Numerical Simulation Of Fluidization And Combustion Characteristics Of Multicomponent Particles And Oxygen-enriched Combustion In CFB

Circulating Fluidized Bed(CFB)contains a variety of particles of different diameters and densities,leading to the different fluidized behavior and chemical reaction rates.These behavir effects on performene,and optimum of design as well as operating control of CFB combustors.Therefore,the invesigations of the effect of the different diameters and denistyies on flow behavir and chemical rection are urgently nedded by means of numerical simulations in a circulating fluidized bed boiler.In this paper,in order to truly reflect the fluidization characteristics in the circulating fluidized bed,four different particles are set up: bed material particles,coarse coal particles,fine coal particles and recycled particles.The coal particles(course and fine particles)and recycled particles are respectively transported through the feed port and the return port.Using the multi-component particle dynamics theory,the flow characteristics of the circulating fluidized bed with multi-component particles were simulated by simulation software,and the main characteristics of complex gas-solid flow were obtained,for example,the instantaneous contour distribution of the particles of each component,the time-average distribution of the particle concentration and the velocity,the radial distribution of the granular temperature of each component particle,etc.In addition,the effects of specularity coefficient and fluidization velocity on the flow characteristics of each particle were also investigated.In the process of exploring oxy-combustion,the model test of the pilot-scale circulating fluidized bed combustor was first carried out.By comparing the temperature field distribution in the furnace and the volume concentration of the main gas components at the exit of the furnace,it is found that the numerical simulation results are in good agreement with the experimental results.The feasibility and correctness of the model were verified.Then,based on the verified chemical reaction theory,a mathematical model of coal combustion in a two-dimensional two-component particle circulating fluidized bed boiler was established,and the combustion characteristics of coal in air combustion mode and oxy-fuel combustion mode were explored.In this paper,the chemical reactions mainly consider moisture evaporation,volatile pyrolysis,char gasification(CO2,H2O)and oxidation,pyrolysis gaseous product oxidation,and the formation of pollutants(NOX,SO2).The model was used to analyze the fluid dynamics,temperature field distribution,gas composition distribution characteristics,combustion chemical reaction rate distribution,and volume concentration of main gas components at the exit of the furnace and pollutant emissions.In particular,the comparison of coal combustion characteristics in air combustion mode and oxy-fuel combustion mode illustrates the unique advantages of oxy-combustion in CO2 capture and pollutant emissions.In addition,the effects of different O2 concentrations on the overall performance of the circulating fluidized bed under oxy-combustion mode were also explored,and the combustion process of coal at high oxygen concentration was successfully predicted.