Numerical Simulation On Low NO_x Emission Modification Of A 220t/h Pulverized-coal-fired Boiler

The environmental negative impact of economic and social development is a serious problem that human beings are facing,and the pollution caused by NOx emissions has been one of the most important topics in recent years.At present,there are many researches on NOx emission of 300MW~1000MW tangentially coal-fired boilers,but the low nitrogen modification and research on the small thermal power plant boiler for heating equipment are not very common.Based on this background,this paper depends on low nitrigen combustion modification project of a 220t/h tangentially coal-fired boiler which is in a thermoelectric center that is participated by the institute of thermoelectric design of southeast university.The boiler is used as the research object,and the boilers before and after the modification of low nitrogen combustion are simulated numerically under different working conditions,in addition,the distribution of flow field,temperature field,component concentration and NOx pollutant in the furnace is compared and analysed,so as to verify the results,predict the operation characteristics of the boiler and seek better operation modes.The results are compared in order to provide the guidance for further operation optimization and commissioning of the boiler which is after modification.In addition,the results of the study on the selection of running parameters can be used to provide suggestions for the modification and operation of low nitrogen combustion in the same type boilers.In this paper,the generation mechanism of NOx,control method of NOx,the research and development of low nitrogen combustion in China and abroad are introduced firstly.Secondly,the mathematical model,boundary condition,operation parameters of the boiler and modification scheme of the numerical simulation process are introduced in detail.Thirdly,the Fluent software was applied to simulate the boilers before and after modification numerically,and the distribution of flow field,temperature field,component concentration and NOx pollutant in the furnace is compared and researched,so as to verify modification results.And the results of numerical calculation are compared with the actual running data collected to verify the feasibility of the model.The results show that the calculation results are basically fitting with the actual operation data,and in accordance with the actual operation rule of the tangentially coal-fired boiler.The velocity field and temperature field of the boiler which are after modification are more reasonable and well-distributed than the boiler which are before modification,and the modification relieves the problems,such as coking in the furnace,high temperature corrosion,and flame scouring the wall,effectively.At the same time,the combustion in the furnace has been improved better,and the product of NOx has been greatly reduced,the reduction rate of NOx has reached about 60%,and the low-nitrogen combustion modification has remarkable effect.Finally,the boiler after modification is used to be the research object,which is simulated numerically under different working conditions,such as different excess air coefficients,different rate of over-fire air,different air distribution way of secondary air,and different nozzle height of over-fire air.And the distribution of flow field,temperature field,component concentration and NOx pollutant in the furnace is compared and analysed,so as to predict the operation characteristics of the boiler and seek better operation modes.The results show that the optimal excess air coefficient of the boiler is 1.2,and the optimal rate of over-fire air in boiler is 30%,which can not only achieve better NOx emission reduction,but also guarantee the economic security of the operation in boiler;and the positive pagoda distribution of secondary air is the the most reasonable air distribution way of secondary air;keeping the nozzle height of over-fire air in original condition the same or increasing the nozzle height of over-fire air reasonably in the range of 2m can not only achieve better NOx emission,but also guarantee the combustion efficiency of furnace.