| With the increasingly strict emission standards of national environmental protection,the problem of NO_x emission exceeding the standard appeared in the actual operation of an old circulating hydrogen heating furnace of a hydrocracking unit.By reducing the amount of air into the furnace and reducing the oxygen content of the flue gas,the NO_x control is barely qualified.However,the reduction of oxygen content leads to inadequate combustion,the increase of CO content in the flue gas,black smoke from the chimney,the heating furnace thermal efficiency is always unable to meet the standard,the need to transform the heating furnace.The burner is the core component of the whole set of heating furnace.Because the burner of the old heating furnace is not considered in the design of NO_x emission,the combustion state of diffusion and the high core temperature lead to a large number of NO_x generation,so the design and transformation of the burner is the core link of the whole set of heating furnace transformation project.This topic combined with air/fuel classification technology and gas recirculation technology,with the help of numerical simulation method to optimize the structure of the burner,and using numerical simulation method to study the velocity field,component field,temperature field and NO_x distribution field in the burner,the main research content is as follows:Firstly,this thesis takes the burner of circulating hydrogen heating furnace as the prototype,carries on the three-dimensional modeling by changing the nozzle structure of the burner,carries on the grid division for the burner model,and sets the boundary conditions and the model.On this basis,the mesh independence of the burner is verified,and the different turbulence models in the Fluent model are also verified.Finally,the simulation results are compared with the actual operating parameters to verify the feasibility and accuracy of the simulation model and the calculation grid,and the model is used as the basis for the subsequent burner optimization simulation.Secondly,on the basis of determining the simulation model and calculation grid,this thesis designs two new burner structures according to changing the number and layout of nozzles and using air/fuel classification technology.The combustion reaction is simulated and analyzed respectively.In terms of velocity,composition,temperature and NO_x distribution,it is compared with the simulation results of the prototype burner.Increasing the number of nozzles(increasing fuel classification)can make the flow field more uniform,the lower speed area of the nozzle downstream more regular,smaller size,more uniform fuel layout,and larger diffusion area.At the same time,it is easier to form the "lifting flame",the combustion temperature is lower,the NO_x emission can be reduced by more than 90%,but there is the possibility of increasing the combustion instability.Finally,in order to realize the stable combustion of fuel in the burner,on the basis of the air/fuel classification structure,the high temperature flue gas after combustion is pressurized and re-introduced into the burner through the pipeline.In this thesis,two different flue gas reflux ratios are designed for simulation analysis,and the simulation results of the basic model in the velocity field,component field,temperature field and NO_x distribution field are compared.The research finds that,Flue gas recirculation can pressure the inner flow of the burner,shorten the low speed area,make the fuel distribution in the inner layer more concentrated,fuel concentration is relatively larger,at the same time,increase the oxygen concentration in the combustion zone,reduce the combustion temperature,reduce NO_x emissions.Based on the optimized burner structure,NO_x emission can be reduced by more than 90%,and the highest temperature of the burner outlet can be reduced by more than100 K,which can meet the emission requirements of the current standards,and provides a reference basis for the design and transformation of the burner. |
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