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 [small nu, Greek, tilde] = 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/(cm(3) mol(-1) s(-1)) = 4.1 × 10(13) 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 NOx 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.

Shock Tube Measurements of the Rate Constant of the Reaction NCN + O2

“…In recent years, using NCN 3 thermal decomposition as a quantitative source of NCN radicals and sensitive time‐resolved UV absorption spectroscopy to detect NCN concentration–time profiles, we have performed several direct shock tube measurements of bimolecular NCN rate constants at high temperatures. These studies include the reactions NCN + NO, NO 2 , NCN + NCN, O, and M as well as NCN + H and NCN + H 2 . The only other direct study reported in the literature, which is in excellent agreement with our work, was concerned with the unimolecular decomposition of NCN using the C‐ARAS technique .…”

“…These studies include the reactions NCN + NO, NO 2 [4], NCN + NCN, O, and M [5] as well as NCN + H [6] and NCN + H 2 [7]. The only other direct study reported in [14].…”

“…Consequently, also the absolute value of k 6 could not be determined but only its value relative to k 5 . Experimental conditions and fitting results for k 7 and φ = k 6 /(k 6 + k 5 ) are listed in Table II and plotted in Figs. 3a and 3b.…”

“…The upper accessible temperature limit was determined by the onset of NCN thermal decomposition, NCN + M; the lower temperature limit was set by the reaction NCN + NCN, which becomes dominant at temperatures below 1700 K. For modeling purposes, NCO + NO have been assumed as the sole products of reaction (2), hence small contributions from the minor CNO + NO channel are included in k 7 An example for a typical NCN concentration-time profile at initial reaction conditions of T = 2036 K and p = 264 mbar (ρ = 1.62 × 10 −6 mol cm -3 ) is presented in Fig. 4a.…”

Collisional Relaxation of NCN: An Experimental Study with Laser-Induced Fluorescence

“…In combustion chemistry, the kinetics of the cyanonitrene radical, NCN, has recently attracted attention [4][5][6][7][8][9][10][11] because NCN is considered an important intermediate in the formation of so-called prompt NO in hydrocarbon flames under fuel-rich conditions [12][13][14][15][16][17][18][19][20]. Cyanonitrene is a linear diradical with a 3 − g electronic ground state [21,22] and was shown to be formed under combustion conditions in the spin-allowed reaction CH(…”