Numerical Simulation Of Biomass Fast Pyrolysis In Fluidized Bed Reactor

As a new energy resource with great potential for development,biomass is widely distributed in the world and is abundant and easy to be exploited.It can be processed to produce fuel and chemical raw materials,which can not only reduce human dependence on fossil energy,but also reduce air pollution.Fast pyrolysis of biomass in fluidized bed is a new process for converting biomass into high value-added bio-oil.However,this process involves gas-solid multiphase,such as inert nitrogen,biomass and sand.Gassolid multiphase flow,mixing and heat transfer process will greatly affect the product yield.Therefore,a comprehensive understanding of gas-solid multiphase flow,mixing and reaction is of great significance to the industrial application of biomass fast pyrolysis process.Firstly,based on Eulerian-Eulerian scheme,a gas-solid-solid multiphase flow model was established for gas-biomass-sand in fluidized bed.Validation against experimental data indicates that the model is accurate in describing the gas-solid-solid flow behaviors.The effects of different particle properties on the bubble distribution and on the mixing characteristics of binary particles in the fluidized bed were investigated.Results show that large and small bubbles can coexist in the fluidized bed when the properties of binary particles becomes similar,which is beneficial for the mixing of binary particles in the whole bed.Larger difference in the properties of binary particles can lead to worse mixing of binary particles.The increase of operating gas velocity and the decrease of particle properties are beneficial to the good mixing of binary particles in fluidized bed,and then to the uniform dispersion of biomass particles.Secondly,a comprehensive flow-reaction mathematical model for biomass pyrolysis process in fluidized bed was developed by coupling the multiphase flow model and the appropriate kinetic model of biomass pyrolysis.Compared with the experimental data,the accuracy of the model was verified,and the flow and biomass pyrolysis characteristics in the reactor were analyzed.Results show that,when biomass particles enter the reactor from one side,they mix with sand in the bed and move upward under the combined action of bottom carrier gas and sand in the bed.The biomass particles are heated rapidly and pyrolysis reaction occurs,resulting in tar and non-condensable gas.Secondary conversion of tar would reduce the yield of tar.During the process of biomass moving along the axial direction,its size gradually decreases.Finally,the comprehensive flow-reaction mathematical model was used to optimize the pyrolysis process of biomass in fluidized bed.The distribution of biomass pyrolysis products under various conditions was investigated.By exploring the composition of biomass and the operating conditions,it was found that the yield of tar from cellulose pyrolysis is the highest,and that solid product(charcoal)of lignin is the highest.The optimum temperature for biomass pyrolysis to obtain the maximum tar yield is 773 K.When the temperature decreases,the tar yield decreases,and when the temperature increases,the secondary conversion of tar is obvious,which all will not be conducive to the maximization of tar yield.With the increase of lignin content in biomass,the optimal operating gas velocity needed to maximize tar yield decreases.Increasing the initial accumulation of sand in fluidized bed will increase the residence time of biomass in the bed and increase the gas velocity in the bed,thus increasing the tar yield.