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 (∆ T c. f. ) was found to be constant at a constant ambient temperature ( T 0 ) on varying the mixture composition and total pressure and on addition of inert gases, but probably increased slightly with increasing T 0 . When only slow oxidation occurred the maximum temperature rise was less than Δ T c. 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 T 0 and varied with total pressure and on addition of inert gases. Addition of He, CO 2 and H 2 O increased the cool flame pressure limit at constant T 0 , 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.