The cool flame combustion of ethanol

Abstract
The kinetics and products of the cool flame combustion of ethanol between about 280 and 330°C have been studied using a static system. The oxidation exhibited a long induction period, during which very little reaction occurred, followed by a period during which the rate, as measured by temperature and pressure increase and by consumption of reactants, accelerated exponentially. The passage of a cool flame was marked by a sharp increase in rate. During the acceleration period before a flame, the main products were hydrogen peroxide, water, carbon oxides, acetaldehyde, formaldehyde and methanol, the concentrations of which increased continuously except for acetaldehyde which rose to a constant value or even a maximum. Added acetaldehyde did not completely remove the induction period and did not reduce the acceleration period unless the amount added was greater than that normally present just before a cool flame passed. Coating the vessel with potassium chloride markedly inhibited cool flame formation. The temperature rise in a mixture up to the passage of a cool flame (∆Tc. f.) was found to be constant at a constant ambient temperature (T0) on varying the mixture composition and total pressure and on addition of inert gases, but probably increased slightly with increasing T0. When only slow oxidation occurred the maximum temperature rise was less than ΔTc. f.. In contrast, the amount of alcohol consumed up to the cool flame was much less for a 1:2 than for a 1:1 ethanol/oxygen mixture at constant T0 and varied with total pressure and on addition of inert gases. Addition of He, CO2 and H2O increased the cool flame pressure limit at constant T0, whereas Ar and Xe reduced it. The results indicate that branching was probably due to the decomposition of an acetyldehyde-hydrogen peroxide compound and that thermal factors, particularly the thermal conductivity of the mixture, were of predominant importance in determining whether a cool flame occurred. The variation in the consumption of ethanol up to the cool flame with conditions has been explained qualitatively by considering the oxidation simply as a self-heating reaction. The sudden increase in rate when a cool flame occurred may have been due to spontaneous ignition of a peroxidic intermediate.