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I understand that the byproducts of burning methane to be carbon dioxide and water, and in low oxygen situations, carbon monoxide.

$\ce{CH4 + 2 O2 -> CO2 + 2 H2O}$

$\ce{2 CH4 + O2 -> 2 CO + 4 H2}$

Where does nitrogen come into this? How does burning methane produce nitrogen oxides?

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    $\begingroup$ $\ce{N2 + O2 <<=>[high T] 2 NO}$ $\endgroup$
    – Poutnik
    Jan 30, 2023 at 14:45
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    $\begingroup$ You are assuming the composition of natural gas only includes methane, although in reality it includes a variety of other gases like nitrogen in smaller quantities. $\endgroup$
    – Sam202
    Jan 30, 2023 at 14:47
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    $\begingroup$ @Sam202 .... and in air used for burning in big quantities. $\endgroup$
    – Poutnik
    Jan 30, 2023 at 14:48
  • $\begingroup$ @Poutnik so just High temps? Does an electric stove also produce NO? Just trying to understand the politics going on in NY right now. $\endgroup$
    – rtaft
    Jan 30, 2023 at 14:52
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    $\begingroup$ High as very high. Like in combustion engines or gas burners used for melting glass, to get the picture like t > 1500 deg C. At low T like in stoves is the reaction unfavoured and mainly kinetically frozen. $\endgroup$
    – Poutnik
    Jan 30, 2023 at 15:00

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Because air contains nitrogen

One of the problems with looking at a real world chemical reaction by looking at the reaction formula for the major reaction going on is that you miss the possibility of side reactions.

In this case it looks like you are reacting methane with pure oxygen. But very few things that burn methane use pure oxygen. Most devices that burn methane (bunsen burners, gas cookers, boilers, some vehicles and plenty of other devices) use air not pure oxygen and air is ~80% nitrogen.

Nitrogen isn't that reactive under most normal circumstances. But it is a little reactive and can form nitrogen oxides if it gets hot enough. In low temperature flames the level is usually fairly minimal (like the flames in a gas cooker or a bunsen burner). In hotter flames the yield of nitrogen oxides (NOx is an often used all-encompassing term) can be significant. For example internal combustion engines have much hotter combustion than simple flames and tend to produce worrying amounts (though there are not many vehicles that use methane, but typical petrol (gasoline) engines produce a lot of NOx in the same way as a methane powered engine would). This is a big reason why most vehicles have to have catalytic converters.

Basic point: flames are hot and can encourage side reactions. If you are using air, those reactions will often involve nitrogen.

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  • $\begingroup$ Here is a starting point for further research. If your fuel is coal or oil, nitrogen-containing compounds in the fuel contribute substantially to the NOx load. Natural gas also contains dinitrogen, so even burning it in pure oxygen would produce NOx. The amount of NOx depends on various parameters of combustion, not just the temperature. $\endgroup$
    – Karsten
    Jan 31, 2023 at 1:34
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    $\begingroup$ @Karsten Raw natural gas sometimes contains small amounts of nitrogen but CO2 is usually higher. But the gas we use does not, especially if it has been liquefied fro transport. The production process usually leaves fairly pure methane. So the nitrogen content is really not relevant not relevant to this question. $\endgroup$
    – matt_black
    Jan 31, 2023 at 9:20

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