CFD-DEM Study Of A Down-Tube Pyrolysis Reactor:Flow And Heat Transfer Characteristics Between Heat Carrier And Biomass

As a green and renewable energy source,biomass energy is of great significance to reduce environmental pollution.Among them,rapid thermal cracking of biomass is a thermochemical conversion technology to achieve higher energy density of biomass and higher value of products,which has the characteristics of fast and efficient,large processing capacity and diversified products.Shandong University of Science and Technology has proposed a technical solution for direct catalytic biomass thermal cracking with circulating heat carriers,i.e.,using ceramic ball heat carriers at high temperatures to achieve rapid biomass thermal cracking by rapid mixing with cold or preheated biomass feedstock,with intense mixing and contact between heat carriers and biomass particles,and enhanced heat and mass transfer.In the pyrolysis process,the product yield and distribution are greatly influenced by the pyrolysis conditions(especially temperature,heating rate,etc.),where the ceramic ball particle size and the mass ratio of ceramic ball and biomass greatly affect the flow and heat transfer of binary particles in the drop tube.In addition,it is difficult to experimentally study the temperature distribution inside the V-shaped down tube reactor,the particle heating process and the particle heat transfer under high temperature environment due to the limitation of experimental measurement conditions and errors.These problems can be effectively avoided by using numerical simulation.However,the traditional two-fluid model cannot obtain the flow and heat transfer information of individual particles,so this paper adopts a coupled numerical calculation method of computational fluid dynamics and discrete element method(CFD-DEM)to investigate the flow and heat transfer characteristics of the heat carrier and biomass in the down tube reactor,and the coupled CFD-DEM model can observe the flow and heat transfer processes of particles and fluids in real time.The theoretical model was created to reveal the influence law of different ceramic ball particle sizes and different mass ratios on the flow and heat transfer process of biomass particles and ceramic balls.Firstly,a coupled CFD-DEM computational framework is established,and the fluid part is computed in Fluent using CPU multi-core parallel computation,while the solid phase particles are computed using GPU CUDA solver,and GPU accelerated computing technology has been proven and applied.The Ranz-Marshall model is used for the heat transfer between the particles and the fluid,and the heat transfer between the particles is considered.The Gidaspow model is used for the fluid-solid traction model,and the HertzMindlin slip-free contact model is used for the particle contact model.The flow and heat transfer of the particles were experimentally verified and the errors were within acceptable limits.Then,the flow and mixing patterns of biomass and ceramic sphere particles within the V-shaped drop tube pyrolysis reactor were investigated using CFD-DEM method.The motion characteristics and residence time of biomass particles and ceramic spheres were simulated for different ceramic sphere particle sizes and different mass ratios.Different ceramic ball particle sizes and different mass ratios have important effects on the flow behavior of binary particles.At the same mass ratio,the smaller the ceramic ball particle size,the greater the promotion of biomass particle motion and the shorter the average residence time of biomass particles.The biomass residence time increases with the increase of mass ratio when the ceramic ball particle size is constant.The larger the mass ratio,the greater the number of biomass particles,and the more obvious its fluid-like motion effect.The fluidlike motion promoted the motion of biomass particles in the vertical section,but in the inclined section it was mainly influenced by wall friction and ceramic ball collision.Finally,the heat transfer characteristics of the heat carrier and biomass particles inside the falling tube were investigated.The mass ratio has a significant effect on the temperature distribution inside the falling tube reactor;the larger the mass ratio,the higher the temperature inside the tube.The heating rate and final temperature of biomass pellets increased with the increase of mass ratio.The cooling rate and final temperature of ceramic balls decreased with the increase of mass ratio.The simulation results showed that the heating rate and final temperature of biomass pellets were better than other mass ratios(10:1,15:1,30:1 and 40:1)for a mass ratio of 20:1.When the mass flow rate of binary particles and the initial temperature are the same,the smaller the particle size of ceramic balls,the faster the heating up of biomass particles and the faster the cooling down of ceramic balls.Based on the results of this paper to guide the process production,it can be determined that the trial run of biomass pyrolysis process with 2 mm ceramic ball particle size and 20:1 mass ratio can be carried out to obtain the best heat and mass transfer effect,and can guide the subsequent parameter optimization,which provides valuable reference value for further design and optimization of the reactor.