EFFECTS OF OXYGEN ON SOOT FORMATION IN METHANE, PROPANE, AND N-BUTANEDIFFUSION FLAMES

Overventilated coflow axisymmetric laminar diffusion flames of methane , propane, and n-butane were used to study the influence of oxygen add ition to the fuel side on soot formation. The line-of-sight soot volum e 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 adiaba tic flame temperature. The relative influences of dilution and direct chemical interaction effects of oxygen in the fuel gas mixture were qu antified. It was found that, when allowance was made for the influence of dilution and thermal effects, the addition of oxygen to the methan e diffusion flame chemically suppressed soot formation. This suppressi on was argued to be due to the reduction in acetylene concentration in the pyrolysis products as the oxygen mole fraction in methane was inc reased. 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 o ther hand, oxygen addition chemically enhanced soot formation. The deg ree of enhancement was small for low mole fractions of oxygen, but inc reased with increasing oxygen. When oxygen is added to the fuel side o f a diffusion flame, two counteracting chemical effects are expected: Oxygen promotes fuel pyrolysis and hence production of hydrocarbon rad icals and H atoms which enhance soot formation. On the other hand, aro matic 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.