Suppression of Soot in Flames by Alkaline-Earth and Other Metal Additives

Experiments were performed to clarify the role of metallic fuel additives in relation to soot suppression in a well defined laboratory-scale diffusion flame. Principal emphasis was given to three alkaline-earth metals; namely, Ba, Sr and Ca. The experiments included Cu, Sn, Li, Na and K as well. but only to the extent that the latter contributed to understanding the details of alkaline-earth behavior. The additives were in the form of aqueous solutions of salts of the preceding metals. The solutions were aspirated into the oxidant flow of a nearly two-dimensional laminar C:H/air flame emanating from a symmetric Wolfhard-Parker burner. Soot size, number density and volume fraction were determined from Mie scattering. Metal species concentrations were determined semiempirically by combining H-measurements with known equilibrium constants for the metal/radical reactions occurring in the flame. The flame temperature was measured via sodium line reversal. The alkaline-earth metals were observed to alter the three soot parameters above in accordance with a Ba>Sr>Ca order of effectiveness and, moreover, the effectiveness was a strong function of measurement position. The temperature and species measurements were used to relate this variation to a specific metal combustion product. Following this approach, a conclusion is reached which strongly implicates the charged MOH + (M = Ba, Sr. Ca) species as responsible for soot suppression. In addition, since significant suppression may occur with only a slight size change, the metal principally reduces the soot number density, which suggests that inhihitory intervention is most significant at an early soot formation stage.