Some reactions of neutral species in flames

Abstract

The combustion of trace additives in hydrogen-nitrogen-oxygen flames has been investigated experimentally by mass spectrometric and spectroscopic analysis of the burnt gas stream and theoretically by computer simulation. Breakdown of cyanogen in a flame of composition H₂ : N₂: O₂ = 4.5 : 8.0 : 1.0 gives HCN and carbon oxides. The mass spectrometric ratio of HCN to CO/CO₂ mixture in the burnt gases emerging from the reaction zone is dependent on the percentage cyanogen added to the flame, showing a slow rise from 0.25 at 0.1% to 0.30 at 1.0% cyanogen, followed by a more rapid increase to a value of approximately unity at an additive concentration of 1.5%. Computer modelling on the basis of a main primary reaction: H + C₂N₂→ HCN + CN, is inconsistent with these results. It is concluded that primary decomposition of C₂N₂ in hydrogen flames probably takes place by reaction with atomic oxygen: O + C₂N₂→ NCO + CN, carbon oxides being formed from CN and NCO radicals by the fast processes: CN + O₂→ NCO+ O, NCO + O→ CO + NO, NCO + H→ CO + NH. Production of HCN would then be limited to relatively slow reactions, giving calculated yields of HCN lower than the corresponding yields of carbon oxides, as observed experimentally. Mass spectrometric measurement of HCN and NO profiles in a flame containing 1.1% cyanogen and 2.0% nitric oxide is used to determine a value for the rate of reaction of NO with CN radicals at the burnt gas temperature: CN + NO→ N₂ + CO, k = 7.5 x 1⁹m⁳ kgmol -ⁱ s -ⁱ at 1500 K. The reaction of NO with NH radicals is studied in a flame containing 0.90% nitric oxide and up to 1.15% ammonia. Results indicate that the reaction rate is sufficiently fast to reach equilibrium within the combustion zone, while isotopic labelling of the reactant NO and mass spectrometric analysis of the burnt gas stream gives evidence for the formation of molecular nitrogen as the predominant stable product. Metal cyanide dissociation energies are determined for the alkali metals Li, Na, K, Rb and Cs. In the presence of known concentrations of HCN and H atoms, spectroscopic measurement of the proportion of metal combined to form metal cyanide enables a value to be calculated for the equilibrium constant of the reaction: M + HCN ⇋ MCN + H. Statistical methods are then used to obtain the enthalpy of reaction at absolute zero and thus the dissociation energy of the metal cyanideo