Cool flames and oscillations in hydrocarbon oxidation

Abstract

Stabilized cool flames and oscillations of propane and butane have been studied. An experimental technique has been developed, which enabled for the first time the detection of free radicals in stabilized cool flames of hydrocarbons. For this purpose, a two-section flow reactor has been constructed. Cool flame appears in the second section and stabilizes there. In order to detect free radicals and examine their behaviour, a small part of the gas flow was sampled from each section continuously and directed at low pressure to a freezing pin at liquid nitrogen temperature inside the cavity of an electron paramagnetic resonance (EPR) spectrometer. It has been shown that when passing from the first section (preflame zone) to the stabilized cool flamezone, the concentration of radicals increases aruptly. For example, in the case of propane (C3H8:O2=1:1; P=270 torr; T=320°C), the radical concentration makes up 6·1012 molecules/cm3, whereas in the cool flame zone it is 3·1014 molecules/cm3 at the same temperature. This abrupt change of radical concentration is caused by the self-accelerated chain process. The temperature increase in the cool flame zone results in an increase of the radical quantity. Thefurther temperature increase leads to the decrease of radical quantity, and the flame fades at T=370°C to 380°C. Asymmetric profiles of temperature, radical concentration, and stable products appear in the cool flamezone along the vertical cross section of the reactor. It is shown that the regularities observed are the results of nonlinear processes with the participation offree radicals. The same results are obtained for other hydrocarbons and mixtures. By changing the process parameters, damped and stable oscillations are observed, which are correlatedto cool flame characteristics. Stabilized cool flames of propane and butane have been studied in a special two-section reactor. Direct experimental data have been obtained on the behaviour and nature of free radicals in the stabilized cool flame. It has been ascertained that the appearance and stabilization of the cool flame are accompanied by an abrupt increase of radical concentration caused by the rapid self-accelerated chain process. The temperature increase above 350°C to 360°C adversely affects the process development. The radical concentration is decreased, and the cool flame is damped. The oscillation regimes of the oxidative process have also been studied in the two-section reactor. A correlation between the cool flame phenomenon and the oscillations has been verified.