Influence of Alkaline-Earth Additives on Soot and Hydroxyl Radicals in Diffusion Flames

摘要

When alkaline-earth metals are introduced as fuel additives in sooting premixed or diffusion flames, a significant reduction in the soot loading commonly occurs. There have been recurrent, unsubstantiated speculations which have related this reduction to an hydroxide mechanism. In it, the role of the metal is to catalyze an increase in the OH concentration, which then accelerates soot removal via increased oxidative reaction of soot with OH. Measurements have been made which represent the first quantitative test of the OH mechanism. Spatially-precise nonperturbing laser techniques have been used to map OH and soot throughout a hydrocarbon/air diffusion flame with and without Barium, Strontium, Calcium salt additives present. Specifically, the OH concentrations were inferred from saturated laser-induced fluorescence measurements, whereas soot size, number density and volume fraction were inferred from Mie scattering measurements. In addition to soot and OH measurements, the flame temperature was determined from sodium line reversal. Results have been obtained which demonstrate convincingly that in the vicinity of the peak soot concentration the added metals cause the OH concentration to decrease significantly. The decrease becomes less pronounced for measurement points in the vicinity of the main reaction zone. This catalytic decrease of OH, including its spatial dependence, is very similar to that observed for H radicals in premixed H2/O2/N2 flames with added alkaline-earth metals present. The results of this experiment do not support the often quoted OH related hypothesis of soot removal by metal additives.