Effects of oxygen on soot formation in methane, propane, and n-Butane diffusion flames

Abstract

Overventilated coflow axisymmetric laminar diffusion flames of methane, propane, and n-butane were used to study the influence of oxygen addition to the fuel side on soot formation. The line-of-sight soot volume fractions and the visible flame profiles were measured as a function of axial location along the centerlines of pure fuel flames, and the flames in which the fuel was diluted either with oxygen or nitrogen at selected temperatures of the reactants, to maintain a constant adiabatic flame temperature. The relative influences of dilution and direct chemical interaction effects of oxygen in the fuel gas mixture were quantified. It was found that, when allowance was made for the influence of dilution and thermal effects, the addition of oxygen to the methane diffusion flame chemically suppressed soot formation. This suppression was argued to be due to the reduction in acetylene concentration in the pyrolysis products as the oxygen mole fraction in methane was increased. The chemical influence of oxygen addition to methane decreased when the adiabatic flame temperature was decreased by decreasing the temperature of the reactants. In propane and n-butane flames, on the other hand, oxygen addition chemically enhanced soot formation. The degree of enhancement was small for low mole fractions of oxygen, but increased with increasing oxygen. When oxygen is added to the fuel side of a diffusion flame, two counteracting chemical effects are expected: Oxygen promotes fuel pyrolysis and hence production of hydrocarbon radicals and H atoms which enhance soot formation. On the other hand, aromatic radicals and critical aliphatic hydrocarbon radicals are removed by reactions with molecular oxygen and oxygen atom. The net chemical influence is the difference of these two counteracting effects.