The Sulfur Catalyzed Recombination of Atomic Oxygen in a CO/O2/Ar Flame

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Abstract
Flame structure has been determined for two fuel-lean, premixed laminar CO/O2 Ar flames, one of which contained 1.2 percent SO2 by volume. These flames were fairly “dry” (∼75ppm hydrogen as H2O and H2 impurities), but could be stabilized at 26.5 kPa with 10 percent diluent. Concentration profiles were measured for atomic oxygen and SO2 as well as all major species (CO, O2, CO2) by molecular beam-mass spectrometry. Some elements of the calibration and data reduction procedures are discussed. Temperature profiles were measured with a coated thermocouple. At distances greater than 0.3cm from the flameholder the structure of the SO2 doped flame was found to be consistent with a state of balance between the reactions. The catalytic recombination of O-atoms via reactions (4) and (5) practically eliminated the ∼30 percent O-atom superequilibrium overshoot for the sulfur-free flame. Sulfur addition resulted in a considerable increase in temperature, ranging from 50K. near the flameholder to ∼200K. 1cm downstream. This change is far too large to be caused by reactions (4) and (5), and is primarily due to the method of flame stabilization utilized. Flame structures were modeled by solving simultaneously the one dimensional, unsteady conservation equations using an oxidation mechanism including 14 species and 24 elementary reactions. Considering the crude method used in the calculation to account for heat losses, very good agreement was obtained with the experiment. Both the experimental measurements and calculations indicate that sulfur addition has an inhibiting effect on the flame by catalyzing the recombination of chain carriers. Reaction 8 is responsible for most of the chain branching, even in our fairly ‘dry’ CO flames. It therefore strongly influences both flame speed and thickness. Reaction 12 (along with the reverse of reaction (8) at high temperatures) is the major channel for consumption of atomic oxygen. This reaction also produces about half of the CO2. Our investigation indicates that it will probably be necessary to include HSO2 chemistry to accurately predict O, OH, or H profiles in the presence of sulfur, even for a flame with as little as 150 ppm H.