Formation of nitric oxide in flames

The aim of this work was the investigation of the production of the important pollutant nitric oxide in hydrogen-rich flames in the presence of both nitrogenous compounds and hydrocarbons, and also the effect of the type of hydrocarbon on the mechanism of disappearance of these nitrogenous compounds. The project is thus related to that of ascertaining the mechanism and rate with
which a nitrogen compound in a fuel such as coal ultimately ends up as NO or N2 on combustion. A variety of laminar flames were burned with different [Hc]/[O2] ratios in the burner supplies, and argon as diluent in such an amount that an adiabatic temperature of 1900 K was reached. Nitric oxide was added to the unburnt gases of these flames in varying amounts. Concentrations of NO were measured with a chemiluminescent N0x Analyser, whereas those of the cyanides and NH1. species formed were measured with ion-specific electrodes. Disappearance of NO was only observed when hydrocarbons were added to a flame together with the NO, and this decay of NO only took place in the reaction zone, in contrast to the cyano and NH1 species, which were at their highest concentrations in the reaction zone. Nitric oxide was then re-formed downstream with the highest rate of production being near the reaction zone. The initial loss of NO in the reaction zone appeared to be proportional to the
number of carbon atoms in a molecule of the hydrocarbon. Downstream, both the cyano pool and NH. pool decayed; the cyano pool disappeared via HCN + OH → HOCN + H or CN + H2O → HOCN + N as the rate-determining steps. Since NH. species were formed in flames to which NO and a hydrocarbon were added, in order to have a better understanding of the mechanism of production of NO from NH1, flames were studied with various [H2]/[02l ratios, temperatures and quantities of added ammonia. The reactions which appeared to determine the production of NO from the various NH1 species formed from NH3 were
N + OH → NO + H ..... (A) in the reaction zone, and NH2 + OH - - - → NO + H2 + H ..... (B) downstream. In addition, NO reacted to form N2 in: N + NO → N2 + O ..... (C) in the reaction zone, and subsequently downstream in one or other of NH2 + NO - - - → N2 + H20 ..... (D) NH + NO - - - → N2 + OH ..... (E) The extent to which reactions (D) and (E) occur depends upon [H21/[021 ratio
in the unburnt gases and the temperature of the flame. The addition of a hydrocarbon to a flame with ammonia in the burner supplies, only seemed to affect the reaction zone, where again a cyano pool was observed. From the experimental observations, rate constants for reaction (B), which produces NO, were determined. In addition, velocity constants were measured
for (D) and (E) as well as for the oxidation of HCN by OH and, CN by H2O.