Study On Intensification Mechanism And Density Regulation In Gas-solid Fluidized Bed Beneficiator Using Geldart A~- Fine Composite Dense Medium

As the"ballast"and"stabilizer"of China’s energy security,coal is the most economical,reliable and basic guarantee of China’s energy security and energy strategy.Through efficient coal beneficiation and processing,thereby promoting the classified and qualitative coal utilization,it can be realized the low-carbon transition and utilization from high-carbon coal.The fluidization dry separation technology,which is an important part of coal beneficiation and processing,provides an effective way for the clean and efficient utilization of coal in arid and frozen areas.However,in the industrial application of gas-solid fluidized dry beneficiation,the fluidization process using the traditional Geldart B/D binary dense medium particles has bottleneck problems,such as sharp bed density fluctuations,narrow density regulation range,and difficulty in achieving fine grain coal beneficiation,which restrict the industrialization process and commercialization promotion of the gas-solid fluidization dry coal beneficiation technology.In response to the above problems,this thesis designs and develops a new Geldart A~-fine composite dense medium particle system for gas-solid fluidized bed beneficiator,to improve the fluidization process and to intensify bed density regulation,thereby providing the theoretical foundation for efficient dry coal beneficiation in industry.The modulation process of Geldart A~-fine composite dense medium particles,including the dispersive adhesion of ultrafine coal powder,the adhesion of ultrafine coal agglomerates,and the gap filling of ultrafine coal particles/agglomerates,was systematically studied from the perspective of the force analysis between Geldart A~-fine composite dense medium particles with the aid of the Scanning Electron Microscopy(SEM),thereby revealing in-depth the mechanism of modulation of ultrafine coal particles.The incipient fluidization gas velocities of Geldart A/A~-fine composite dense medium particles with different properties were measured to reveal the variation of incipient fluidization velocities with operating parameters.Results indicated that the incipient fluidization gas velocity could be significantly reduced by using Geldart A~-fine composite dense medium to reduce the energy consumption in the industrial process.Moreover,the theoretical derivation of the incipient fluidization state of Geldart A/A~-fine composite dense medium was carried out,and the theoretical prediction model for the incipient fluidization gas velocity of Geldart A/A~-fine composite dense medium was established by combining a large amount of experimental data and literature data for correction and calibration,the prediction error was controlled below±30%,indicating high prediction accuracy in comparison of traditional correlations.The bed expansion behavior and two-phase distribution characteristics of Geldart A~-fine composite dense medium particles in gas-solid fluidized bed beneficiator were studied by bed expansion test and bed collapse test.The dimensionless subsidence time was introduced to quantitatively evaluate the bed particulate expansion characteristics.The effects of bed particulate expansion characteristics on the improvement of bed fluidization quality and the optimization of the two-phase distribution of dense phase and bubble phase in the bed in gas-solid fluidized bed beneficiator under the modulation of Geldart C ultrafine coal particles were explained in detail.Based on the derivation of the two-phase theory of fluidization,a prediction model for the volume fraction of the bubble phase was developed and modified with the prediction error controlling below±25%.Moreover,based on the particulate fluidization theory,a predictive model for the dense phase voidage of the bed was developed for Geldart A~-fine composite dense medium.The prediction error was controlled within±10%,and the prediction accuracy was significantly improved compared with the traditional prediction model.The evolutionary variations of spatial bed density with the volume fraction of ultrafine coal powder,operating gas velocity,and spatial location of the bed were investigated.The results showed that the adjustment range of bed density can be extended to 1500～2000 kg/m~3 and the fluctuation deviation of spatial bed density distribution can be controlled below 30 kg/m~3 in the Geldart A~-fine composite dense medium gas-solid fluidized bed.Meanwhile,the intensification mechanism of uniform bed density distribution and wide density regulation performance using Geldart A~-fine composite dense medium was revealed by combining with the dense phase expansion characteristics.Moreover,the bed density regulation performance of different dense medium particles was comparatively studied.The results showed that Geldart A~-fine composite dense medium particles can effectively regulate the bed density and flow uniformity in comparison to traditional binary dense media.In the suitable range of Geldart C ultrafine coal particles addition,the bed density can be effectively reduced,and the uniformity of spatial bed density distribution can be significantly improved.In addition,based on the two-phase theory of fluidization,the calculated correlation for accurately predicting the average bed density of Geldart A/A~-fine composite dense medium gas-solid fluidized bed beneficiator was derived by considering the influential effects of particulate expansion characteristics and bubble behavior.Furthermore,based on the feed-forward neural network algorithm,a prediction model for the spatial bed density was established,which can be used to provide a theoretical basis for guiding the industrial design and evaluating the effects of process variables on bed density to determine operating parameters.Based on the time-series distribution characteristics of the instantaneous bed density,the influence of parameters,such as the volume fraction of ultrafine coal,the spatial position of the bed,and the operational gas velocity,on the bed density stability is systematically investigated.The results showed that under different operating conditions,the standard deviation of local bed density fluctuation was less than100 kg/m~3,while it was controlled below 50 kg/m~3 using Geldart A~-fine composite dense medium particles,which revealed the optimization mechanism of the modulation effect of a satisfactory amount of ultrafine coal addition on the bed density stability.Moreover,based on the theoretical analysis of the bed density stability,it was closely related to the bed dense phase expansion,bubble phase expansion and its distribution,and thus the time-series bed density distribution was divided into the dense phase stability zone,the bubble-entry disturbance zone and the bubble-escape disturbance zone,thereby proposing an accurate and reliable non-factorial quantitative evaluation index of the bed density stability.Furthermore,the synergistic mechanism of the operating parameters on the bed density stability was analyzed in detail.This provided the theoretical guidance for the regulation of the bed density stability of Geldart A~-fine composite dense medium gas-solid fluidized bed.The effects of different operating conditions,including volume fraction of ultrafine coal,operational gas velocity,feed size,and separation time on the separation performance of Geldart A~-fine composite dense medium gas-solid fluidized bed was investigated by using tracer feed objects with different sizes and densities.The results showed that in the medium operational gas velocity and separation time,the separation density of the gas-solid fluidized bed beneficiator could be regulated by adjusting the volume fraction of the ultrafine coal particles,to achieve high-precision and high-efficiency separation of the feed objects with wide-size fraction,especially for–6 mm feed objects.The probable errors were less than 0.11 g/cm~3,indicating satisfactory separation accuracy.The difference in separation accuracy between Geldart A/A~-fine composite dense medium fluidized bed beneficiator and Geldart B/D dense medium fluidized bed beneficiator before and after the introduction of different external energy was comparatively analyzed.This clarified the advantages of Geldart A/A~-fine composite dense medium fluidized bed beneficiator to separate feed objects,including wide size fraction,high accuracy and low energy consumption without the introduction of external energy.Moreover,based on the force analysis of the feed object,the theoretical calculation models for the separation density of the gas-solid fluidized bed beneficiator using different dense medium were derived,revealing the intrinsic mechanism of the Geldart A~-fine composite dense medium gas-solid fluidized bed beneficiator that can be used for separating fine-grained objects.The accuracy and reliability of the predictive model for separation density were verified by combining the experimental data and literature data.This thesis provides technical and theoretical support to promote the large-scale and practical industrialization of Geldart A/A~-fine composite dense medium gas-solid fluidized bed beneficiator.The dissertation consists of 88 figures,12 tables,and 251 references.