Study On The Formation Route Of Nitrogen Oxides In Methane MILD Combustion

MILD（Moderate&Intense Low Oxygen Dilution）combustion can significantly reduce the production of NO during natural gas combustion.It is one of the most potential low-nitrogen combustion technologies in the past 20 years.However,there are still some controversies at domestic and foreign about the NO formation mechanism under the methane MILD combustion mode.To solve this problem,this paper uses a combination of numerical simulation and reaction kinetic analysis to evaluate,improve and perfect the existing different classification and calculation methods of NO formation routes under methane MILD combustion,which not only deepens the understanding of the route and mechanism of NO formation under methane MILD combustion,but also proposes measures to further reduce NO emissions on this basis.First,based on the GRI-2.11 mechanism,this paper evaluates the performance of the three existing methods for calculating NO routes in the methane MILD combustion mode,and proposes an improvement plan on the basis of the existing calculation methods.The results show that none of the three existing methods can fully describe the NO formation route of methane in the MILD combustion mode;and the improved calculation method not only retains the advantages of the existing calculation method,but also optimizes the decoupling of the NO route,which greatly improves the accuracy of the prediction result.Furthermore,the above calculation method is improved by combining with the most detailed NO_x chemical reaction model（PG2018 mechanism）,and compared with the calculation result of GRI-2.11 mechanism.The results show that the decoupling error calculated by the improved calculation method is below 3%.The reaction rate of the HNO→NH→NH₂→NH₃ route calculated by the GRI-2.11 mechanism is too large,so that the GRI-2.11 mechanism will overestimate the concentration of NH₃ groups and underestimate the concentration of HCN groups,while the PG2018 mechanism can predict NH₃ and HCN groups well.Within the range of the total research conditions,the effect of the NNH route under the PG2018 mechanism is negligible,while the NNH route under the GRI-2.11 mechanism still contributes 10-20%.Finally,the method of coupling MILD combustion and staged combustion is proposed to further reduce NO emission concentration,and the improved calculation method is used to analyze the NO formation route.The results showed that the NO emission concentration of air-staged MILD combustion and fuel-staged MILD combustion were reduced by 15%and 46%,respectively,compared with unclassified ones.The main reason was the significant suppression of the prompt route.