# NOx Formation Mechanism and Emission Prediction in Turbulent Combustion: A Review
## 5 The Chemical Reaction Network Method  
Although the CFD method coupled with the finite rate chemical reaction has achieved good results in the prediction of temperature and the results of infinite reaction rate, for the NOx generation process, which is a finite chemical rate process, the CFD method was limited by its turbulence reaction model, which showed the characteristics of huge calculation and accuracy problems. These drawbacks are deeply ingrained in the CFD method and limit the CFD method applied to the practical issue. Unlike the CFD method, which needed to introduce reaction models to approach the concentrations, the chemical reaction networks (CRNs) method completely predicts the NOx generation via detailed mechanisms. In the CFD method, the concentrations of each point were calculated. But in the CRNs method, only the values at the exit are of concern. By sacrificing the accurate flow characteristics, the CRNs method could ensure the accuracy of concentrations. Also, with the decoupling of the flow, the calculation would be decreased [49]. Hence, the CRNs approach method is an economical and convenient method to evaluate NOx production compared with the other methods. Except for the simulation of the results of combustion, the CRNs method could also be applied to research the effect of the coefficients. [PERSON] et al. [50] built a skeletal chemical reactor network, which consists of two PSR units, to research the NO prediction for pure dimethyl ether by the effect of the equivalence ratio. Through applying two different mechanisms, the PSR model could approach the experiment date with 7.1 NOx dry ppm to 7.0 dry ppm at 3% O\({}_{2}\) at the equivalence ratio of 1.0. [PERSON] et al. [51] applied a Mixer and a PSR model to build a model for researching the relationship between adiabatic combustion temperature and heat loss. The results showed that, considering the heat loss, the PSR model showed NOx emissions increased slightly with increasing hydrogen content, which has a similar tendency to the experimental results. [PERSON] et al. [52] applied the CRNs model to simulate the NOx emission of a syngastro fueled gas turbine combustor using rich-burn, quick-mix, lean-burn (RQL) architecture, and the impacts of combustor design/operating parameters on NOx were investigated. The most significant parameter for NOx control is temperature. Following the mixing in the rich-burn zone, the airflow split emerges as a highly influential parameter. However, with the combustor outlet temperature increase, the residence time split and the mixing in the quick-mix zone become important. In most cases, the chemical reaction networks (CRNs) model is widely applied to overcome the drawbacks of the CFD method, which showed a good agreement with the experiment results. For example, [PERSON] et al. [53] applied the CRNs model to simulate modern aircraft engine combustion chambers. The final result showed that the EI NOx predicted by the CRNs model approached the experiment results, especially at take-off and climb conditions. With the approach and idle condition, even the biggest deviation between them is under 12%. [PERSON] et al. [54] applied the CRNs model to a rectangular section of a gas turbine combustor. The tolerance between experiment and simulation at two full-power and idle conditions is smaller than 10.13%. The results showed a good agreement with the practical. [PERSON] et al. [55] simulated the emission of the diffusion flame combustion chamber. The emission index of NOx through simulation was 42, and the experimental result was 44 at take-off conditions. The simulation result could match well with experiment one. [PERSON] and [PERSON] [56] used PSR and PFR models to simulate different types of combustion chambers, and the simulation results showed good accuracy compared with the experiment results. For the single-chemical reaction networks (CRNs) method, [PERSON] et al. [57] divided the combustor into three sections to predict the accurate pollutant emission and combustion efficiency estimations.