Fundamental Mechanisms for CO and Soot Formation. Final Report.
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.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899