Numerical Simulations Of Coal Hydrodynamic And Combustion In Furnace Of Pulverized-fluidized Bed Boilers

Pulverized coal(PC)combustion has the advantages of higher combustion efficiency,but more flue air pollutants and higher cost of desulfurization and denitrification.Fluidized bed(FB)combustion has wider applicability of fuels,lower concentration of air pollutants,but lower combustion efficiency.The advantages of PC and FB combustion are combined to give full play to their respective advantages.The bottom of the furnace is dense phase FB combustion and the upper part of the furnace is dilute phase PC combustion,which is known as PC-FB combustor.It anticipates that it has higher combustion efficiency,wider load regulation range,wider coal adaptability and lower air pollutants generated by combustion.However,the coupled flow and combustion between the FB dense phase and the PC dilute phase will directly affect the flow,combustion reaction and heat transfer processes in boiler.Hence,the research on the coupling combustion process of PC-FB combustors using numerical simulation technology will help to deepen the understanding and mastery of the flow,combustion reaction and heat transfer processes of coal particles,and provide theoretical basis for engineering application.In this paper,two kinds of PC-FB boiler structures,single bed and double bed,are proposed combining with the mature PC and FB combustion technologies.The hydrodynamics and combustion characteristics of PC,FB and PC-FB boilers are studied by means of numerical simulations.The influence of the interactions of between the fluidized bed particles and the pulverized coal particles and between the gas phase and the different particles components on hydrodynamics of gas phase and solids phase is predicted by means of kinetic theory of granular mixture flow.The heat transfer between the gas-particles mixture and the water walls is modeled using the convective and radiative heat transfer models.The chemical reaction mechanism of fluidized bed coals and pulverized coal particles includes the process of volatile pyrolysis and char combustion incorporating the homogeneous and heterogeneous reaction models and sorbent models.The distributions of velocity,concentration,and temperature and gas species concentration are predicted using Euler-Euler multi-fluid model in the single bed and double bed PC-FB boilers.The gas-solid flow characteristics of the single bed and double beds PC-FB boilers are simulated at the different fluidizing velocities.The influence of the bottom fluidizing gas velocities on gas-particle jets from pulverized coal burners and the effect of the upper multi-layer transverse air on the fluidization of FB are discussed,respectively,in order to analyze whether the two combustion modes of the FB and the PC can combine organically to promote each other.The results show that when the fluidizing gas velocity is less than 1.5 m/s,the upstream fluidizing air will not affect the formation of tangential circular shape of the primary and secondary air of the PC burner.The larger the fluidizing gas velocity is,the larger the tangential circular size is,and the tangential radius of the secondary air port is smaller than the primary air port.The tangential radius of primary and secondary air of single bed is larger than that of double beds at the same fluidizing gas velocity.Through comparative analysis,it is found that when the fluidizing gas velocity is1.3 m/s,the dilute flow of PC and the dense flow of FB can couple and promote each other most effectively.At this fluidizing gas velocity,the best ratio of two furnace loads is 70:30 according to the analysis of gas phase velocity and particle phase concentration.The combustion characteristics and pollutant generation mechanism are simulated by the kinetic theory of granular flow with the chemical reaction models in two kinds of PC-FB boilers.The results show that the temperature distribution of gas-solid is uniform and high heat transfer performance in the furnace.Simulated results show that the high solids volume fraction is at the dense zone,and decreases along furnace height.The peak values of gas-solid temperatures are located in the pulverized coal combustion region.Two combustions modes are coupled that some small particles fall into the FB under the action of gravity and participate in FB combustion,while some of medium coal particles participate in PC combustion under the support of upward fluidization gas.The gas-particles mixture temperatures are close to constant,and increase toward the pulverized coal combustion region.Through the analysis of the concentration of solid particles and the corresponding reaction rate,the simulated results show that the gas species CO mainly comes from incomplete combustion of carbon,and oxygen concentration has a great influence on the combustion reaction rate of CH₄ gas species and tar.The gas species NO is extremely sensitive to temperature.The gas species NO concentration is the highest in the high temperature region.Limestone in the fluidized bed can play a role in desulfurization.The combustion characteristics of the PC and the FB boilers are studied under different loads by means of performance test and numerical simulation.The performance test shows that the thermal efficiency of PC and FB boilers is the highest under boiler maximum continuous rating(BMCR),and decreases with decreasing load rate.The experimental data and numerical simulation results of PC and FB boilers are compared under full load.The distributions of temperature and gas phase and solid phase in the PC and FB boiler are obtained by means of numerical simulation.The water wall heat flux is reduced in the PC-FB boiler in comparison to PC boiler.On the other hand,the gas and particles temperature is higher in the PC-FB boiler than that in the furnace bottom of the PC boiler.Comparing the special thermal parameters of PC,FB and PC-FB boilers,the design method of PC-FB boilers can greatly increase the range of boiler load with high operation efficiency.