MECH6019 Chap.10 Nitrogen Oxide Formation and Control
Nitrogen Oxide Formation and Control
Nitrogen oxide control connects formation in combustion equipment, transformation after release and chemical treatment in exhaust. Nitric oxide and nitrogen dioxide are related but not interchangeable, and emitted species can participate in later ozone and secondary-particle chemistry. Stack control uses a source mass balance, while ambient interpretation adds transport, reaction and exposure.
Keep those boundaries separate.
Thermal NOx begins mainly with atmospheric nitrogen in sufficiently hot regions and responds to peak temperature, oxygen and hot residence time. Prompt NOx arises rapidly near flame fronts through radical chemistry. Fuel NOx begins with nitrogen bound in the fuel.
These pathways may occur together, but their relative importance determines whether fuel choice, staging, recirculation or another formation control can reach the cause.
Combustion modification acts before NOx exists. Staged combustion redistributes oxygen and reaction zones. Flue-gas recirculation changes flame temperature and composition.
These changes can affect carbon monoxide, unburned fuel and efficiency, so verification must be multi-pollutant and tied to equal useful output. Raw concentration also needs oxygen correction or mass flow when total exhaust changes.
Selective reduction acts downstream after formation.
A reducing reagent reacts with nitrogen oxides, with catalytic and non-catalytic approaches operating over different useful temperature ranges. Too cold can give slow reaction or slip; too hot can promote unwanted chemistry. Catalysts shift the window but add activity loss, poisoning, pressure drop and replacement considerations.
Mixing and residence time remain essential.
Convert ppmv to molar flow before sizing reagent. Multiply total compatible dry-gas flow by NOx mole fraction to obtain inlet and outlet molar rates, then subtract for removal. Apply the stoichiometric reagent ratio for the selected reaction and any justified operating excess. Compare calculated feed with measured slip.
If more reagent raises slip without improving removal, temperature, catalyst or distribution may be limiting.
A complete recommendation compares outlet NOx, reagent use, slip, energy, catalyst or waste burden and co-pollutant changes across the operating range. It names the formation route addressed by upstream measures and the reaction window required by downstream treatment.
This prevents one compliant concentration from being treated as proof that the whole control system performs as intended.
What this chapter covers
- 01
NO and nitrogen dioxide
- 02
Atmospheric reactions
- 03
Thermal NOx
- 04
Prompt and fuel NOx
- 05
Combustion modification
- 06
Selective reduction
Worked example · free
Convert ppmv to NOx molar removal
- 1Find 0.0060 kmol/h inlet.
- 1Find 0.00096 kmol/h outlet.
- 1Subtract 0.00504 kmol/h.
- 1Verify 84% reduction.
Key terms
- Selective reduction
- Reaction that converts nitrogen oxides using a reducing reagent.
- Reagent slip
- Unreacted reagent emitted after a control process.
- Temperature window
- Range in which a control reaction has useful rate and selectivity.
- Flue-gas recirculation
- Return of exhaust to modify flame temperature and composition.
Nitrogen Oxide Formation and Control FAQ
How do formation and treatment controls differ?
Staging and recirculation alter flame conditions before NOx forms, while selective reduction reacts with NOx downstream after formation. The first group can change carbon monoxide or efficiency; the second introduces reagent, temperature-window, slip and catalyst considerations that require separate verification.
Why is injection temperature important?
Outside the useful reaction window, conversion falls or unwanted reactions and reagent slip increase. A technology label alone does not define performance. Mixing, residence time, catalyst condition and the reagent-to-NOx ratio determine whether the reaction can use the nominal temperature effectively.
What should verification include?
Measure NOx on a consistent basis alongside reagent slip, energy, catalyst or waste impacts and any co-pollutant change caused by combustion modification. Convert concentration to molar flow at each load so chemical duty and reagent performance remain comparable.
Exam move
Sort every control into formation change or post-combustion reaction. Convert reported ppm to molar flow before calculating reagent demand. Build a pathway table for thermal, prompt and fuel NOx with nitrogen source, reaction zone and sensitive variable. Use it to diagnose cases before selecting staging, recirculation, fuel intervention or downstream reduction.
For combustion changes, track corrected NOx, carbon monoxide, oxygen and useful output across load. For selective reduction, draw the temperature window and explain what happens when operation is too cold or too hot. Add reagent mixing, residence time, catalyst activity and slip as separate constraints. Practise converting dry gas flow and ppmv to inlet and outlet NOx kilomoles per hour, then subtract for removal.
Only after that should the reaction stoichiometry determine reagent demand. Compare additional reagent with measured slip; if slip rises without improved conversion, diagnose temperature, catalyst or distribution rather than simply increasing feed. Keep stack chemistry separate from ambient transformation: source control uses the stack balance, while air-quality effect adds reaction, transport and exposure.
In final revision, write one upstream and one downstream recommendation for the same source and identify what each can and cannot control. Close with a verification set that includes target removal, co-pollutants, reagent, energy and residual or catalyst burden.
