THE ROLE OF NITROUS-OXIDE IN THE MECHANISM OF THERMAL NITRIC-OXIDE FORMATION WITHIN FLAME TEMPERATURE-RANGE

The thermal nitric oxide formation in a one-dimensional tubular flow r eactor has been investigated. A mixture of oxygen and nitrogen was con tinuously supplied into an electrically heated reactor of wall tempera ture within the range 1653-1798K. Five different reactor diameters mad e of two materials were used. The measured rate of nitric oxide format ion was much higher than the calculated by the Zeldovich thermal mecha nism with the available rate constants. During the experiments no fuel was supplied into the reactor, thus the concept of the Fenimore promp t NO could not be used to clear up these discrepancies. Also the nitro us oxide mechanism with the recommended rate constants could not expla in the results. The observed rate of nitric oxide formation has been d escribed by a sequence of five reactions formed of the Zeldovich mecha nism and the proposed by Malte and Pratt N2O mechanism: N-2 + O revers ible arrow NO + N, (1) N + O-2 reversible arrow NO + O, (2) N-2 + O M reversible arrow N2O + M, (6) N2O + O reversible arrow NO + NO, (7) N2O + O reversible arrow N-2 + O-2, (8) For which the rate constants o f reactions (1) and (7) with the assumption of O radicals in equilibri um were tested. Because these reactions proceed independently of the c ombustion process it is proposed to call them as an extended thermal m echanism. The rate of nitric oxide formation through the extended ther mal mechanism is much faster than by the Zeldovich thermal mechanism ( reactions 1 and 2) within the flame temperature range. The role of nit rous oxide for the thermal NO build up is crucial in temperatures belo w 1800K, while of the Zeldovich mechanism (reactions 1 and 2) above 20 00K.