[audiofish]

There are multiple NOx sources during combustion.

Fuel NOx is only one of them, which you quite rightly point out is not dominant in methane combustion due to the rarity of nitrogen in the fuel.

The dominant source in methane combustion is thermal NOx, which forms due to the extreme temperature of the combustion, causing atmospheric nitrogen to decompose and react with atmospheric oxygen.

Source: https://www.epa.gov/sites/default/files/2020-09/documents/1....