A study is made of the influence chemical structure of fuel has
upon combustion performance through tracing the generation of carbon
monoxide and oxides of nitrogen in lean, premixed, hydrocarbon-air
flames. The study includes both analytical and experimental investi-
gations.
On the analytical side, a kinetic model is developed to predict
both CO and NO time-histories in one-dimensional, premixed flames.
The model is based upon the assumption of partial equilibrium in the
post-flame zone while the fuel oxidation in the main reaction zone is
allowed for by using a global oxidation rate equation. NO formation
is assumed to be entirely via the Zeldovitch mechanism and to start
in the post-flame zone. The utility of the model is judged through
comparison between theoretical results and experimental data.
On the experimental side, a simple burner system, supporting a
one-dimensional premixed flame was designed and built. All fuels
selected for investigation were pure hydrocarbons representing the
main hydrocarbon types usually found in practical fuels; namely
paraffins, olefins, naphthenes and aromatics. The hydrogen-to-carbon
ratio ranged from 1 to 2.67 and the carbon number from 3 to 12. The
experiments were performed at 1,2 and 3 atm pressure levels and 140°C
inlet temperature, while the equivalence ratio was in the range 0.6
to 0.9. Flames were sampled for most stable species by a water-
cooled stainless steel sampling probe.
The experimental results show that the fuel structure signifi-
cantly affects CO time-histories in the investigated flames mainly
through influencing its generation rather than its burnout. CO
burnout is shown to be mainly controlled by radical recombination
processes, and the experimentally derived CO global oxidation rate
equations are found not to be universally applicable. The results
also show that the fuel structure influences prompt NOx formation
within, and very near, the main reaction zone but that it does not
influence post-equilibrium NOx formation if account is taken of
differences in the flame temperatures. N02 is found to constitute a
large percentage of total NOx measured especially at lower temperature
and equivalence ratios.
Comparison between experimental and theoretical results show
that the prescribed kinetic model can satisfactorily predict CO levels
for different fuels and under different conditions if the fuel oxidation
global rate equation is correctly defined for different fuels. On the
other hand, agreement between predicted and measured NO profiles has
been obtained at atmospheric pressure only. At high pressure, the
predicted levels were much smaller than those measured experimentally,
and this disagreement is attributed to the fact that proper account is
not taken of the NO and N02 formation kinetics in the main reaction
zone
