| With the implementation of "environmental protection law","air pollution prevention law" and other regulations,the emission standard for air pollutants fro m thermal power plants has reached to a newly high level.The research object is a 600 MW supercritical boiler in a power plant,which is designed and manufactured by Babcock & Wilcox Beijing company ltd with the type of B&WB-2090/25.4-M.Because the NO_x emission concentration of the boiler is difficult to meet the requirements of the national standard,the demand for low nitrogen retrofit for the boiler is put forward.According to the original design features and the actual operating conditions of the boiler,the retrofitting scheme is to replace the burners and adjust the elevation of the OFA nozzles.There is a lack of systematuic research about the effects of OFA location on combustion characteristics and NO_x emission.Therefore,in order to determine the optimal location of the OFA nozzle,this paper adopts the Fluent software and carries out a numerical simulation of the combustion process when the OFA nozzles is on different elevation.Firstly,the complexity of the combustion process in the furnace was analyzed,which presented the theoretical principles of the numerical simulation in the field of combustion furnace,basic equations and a variety of numerical models included.And then the feature and application of the equations and models were described.By analyzing the information above,the main calculation methods and models were determined in this paper.Secondly,the design and operation of the boiler before the reform were investigated,and the reconstruction scheme was established.Before using the Fluent simulation calculation,the Gambit is used to model the boiler furnace,and the grid is divided reasonably,then the combustion process in the furnace is simulated and calculated.The flue gas temperature of furnace outlet,O2 volume concentration and NO_x volume concentration obtained in the computation were compared with measured operation data of boiler.The comparison results show that the simulation calculation is accurate and reasonable.The velocity field,temperature field,components and distribution characteristics of NO_x in the furnace obtained from simulation are analyzed,which proved that the model and calculation method used in this paper can be used to simulation of modification conditions.Finally,four kinds of modification conditions in different situations that OFA nozzles at different elevations while other conditions are the samesimulated.The velocity field,temperature field,components and distribution characteristics of NO x in a vertical section or cross section of a furnace under four conditions are analyzed.By comparing the values of NO_x concentration change rate of furnace outlet,CO concentration variation and superheater bottom mean temperature variation,conclusion can be drawn: When the OFA nozzle height increased 2000 mm based on the design elevation,air gradation efficiency and nitrogen oxide emissions have been significantly improved,and the negative impact on the safe operation of the boiler is also within the acceptable range.Therefore,this scheme is chosen for the final transformation scheme.In the process of determining the transformation scheme,the simulation results in this paper are played a decisive role,it also provides an important theoretical basis for the optimization and improvement of the staged-air combustion system of the same type boiler in our company.It has a high reference value for the future work of low nitrogen transformation. |
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