| The greenhouse effect and environmental pollution caused by sustainable consumption and conventional combustion of fossil fuels limit its sustainable utilization.As an environmentally friendly renewable energy source,biomass can well make up for the shortage of fossil fuels.Conventional fluidized bed gasification technology can achieve high-efficiency thermochemical utilization of biomass energy.However,the syngas produced by the conventional biomass gasification process usually contains a high concentration of CO2,which will dilute the concentration of combustible gas,and reduce the system economy and the usable value of syngas.In contrast,biomass gasification combined with the CO2absorption enhanced reforming（abbreviated as AER gasification）technology can produce hydrogen-rich syngas by absorbing the CO2generated in the biomass gasification process,which has a very broad application prospect.However,the complex micro-scale gas-solid interaction,meso-scale bubble evolution,multi-scale characteristics of macroscopic reactor performance,and multi-physical/chemical coupling characteristics of flow-transfer-reaction make experimental measurements can only obtain limited data under typical working conditions,which limits people’s understanding of the complex coupling mechanism in the biomass gasification process and poses challenges for reactor design and process optimization.in the biomass gasification process and poses challenges for reactor design and process optimization.As an important supplementary means of experimental measurement,computational fluid dynamics（CFD）can efficiently and intuitively obtain the hydrodynamics and heat and mass transfer characteristics of the biomass gasification process,understand the gas-solid interaction mechanism and assist in the optimal design of the reactor,which is beneficial to engineering practice.Based on the Eulerian-Lagrangian framework,this thesis develops a massively parallel computing platform for biomass gasification systems with the sub-models（including the drag model,collision model,heat transfer model,pyrolysis model,shrinkage model,char gasification/combustion reaction model,and carbonation/calcination reaction model）incorporated.Numerical simulation studies are carried out on the conventional biomass gasification process and the AER gasification process in the bubbling fluidized bed（BFB）and dual fluidized bed（DFB）.Based on the 1MW DFB thermal test platform developed by the Institute of Thermal Engineering,Zhejiang University,the optimized design scheme is proposed to significantly improve H2production and reduce CO2emission in produced syngas,which has good social and economic benefits,so as to serve the goal of carbon neutrality in China.The contents of this thesis mainly include the following four parts.In the first part,the biomass gasification process in a lab-scale quasi-3D BFB reactor is numerically studied based on a computational fluid dynamics-discrete elementmethod（CFD-DEM）featuringpolydispersityeffects and coupling the detailed reaction mechanism,focusing on the particle behaviors in the gasification process.First,the reaction model is validated,demonstrating the rationality of the model.Subsequently,the particle behaviors（particle motions,mixing,and heat transfer）during the gasification process are explored.The effects of several important operating parameters on the particle behaviors during the gasification process are investigated in depth.In the second part,the AER gasification process in a lab-scale 3D bubbling fluidized bed（BFB）reactor is numerically studied based on the MP-PIC method.First,the AER gasification model is established,and the rationality of the AER model is proved through model validation.Subsequently,the gasification performance of the AER gasification process and the conventional gasification process is evaluated by introducing gasification performance evaluation parameters,and the intensification effect of CO2absorption on the biomass gasification process is proved.An innovative meso-scale bubble detection algorithm is proposed to quantitatively characterize the behavior and characteristics of bubbles in the BFB,and the relationships between microscale particle behaviors,mesoscale bubble dynamics,and macroscale reactor performance are revealed.Moreover,the effects of operating temperature,operating pressure,steam-to-biomass ratio,and bed material composition on microscale particle behaviors,mesoscale bubble dynamics,and macroscale reactor performance of the AER gasification process are thoroughly explored.In the third part,the biomass gasification process in a pilot-scale dual fluidized bed（DFB）reactor with a complex geometric structure is numerically studied based on the MP-PIC method with thermochemical and polydispersity effects considered.After model validation,the optimal grid parameters for the simulation are determined by grid independence tests.The characteristics of the gas-solid flow pattern,the temperature distribution of gas and solid phases,and pressure and gas components distribution during the gasification process in the DFB are deeply explored,and the effects of operating temperature and steam-to-biomass ratio on gasification performance are discussed.In the last part,a numerical simulation study is carried out on the conventional biomass gasification process and the AER gasification process in an industrial-scale DFB reactor.The characteristics of the gas-solid flow pattern,the temperature distribution of the gas phase and solid phase,particle transport between reactors,and gasification performance under the two gasification conditions are explored.The effects of different operating parameters（gasification temperature,steam/biomass ratio,particle size distribution,etc.）on meso-scale bubble characteristics,particle transport,and macro-scale reactor performance in the AER gasification process are quantitatively analyzed.In addition,based on the 1MW DFB thermal test platform developed by the Institute of Thermal Engineering,Zhejiang University,the optimal design of the DFB reactor based on the AER gasification process is carried out,the particle mixing efficiency and gasification performance of the original 1MW DFB reactor and the design-optimized 1MW DFB reactor are compared and evaluated. |
