The suppression of soot emission from flames by metal additives

A propane diffusion flame has been used to examine the mechanism of soot removal by metallic additives. Forty metals have been studied, and their efficiencies of soot removal in this flame are given. A semiquantitative mechanism is proposed for the alkaline earth metals; its basis is that these metals undergo a homogeneous, gas-phase reaction with flame gases to produce hydroxyl radicals which rapidly remove soot or soot precursors. The mechanism is supported by measurements of free-radical recombination catalysis by the alkaline earths in premixed hydrogen-oxygen-nitrogen flames. Soot removal by most other metals studied is thought to occur by different mechanisms, and possible ones are discussed qualitatively.