Evidence for a new way of producing NO via NNH in fuel-rich flames at atmospheric pressure

Concentration profiles of NOX have been measured along the axes of laminar, premixed, flat flames of H-2 + O-2 + N-2 and CH4 + O-2 + N-2 burning at 1 atm, with final flame temperatures in the range of 2500 to 1850 K. While the Zel'dovich mechanism [1] explains the rate of production of NO in the burnt gases of the fuel-lean flames studied, it was found that, downstream of the reaction zone in fuel-rich names, NO is produced faster than predicted by the Zel'dovich mechanism alone, especially in flames cooler than 2100 K. Careful examination of the observations suggests there is an additional mechanism forming NO in the burnt gases of these fuel-rich flames. The route suggested by Bozzelli and Dean [2] via NNH as an intermediate, i.e., N-2 + H = N2H (1) N2H + O --> NO + NH, (2) accounts, both qualitatively and quantitatively, for the downstream measurements of the rate of generation of NO exceeding the rate of the Zel'dovich mechanism in these names. The measurements here indicate that for reaction (1) at similar to 2000 K: Delta H-1(0) = 23 +/- 7 kJ mol(-1) and Delta S-1(0) = -94.4 +/- 2.0 J mol(-1) K-1. Reaction (2) has a constant rate coefficient equal to 2 X 10(-10) ml molecule(-1) s(-1), to within a factor of 2.5, from 1800 to 2500 K. (C) 1998 by The Combustion Institute.