Discussion
Consider the following statements about nitrogen oxides from combustion:
1. Thermal NOx forms from atmospheric nitrogen at high flame temperature.
2. Fuel NOx arises from nitrogen bound in the fuel.
3. Lowering the peak flame temperature increases thermal NOx.
Which of the statements given above is or are correct?
Answer: A
Correct answer: 1 and 2 only
Statement 1 is correct: thermal NOx forms from atmospheric nitrogen and oxygen at high flame temperature, by the Zeldovich mechanism. Statement 2 is correct: fuel NOx arises from nitrogen chemically bound in the fuel, and matters for coal and heavy oil rather than for gas.
Statement 3 is false and inverts the dependence. Lowering the peak flame temperature decreases thermal NOx, sharply, which is exactly why staged combustion, flue gas recirculation and low-NOx burners all work by suppressing the peak temperature.
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