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Hydrocarbon cool flames and the influence of hydrogen bromide

Small amounts of hydrogen bromide added to n-pentane + 1.33 O2 mixtures lower both the limiting pressure for the onset of two-stage ignition and also, to a smaller extent, that for the appearance of cool flames. The induction period preceding the first cool flame, ז1, is shown to be related to the initial pressure, P0, by a relation of the form ז1 = kP-n0 + c, where k, n and c are constants for a given set of initial conditions. The results show that c ≠ 0 and that the addition of hydrogen bromide reduces both ז1 and c. However, since ז1 ≽ c and c is always finite, it is clear that, even at temperatures above the ignition profiles, ignition continues to take place as a two-stage process. Plots of lg ז1 against reciprocal temperature are invariably characterized by a well-defined change in slope and the temperature at which this occurs decreases with increasing concentration of hydrogen bromide, eventually reaching a limiting value of ca. 270 °C. Above this temperature the slopes of the plots are more or less independent of hydrogen bromide and correspond to an overall activation energy of 82 kJ mol-1. Below this temperature the apparent energy of activation decreases from 206 to 113 kJ mol-1 as the concentration of hydrogen bromide is increased. Similarly there is a limiting concentration of the additive above which the pressure change accompanying the first cool flame is not appreciably increased except at low temperatures. In systems which exhibit multiple cool flames, the second and fourth cool flames are generally too indistinct for their characteristics to be measured with any accuracy. However, the third cool flame appears as a well-defined but relatively weak pressure pulse. In striking contrast to the behaviour of the first cool flame, neither the third cool flame nor the induction period preceding it is appreciably affected by the presence of hydrogen bromide. It thus appears that, although the halogen compound is presumably involved in the chemical reactions leading to the first cool flame, the third cool flame is propagated by intermediates whose mode of formation is independent of the additive.