Numerical Simulation Of The Flow And Combustion Characteristics Of The 660MW Circulating Fluidized Bed

The flow state in the circulating fluidized bed lies between concurrent combustion and suspension combustion.It has advantages of high efficiency of combustion,high intensity,high adaptability,low Emission of SO₂ and NO_x and easier load control in combust inferior coal such as washed-coal,coal and mixture of them,gradually becoming an efficient and clean power generation technology.With the increase of installed capacity and the power generation of circulating fluidized bed,the simulation research of circulating fluidized bed is gained more and more attention.At present,there are two dominating models for the simulation of the flow state of coal particles in a circulating fluidized bed the Lagrange stochastic trajectory model and EulerianEulerian gas-solid multiphase flow.The simulation for the combustion of circulating fluidized bed is infrequent and generally non-premixed model based on infinite fast chemical reaction or finite rate-eddy dissipation model is adopted,neglecting the influence of turbulence in the CFB upon the reaction rates.The Eulerian-Eulerian gassolid-solid multiphase model is adopted,treating air as gas phase,coal particles and ash-coal particles as solid phase,limestone,claimed lime and calcium sulfate as the solid phase.All three phases are treated as continuous.The model of coal combustion is divided into moisture volatilization,raw coal devolatilization,volatile gases oxidation,char combustion,limestone decomposition,desulfurization reaction,formation and deformation of NO_x.the user defined functions code is compiled to realize the circulated combustion of coal particles by returning the bed materials out from the outlet to the coal feed inlet.For the homogeneous chemical reactions in the furnace,this paper adopts the eddy dissipation concept based on the global reactions,incorporating the turbulent flow mechanisms the detailed reaction mechanisms.Simulated results of 3MW circulating fluidized bed verifies the present EulerianEulerian gas-solid multiphase model where homogeneous combustion reaction rates are calculated by eddy dissipation concept based on global reaction mechanisms.the simulated gas components are in agreement with the experimental results,better than the simulated ones adopting finite rate-eddy dissipation.besides,the axial velocity and concentration of solid particles are analyzed and distribution of gases is obtained.The curve of reaction rates along furnace height shows that in the dense zone CH4 reacting with O₂ is faster than CO reacting with O₂ and C reacting with O₂ is faster than C reacting with CO.The mass source of gases along furnace is discussed and the relationship between reaction rate and solid phase volume fraction is obtained.the principal ways of formation and deformation of NO_x is studied.After being verified by the experimental data of 3MW circulating fluidized bed,the model is used to simulate the flow state and combustion of 660 MW circulating fluidized bed.The temperature of furnace,solid particles volume fraction,axial velocity of solid particle is obtained.The solid particles is dense in the bottom zone and dilute in the upper zone.The axial velocity of solid particles in the middle zone is higher than that near the wall.the axial velocity near the wall in the upper zone is negative and the flow structure appears core-annulus.The variation of volume fraction of solid particle under various primary air velocities is simulated.The influence of excess air ratio,primary air ratio and ration between calcium and sulfur upon the emission concentration of SO₂ NO and N2 O is analyzed.The increase of primary air ratio and ration between calcium and sulfur and decrease of excess air ratio will increase the SO₂ concentration.The increase of excess air ratio and primary air ratio will increase the NO concentration.The decrease of primary air ratio will increase the N2 O concentration,the excess air ratio affects N2 O concentration slightly.The Eulerian-Eulerian gas-solid-solid multiphase model incorporating eddy dissipation concept based on global reaction mechanisms in this passage characterizes the flow state and combustion well,providing an effective method to simulate the circulating fluidized bed.