Melte and Pratt [426] proposed the first intermediate mechanism for NOx formation from molecular nitrogen (N2) via nitrous oxide (N2O). Nitrogen enters combustion systems mainly as a component of the combustion and dilution air. Under favorable conditions, which are elevated pressures and oxygen-rich conditions, this intermediate mechanism can contribute as much as 90% of the NOx formed during combustion. This makes it particularly important in industrial application such as gas turbines and compression-ignition engines. Because these devices are operated at increasingly low temperatures to prevent NOx formation via the thermal NOx mechanism, the relative importance of the N2O-intermediate mechanism is increasing. It has been observed that about 30% of the NOx formed in these systems is mainly related to the N2O-intermediate mechanism.
The N2O-intermediate mechanism may also be of importance in systems operated in flameless mode (for example, diluted combustion, flameless combustion, flameless oxidation, and FLOX systems). In a flameless mode, fuel and oxygen are highly diluted in inert gases so that the combustion reactions and resulting heat release are carried out in the diffuse zone. As a consequence, elevated peaks of temperature are avoided, which prevents thermal NOx. Research suggests that the N2O-intermediate mechanism may contribute about 90% of the NOx formed in flameless mode, and that the remainder can be attributed to the prompt NOx mechanism. The relevance of NOx formation from N2O has been observed indirectly and theoretically speculated for a number of combustion systems, by a number of researchers [40], [122], [207], [632], [641].
The simplest form of the mechanism [426] takes into account two reversible elementary reactions:
 | (9–65) |
 | (9–66) |
Here, 
is a general third body. Because the first
reaction involves third bodies, the mechanism is favored at elevated
pressures. Both reactions involve the oxygen radical 
, which makes the mechanism
favored for oxygen-rich conditions. While not always justified, it
is often assumed that the radical 
atoms originate solely from
the dissociation of molecular oxygen,
 | (9–67) |
According to the kinetic rate laws, the rate of NOx formation via the N2O-intermediate mechanism is
 | (9–68) |
To solve Equation 9–68, you will need to have first calculated [O] and [N2O].
It is often assumed that N2O is at quasi-steady-state (that is,
), which implies
 | (9–69) |
The system of Equation 9–68 – Equation 9–69 can be solved for the rate of NOx
formation when the concentration of N2, O2, and
, the kinetic rate constants for Equation 9–65 and Equation 9–66, and the equilibrium constant of Equation 9–67 are known. The appearance of NO in Equation 9–66 entails that coupling of the N2O
mechanism with the thermal NOx mechanism (and other
NOx mechanisms).
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In the above expressions, 
and 
are the forward rate constants of Equation 9–65 and
Equation 9–66, and 
and 
are the corresponding reverse rate constants. The units for 
, 
, and 
are m3/mol-s, while 
has units of
m6/mol2-s.