Formation of Nitrogen Oxides
Nitric oxide is the major component of NOx emissions from the internal combustion engines. During combustion, three probable sources of NO formation are:
| (i) |
Thermal NO |
: |
By oxidation of atmospheric (molecular) nitrogen at high temperatures in the post-flame burned gases. |
| (ii) |
Prompt NO |
: |
Formed at the flame front within the flame reaction zone. |
| (iii) |
Fuel NO |
: |
Oxidation of fuel-bound nitrogen at relatively low temperatures |
Thermal NO is the dominant source of nitrogen oxides in IC engines.
Thermal NO
NO is formed in the high temperature burned gases behind the flame front. The rate of formation of NO increases exponentially with the burned gas temperature although, it is slower compared to the overall rate of combustion.
Kinetics and Modelling of Thermal NO Formation
The following three reactions commonly referred to as the extended Zeldovich mechanism govern the formation of thermal NO
k1, k2 and k3 are the reaction rate constants for the forward reactions and k-1, k-2 and k-3 are for the reverse reactions
The original Zeldovich mechanism consisted of the first two reactions (2.1) and (2.2) and the third reaction (2.3) was added by Lavoie. The forward part of the first reaction (2.1) is highly endothermic with high activation energy of about 314 kJ /mol and is a rate determining reaction in NO formation.
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