Research On Heat Transfer Characteristics In Lignite Drying Process By Vibration Fluidized Bed

The reserve of lignite in China accounts for about1/8of the total reserves in theworld. There exists a high moisture content (30%to50%), high ash content (15%to30%), and low heat (12.56~14.65MJ/kg), all of which are the main reason of ahigher reserve but a lower utilization efficiency of Chinese lignite resource. Therefore,how to enhance the development of lignite and achieve its clean and efficient use is atopic worthy of further study. A large number of experimental results show that thevibration can promote heat transfer during the drying process, and based on this,widely used for fluidized bed drying process has been studied. There lies importanttheoretical significance and application value on the heat transfer characteristicsresearch of lignite drying, which has higher moisture content, by use of the externalvibration.We investigated the effects of four factors, particle size, temperature, ash content,and moisture content, respectively, in the process of coal isothermal drying, under thecondition of80℃～160℃.Six kinds of thin layer drying models are selected to fitdrying characteristics, by using the correlation coefficient R2, chi-square. The resulthad showed that the Logarithim model (MR=c+aexp(-kt))had a best fitting effect. Thetest of thermal conductivity and calculation of the effective moisture diffusivity ondifferent ash content of coal sample had been experimented, from which we foundthat the influence of ash content in the drying process was not significant, giving aninstruction for the design of fluidized bed dryer and providing the basis of dryingdynamics analysis. The function of magnetite power in the drying process had beeninvestigated preliminarily, and the result showed that magnetite power is helpful tocarry out the drying process. When the amount of the magnetite power was30%, thedrying rate had the maximum0.70kg·min-1·kg-1, but the drying rate without magnetitepower had a very low heat transfer coefficient0.45kg·min-1·kg-1.To make a study on the contribution of vibration energy during the heat transferprocess, the vibration fluidized bed was used to do the single factor experiments onthe high moisture content samples. The factors to be examined on the heat transfercoefficient contained the moisture content, temperature, superficial air velocity andvibration intensity. The test of four factors and three levels had been put into practicewith the design-expert software for1～3mm fraction following the determination ofthe optimal experimental condition in the single-factor test. Making the multiplelinear regression analysis on the result can help us find the heat transfer correlation about Nusselt number and Reynolds, vibration intensity and temperature, moisturecontentNu Re0.4300.635MR0.201. By comparing the index in the correlation, we couldfind that the vibration intensity had the maximum heat transfer coefficient, in otherword, it had a most significant effect.The role of magnetite power in vibration fluidized bed drying lignite in theprocess of heat transfer characteristics are studied, the results show that the influenceof four factors (the vibration intensity, wind velocity, magnetite powder addingquantity and the moisture content of lignite) on the heat transfer coefficient had asignificance order: vibration intensity＞magnetite powder adding quantity＞windvelocity＞moisture content.