Discussion
Statement I: Thermal nitrogen oxide formation is insensitive to flame temperature.
Statement II: The reaction between atmospheric nitrogen and oxygen becomes significant above about 1300 degrees.
In the light of the above statements, choose the correct answer.
Answer: A
Correct answer: Statement I is incorrect but Statement II is correct
Statement I is incorrect, and the reason shows why. Thermal NOx formation is extremely sensitive to flame temperature: the Zeldovich mechanism has a high activation energy, so the rate rises steeply above about 1300 degrees Celsius and roughly doubles for each further increment of a few tens of degrees.
Statement II is correct as stated. That sensitivity is what makes temperature the primary control variable - low-NOx burners, staged combustion, flue gas recirculation and water injection all work by lowering or flattening the peak flame temperature rather than by treating the exhaust.
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