Optimization Design Of Air Distribution System In Fluidized Bed Pyrolysis Reactor

Biomass has a very broad application prospect,with some advantages of being widely and considerably obtained,renewable and environmentally friendly.Biomass can be used in a variety of ways,of which the most concerning technologies is pyrolysis liquefaction.The fluidized bed,with simple structure,reliable operation,convenient regulation and high heat and mass transfer efficiency,is often applied as biomass pyrolysis reactor to product bio-oil.Air distribution effect of the air distribution system in the fluidized bed is one of the main factors influencing biomass pyrolysis.The improvement of air distribution effect makes for strengthening the fluidization of biomass particles,promoting the efficiency of pyrolysis reaction and enhancing the quality of bio-oil.The uniformity of air distribution of the air distribution system in the fluidized bed is not only related to the structure of the air distribution plate,but also affected by the state of the flow field in front of the air distribution plate.In order to prevent the bed material from falling directly into the fan,lateral air intake is usually adopted.However it will cause the airflow to deflect.In this paper,aiming at the problem of airflow deflection caused by lateral air intake,the research on air distribution system in the biomass pyrolysis fluidized bed was carried out.The main factors affecting the airflow deflection and wall attachment were explored by numerical simulation,and then corresponding measures improving air distribution effect of the air distribution system of the biomass pyrolysis fluidized bed were taken.1.Analysis of the factors affecting airflow deflectionLateral air intake is the direct reason for airflow deflection.Compared with the initial structural model,when maintaining the original diffusion pipe in the vertical direction,by appropriately increasing the inner diameter of the air delivery pipe and correspondingly adding a diffusion pipe in the horizontal direction,the high-velocity airflow introduced from the fan could be fully pre-diffused,and thereby reducing the inhomogeneity of velocity distribution of the airflow inside the wind chamber.Merely increasing the inner diameter of the air delivery pipe had little effect on reducing the airflow deflection,but reserved a certain space for further optimization of the structural design of the air delivery pipe.When from the horizontal into the vertical,the airflow produced by the fan in the right side was easy to form a jet deflecting to the left side.When entering the vertical diffusion pipe,the jet would deflect heavily to the left side of the pipe wall.A large amount of air was drawn from the right side to form a larger backflow while lacking air from the left side,causing the jet to shift to the left side,which is called Coanda effect(also named by wall attachment effect).With it rising in the wind chamber,the airflow gradually shifted to the left side and even attached to the wall,which was not conducive to air distribution of the air distribution plate,seriously decreased the uniform distribution of the flow field,and thus affected the flow field characteristics and effect of heat and mass transferring inside the bed.2.Study on improvement of flow field characteristics by rectifying pipe structureWhen the airflow introduced from the right side was diverted into the vertical air delivery pipe,there was a left component velocity.A small-diameter pipe was arranged in the vertical air delivery pipe to force the deflected airflow to turn right,which means reducing the leftward velocity and increasing the rightward velocity,and thus the airflow deflection was reduced.To decrease and even avoid the Coanda effect,the diffusion pipe was not connected directly with the outlet of the rectifying pipe.Compared with other improved models,there was the most obvious airflow deflection in the Ml model,but the airflow was no longer adherent to the left wall compared to the original model.The two-stage rectifying pipe structure worked better in improving the airflow characteristics,with lower peak gas velocity,less deflection,faster diffusion and lower inhomogeneity.Increasing the length hi,the deflection of the airflow was heavier,bad for the flow field characteristics inside the wind chamber.3.Study on improvement of flow field by the inner diameters of air delivery pipe and rectifying pipeThe inner diameters of air delivery pipe and the rectifying pipe will influence the rectification effect.The larger the inner diameter of the air delivery pipe is,the more the airflow diffuses before entering the air chamber,and the smaller the average velocity is.To make the intake air fully diffuse at the moment,the diffusion pipe in the horizontal direction should be appropriately lengthened.Thus,the air delivery pipe becomes bigger and bigger.The smaller the inner diameter of air delivery pipe is,the less the airflow diffuses before entering the wind chamber,and the higher the average velocity is.If the inner diameter of the rectifying pipe is too large,the airflow deflection will not be significantly reduced.The smaller the size of the rectifying pipe is,the larger the outlet jet velocity is,which is not conducive to airflow diffusion.When the inner diameter of the rectifying pipe was half the one of the air delivery pipe,the rectification effect and the flow field characteristics were generally best.The M80-40 model had the best general effect on improving the flow field characteristics.According to the numerical simulation results,adopting the M80-40 model to optimize the original device with the air distribution plate,could significantly improve the flow field characteristics inside the bed.