Meso-scale Simulation Of Complex Systems Involving Gas-Solid Reactive Flows Using Lattice Gas Cellular Automata

Gas-solid two-phase reactive flows exist extensively in natural phenonema and industrial processes and are prevailing and key problems in the field of process engineering such as metallurgy,chemistry and energy,involving complex processes such as mass transfer,momentum transfer,heat transfer and chemical reaction?'Transport phenomena and chemical reaction' for short?.Therefore,it is of great significance to investigate these physical and chemical phenomena in gas-sold reactive flows at macro-scale and meso-scale for revealing the fundamental rules and coupling mechanism of the transport phenomena and chemical reaction.Fluidized iron oxide reduction,as a complex process with multistep reduction kinetics,is one of the most typical gas-solid reactive flow problems.Thus the development and optimization of gas-solid reactive flow model will benefit from further investigation of the multistep reaction kinetics of fluidized iron oxide reduction,which is also vital to the understanding of the intrinsic properties lying in relevant industrial processes,as well as to the application and further development of engineering technologies.In this thesis,a simulation method using cellular automata model was proposed based on micro-scale?or meso-scale?interaction between gaseous particle and solid particle,spontaneous evolution,and meso-scale regulation of mass transfer,momentum trnasfer,heat transfer and chemical reaction,with respect to the micro-scale?meso-scale?physical rules and macro-scale behavior of physics and chemistry involved in gas-solid reactive flows.The properties of gaseous particle were described by several discrete species and energy states in this method,thus macro-scale physical and chemical parameters were evaluated by statistics of the variations of species,momentum and energy states of gaseous and solid particles.Based on fundamental principles such as mass conservation,momentum conservation,energy conservation and component conservation,a lattice gas cellular automata model featured with single velocity,multiple species,multiple energy states and chemical reaction was developed at meso-scale using the idea of cellular automata and theory of lattice gas automata,thereby reasonable evolutions of transport phenomena and chemical reaction can be obtained without solving macro-scale constitute equations.The proposed lattice gas cellular automata model was validated and verified by its application to processes such as multi-component diffusion,flow and heat transfer over a circular cylinder,classical homogenous reaction kinetics,gas-solid reaction without solid product,gas-solid reaction with solid product and porous solid-gas reaction etc.,with controlling parameters of gas-solid reactive flow introduced with respect to thermodynamics and reaction kinetics.The effects of reaction probability at interfaces,gas diffusion probability in solid product,gaseous concentration and gaseous temperature,etc.,on the evolution characteristics of gas-solid reaction were discussed,and simulation results were also compared with those obtained by well-developed mathematical models.Porous media were constructed by the quartet structure generation set,and the generation and characterization of complex structure at meso-scale were discussed together the influence factors.The flow,heat transfer and chemical reaction of gaseous reactant and product particles in conjunction with solid particle surface and inner structure were investigated to reveal the basic features and coupling mechanism between transport phenomena and chemical reaction.The reductions of iron ore and pure iron oxide were further investigated using a micro fluidized bed connected to a fast mass spectrometer,taking into account the effects of reducing gas concentration,superficial gas velocity and temperature.Firstly,experimental data was analyzed by conventional kinetic analysis methods such as iso-conversional method and model matching method.It was found that the reductions of iron ore and iron oxide could not be described by classical kinetic models in detail and reasonably.Apparent activation energy during reduction was observed to vary with increasing conversion,which indicates the reduction of iron oxide is a multistep reaction process.Therefore,the experimental data of isothermal reduction process of iron oxide was further analyzed by a multistep reaction model on the basis of Johnson-Mehl-Avrami?JMA?equation using SPSS.Meanwhile,SEM/EDS and XRD technologies were used to evaluate the solid products at different reduction stages,which verified the parallel reaction feature of Fe₂O₃?Fe₃O₄,Fe₃O₄?FeO and FeO?Fe during reduction process.The kinetics parameters of individual reactions and their contributions to the overall reduction process were also determined,interpreting the physical chemical phenomena during the multistep reduction of iron oxide more reasonably from a new viewpoint.On top of experimental investigations of iron ore and iron oxide reduction kinetics,kinetic parameters of iron oxide reduction were designed for the lattice gas cellular automata model with respect to gas-solid reactive flows which was further used to investigate the evolution features of concentration of gaseous product around solid particle,morphology of solid particle and conversion and metallization inside solid particle,etc..The effects of reaction parameters such as gaseous component,individual reaction probability,controlling regime on the reduction characteristics were analyzed.The reduction of iron oxide in fixed beds was also investigate taking into account multi-component reducing gas and multi-layer of non-porous and porous iron oxide particles.Results indicate that chemical reactions occur at a sharp interface similar to that described by shrinking unreacted core model when the reaction probabilities of individual reactions equal to each other and internal diffusion becomes the rate-limiting step.On the other hand,chemical reactions take place in an annular transition domain containing Fe₂O₃,Fe₃O₄,FeO and Fe when the reaction probabilities of individual reactions differ from each other and interfacial chemical reaction is the rate controlling regime.These are consistent with the reduction behaviors of iron oxide under certain experimental conditions,thus the formation and evolution of the multistep reduction process of iron oxide can be reasonably described by the lattice gas cellular automata model,which is capable of obtaining both macro-scale features and micro-scale interactions with advantages such as parallel computing and absolute numerical stability,providing a new method and strategy for simulating gas-solid two phase reactive flows.