Combustion Phenomena of High Temperature Air Combustion - High Temperature Air Combustion: From Energy Conservation to Pollution Reduction - page 78

the scheme employed here, but NO can be thought to be formed exclusively through
the extended Zel'dovich mechanism:

N 2 + O → NO + N (R224)

O 2 + N → NO + O (R225)

N + OH → NO + H (R226)

because reactions other than these three occur with much smaller rates.
Although the NO formation rate increases as α decreases, the NO concentration
at α = 0.06 is 57.0 ppm, while at α = 0.15 it reaches 114 ppm. The NO concentration
is the product of the NO production rate by the reaction time. The reaction time is
the ratio between the thickness of the NO production zone (because the concentration
is that at 10 mm after the flame front, this thickness is 10 mm in all cases) and the
flow velocity. Therefore, the low NO concentration in the α = 0.06 flame is due to
the large burning velocity at that condition.
The above discussion on NO formation is valid for other values of T a . The same
three reactions are the main NO-forming steps, and NO concentration is lower for
smaller α. In Figure 2.53 the amount of NO emitted per unit of supplied fuel is
plotted for various values of T a and α. It can be seen that when an adiabatic flame
temperature of 2200 K or higher is required, the NO emission is very sensitive to
the dilution of the mixture with combustion gases, with optimum values of α being
around 0.10 or lower. The same analysis was done for mixtures of equivalence ratio
of 0.8 and 1.4. Fuel flux and NO emission varied with α and T a in a similar way as
the conditions of the equivalence ratio = 1.

2.3.2.3 Summary

The combustion of preheated and diluted mixtures of methane and air was simulated
through a flat flame model with the following conclusions:

1. For cases where the adiabatic flame temperature is 2200 K or higher, the

dilution of the mixture with combustion gases and the preheating to high
initial temperatures were found to be effective in enlarging the fuel flux,
the amount of fuel burned per unit area and time. However, to minimize
the NO emission an optimization of the values of both the dilution ratio
and preheating temperature is required, to have a large fuel flux with a low
NO emission. In general, the reason preheated and diluted flames show high
performance is that preheating increases the burning velocity by allowing
self-ignition of the mixture. Dilution inhibits the combustion reactions but
this effect is compensated by enlargement of the reaction zone thickness.

2. For cases where the adiabatic flame temperature is 2000 K or lower, the

fuel flux and NO emission are virtually unaffected by preheating and
dilution of the mixture. In these flames, the effect of preheating and
dilution is just that of stabilizing the combustion of mixtures that could
not burn stably under standard conditions.