Numerial Studies On Computational Fluid Dynamic Simulations Of Flue Gas DeNO_x Process In A Waste Incinerator

Nitrogen oxides are one of the major pollutants in the atmosphere and have greatharm, while in the waste incineration flue gases contain nitrogen oxides and otherharmful gases. In the "12th Five-Year Plan", it is proposed to increase the proportion ofwaste incineration clearly, so the total amount of emissions of nitrogen oxides will beincreased. At the same time, more severe regulation trend of nitrogen oxides in the fluegases from the waste incinerator will be established. And the existing regulation ofnitrogen oxides from the waste incineration would exceed the limit of the forthcomingrevised regulation. Therefore, making nitrogen oxides emissions from the existingincineration plant to meet the forthcoming regulation is an urgent problem to be solved.Studies have shown that setting the selective non-catalytic reduction (SNCR) process atthe end of the waste incineration plants is more economical and effective for theremoval of nitrogen oxides. As a result, additional specific DeNO_xprocess-SNCRprocess is introduced for the transformation of incineration plants.In such a case, computational fluid dynamics (CFD) software is used for modelingand computational analysis of the waste incinerator with the capacity of400t/d. Thenumerical simulation method can provide reference for the design of the additionalSNCR DeNO_xprocess which is the transformation of the waste incinerator. In order tosimplify the calculation, the numerical study process develops to the following threesteps. First, establishing a combustion model for the incinerator is to study the effects ofthe combustion conditions in the incineration furnace, compared with the actualoperating data of the incineration plant to verify that whether the established model isreliable or not. Secondly, establishing a reducing agent injection model is to study theevaporation and mixing law of the reducing agent which is injected into the incinerator,to select suitable parameters for ammonia injection of the incinerator as well asarrangement of nozzles, and intended to be a basis of ammonia injection parameters forother incinerators to select. Finally, we take the combustion conditions in the incinerator,the ammonia injection parameters and the nozzle arrangement which are already chosenas the boundary condition of SNCR reaction in the final part. Then, the simulation iscarried out to obtain a final DeNO_xrate. In addition, a series of study of the influence ofsecondary air to the incinerator is also conducted.Summaries of the study are as the following. In this study a number of soundassumptions have been made for simplifying, modeling and calculating of theincinerator. While comparing the calculated results to the data of the actual incineratorshows that analog temperature, component are in good agreement with the actualmeasured datas, which shows that assumptions is reasonable, the established model is reliable as well, and a height of10-32m is suitable for SNCR reduction reaction. Inorder to study the ejection distance, the evaporated and mixing law of the reducingagent in the incinerator, calculation of the droplet size, the ejection speed, injectionangle, the nozzle temperature, air speed and the injection amount of the reducing agenthas been made. While in a finite model, to increase the droplet size, the ejection speedand the injection amount appropriately helps improve the incident depth and the mixingeffect of the reducing agent with the flue gas. And optimal parameters for SNCRreaction are the droplet size of300μm, the ejection speed of30m/s, the ejection angle of20°, and six nozzles arranged symmetrically in the right and left walls in the ammoniainjection. After the final calculation of SNCR reduction reaction, the NO concentrationof the incinerator outlet is182.39mg/m3, and the DeNO_xrate is50.97%, satisfied theEU2000and the forthcoming nitrogen oxides control regulations that is200mg/m3.It’sfound in the study of the influence of secondary air in the incinerator that the faster thesecondary air inlet velocity, the more uniform distribution of O2in the furnace, and thefaster the speed of flue gas gets to stable.