How significant is $\ce{NO$_x$}$ formation from $\ce{O2}$ (reactant) and $\ce{N2}$ decomposition from simply heating the two as opposed to from $\ce{CO2}$, $\ce{H2O}$, (and $\ce{N2}$) dissociation? In other words, which of the two pathways for $\ce{NO$_x$}$ formation is more significant (I imagine this will vary with equivalence ratio):

  1. $\ce{O}$ and $\ce{N}$ radical from $\ce{N2}$ and $\ce{O2}$ dissociation
  2. $\ce{O}$ radical from $\ce{CO2}$ and $\ce{H2O}$ dissociation with $\ce{N}$ radical

My feeling is that the second becomes more significant as equivalence ratio increases (more fuel means more $\ce{O2}$ is consumed in combustion).

  • $\begingroup$ I have updated your post with chemistry markup. If you want to know more, please have a look here and here. Please do not use markup in the title field, see here for details. $\endgroup$ – Martin - マーチン Dec 3 '14 at 5:44

In situations where NOx originiates form $\ce{N2}$, such as lightning or a high temperature furnace, the Zeldovich mechanism dominates:

$$\ce{O2 -> O +O}$$

$$\ce{O + N2 -> NO +N}$$

$$\ce{O2 + N-> NO +O}$$

If a combusting fuel actual contains nitrogen, other mechanisms can be involved.

  • $\begingroup$ Generally speaking, would the proportion of NOx formed from $N_2$ and $O_2$ above (air decomposition) decrease as we increase the equivalence ratio (oxygen will favour reaction with fuel as opposed to decomposition)? $\endgroup$ – Yandle Dec 6 '14 at 19:53

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