Oxidation of Soot and Carbon Monoxide in Hydrocarbon Diffusion Flames.
Oxidation of Soot and Carbon Monoxide in Hydrocarbon
Puri, R.; Santoro, R. J.; Smyth, K. C.
Combustion and Flame, Vol. 97, 125-144, 1994.
Sponsor:National Institute of Standards and Technology,
soot; carbon monoxide; diffusion flames; oxidation;
particle size; concentration measurement
Quantitative OH concentrations and primary soot particle
sizes have been determined in the soot oxidation regions
of axisymmetric diffusion flames burning methane,
methane/butane, and methane/1-butene in air at
atmospheric pressure. The total carbon flow rate was
held constant in these flames while the maximum amount
of soot varied by a factor of seven along the
centerline. Laser-induced fluorescence measurements of
OH were placed on an absolute basis by calibration
against earlier absorption results. The primary size
measurements of the soot particles were made using
thermophoretic sampling and transmission electron
microscopy. OH concentrations are greatly reduced in
the presence of soot particles. Whereas large
super-equilibrium ratios are observed in the
high-temperature reaction zones in the absence of soot,
the OH concentrations approach equilibrium values when
the soot loading is high. The diminished OH
concentrations are found to arise from reactions with
the soot particles and only to a minor degree from lower
temperatures due to soot radiation losses. Analysis of
the soot oxidation rates computed from the primary
particle size profiles as a function of time along the
flame centerlines shows that OH is the dominant oxidizer
of soot, with O2 making only a small contribution.
Higher collision efficiencies of OH reactions with soot
particles are found for the flames containing larger
soot concentrations at lower temperatures. A comparison
of the soot and CO oxidation rates shows that although
CO is inherently more reactive than soot, the soot
successfully competes with CO for OH and hence
suppresses CO oxidation for large soot concentrations.