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20.1.1 Overview

NOx emission consists of mostly nitric oxide (NO), and to a lesser degree nitrogen dioxide (NO $_2$) and nitrous oxide (N $_2$O). NOx is a precursor for photochemical smog, contributes to acid rain, and causes ozone depletion. Thus, NOx is a pollutant. The FLUENT NOx model provides a tool to understand the sources of NOx production and to aid in the design of NOx control measures.



NOx Modeling in FLUENT


The FLUENT NOx model provides the capability to model thermal, prompt, and fuel NOx formation as well as NOx consumption due to reburning in combustion systems. It uses rate models developed at the Department of Fuel and Energy at The University of Leeds in England as well as from the open literature. NOx reduction using reagent injection, such as selective noncatalytic reduction (SNCR), can be modeled in FLUENT along with an N $_2$O intermediate model which has also been incorporated.

To predict NOx emissions, FLUENT solves a transport equation for nitric oxide (NO) concentration. When fuel NOx sources are present, FLUENT solves additional transport equations for intermediate species (HCN and/or NH $_3$). When the N $_2$O intermediate model is activated, an additional transport equation for N $_2$O will be solved. The NOx transport equations are solved based on a given flow field and combustion solution. In other words, NOx is postprocessed from a combustion simulation. It is thus evident that an accurate combustion solution becomes a prerequisite of NOx prediction. For example, thermal NOx production doubles for every 90 K temperature increase when the flame temperature is about 2200 K. Great care must be exercised to provide accurate thermophysical data and boundary condition inputs for the combustion model. Appropriate turbulence, chemistry, radiation and other submodels must be employed.

To be realistic, one can only expect results to be as accurate as the input data and the selected physical models. Under most circumstances, NOx variation trends can be accurately predicted but the NOx quantity itself cannot be pinpointed. Accurate prediction of NOx parametric trends can cut down on the number of laboratory tests, allow more design variations to be studied, shorten the design cycle, and reduce product development cost. That is truly the power of the FLUENT NOx model and, in fact, the power of CFD in general.



NOx Formation and Reduction in Flames


In laminar flames, and at the molecular level within turbulent flames, the formation of NOx can be attributed to four distinct chemical kinetic processes: thermal NOx formation, prompt NOx formation, fuel NOx formation, and intermediate N $_2$O. Thermal NOx is formed by the oxidation of atmospheric nitrogen present in the combustion air. Prompt NOx is produced by high-speed reactions at the flame front, and fuel NOx is produced by oxidation of nitrogen contained in the fuel. At elevated pressures and oxygen-rich conditions, NOx may also be formed from molecular nitrogen (N $_2$) via N $_2$O. The reburning and SNCR mechanisms reduce the total NOx formation by accounting for the reaction of NO with hydrocarbons and ammonia, respectively.

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The NOx models cannot be used in conjunction with the premixed combustion model.


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