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NOx Formation: The NCN Pathway

 

Nitrogen oxides, NOx, are major atmospheric pollutants generated in combustion processes by different mechanisms. For example, under rich combustion conditions, prompt-NO formation is initiated by the reaction of small hydrocarbon radicals with nitrogen stemming from the combustion air. In contrast to textbook knowledge, it has been theoretically and experimentally shown in recent years that the cyanonitrene radicals (NCN) plays a key role in the reaction sequence

 

CH + N2 → NCN + H → … → NOx .

 

ncn_source_webIn order to model NOx formation via the NCN pathway, NCN chemistry needs to be implemented into combustion mechanisms. However, as rate constant data for NCN high temperature chemistry were only scarcely available, first implementations mostly relied on theoretical or estimated data. In our group, we use the thermal decomposition of the very toxic and highly explosive NCN3 as a quantitative source for NCN radicals allowing us to directly measure the rate constants of NCN reactions at combustion relevant temperatures for the first time. Concentration-time profiles of NCN radicals were detected at λ = 329.1302 nm by UV difference laser absorption spectroscopy in shock tube experiments [1,3]

 

ncn_rates_web

 

By simulating the experimental NCN concentration-time for experiments with different reaction mixture composition the rate constants of several bimolecular NCN reactions could be directly measured [1-7]

 

Especially the fast reactions NCN + O and NCN + H (and its product branching ratios) are very important to reliably model overall prompt NO concentrations in combustion processes.

 

 

 

 

 

 

In collaboration with N. Lamoureux and P. Desgroux (Lille University), a detailed NCN submechanism including the reactions shown in the Arrhenius plot has been developed and implemented in a global combustion mechanism [6]. Based on this new mechanism, it is possible to simulate prompt NOx formation more reliably, which is important for the design and testing new combustion engine concepts.

ncn_reaction_scheme_web

 

 

[1] "The Thermal Decomposition of NCN3 as a High Temperature NCN Radical Source: Singlet-Triplet Relaxation and Absorption Cross Section of NCN(3Σ)", J. Dammeier, G. Friedrichs, J. Phys. Chem. A 114 (2010) 12963–12971.

[2] "Direct Measurements of the Rate Constants of the Reactions NCN + NO and NCN + NO2 Behind Shock Waves", J. Dammeier, G. Friedrichs J. Phys. Chem. A 115 (2011) 14382–14390.

[3] "Direct measurements of the high temperature rate constants of the reactions NCN + O, NCN + NCN, and NCN + M", J. Dammeier, N. Faßheber, G. Friedrichs Phys. Chem. Chem. Phys. 14 (2012) 1030 – 1037.

[4] "A consistent model for the thermal decomposition of NCN3 and the singlet-triplet relaxation of NCN", J. Dammeier, B. Oden, G. Friedrichs, Int. J. Chem. Kinet. 45 (2013) 30-40.

[5] "Direct measurements of the total rate constant of the reaction NCN + H and implications for the product branching ratio and the enthalpy of formation of NCN", N. Faßheber, J. Dammeier, G. Friedrichs, Phys. Chem. Chem. Phys. 16 (2014) 11647-11657.

[6] "Rate constant of the reaction NCN + H2 and its role for NCN and NO modeling in low pressure CH4/O2/N2-flames", N. Faßheber, N. Lamoureux, G. Friedrichs, Phys. Chem. Chem. Phys. 17 (2015) 15876 – 15886.

[7] "Shock Tube Measurements of the Rate Constant of the Reaction NCN + O2" N. Faßheber, G. Friedrichs, Int. J. Chem. Kinet. 47 (2015) 586-595.

 

Contributing Researchers: N. Faßheber, S. Hesse, G. Friedrichs and (formerly) J. Dammeier, B. Oden

 


 

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