The formation and destruction of hydrogen cyanide from atmospheric and fuel nitrogen in rich atmospheric-pressure flames

Abstract

A novel method involving direct sampling of natural flame ions into a mass spectrometer has been used to measure the concentration of HCN, NH 3 and NO in atmospheric-pressure flames. It is found that HCN is formed from N 2 in the reaction zone of rich hydrocarbon flames and is subsequently completely oxidized within the flame, in part to NO. This corresponds to Fenimore's ‘prompt NO’. The amount of HCN formed is proportional to the N 2 concentration, but is not very dependent on the nature of the fuel (CH 4, C 2H 4 and C 2H 2) or on the temperature (between 2000 and 2560 K). It varies strongly with the equivalence ratio and is greater, the richer the flame. In contrast to N 2, nitrogen compounds in the fuel, whatever their nature (NO, NH 3, CH 3CN or pyridine), are converted quantitatively to HCN in the reaction zone of rich hydrocarbon flames. The subsequent disappearance of HCN within the flame has been studied, and between 2300 K and 2560 K the rate is consistent with the reaction CN + OH → NCO + H being the rate-limiting step with a rate constant of approximately 1 × 10 −10 cm 3 molecule −1 s −1, although other mechanisms may be important at lower temperatures.