Multi-scale Simulation Of Hydrodynamics And Reactions In Chemical Looping Combustion Process

Nowadays,it is generally accepted that the carbon dioxide（CO₂）produced from fossil fuel combustion in the energy and chemical industrials is the main contributor to global climate change and greenhouse effect.A series of environmental problems caused by the CO₂ emission has triggered a broader concern by the public.Accordingly,China will aim to hit CO₂ emissions peak before 2030 and achieve carbon neutrality by2060.Chemical looping combustion（CLC）is emerged as an attractive combustion technology with an inherent characteristic of CO₂ separation.Due to the complex nonlinear coupling of multi-field（flow field,temperature field,and species field）,multi-scale（particle-scale,cluster-scale,reactor-scale）,and multi-physical processes（gas-solid hydrodynamics,collision,heat and mass transfer,homogeneous and heterogeneous reactions）in the CLC process,deep insights into the CLC system challenge the capacity of measuring techniques by the experimental method.Comparatively,computational fluid dynamics（CFD）has been regarded as a powerful tool for comprehensively and systematically understanding the interplays among the gas-solid hydrodynamics,heat transfer,and chemical reactions in the CLC system.Aiming to construct a multi-scale parallel simulation platform for the CLC process,the sub-models of poly-dispersed drag model,collision model,heat transfer model,pyrolysis model,shrinking core model,homogeneous and heterogeneous reaction models are incorporated into the Eulerian-Lagrangian framework.Specifically,the CFD-DEM,coarse-grained CFD-DEM,and MP-PIC methods are employed to handle the CLC systems of different scales.An augmented coarse-grained CFD-DEM approach which combines the coarse-grained CFD-DEM and the reduced particle stiffness model is originally proposed to reduce the computation load multi-dimensionally.Firstly,the poly-dispersed drag model is validated in a binary particle system.The influences of superficial gas velocity,Sauter mean diameter,and the distribution range of particle size on the performance of mixing index are quantitatively explored,which lay the foundation for the simulation of CLC with poly-dispersed particle system.Subsequently,the multi-scale numerical simulations of CLC process are carried out.Firstly,the effects of coal particle feeding rate and oxygen carrier diameter on the coal-direct CLC system are investigated at particle-scale.The detailed information of coal particle temperature,moisture in coal particle,and particle residence time is obtained.Meanwhile,the effect of coal particle feeding arrangement on the non-uniform distribution of gas product species in the CLC system is analyzed.Based on the bubbling-scale,the influence of the bubbling phenomenon on the chemical reactions between reactive gas and oxygen carrier is investigated.Then,the effect of oxygen carrier polydispersity and fuel reactor internal diameter on the CLC process is deeply revealed at bubbling-scale.Finally,the configuration optimization and scale-up effect of the CLC reactor are proceed based on the chemical mechanism revealed at particle-scale and bubbling-scale.The coarse-grained CFD-DEM is validated via the simulation of a gasifier on a lab-scale chemical looping gasification（CLG）system located in the Institute for Thermal Power Engineering of Zhejiang University.The influence of riser reactor height and diameter on the CLG process is assessed.The optimum height-to-diameter ratio of riser reactor is proposed according to the simulation.Moreover,the reactive MP-PIC method is adopted to reconstruct the CLC process in a 1 MWth pilot-scale unit.The numerical results are compared with the experimental measurements.The characteristics of gas-solid hydrodynamics,heat transfer,and chemical reactions are comprehensively discussed.