in the fluidized bed is mainly composed of large circulation and small circulation.The large cycle includes the upper layer of clean coal flow and the lower layer of gangue flow.The main operating power of the upper flow is provided by the overflow of the clean coal end and the medium particles backflow of the gangue end,and the lower flow is caused by the transverse belt action of the scraper.During the formation of large circulation migration of coal particles,due to the existence of vortices caused by the scraper movemen,a vortex of the middle and lower regions,i.e.,a small circulation,is formed in the vicinity of the scraper.The coal particles finally complete the separation process in the entire fluidized bed under the action of large circulation and small circulation.Finally,orthogonal test is used to analyze the separation effect of coal in fluidized bed under multi-factor coordination,which provided a basis for further improving the separation precision of fluidized bed.There are totally 103 figures,21 tables and 209 references in this paper.