Discussion
Oxides of nitrogen from combustion arise chiefly as
Answer: B
Correct answer: Thermal NOx from atmospheric nitrogen
Most oxides of nitrogen from combustion are thermal NOx, formed when nitrogen and oxygen in the combustion air react at high temperature by the Zeldovich mechanism. The reaction becomes significant above about 1300 degrees Celsius and rises steeply thereafter, which is why the formation depends on flame temperature far more than on the fuel.
Fuel NOx, from nitrogen chemically bound in the fuel, matters for coal and heavy oil but is secondary for gas. The emission leaves the stack largely as nitric oxide, which oxidises to nitrogen dioxide in the atmosphere, so it is not emitted directly as \( \mathrm{NO_2} \).
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