The rate constant of the reaction NCN + H2 and its role in NCN and NO modeling in low pressure CH4/O2/N2-flames

Abstract

Bimolecular reactions of the NCN radical play a key role in modeling prompt-NO formation in hydrocarbon flames. The rate constant of the so-far neglected reaction NCN + H₂ has been experimentally determined behind shock waves under pseudo-first order conditions with H₂ as the excess component. NCN₃ thermal decomposition has been used as a quantitative high temperature source of NCN radicals, which have been sensitively detected by difference UV laser absorption spectroscopy at = 30383.11 cm⁻¹. The experiments were performed at two different total densities of ρ ≈ 4.1 × 10⁻⁶ mol cm⁻³ and ρ ≈ 7.4 × 10⁻⁶ mol cm⁻³ (corresponding to pressures between p = 324 mbar and p = 1665 mbar) and revealed a pressure independent reaction. In the temperature range 1057 K < T < 2475 K, the overall rate constant can be represented by the Arrhenius expression k/(cm³ mol⁻¹ s⁻¹) = 4.1 × 10¹³ exp(−101 kJ mol⁻¹/RT) (Δlog k = ±0.11). The pressure independent reaction as well as the measured activation energy is consistent with a dominating H abstracting reaction channel yielding the products HNCN + H. The reaction NCN + H₂ has been implemented together with a set of reactions for subsequent HNCN and HNC chemistry into the detailed GDFkin3.0_NCN mechanism for NO_x flame modeling. Two fuel-rich low-pressure CH₄/O₂/N₂-flames served as examples to quantify the impact of the additional chemical pathways. Although the overall NCN consumption by H₂ remains small, significant differences have been observed for NO yields with the updated mechanism. A detailed flux analysis revealed that HNC, mainly arising from HCN/HNC isomerization, plays a decisive role and enhances NO formation through a new HNC → HNCO → NH₂ → NH → NO pathway.