Gas-solid Hydrodynamics And Design On Juxtaposed Dual-CFB Of Chemical Looping

Chemical looping technology is a new carbon capture technology.As it continuously develops,the design and scale-up of chemical looping systems have become increasingly important.This paper mainly investigates the design and the industrial application of the large-scale chemical looing dual fluidized bed systems.Three aspects are included:design methodology of the chemical looing dual fluidized beds,experiments on gas-solids hydrodynamics in dual fluidized bed test unit and numerical simulation.The main works are:1)Research on the design methodology of the chemical looing dual fluidized bed systems;2)Experimental and modelling research on the hydrodynamic characteristics and bed-inventory balance as well as stability in a dual circulating fluidized bed system;3)Design and numerical prediction of a large-scale chemical looping gasification dual circulating fluidized bed system.The present investigation provides reference for the design as well as safe,stable and efficient operation of a large-scale chemical looping dual fluidized bed system.In terms of the design methodology for the chemical looping dual fluidized bed systems,more than 200 publications on the design and operation of the chemical looping dual fluidized bed systems were reviewed and summarized,and the proposed design methodology includes:determination of the key issues（the combination of the two reactors,fuels,oxygen carriers,oxygen-to-fuel ratio,steam-to-fuel ratio,operating temperature and velocity）,mass and energy balance analysis（calculation of flue gas composition,oxygen transport,energy balance）,the design of the reactors and the main components（dual reactors,gas-solids separation devices,solid material return devices,air distributors and heat exchange surfaces）,and the pollutant control methods,etc.In terms of the experimental research on the gas-solids hydrodynamics of the bed-inventory balanced and stability of the dual fluidized bed,a 15.5-meter juxtaposed dual circulating fluidized bed cold test unit was constructed to investigate the effects of operating parameters on the gas-solids hydrodynamics.The results show that the operating velocity,the height of the static bed material and the type of solids returning mode are important parameters which affect the bed-inventory balanced and stability of the juxtaposed dual circulating fluidized bed.A higher static bed height and the use of cross-returning coupled with self-returning mode are beneficial to improve the balance and stability of the system.Results from a fluid-dynamic model for solids circulation and residence time distribution of the particles in the dual fluidized bed system show that:as the size of the reactors scales up,the gas-solids concentration in the dense region and the dilute region of the fluidized bed decreases,the solids circulation between the dual fluidized beds decreases,and the mixing uniformity of the particles is reduced,but the residence time of the particles is prolonged.Combined with the results of theoretical,experimental and numerical conclusions,the paper investigates the design of the dual fluidized bed systems for chemical looping gasification.It is concluded that the operating parameters may affect the auto-thermal and stable operation as well as the mass and energy balance of the dual fluidized beds.Considering the gas-solids hydrodynamics,residence time of the particles,oxygen-carrying and heat-carrying balance in the dual fluidized bed comprehensively,the paper presents a large-scale design scheme for a 200 MW dual circulating fluidized bed system of chemical looping gasification.The solid fuel and oxygen carrier are biomass and natural iron ore,respectively;the sizes of the two circulating fluidized bed are identical,with a depth of 10 meters,a width of 4 meters,and a height of 38 meters.The solids flux across the circulating fluidized bed is 4.2～6.3 kg/m²?s,the conversion rate of solid fuel η_C=100%,and the cold gas efficiency η_gas=94～99%.In the end,the paper develops a three-dimensional numerical simulation model for the large-scale chemical looping dual fluidized bed systems,and selects a 1 MW dual fluidized bed"combustion-gasification"test unit as the validation object.The results of the gas-solids flow,temperatures and the gaseous compositions are in good agreement with the experimental results.Mass-energy transfer and balance in the dual-fluidized bed system were analyzed,and the auto-thermal and stable state were obtained,which verified the feasibility and accuracy of this model.