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Fundamental Mechanisms for CO and Soot Formation. Final Report.


pdf icon Fundamental Mechanisms for CO and Soot Formation. Final Report. (6483 K)
Santoro, R. J.

NIST GCR 94-661; 167 p. November 1994.

Sponsor:

National Institute of Standards and Technology, Gaithersburg, MD

Available from:

National Technical Information Service
Order number: PB95-143160

Keywords:

carbon monoxide; diffusion flames; fire research; oxidation; soot; global equivalence ratio

Abstract:

Studies investigating the oxidation of soot and carbon monoxide (CO) have been conducted in a series of laminar diffusion flames. Both overventilated and underventilated conditions have been examined. For the overventilated studies, the production and destruction of CO has been found to be influenced by the amount of soot present in the flame. Measurements of the hydroxyl radical (OHDT) have demonstrated that soot can compete for OHDT when undergoing oxidation and, thus, impede the oxidation of CO to CO2. Absolute concentration measurements for OHDT have shown that superequilibrium values of OHDT are achieved in the upper region of these diffusion flames. In these situations, equilibrium estimates for OHDT are in error, underestimating the OHDT concentration significantly. However, as soot concentration increases to a point where soot is emitted from the flame, rapid reactions between soot particle and OHDT result in concentration levels close to equilibrium values. These results clearly demonstrate that soot particles are far from passive species in flames and can directly affect the chemical pathways involved in the oxidation process through radiative effects on temperature and soot particle reactivity effects on radical concentrations. The CO and smoke yields were observed for underventilated laminar diffusion flames burning methane and ethene for global equivalence ratio over the range 0.5 to 4.0. A Burke-Schumann type burner with fuel in the center tube and air in the annular region was used. The peak CO yields for methane and ethene, 0.37 and 0.47 respectively, are at least a factor of 100 greater than for overventilated burning. The ratio of CO/CO2 versus for the methane flame is compared with local measurements of this ratio for both overventilated and underventilated laminar diffusion flames and with the results for turbulent natural gas flames quenched in an upper layer. The peak smoke yields for methane at a flow rate of 10 cm3/s and for ethene at a fuel flow rate of 6.4 cm3/s are 0.01 and 0.05, respectively, compared to yields of 0. and 0.028 for the overventilated case. The proportionality between smoke yield and CO yield observed for overventilated burning for a wide range of fuels is found not to be valid for the underventilated case. The chemical makeup and structure of the smoke produced at high equivalence ratio is qualitatively different from smoke produced under overventilated conditions; the smoke is mainly organic rather than graphitic and it has an agglutinated structure rather than an agglomerate structure with distinct primary spheres usually observed in overventilated burning.