CFD Simulation Study Of Oxy–fuel O₂/CO₂ Combustion In A Circulating Fluidized Bed

Recently,the environmental issue caused by the CO₂ emission has gained great attention.In China,the coal-fired power plant of the electric industry is the main contributor to the increased CO₂ emission,and it is of great significance to reduce CO₂ emission in the coal-fired power plant.The circulating fluidized bed(CFB)O₂/CO₂ oxy-fuel combustion is considered as one of the most promising technologies for CO₂ capture.However,the CFB combustion is an extremely complicated process coupled with multiphase flow,heat transfer,mass transfer,as well as various homogeneous and heterogeneous chemical reactions,which brings great difficulty for the theoretical analysis and experimental study of the CFB system.With the fast development of the computer science,the numerical simulation based on the computational fluid dynamics(CFD)has become a significant way to study the gas-solid fluidization.With the help of the CFD simulation,the mechanism of the fluidized bed reactor can be further studied,which exhibits great significance for the development of the CFB O₂/CO₂ oxy-fuel combustion technology.(1)Based on the CFD simulation methods,this thesis develops a three-dimensional CFB coal combustion comprehensive model under both the Eulerian-Eulerian framework and the Eulerian-Lagrange framework,and analyzes its flow,heat transfer,combustion and pollutant emission characteristics,which enriches the understanding of the CFB O₂/CO₂ oxy-fuel combustion.The main results obtained are summarized as follows:A three-dimensional CFB coal combustion comprehensive model is developed with the two fluid model(TFM)under the Eulerian-Eulerian framework.The gas-solid flow structure,heat and mass transfer,as well as the various homogeneous and heterogeneous chemical reactions are coupled.The simulated results successfully show the gas-solid flow,temperature distribution,species distribution and combustion characteristics.For the gas-solid flow,by adopting the EMMS drag model,the fluidized bed is separated into the dense zone and the dilute zone with the core-annulus flow structure observed.In terms of the combustion characteristics,the axial bed temperature is well validated by experimental data and the radial bed temperature at different bed heights varies greatly.The gas temperature is close to the solid temperature,indicating an excellent gassolid heat transfer in the CFB.The axial gas distribution(including the gas pollutants)along the bed height and the radial gas distribution at different bed heights are quantitatively illustrated,enriching the research data of the CFB combustion.All the simulated results show a good agreement with the experimental data,proving the feasibility of the established CFD model.Besides,the simulated results with two different gas-solid drag models are compared.Results show that the gas-solid drag model is in close connection with the solid phase distribution,thus exhibits great effects on the combustion fraction in the furnace.The simulated results with two different gas-solid drag models varies greatly in the gas-solid flow,temperature distribution and gas emission,which illustrated that the feasibility of the gas-solid drag model lays a solid foundation for the comprehensive simulation of the CFB combustion process,not only in the gas-solid flow structure.(2)Based on the established two fluid model(TFM),the O₂/CO₂ oxy-fuel combustion with warm flue gas recycle is conducted.The feasibility of the model is verified by comparing the flue gas recirculation ratio,pressure,temperature and pollutant emission with the experimental data.Under the warm flue gas recycle condition,the gas accumulation of H₂O(g),NO and SO₂ is obvious.However,accumulation of CO is not obvious due to large difference between the outlet concentration and the dense concentration.It is also found that the warm flue gas recycle is beneficial for reduction of pollutant emission.By comparing the kinetics and diffusion coefficient in the char combustion model,the char combustion is controlled by both the kinetics resistance and the diffusion resistance,and the kinetics coefficient is more sensitive to temperature than the diffusion coefficient.Besides,the established model predicts the O₂/CO₂ oxy-fuel combustion characteristics under different O₂ concentrations(21% O₂,30% O₂ and 40% O₂),and quantitatively conduct effects of the O₂ concentration on the oxy-fuel CFB combustion characteristics.Results show that,with the increase of the O₂ concentration,the fluidization gas velocity reduces and the dense bed height reduces on the premise that the cross section remains unchanged.Besides,the increased O₂ concentration increases the coal combustion rate and the flue gas temperature.For CO₂ and H₂O(g),the emission concentration remains at a high level.For the pollutant emission,the CO emission reduces whereas the NO and SO₂ emission increases with the increased O₂ concentration.(3)By adopting the two fluid model(TFM)under the Eulerian-Eulerian framework and the dense discrete phase model(DDPM)under the Eulerian-Lagrangian framework,the three-dimensional gas-solid hydrodynamics in the CFB was studied and compared.Besides,sensitivity analysis of some key parameters is conducted.Results show that DDPM can predict the gas-solid hydrodynamics in the fast fluidized bed.There are differences of the simulated results between TFM and DDPM,especially in the radial distribution profiles.In terms of the radial solid velocity,there is a reduction for the solid velocity at the wall,but the solid velocity exhibits the symmetric flow structure with a high value at the center but a low value at the wall for DDPM whereas the symmetry in the flow structure is not achieved for TFM.Besides,the solid velocity magnitude and the solid flux of DDPM are obviously higher than TFM.For the sensitivity analysis of the key parameters,the gas-solid drag model exhibits great effects on the results for both the two models,thus the verification of the proper drag model is of great significance in the simulation.The specularity coefficient and restitution coefficient in TFM,as well as the reflection coefficient and the parcel number in DDPM,exhibit little impact compared to the drag model,and the consideration of these parameters is not as pronounced as the drag model.The simulated results of the pressure drop and solid volume fraction by the two models are close to each other.However,due to the overestimation of the conventional drag models,the simulated results deviate from the experimental data to some extent,and more solid particles are delivered to the upper region of the fluidized bed.(4)Based on the dense discrete phase model(DDPM)under the Eulerian-Lagrangian framework,a CFB comprehensive model is developed to conduct the O₂/CO₂ oxy-fuel combustion process by considering the particle size distribution.The simulated results of the particles(such as the particle position,temperature and composition)with different diameters are tracked in the Lagrangian way.The simulated results agree well with the experimental data.Results show that the typical‘core-annulus'flow structure can be observed in DDPM.The position,temperature and the fixed carbon fraction of the coal particles with different diameters varies greatly.Compared to large coal particles,the small particles exhibit a smaller residence time and combustion time,but a higher heat transfer and combustion rate.Due to differences of flow structure,heat transfer and combustion rate,the char combustion fraction of particles under different bed heights vary with different diameters.The char combustion fraction of small particles is mainly located at the upper region whereas the char combustion fraction of large particles is mainly located at the lower region.Besides,the char combustion fraction of particles shows a different distribution under different O₂ concentrations.With the increase of the O₂ concentration,the combustion fraction of the lower zone increases whereas the combustion fraction of the upper zone decreases,leading to a different O₂ distribution along the bed height.Finally,a comparative study is conducted between the simulation with and without consideration of particle size distribution.The simulated results show that the gas-solid flow,temperature distribution and the gas composition distribution varies greatly between the two conditions,indicating the significance of the particle size distribution on the simulation of the multi-particle system.