The influence of methyl bromide on flames. Part 2.—Diffusion flames

Abstract

The concentrations of methyl bromide required to extinguish fuel + air diffusion flames have been determined and its effect on the structure of these flames studied spectroscopically. It was found that the concentration of methyl bromide required in the air to extinguish the flame was of the same order as for the pre-mixed flame, but considerably higher concentrations were required in the fuel. The structure of diffusion flames was drastically altered by the addition of methyl bromide. When added to the air an extra reaction zone forms on the air side in which the methyl bromide reacts, and the quenching of the flame involves an interaction of this extra zone and the main reaction zone. When methyl bromide is added to the fuel, carbon formation is greatly increased, and hydrogen and carbon monoxide flames obtain the appearance of hydrocarbon flames.The effect of methyl bromide and bromine on pre-mixed flames has been described in part 1,1 and its effect on diffusion flames is described in the present paper. Probably the main conclusion which arises from the work described in part 1 is that the action of methyl bromide in pre-mixed flames can be explained in terms of the addition of both inhibitor and additional fuel, and it will be seen that this is also true for diffusion flames. The investigation on diffusion flames consists firstly of a study of the concentrations of methyl bromide required to extinguish diffusion flames, and secondly, a spectrographic investigation. EXPERIMENTAL DETERMINATION OF CRITICAL CONCENTRATIONS OF METHYL BROMIDE

Molecular basis of flame inhibition

“…Friedman also notes that a diffusion flame does contain a region of premixing at the base and it has not been generally established whether the inhibitor functions in this region or not. It is also found, with CHaBr, for example, that a similar amount of inhibitor can extinguish both a diffusion and a premixed flame and this strongly suggests the premixed region to be the relevant area of inhibition [38].…”

A literature survey of the chemistry of flame inhibition

“…In the case of halogenated organic inhibitors the most active fragment is believed to be the halogen (Cl, Br, or I) atom. Some support for this belief is given by the observation [11] of emission bands associated with the recombination of bromine atoms on the air side of diffusion flames inhibited by CH3Br. When Brz was substituted for the CHaBr, no Br2 emis· sion was seen, but inhibition was still evident.…”

“…For fuel side injection of methyl bromide and POCb, the decrease in OH emission was more marked than for air side injection [25]. However, air side injection is expected to produce greater amounts of inhibition [11,13]. Unexpectedly, CCl4 enhanced, rather than reduced the OH e mission with air side injection [25].…”

“…For the reaction X+X+M~X2+M (34) the rate constants may be calculated from log k=-31.26-2 .7 log (T/300) [74] for X= Cl, M = Cb; log k=-30.67 -3.01 log (T/300) [75] for X=Br, M=Br2; log k=-35 .54+10g T+1l50/T' [ 76] The structure of inhibited diffusion flames gives clues to the chemical processes going on in them. The "auxiliary reaction zone," which forms outside the main reaction zone of some inhibited flames when the inhibitor is added to the air side, has been identified as due to the recombination of halogen atoms [11].…”

Flame inhibition with electron attachment as the first step

“…Simmons and Wolfhard [13] report that in CO and air flames, "the amount of methyl bromide which can be introduced into carbon monoxide before the flame is extinguished is very large." Creitz [ 8 ] agrees that CH^Br is ineffective in CO flames, but found CF^Br to be quite effective.…”